14                 Hazard to life

 

PART C            Assessment for Disinfection Facilities

 

Introduction

Background

14C.1            The Stonecutters Island Sewage Treatment Works (SCISTW), with a design capacity of 1.7 million cubic metres per day, is one of the key components of the Habour Area Treatment Scheme (HATS).  At present, sewage collected from the urban Kowloon, Kwai Tsing, Tseung Kwan O and north-eastern Hong Kong Island is treated at SCISTW by chemically enhanced primary treatment (CEPT) before discharge into the western harbour through a submarine outfall.

14C.2            Under HATS Stage 2A, part of the Project is to expand the existing chemical treatment facilities at the Stonecutters Island Sewage Treatment Works (SCISTW), so to cope with the projected flow and load under the ultimate development scenario for the whole HATS catchment and provides disinfection facilities for the sewage collected before discharging into the harbour.  With reference to the Agreement No. CE 7/2005 (EP) “Harbour Area Treatment Scheme Environmental Impact Assessment Study for the Provision of Disinfection Facilities at Stonecutters Island - Investigation” (HATS ADF EIA), the option of purchasing sodium hypochlorite solution for chlorination and sodium bisulphite for dechlorination would be adopted.

14C.3            In accordance with Clause 3.4.8.4 of the EIA Study Brief of the project, a hazard assessment of manufacture, storage, use and transport of dangerous goods (for the disinfection facilities operation) shall be carried out following the criteria for evaluating hazard to life as stated in Annexes 4 and 22 of the Technical Memorandum (TM).  Under the scope of the disinfection facilities operation under the HATS 2A project, two dangerous goods (DG) classified under the Dangerous Goods Ordinance in Hong Kong, namely sodium hypochlorite and sodium bisulphite, are identified.  There will be no on-site generation of DG and hence the risk of the DG manufacturing does not exist.  The potential risk of storage, use and transport of the DG will be assessed in the hazard assessment for the Project under this section.

Legislations, Policies, Plans, Standards, and Criteria

14C.4            As set out in Annex 4 of the EIAO TM, the risk guidelines comprise two components:

·           Individual Risk Guideline: the maximum level of off-site individual risk should not exceed 1 in 100,000 per year, i.e. 1 x 10-5 per year

·           Societal Risk Guidelines is presented graphically in Figure 14C.1, in terms of the “frequency” (F) of “number deaths in the population” (N) from accidents at the facility of concern.  There are three areas:

-           Acceptable, where risks are so low that no action is necessary

-           Unacceptable, where risks are so high that they should usually be reduced regardless of the cost or else the hazardous activity should not proceed

-           ALARP (As Low As Reasonably Practicable), where the risks associated with the hazardous activity should be reduced to a level “as low as reasonably practicable”, in which the priority of measures is established on the basis of practicability and cost to implement versus risk reduction achieved.

General Description of Disinfection Facilities

14C.5            Under Stage 2A, four chlorine contact tanks and a new dechlorination plant (to replace the temporary one provided for HATS advance disinfection facilities (ADF)) will be constructed. The maximum sodium hypochlorite and sodium bisulphite solution storage on-site is about 2,665,000L and 175,000L respectively (compared to 1,800,000L and 150,000L in HATS ADF).  The key elements of the proposed disinfection facilities (Figure 14C.2), of which their operations involve sodium hypochlorite and sodium bisulphite, for the Project include:

·           Chlorination system – (apart from the chlorine contact tanks, will be constructed under the consultancy of HATS ADF project) provision of a sodium hypochlorite solution storage plant and associated dosing system, including

-           Six sodium hypochlorite storage tanks (8m in diameter, 12.5m in height)

-           One day tank for sodium hypochlorite storage (capacity of about 100m3)

-           Pipes in pipe trenches

-           Switch room

-           Chemical feeding and transfer system

-           Bund wall (2m in height)

-           ISO tank unloading area

-           Sodium hypochlorite barge unloading facility

-           Four chlorine contact tanks

 

·           Dechlorination system - provision of a sodium bisulphite solution storage plant and associated dosing system, including

-           Two sodium bisulphite storage tanks

-           Chemical feeding and transfer system

-           Bund wall (1.5m in height)

-           ISO tank unloading area

-           Security fence and gate

 

14C.6            Sodium hypochlorite solution (v/v 10-12%) will be purchased and delivered to SCISTW by sea or by road.  For sea transport, a specially designed barge will be designated for delivery of sodium hypochlorite solution and the sodium hypochlorite solution will be unloaded at the berthing facility at SCISTW at an interval of 2 to 3 times a week.  When weather or other conditions do not allow chemicals delivery by sea, the sodium hypochlorite solution will be delivered by road tankers.

14C.7            Sodium bisulphite solution (v/v 38%) will be purchased and delivered to SCISTW at an interval of 1 to 2 time(s) a week by road tankers and then unloaded to the storage tanks.

14C.8            As shown in Figure 14C.2, road tankers delivering different chemicals will travel along different routes, as follows:

·           Road tanker delivering sodium bisulphite will travel to the dechlorination plant direct via Ngong Shuen Road and will not enter the SCISTW main compound

·           Road tankers delivering ferric chloride will enter the SCISTW main compound via the main entrance located at northeast corner of SCISTW main compound, then travel along the road separating the existing sedimentation tanks and the sludge treatment facilities and then to the ferric chloride tank farm

·           Any road tankers delivering sodium hypochlorite will enter the SCISTW main compound via another entrance located at southern part of the SCISTW main compound, then travel along the road near the Northwest Kowloon Pumping Station and then to the sodium hypochlorite storage tank farm 

 

14C.9            There are other chemicals stored in SCISTW.  These chemicals includes sodium hydroxide[1] (storage amount: 7,500L), ferric chloride[2] (storage amount: 1,900,000L), anionic polymer (storage amount: 31 tonnes) and cationic polymer (storage amount: 82 tonnes).  The hydrochloric acid contained in ferric chloride is incompatible to sodium hypochlorite and sodium bisulphite; chlorine gas and sulphur dioxide gas would be evolved when ferric chloride (hydrochloric acid inside) mixes with sodium hypochlorite and sodium bisulphite respectively.  According to the Material Safety Data Sheet (MSDS) of the chemicals, these chemicals would not generate toxic gas when mix with other chemicals on site.

14C.10            As sodium hypochlorite solution will be purchased from off-site manufacturers rather than produced on-site by electrochlorination process, hydrogen, which is a by-product of the electrochlorination process, will not be involved in the Project and hence are not be considered in the hazard assessment.

14C.11            Apart from ferric chloride that would interact with sodium hypochlorite or sodium bisulphite to produce toxic gas (the risk of such interaction was assessed in this hazard assessment and presented below), sodium hydroxide, anionic polymer and cationic polymer does not have hazardous interaction with sodium hypochlorite or sodium bisulphite.

14C.12            Based on the above, the potential risk of storage, use and transport of sodium hypochlorite and sodium bisulphite will be assessed in the hazard assessment for the Project.  Ferric chloride, which is a chemical used in the sewage treatment process at SCISTW, is considered in the hazard assessment because its hazardous interaction with sodium hypochlorite and sodium bisulphite.

14C.13            It should be noted that there are other new plant facilities to be constructed under Stage 2A (Figure 14C.2 and Figure 14C.7).  These facilities includes:

l        Influent pumping station with eight centrifugal pumps

l        Chemical enhanced primary treatment (CEPT) facilities

-           Two rapid mixing chambers

-           Four flocculation tanks

-           Eight double-deck sedimentation tanks

l        Sludge treatment facilities

-           One sludge holding tank

-           Six sludge dewatering centrifuges

-           Two sludge cake storage silos

l        Effluent tunnel and its associated chambers

l        Odour control facilities – provision of a two-stage odour treatment (i.e. biofilter plus activated carbon)

l        Electrical substation and switch gear building

 

14C.14            The proposed influent pumping station, CEPT facilities and the sludge treatment facilities are the extensions of the existing SCISTW facilities to cope with the increased flow from Stage 1 to Stage 2A.  Operation of these new facilities would be the same as the existing SCISTW facilities and the chemicals involved in the operation of these new plant facilities are also the same and are stated in Section 14C.9.

14C.15            There is no special concern on the effluent tunnel and its associated chambers during normal operation as no chemicals or dangerous goods would be involved.  However, there may be concern on the construction activities of the effluent tunnel, especially for drill and blast method.  The risks associated with the construction of the new plant facilities to the disinfection facilities are further addressed in Sections 14C.178 to 14C.182.

14C.16            The proposed odour control facilities consist of a blower house and deodorization units.  Two-stage odour treatment (i.e. biofilter plus activated carbon) are adopted in the deodorization units.  Biofilter is not a chemical while activated carbon is a very inert chemical that does not have hazardous reactions with sodium hypochlorite or sodium bisulphite.  No other chemicals are anticipated.

14C.17            The proposed electrical substation (132kV / 11kV) houses the high voltage equipment and the switch gear building mainly serves the new influent pumping station.  No chemicals or dangerous goods are anticipated in the operation of the substation.  The risks associated with the operation of the electrical substation are further addressed in Sections 14C.177. 

14C.18            Hazard identification exercises and reviews of DG hazardous properties were conducted under the hazard to life assessment in the EIA Study for the Provision of Disinfection Facilities at Stonecutters Island Sewage Treatment Works (referred to as HATS ADF EIA Study) and the current HATS Stage 2A EIA Study.  The hazard identification results show the identified hazardous scenarios to be considered in the hazard to life assessment.  The review of the hazardous properties of DG would help evaluate the potential of various identified hazardous scenarios to cause off-site fatality under the conditions of the current Project.

Assessment Approach

14C.19            The hazard to life assessment consists of the following four tasks:

·           Data / Information Collection: collects relevant data / information which is necessary for the hazard to life assessment

·           Hazard Identification: identifies hazardous scenarios associated with the use of sodium hypochlorite and sodium bisulphite solutions for the HATS 2A.

·           Frequency Estimation and Consequence Analysis: estimates the frequencies of the identified hazardous scenarios, with consideration of the HATS 2A specific conditions including the proposed precautionary / mitigation measures.  Then, analyses the consequences of the identified hazardous scenarios

·           Risk Evaluation: evaluates the risks associated with the identified hazardous scenarios.  The evaluated risks will be compared with the Criteria for Evaluating Hazard to Life stipulated in Annex 4 of the TM to determine their acceptability.  Risk mitigation measures will be identified if they are needed for the compliance of the Criteria.

 

14C.20            These are elaborated below.

Data Collection

I.            Overview

14C.21            The following relevant data / information were collected:

·           Layout plans of the proposed chlorination and dechlorination systems

·           Operations associated with the use of sodium hypochlorite and sodium bisulphite for disinfection at SCISTW

·           Chemical properties of sodium hypochlorite and sodium bisulphite

·           Population data around SCISTW

·           Meteorological data (including atmospheric stability class, wind speed and wind direction)

 

14C.22            The layout plans of the SCISTW, chlorination system and dechlorination system are shown in Figures 14C.2, 14C.4 and 14C.5.  The other pieces of information are summarised below.

II.          Activities involving Sodium Hypochlorite

14C.23            Sodium hypochlorite solution (v/v 10-12%) will be used as the disinfection agent.  It will be purchased and delivered to SCISTW by a specially designed delivery barge and unloaded at the berthing facility.  It is estimated that there would be 165 barge deliveries per year; sodium hypochlorite solution will be delivered by vehicles when weather or other conditions do not allow chemicals delivery by sea.  Sodium hypochlorite solution will be transferred to the storage tanks through a designated and isolated piping system laid within pipe trench.  Then the chemical will be pumped from the storage tanks to a day tank next to the flow distribution chamber.  The sodium hypochlorite solution from the day tank will then combine with carrier water (effluent) and dose to the flow distribution chamber through a diffuser installed below effluent surface.  The maximum storage amount for sodium hypochlorite solution on-site for the 2A operation is estimated to be about 2,665,000L.

14C.24            If all the sodium hypochlorite deliveries to SCISTW are made by barge, there will be 165 barge deliveries per year.  The chemical will be delivered by road tankers when weather or other conditions do not allow delivery by barge.  In the past 10 years, there were 146 days that typhoon signal (signal no. 1 of higher) was issued.  This record indicates that poor sea conditions may occur at about 4% of time in a year.  Also, regular maintenance and repair would be needed for the specially designed delivery barge.  In order to maximize the use of barge for sodium hypochlorite delivery, such maintenance and repair will be scheduled at periods between two delivery operations.  Therefore, it is assumed that the barge would be unavailable for delivery operation for only 1% of time in a year due to maintenance and repair.

14C.25            Based on the above, it is expected that only about 5% of the sodium hypochlorite (equivalent to 6 barge deliveries) used in the disinfection operation would need to be delivered by road tanker.  Nevertheless, a conservative assumption of 10% of the sodium hypochlorite (equivalent to 17 barge deliveries) being delivered by road tanker is adopted in this assessment to cover events of temporary suspension of barge services for reasons, such as large-scale emergency repair while a barge replacement is not readily available, even though such events are remote with proper barge maintenance.  Each barge delivery would carry about 500m3 of sodium hypochlorite and this is equivalent to the volume carried by about 30 road tankers.  Therefore, the number of sodium hypochlorite deliveries by road tanker is estimated to be 30 x 17 = 510 per year.

III.        Activities involving Sodium Bisulphite

14C.26            Sodium bisulphite solution (v/v 38%) will be used as the dechlorination agent and dosed at the downstream end of the culvert (Chamber No. 15) before the effluent is discharged into the outfall system.  It will be purchased and delivered to SCISTW at an interval of 1 to 2 time(s) a week only by road tankers (6 road tankers in total) and then unloaded to the storage tanks.  Therefore, the number of sodium bisulphite deliveries by road tanker will be 6 x 52 = 312 per year.  The maximum storage amount for sodium bisulphite solution on-site for 2A operation will be about 175,000L.

14C.27            It should be noted that the above activities involving sodium hypochlorite and sodium bisulphite are commonly practiced in many sewage treatment works in other countries such as United States and Japan, including the Deer Island Sewage Treatment Plant located at Boston, US, which has a comparable capacity and scale to SCISTW.

IV.        Chemical Properties of Sodium Hypochlorite and Sodium Bisulphite

14C.28            Both sodium hypochlorite and sodium bisulphite are corrosive substances; contact with either chemicals would cause the following effects for the following potential routes:

·           Dermal: burning, blistering and tissue destruction for prolonged contact

·           Eyes: corrosion, severe damage

·           Ingestion: chemical burns to oesophagus and to stomach lining

 

14C.29            Sodium hypochlorite and sodium bisulphite are not acutely toxic, flammable, or explosive substances.  However, hazardous gas would be generated if sodium hypochlorite or sodium bisulphite accidentally mixes with incompatible chemicals.  Ferric chloride is the only chemical stored in SCISTW that is incompatible with sodium hypochlorite or sodium bisulphite, due to the hydrochloric acid inside the ferric chloride solution.

14C.30            The following scenarios may occur in the SCISTW disinfection operation if the incompatible chemicals are mixed together:

·           Sodium hypochlorite mixes with ferric chloride: generate chlorine gas

·           Sodium bisulphite mixes with ferric chloride: generate sulphur dioxide gas

 

14C.31            If sodium hypochlorite mixes with sodium bisulphite, heat would be evolved, but no toxic gas would be generated. 

14C.32            The only identified hazardous scenario having potential to cause off-site fatality is mixing of incompatible chemicals.  Under the circumstances of this Project, errors in chemical unloading, which will only be conducted during daytime, is the only possible cause for this hazardous scenario (detailed discussion on the hazardous scenario identification and assessment is presented in later sections).  Since the hazardous scenario can only occur during daytime, daytime population and meteorological data was collected and presented below.

V.          Population Data

14C.33            The existing ferric chloride tank farm and the sodium hypochlorite tank farm are located at the northeast and southeast parts of the SCISTW main compound respectively.  At northeast of the ferric chloride tank farm is the sea.  The land use to the north of the SCISTW is industrial in nature and comprises the Container Terminal no. 8, the COSCO HIT Terminal Building, bus depot, shipyards and storage areas.  Adjacent to the south-eastern boundary of the SCISTW main compound is the Government Dockyard at the seaside.  The FSD Diving Training Centre, which is under construction, is located adjacent to the east boundary of the SCISTW main compound. The Ngong Shuen Chau Barracks are located at the south of Stonecutters Island.

14C.34            For the proposed dechlorination plant accommodating sodium bisulphite storage tanks, it will be located near the Ngong Shuen Chau Barrack and Container Port Road South.

14C.35            Residential Population was estimated based on Population Census data.  Planning Department was contacted to obtain population data at locations accommodating non-residential uses surrounding SCISTW.  However, Planning Department indicated that there was no detailed breakdown for the population figures.  In order to obtain the best estimate of the population, data were collected from the following paths: (i) information obtained from government departments and private companies, (ii) references from previous EIAs or government reports, and (iii) on-site observation and communication with on-site personnel.  Photo documentation of the site survey is presented in Appendix 14C.1.  Populations in vicinity of 2A are listed in Table 14C.1, while the locations of the population are shown in Figure 14C.6.

14C.36            Population will not spend all time staying at the locations in vicinity of the disinfection facilities.  For example, workers will not stay in industrial building for all 12 hours of daytime in all 365 days of a year.  The risk assessment will be over-conservative if it is assumed that the locations are occupied all the time.  Therefore, occupancy factor for population is estimated to provide a more realistic population estimation for the risk assessment.  The estimation of occupancy factor and outdoor population fraction of various locations are presented in Appendix 14C.2.

14C.37            The surveyed locations have covered all the locations could be impacted by the hazardous events occurred in disinfection facilities operations.  Further details are given in later part of this section.

14C.38            Traffic data of roads near SCISTW were obtained from the Annual Traffic Census 2005 of Transport Department to estimate the road population.  For Lai Po Road, Ngong Shung Road, Mei Ching Road and Hing Wah Street West, traffic data was not available from the Traffic Census.  Therefore, traffic population of these roads was estimated based on data obtained from site survey conducted by the Consultants during the peak hour (8am to 9am).

14C.39            The estimated road and railway population is presented in Table 14C.2.

Table 14C.1                  Population in vicinity of 2A

Location

Distance to SCISTW1 (m)

Type of Land Use

Land Use

Estimated

Population during Daytime

Occupancy Factor during Daytime

Fraction of Outdoor Population

Remark

1

288

G/IC

Government Dockyard

18002

0.57

0.1

·       Population data provided by manager of the Government Dockyard

2

658

G/IC

Ngong Shuen Chau Barracks

1

0.5

·       Population data provided by Security Bureau

·       It is assumed that population in barracks needs to stay on site all the time (i.e. occupancy factor = 1)

·       A factor of occupancy factor of 0.5 is assumed since the barracks personnel would be trained outdoor

3

247

Industrial

West Kowloon Refuse Transfer Station

104

0.57

0.1

·       Population data provided in returned questionnaire

4

151

G/IC

FSD Diving Training Centre

50

1

0.5

·       According to the layout plan of the FSD Diving centre, there will be totally 18 beds provided for trainees training at the training centre. Population of 50 people is therefore assumed during daytime taking into account the presence of other staff members such as trainers and other supporting staff

·       A factor of occupancy factor of 0.5 is assumed since the trainee would be trained outdoor

·       A conservative occupancy factor of 1 is assumed since trainee may need to stay on site all the time 


5

370

Industrial

Wang Tak Building

150

0.57

0.1

·       Population data provided in returned questionnaire

·       The estimated gross floor area (GFA) of the Wang Tak Building is 4032m2, and therefore the worker density is estimated to be 0.0372ppl/GFA-m2.  This estimated worker density is used to estimate the population of other industrial buildings nearby, where questionnaire requesting population data was not returned 

260

Industrial

Ocean Ship Building Engineering

533

0.57

0.1

·       Questionnaire to request population data was not returned

·       The estimated GFA of the building is 14,336m2,  taking the worker density as 0.0372ppl/GFA-m2, the population is estimated to be 533

·       The estimated population is considered conservative because the data obtained from communication with on-site personnel indicated a much lower population  (90)

315

Industrial

Choy Lee Shipyard Limited

50

0.57

0.1

·       Questionnaire to request population data was not returned

·       The estimated GFA of the building is 1,344m2,  taking the worker density as 0.0372ppl/GFA-m2, the population is estimated to be 50

·       The estimation population is consistent with the data obtained (population = 40) from communication with on-site personnel indicated 


5

288

Industrial

Yuet Hing Marine Supplies Company Limited

182

0.57

0.1

·       Questionnaire to request population data was not returned

·       The estimated GFA of the building is 4,896m2,  taking the worker density as 0.0372ppl/GFA-m2, the population is estimated to be 182

·       The estimation population is considered conservative because the data obtained from communication with on-site personnel indicated a much lower population (60)

425

Industrial

China Travel Service Parkview Shipyard Building

139

0.57

0.1

·       Questionnaire to request population data was not returned

·       The estimated GFA of the building is 3,744m2,  taking the worker density as 0.0372ppl/GFA-m2, the population is estimated to be 139

·       The estimation population is considered conservative because the data obtained from communication with on-site personnel indicated a lower population (100)

452

Industrial

Hop Hing Marine Industrial (HK) Limited

243

0.57

0.1

·       Questionnaire to request population data was not returned

·       The estimated GFA of the building is 6,528m2,  taking the worker density as 0.0372ppl/GFA-m2, the population is estimated to be 243

·       The estimation population is considered conservative because the data obtained from communication with on-site personnel indicated a much lower population (100)


5

370

Industrial

Kowloon Motor Bus Depot

500

0.57

0.1

·       With reference to the population estimation from previous EIAs; (i) Proposed Headquarters and Bus Maintenance Depot in Chai Wan (2001) -  number of staff working was estimated to be  520 during daytime; (ii) New World First Bus Permanent Depot at Chai Wan (1999) - estimated (maximum) population was 500 persons

·       The population of the Kowloon Motor Bus Depot is estimated to be 500 since this Bus Depot is of similar scale with those in Chai Wan 

6

616

-

Car Park (Open Storage)

30

1

1

·       Based on observation during site survey, the number of people present on site was <10.  As a conservative approach, a population of 30 was assumed

·       Since there is no indoor area at the site, the outdoor population portion is estimated to be 1

7

712

Industrial

Container Terminal 7 and 8

3000

0.57

0.5

·       With reference to the population estimation from previous report South-East Tsing Yi Port Development Planning and Engineering Feasibility Study for Container Terminal No. 9 (1991), it was estimated that there were 1259 workers in Container Terminal 9 

·       Container Terminal 7 and 8 are of similar scale with Container Terminal 9, it is assumed that the population of each Container Terminal is 1500

·       Based on the observation in site survey, it is estimated that there is 50% of population located outdoor.


8

959

-

Car Park (Open Storage)

30

1

1

·       Based on observation during site survey, the number of people present on site was <10.  As a conservative approach, a population of 30 was assumed

·       Since there is no indoor area at the site, the outdoor population portion is estimated to be 1

9

986

Recreational

Lai Chi Kok Park

50

1

1

·       Based on observation during site survey, the number of people present on site was <20.  As a conservative approach, a population of 50 was assumed

·       Since there is no indoor area at the site, the outdoor population portion is estimated to be 1

10

1014

Residential

Mei Foo Sun Chuen Phase 8

989

0.4

0.18

·       Population data obtained from Population Census

11

959

-

Area accommodating Sand Depot and Car Park (Open storage)

30

1

1

·       Based on observation during site survey, the number of people present on site was <10.  As a conservative approach, a population of 30 was assumed

·       Since there is no indoor area at the site, the outdoor population portion is estimated to be 1

12

863

Residential

Hoi Lai Estate

4400

0.4

0.18

·       Population data obtained from Population Census

13

918

Educational

Secondary School

1260

0.56

0.1

·       For Secondary 1 to 5, assume there are 5 classes for each form and 40 students in each class. For Secondary 6 and 7, assume there are 2 classes for each form. Total number of students is (5 x 5 + 2 x 2) x 40 = 1160. Also, it is assumed that there is 100 staff. Total population is estimated to be 1260

14

836

Educational

Primary School

1060

0.56

0.1

·       For Primary 1 to 6, assumed there is 4 classes for each form and 40 students in each class. Total number of students is 6 x 4 x 40 = 960. Also, it is assumed that there is 100 staff. Total population is estimated to be 1060

15

562

Industrial

New World First Ferry

100

0.57

0.1

·       Population data provided in returned questionnaire

16

959

G/IC

Cheung Sha Wan Wholesale Food Market Office

50

0.57

0.1

·       Population of the wholesale food market of 50 is based on on-site observation. The estimation is consistent with the estimation with reference to the previous EIA, Proposed Joint User Complex and Wholesale Fish Market at Area 44, Tuen Mun (2002) - population of 1/3 of the Joint User Complex and Wholesale Fish Market located within the assessment area during night-time and daytime was estimated to be 147 and 20 respectively. The total population of the Tuen Mun Wholesale Fish Market was about 60 during daytime

17

767

-

Open storage

30

1

1

·       Based on observation during site survey, the number of people present on site was <10.  As a conservative approach, a population of 30 was assumed

·       Since there is no indoor area at the site, the outdoor population portion is estimated to be 1

 

1 Distance between the centre point of the location and the ferric chloride tank farm of SCISTW.

2 Due to the confidentiality of population within barracks, the total population of the Government Dockyard and the Ngong Shuen Chau Barracks is shown.

 


Table 14C.2      Road and Railway Population within the Assessment Area

Location

Road Population in Vicinity of SCISTW during Day Time

Container Port Road South

73.0

Hing Wah Street West

9.9

Lai Po Road

37.5

Ngong Shung Road

3.3

Lin Cheung Road

32.4

West Kowloon Highway

198.1

Mei Ching Road

15.6

Sham Mong Road

10.1

KCRC West Rail

78.9

 

14C.40            The raw data of the site survey and detailed calculations of the road population are presented in Appendix 14C.3.

14C.41            A site survey was conducted to gather data for the estimation of marine population in vicinity of SCISTW.  The site survey data and observation, as well as the detailed estimation of marine population are presented in Appendix 14C.4.  Based on the site observation, most of the marine vessels were stationed in three clusters near SCISTW, and about 10% of marine vessels were mobile and travelling at the sea area near the SCISTW.  The mobile marine population was assumed to be evenly distributed at the sea area near the SCISTW.  The estimated population of the population clusters and mobile marine population are presented in Table 14C.3.  The locations of the marine population clusters are shown in Figure 14C.6.

Table 14C.3      Estimated Marine Population in vicinity of SCISTW

Marine Population

Estimated Population

Marine Population Cluster 1 - at the piers of Government Dockyard (location 1)

28

Marine Population Cluster 2 - at seawall near open storage area and New World First Ferry (locations 17 and 15)

53

Marine Population Cluster 3 - at pier of Wholesale Food Market Office (location 16)

29

Mobile marine population

28

Meteorological Data

14C.42            Meteorological data are required for consequence modelling and risk calculation.  Consequence modelling (dispersion modelling) requires wind speed and stability class to determine the degree of turbulent mixing potential whereas risk calculation requires wind-rose frequencies for each combination of wind speed and stability class.

14C.43            Meteorological data at Ching Pak House (Tsing Yi) weather station, which is closest to SCISTW, of year 2005 was obtained from which wind speed, stability class, weather class and wind direction were available.  This data represents the weather conditions for the whole year in 2005 and has already taken into account of seasonal variations, and is therefore considered applicable for the assessment.

14C.44            Table 14C.4a shows the wind speed-stability frequencies.

Table 14C.4a    Stability Category-Wind Speed Frequencies at Ching Pak House Weather Station (day-time)

 

Stability Class

 

Wind Speed (m/s)

A

B

C

D

E

F

G

Total (%)

0.0-1.7

0.71

5.68

0.02

3.33

0.02

4.41

0

14.18

1.8-3.2

0.32

8.26

5.46

5.73

3.36

1.99

0

25.11

3.3-5.2

0

5.71

15.57

14.06

2.21

0

0

37.56

5.3-8.2

0

0

3.56

16.92

0

0

0

20.48

Over 8.3

0

0

0.39

2.28

0

0

0

2.67

All (%)

1.03

19.66

25

42.33

5.59

6.39

0

100

 

14C.45            From the data presented in Table 14C.4a, 6 combinations (2B, 1D, 4C, 7D, 1F and 3E) of wind speed and stability class were chosen as the representative daytime meteorological conditions.  These combinations are considered adequate to reflect the full range of observed variations in these quantities.  It is not necessary and efficient to consider every combination observed.  The principle is to group these combinations into representative weather classes, which together cover all conditions observed.

14C.46            Once the weather classes have been selected, frequencies for each wind direction for each weather class can then be determined.  These frequency distributions are given in Table 14C.4b for the daytime meteorological conditions.

Table 14C.4b    Weather Class-Wind Direction Frequencies at Ching Pak House Weather Station (day-time)

 

Weather Class

 

Direction

2B

1D

4C

7D

1F

3E

Total

0 – 30

5.18

0.55

7.56

3.65

0.46

0.84

18.24

30 – 60

0.66

0.39

0.16

0.02

0.27

0.14

1.64

60 – 90

1.35

0.14

1.14

0.39

0.34

0.09

3.45

90 – 120

1.32

0.34

1.62

0.48

0.16

0.25

4.18

120 – 150

1.71

0.30

3.11

0.34

0.68

1.05

7.19

150 – 180

1.62

0.34

1.53

0.73

0.78

0.59

5.59

180 – 210

0.71

0.21

1.23

0.21

0.18

0.30

2.83

210 – 240

1.12

0.14

4.43

1.76

0.30

1.21

8.95

240 – 270

0.84

0.16

3.93

2.47

0.37

0.91

8.68

270 – 300

0.96

0.21

4.45

4.59

0.09

0.68

10.98

300 – 330

2.72

0.34

8.68

6.78

0.50

1.16

20.18

330 – 360

2.49

0.25

2.99

1.74

0.30

0.32

8.08

All

20.68

3.36

40.82

23.15

4.43

7.56

100.00

Hazard Identification

Overview

14C.47            The hazard identification stage consists of three tasks to identify hazardous scenarios associated with the sodium hypochlorite and sodium bisulphite related activities in disinfection facilities operations, as follows:

·           Incident Review

·           Hazard and Operability (HAZOP) Study

·           Review of Previous Relevant Studies

 

14C.48            The risk of the identified hazardous scenarios was quantitatively assessed in the later stage of the hazard to life assessment.

Incident Review

14C.49            Under the HATS ADF EIA Study, incident review for the advance disinfection facilities operation was conducted.  Since the chemicals and activities associated with the disinfection facilities under HATS 2A are same to those of advance disinfection facilities, the results of the conducted incident review are applicable to the current risk assessment.

14C.50            Forty-six relevant incidents were identified in the conducted incident review.  A summary of these incidents is presented in Appendix 14C.7.  It should be noted that no fatality was reported in all identified incidents.

14C.51            The following scenarios were identified from the incident review exercise:

·           Mixing of incompatible chemicals

·           Spillage of sodium hypochlorite from road tanker

·           Spillage of sodium hypochlorite from delivery pipe

·           Spillage of sodium bisulphite from delivery pipe

·           Spillage of sodium hypochlorite from storage tank

 

14C.52            With respect to mixing of incompatible chemicals, 21 incidents have been identified.  The identified causes of such incidents of mixing of incompatible chemicals are as follows:

·           Error in unloading operation

·           Spillage of incompatible chemicals in different tanks

·           Spillage of incompatible chemicals in two different pipelines

·           Spillage of chemical from valve at soft drinks maker

 

14C.53            In conclusion, the incidents identified can be grouped into two types, which will be further assessed in this hazard assessment.

·           spillage of chemicals

·           accidental mixing between incompatible chemicals and release of toxic gas

HAZOP Study

14C.54            A Hazard and Operability (HAZOP) Study has been conducted under the HATS ADF EIA Study to systematically identify potential hazardous scenarios associated with sodium hypochlorite or sodium bisulphite related activities for the advance disinfection facilities operation.  This has also identified corresponding precautionary or mitigation measures to avoid the occurrence of the hazardous scenarios.  Since the operation of the disinfection facilities under HATS 2A are same to the advance disinfection facilities, the identified scenarios by the conducted HAZOP Study are applicable to this risk assessment.  In addition, another HAZOP Study has been conducted under the current HATS Stage 2A EIA Study in January 2008 to systematically identify the potential hazards scenarios associated with the construction and operation of the proposed new plant facilities of the current EIA study.  

14C.55            The identified scenarios by the HAZOP Study are as follows:

·           Spillage of sodium hypochlorite from delivery barge

·           Spillage of sodium hypochlorite from loading hose

·           Spillage of sodium bisulphite from loading hose

·           Spillage of sodium hypochlorite from road tanker

·           Spillage of sodium bisulphite from road tanker

·           Spillage of sodium hypochlorite from delivery pipelines

·           Spillage of sodium bisulphite from delivery pipelines

·           Spillage of sodium hypochlorite from storage tank

·           Spillage of sodium bisulphite from storage tank

·           Mixing of incompatible chemicals

 

14C.56            Again, the incidents identified can be grouped into two types: (1) spillage of chemicals; and (2) accidental mixing of incompatible chemicals.

Review of Previous Relevant Studies

14C.57            Previous relevant studies involving risk assessment for the operations associated with sodium hypochlorite or sodium bisulphite were reviewed to identify hazardous scenarios.

14C.58            It was found that quantitative risk assessment for the operations associated with sodium hypochlorite or sodium bisulphite is sparse.  It may be because these two chemicals are not particularly hazardous, which may be reflected by the observation that no fatality was noted in the incidents associated with these chemicals.  The following previous studies involving evaluation of risk due to sodium hypochlorite / sodium bisulphite were identified and reviewed:

·           Local Study

-           Construction of an International Theme Park in Penny’s Bay of North Lantau together with its Essential Associated Infrastructures – Environmental Impact Assessment, completed in 2000 (Study A)

-           Provision of Disinfection Facilities at Stonecutters Island Sewage Treatment Works – Investigation, completed in 2007 (Study B)

·           Overseas Study

-           Croton Water Treatment Plant – Final Supplemental Environmental Impact Statement (Study C)[3], a study in the US completed in 2004

-           Yamba Sewerage Augmentation – Environmental Impact Statement[4] (Study D), a study in Australia completed in 2005

 

14C.59            The hazard assessment methodology adopted and hazardous scenarios evaluated in the abovementioned studies are described below.

·           Study A - Hazardous scenarios of sodium hypochlorite spillage as well as mixing of sodium hypochlorite with acids were identified in the study.  Only the risk associated with the latter scenario was considered and assessed quantitatively.

·           Study B – Hazardous scenarios of chemicals (sodium hypochlorite and sodium bisulphite) spillage and mixing of incompatible chemicals were identified in the study. 

·           Study C - Sodium hypochlorite was proposed to be used as one of the chemicals in the water treatment process.  No quantitative risk assessment for chemicals was conducted in the Study.  The Study only mentioned mitigation measures, including installation of containment and storing incompatible chemicals in separate areas, would be implemented and concluded that no potentially significant adverse impacts are anticipated to occur from the transport, storage, or usage of the chemicals.

·           Study D - Sodium hypochlorite was proposed to be used in the sewage treatment plant for recycled water reuse.  The Study did not quantitatively assess the risk due to sodium hypochlorite.  Rather, the Study proposed mitigation measures including installation of bunds and warning signs, worker training and use of personal protective equipment.

 

14C.60            From the review of previous studies, two scenarios are noted:

·           Spillage of chemicals

·           Chemical to be used mixes with incompatible chemicals

Results of Hazard Identification

14C.61            The identified hazardous scenarios from incident review, HAZOP Study, and review of previous relevant studies are listed as follows:

·           Spillage of chemicals

-           Spillage of sodium hypochlorite from delivery barge (hazardous scenario 1)

-           Spillage of sodium hypochlorite from loading hose (hazardous scenario 2)

-           Spillage of sodium bisulphite from loading hose (hazardous scenario 3)

-           Spillage of sodium hypochlorite from road tanker (hazardous scenario 4)

-           Spillage of sodium bisulphite from road tanker (hazardous scenario 5)

-           Spillage of sodium hypochlorite from delivery pipelines (hazardous scenario 6)

-           Spillage of sodium bisulphite from delivery pipelines (hazardous scenario 7)

-           Spillage of sodium hypochlorite from storage tank (hazardous scenario 8)

-           Spillage of sodium bisulphite from storage tank (hazardous scenario 9)

 

·           Mixing of incompatible chemicals on-site (hazardous scenario 10), caused by:

-           Error in chemical unloading operations

-           Spillage of incompatible chemicals in different tanks

-           Spillage of incompatible chemicals in two different pipelines

-           Spillage of chemical from valve at soft drinks maker

 

14C.62            In light of the identified hazards, a range of comprehensive chemical safety measures has been developed, as summarised in Sections 14C.63 to 14C.88.  These hazardous scenarios are further assessed in Sections 14C.89 to 14C.182, with the application of the safety measures in mind.

Proposed Safety Measures for Chemicals-related Operations

14C.63            A package of safety measures was identified under the HATS ADF EIA Study to minimize the risk due to chemicals-related operation.  The applicable safety measures in that package will also be adopted in the current Project.  The safety measures can be categorized into several groups: chemical supply contract arrangement, and design measures, procedures and safety measures.

Special Chemical Supply Contract Arrangement

14C.64            A separate supply contract will be awarded for each of the three chemicals (sodium hypochlorite, sodium bisulphite and ferric chloride solutions).  In each supply contract, the chemical supplier will be required to provide dedicated transport specifically used for delivering the chemical to be supplied, and the road tankers will need to be registered with SCISTW.  In addition, the supply contract for sodium hypochlorite will specify that the delivery barge provided will be dedicated for delivering sodium hypochlorite directly and exclusively from the supplier’s production plant to SCISTW during the contract period. The delivery barge will not be allowed to provide other services, such as carrying other chemical or carrying chemicals to other facilities other than SCISTW.

Design Measures

Separation of Chemical Storage Areas

 

14C.65            As proposed in the HATS ADF EIA Study, locate sodium hypochlorite and ferric chloride tank farms in separate areas of SCISTW, which is about 200m apart, as shown in Figure 14C.2.  At Stage 2A, the area separating the two tank farms will be occupied by existing Chemical Dosing Building and HATS 2A permanent structures such as pumping station and sedimentation tanks (Figure 14C.7 refers).  The design measure can avoid the mixing of spilled sodium hypochlorite and ferric chloride in the situation of storage tanks failure.  More details of this design measure are described in Sections 14C.120 to 14C.126 below.

14C.66            The sodium bisulphite tank farm will be located outside the SCISTW main compound (Figure 14C.2 refers).

Protection and Separation of Chemical Delivery Pipelines

 

14C.67            Each chemical delivery pipeline will be installed in designated and separate service duct or pipe trench to provide additional protection.  The routes of the pipe trenches for sodium hypochlorite pipelines and the service duct that ferric chloride pipelines are laid are shown in Figure 14C.8.  Although it can be seen in Figure 14C.8 that there is an intersection between the routes of the service duct and the pipe trench, the two set of pipelines for ferric chloride and sodium hypochlorite will be installed in separate service duct / pipe trench.  Figure 14C.9A shows the cross section of the service duct accommodating the ferric chloride delivery pipelines and the pipe trench where sodium hypochlorite delivery pipelines are installed.  It can seen that the service duct and pipe trench will be located at different level below ground.

14C.68            Also, it was proposed to install a vibration sensing system at the ferric chloride and sodium hypochlorite tank farm to enable shut down of the chemical pumping system (at storage tank or sodium hypochlorite delivery barge) whenever excessive vibrations are detected, in order to minimize the amount of chemical that can escape in the event of pipeline failure due to excessive vibrations.

14C.69            A section of the pipe trench accommodating sodium hypochlorite feeding pipelines was proposed to be wrapped by heavy-duty impervious membrane to provide an additional barrier to migration of the leaked sodium hypochlorite solution.   Furthermore, a road kerb was proposed to be constructed near the section of hypochlorite pipeline near the barge unloading facility and a U-channel is proposed at the foot of the concrete barrier to facilitate surface drainage into the sea.

14C.70            For the sodium bisulphite delivery pipelines, they will be installed in the dechlorination plant located more than 400m away from pipelines for sodium hypochlorite and ferric chloride.  There is no intersection between the routes of the sodium bisulphite delivery pipelines and the pipeline of the other two chemicals.

Dedicated Chemical Delivery Route and Road Signs

 

14C.71            Specific road tanker transport route will be assigned to each chemical.  Such arrangement provides the most direct route for the road tanker travelling to the intended tank farm.

14C.72            Provide road signs on service road indicating the route to specific chemical storage area.

Security of Chemical Loading Points

 

14C.73            Chemical delivery staff will need to register with SCISTW staff upon entering the site.  Loading points for ferric chloride, sodium hypochlorite and sodium bisulphite will be secured by locks and the keys will be kept by SCISTW staff.  The chemical unloading operation cannot start without presence of SCISTW staff to open the locks.

Unique Colour and Size of Pipelines and Hose Coupling for Different Chemicals

 

14C.74            Delivery pipelines for different chemicals will be in different colours and different sizes.

14C.75            There will be specific hose connection design for each chemical, the type, size and colour of coupling will be specific for each chemical – for loading point at SCISTW, the coupling size, type and colour of the loading point of each tank farm will be unique (different from the other two tank farms).  For the loading hose, the supplier will be required to carry the loading hose specifically used for the delivery to SCISTW in each chemical delivery.  The loading hose should be clearly coloured and labelled with the name of chemical to be delivered, with its coupling matches the size, type and colour of the one at the tank farm loading point.  This measure will be included in the chemical supply contract.

14C.76            There are many specialty couplings available by different manufacturers, not only those couplings are available in a large range of sizes, but they also offer different fittings which would make coupling not of the same type could not be fitted or would render inoperable when forced together.  Examples of different types of hose coupling are presented in Figure 14C.10.  The locking arms or the switch of the coupling can only be fastened or turned when two pieces of matched (i.e. same type and same size) hose coupling are put together, in order to open the seal of both pieces of coupling.  By adopting this mechanism, chemical unloading can only proceed when two pieces of matched hose coupling are put together.

Operation Procedures and Safety Measures

14C.77            Clear Labelling of Chemicals-related Equipment:

·           Provide clear and sufficient signage / labels to indicate the identity (i.e. for which chemical) of each tank farm and associated equipment including pipelines, loading points and loading hoses

 

14C.78            Ensuring Quality of Chemical Supplier:

·           Only appoint chemical suppliers with satisfactory quality system.  Suppliers equipped with good quality system are less prone to errors in chemical delivery operations

·           Request the chemical supplier to employ an independent checker to audit the quality and safety management system of the supplier, in order to ensure the standard of the quality and safety management system.  This measure will be included in the chemical supply contract

·           The chemical supplied to SCISTW can only be produced in designated chemical production plants and delivered directly from designated locations.  This requirement will help ensure the quality of the chemical supplied and reduce the likelihood of errors in chemical delivery operation.  This measure will be included in the chemical supply contract

Procedural Control of Chemical Unloading Operation

 

14C.79            Develop clear procedural controls for barge / road tanker filling and unloading operation.  Procedural controls should include: chemical supplier staff cannot start chemical unloading without authorization and presence of SCISTW staff, checking of the delivery documentation prior to unloading process etc.

14C.80            SCISTW staff will be present at the tank area to receive the barge / road tanker, check barge / road tanker labels, check the transport documents carried by the barge crew / road tanker driver, check type, size and colour of coupling and hose coupler, conduct chemical analysis to check the identity of delivered chemical and only then authorize the driver to unload the content.

14C.81            Chemical supplier needs to fax or electronically transmit copies of delivery bills-of-lading information and barge crew / road tanker driver identification to SCISTW prior to delivery barge / road tanker arriving on-site.  Such information will be in compliance with the supplier’s independently accredited quality assurance system (to ISO:9000 or equivalent), to ensure that the right person driving the right tanker / barge to the right tank farm and further avoid occurrence of delivery operation error.  This measure will be included in the chemical supply contract.

14C.82            Conduct chemical analysis to confirm the right chemical is delivered.  The analysis needs to be conducted by SCISTW staff or independent checker before the chemical is authorized to be unloaded to the tank farm.  HKOLAS accredited analysis methods are currently used in the laboratory in SCISTW to analyze the identity and concentration of ferric chloride (GB4482-93 Section 5.1) and sodium hypochlorite (ASTM D2022-89 cl. 6 to 9).  Accredited analysis method for sodium bisulphite will be adopted in SCISTW for testing of delivered chemical in the future.

14C.83            If the coupling of hose connected to the barge / road tanker is found to be unmatched with the coupling of loading point of tank farm, chemical unloading operation must not proceed and the situation must be reported to the SCISTW management for follow-up actions.

14C.84            Chain-of-custody documentation system will be used to ensure both the supplier (factory) and SCISTW staffs have checked the chemical identity and the consistency of the chemical analysis result.  Such measure will ensure that the identity of the delivered chemical has been checked twice by two different parties before it is unloaded to the storage tank.  This measure will be included in the chemical supply contract.

Measures to Identify and Stop the Chemical Unloading in Error Events

 

14C.85            Error in chemical unloading operation could lead to mixing of incompatible chemicals and toxic gas could be produced in such event.  The abovementioned safety measures were identified to avoid the occurrence of such event.  Further, in order to identify and stop such unlikely event, chlorine gas detectors will be installed near the tank vent for each ferric chloride and sodium hypochlorite storage tank, whereas sulphur dioxide gas detectors will be installed near the tank vent for each ferric chloride and sodium bisulphite storage tank.  When chlorine gas detectors detect a chlorine gas concentration of 3ppm or higher, or sulphur dioxide gas detectors detect a sulphur dioxide gas concentration of 15ppm or higher, alarm will annunciated at the tank farm area and in the central control centre.

14C.86            Emergency shutdown valve will be installed on the inlet of the tank farms, to stop chemical unloading to the storage tank when the valve is closed.  Closure of the emergency shutdown valve can be automatically initiated by the activation of the alarm in the tank farm area or central control centre.  Also, as chemical supplier staff and SCISTW operator will be present throughout the chemical unloading operation, they can stop the chemical unloading operation by turning off the pump for pumping the chemical to the storage tank when they notice the activation of the alarm or other abnormal conditions (e.g. emission of gas from the tank vent).  In such case, it is expected that the wrong chemical unloading operation can be stopped within 3 minute in most cases.

14C.87            In addition, CCTV system will be installed to monitor the situation at the chemical tank farm.  In case the rapid stoppage (within 3 minute) of wrong chemical unloading operation fails, SCISTW operator at the central control centre will be notified of the incident by the CCTV image, and apply appropriate action (e.g. close the emergency shutdown valve) within a short period time, probably 10 minutes.

Special Arrangement of SCISTW Public Event

14C.88            Public evens might sometimes be held in SCISTW which allow access of public to the plant facilities.  As a precautionary measure, chemical delivery operation will be suspended on days of SCISTW public event.  Also, public members visiting the SCISTW will be guided by DSD staff and will not be allowed to visit the area near the chemical storage locations in SCISTW.

Assessment of Chemical Spillage Hazards (Scenarios 1 to 9)

Introduction

14C.89            As mentioned above, sodium hypochlorite and sodium bisulphite are not acutely toxic, flammable, or explosive substances.  It is acknowledged that there is the potential for off-site population to contact and/or ingest the spilled chemicals in these chemical spillage scenarios.  However, there will be no lethal effects on off-site population due to contact with and ingestion of these chemicals.  Further elaboration is given below.

Dermal Contact

14C.90            Under the IMDG Code, both sodium hypochlorite (12%) and sodium bisulphite are assigned to be Class 8 dangerous goods (corrosive substances) with packing group III.  Packing group III is assigned to substances that cause skin tissue damage within an observation period of up to 14 days starting after an exposure time of more than 60 minutes but not more than 4 hours.  As indicated in the assigned packing group, sodium hypochlorite (12%) and sodium bisulphite will not cause chemical burn to skin unless there is a prolonged exposure (i.e. more than 1 hour) to the chemical.

14C.91            Individuals will not be exposed to chemicals for a prolonged period unless they are unable to escape from an area holding considerable depth of chemicals (e.g. chemical storage tank or bund area filled with spilled chemicals).  Such structures that are able to contain chemicals are present within the site boundary but not identified outside the site boundary.

14C.92            For Scenarios 2 (hose connecting delivery pipe and road tanker) to 9, the spilled chemical might spread on land and form a liquid pool, which might extend out of the site boundary.  However, the depth of the liquid pool extended out of the site boundary will not be considerable because no structure, which could contain the spilled liquid, is identified outside the site boundary.  Since the spilled chemical pool will not have considerable depth, off-site population will not have large area of skin exposed to the spilled chemical for a prolonged period.  Therefore, there will be no off-site fatality in these scenarios due to dermal contact of the spilled chemicals.

14C.93            For Scenarios 1 and 2 (hose connecting the delivery barge and delivery pipe), the spilled sodium hypochlorite will be largely diluted by seawater immediately and the corrosivity of the spilled chemical will be greatly reduced due to the dilution.  Therefore, these chemical spillage scenarios will not cause off-site fatality.  Note that the sea area near the delivery barge berthing location is not an area for swimming activities or other water-based recreational activities.

Ingestion

Sodium Hypochlorite

 

14C.94            Literatures have been reviewed to understand the clinical effects of acute exposure of sodium hypochlorite via ingestion.  It is revealed that the vast majority of patients ingesting sodium hypochlorite developed no symptoms or only minor gastrointestinal irritation.  Corrosive damage to the gastrointestinal tract is unusual following ingestion of the chemical unless a large quantity of the chemical is ingested.  It should be noted that no description of “lethal impact due to ingestion” was stated in literatures reviewed.

14C.95            The above findings are consistent with the hazardous properties described in the IMDG Code, a widely used document and the “risk phrases” for dangerous substances defined in the EU Commission Directive (2001/59/EC).  In the IMDG Code, sodium hypochlorite is only classified as a corrosive substance (packing group III, indicating slight corrosivity) but not considered as a toxic substance.  The risk phrase “R22 – harmful if swallowed” (rather than R25 – toxic if swallowed or R28 – very toxic if swallowed) is assigned to sodium hypochlorite.  Such descriptions do not indicate lethal effect due to ingestion of the chemical concerned.

Sodium Bisulphite

 

14C.96            In addition, literatures were reviewed to understand the clinical effects of acute exposure of sodium bisulphite via ingestion.  It is revealed that ingestion of sodium bisulphite will cause swelling of the tongue, angioedema, difficulty in swallowing.  Ocular injury may occur from exposure to high concentrations.  Acute ingestions of 3.5mg/kg produce vomiting in most individuals.  Larger doses may result in gastric irritation, abdominal pain, and gastric haemorrhage.  Again, it should be noted that no description of “lethal impact due to ingestion” was stated in literature reviewed.

14C.97            The above findings are consistent with the hazardous properties described in the IMDG Code, a widely used document and the “risk phrases” for dangerous substances defined in the EU Commission Directive.  In the IMDG Code, sodium bisulphite is only classified as a corrosive substance (packing group III, indicating slight corrosivity) but not considered as a toxic substance.  The risk phrase “R22 – harmful if swallowed” (rather than R25 – toxic if swallowed or R28 – very toxic if swallowed) is assigned to sodium bisulphite.  Such descriptions do not indicate lethal effect due to ingestion of the chemical concerned.

Conclusion

14C.98            In conclusion, an assessment of the consequence of the chemical spillage scenarios (hazardous scenarios 1 to 9) has found that while there is the potential for off-site population to expose to the spilled chemical, there will be no lethal effects on off-site population due to contact with or ingestion of these chemicals.  Hence, the chemical spillage scenarios will not cause off-site fatality under the circumstances of the current Project.

Assessment of Incompatible Chemical Mixing Hazards (Scenario 10)

Overview

14C.99            There would be the potential for off-site fatality when mixing of incompatible chemicals occur on-site leading to generation of toxic gas that eventually migrates off-site. There are four identified causes for the scenario of mixing of incompatible chemicals:

·           Cause 1 - Error in chemical unloading operations

·           Cause 2 – Simultaneous release of incompatible chemicals from their respective storage tanks

·           Cause 3 – Simultaneous release of incompatible chemicals from their respective conveyance pipelines

·           Cause 4 – Simultaneous release of incompatible chemicals from their storage tanks and pipelines

 

14C.100            These four causes are illustrated in Exhibit 14C.1.

 

Exhibit 14C.1     Diagrammatic Illustration of Pathways for Toxic Gas Formation due to Mixing of Incompatible Chemicals

Cause 1 - Error in Unloading Operations

14C.101            There will be separate supply contracts each for ferric chloride, sodium hypochlorite, and sodium bisulphite.  For ferric chloride and sodium bisulphite, these two chemicals will only be delivered by road tanker, as there is no barge-unloading facility for the ferric chloride or sodium bisulphite tank farm.

14C.102            For sodium hypochlorite, barge-unloading facilities will be provided so that the chemical can be delivered by a specially designed delivery barge, unloaded to the sodium hypochlorite storage tank via separate dedicated pipelines.  Road tanker unloading facilities will also be provided at the sodium hypochlorite tank farm, so that the chemical can be delivered by road tanker in the event that conditions do not allow barge delivery.

14C.103            As sodium hypochlorite will be mainly delivered by barge, special conditions will be included in the supply contract for sodium hypochlorite.  In the contract, the use of the delivery barge provided will be restricted for delivering sodium hypochlorite directly and exclusively from the supplier’s production plant to SCISTW during the contract period.  The delivery barge will not be allowed to provide other services, such as carrying other chemical or carrying chemicals to other facilities other than SCISTW.  In addition, SCISTW staff will only accept the chemical delivered by the specially designed delivery barge from the chemical supplier.  Under such circumstances, the delivery barge provided by the sodium hypochlorite supplier will only carry sodium hypochlorite to SCISTW.

14C.104            As mentioned above, no barge-unloading facility will be provided for ferric chloride and sodium bisulphite tank farm.  Therefore, there is no physical pipe connection from the barge unloading facilities to ferric chloride and sodium bisulphite tank farm so it is impossible to unload sodium hypochlorite from delivery barge to ferric chloride or sodium bisulphite tank.  As a result, sodium hypochlorite carried by the specially designed delivery barge will only be unloaded to the sodium hypochlorite tank farm via the dedicated pipeline but not ferric chloride and sodium bisulphite tank farms.  Hence, sodium hypochlorite will not mix with ferric chloride or sodium bisulphite in the unloading operation by delivery barge.  Therefore, the cause “error in unloading operation” is not applicable to barge delivery operation.

14C.105            Nevertheless, error in unloading operations via road tanker delivery of chemicals could be possible in the current Project.  The frequency of occurrence of this event is further assessed in Sections 14C.183 to 14C.211.

Cause 2 - Simultaneous Release Incompatible Chemicals from Storage Tanks

Release Mode

 

14C.106            The incompatible chemical mixing scenario under the cause “release of incompatible chemicals from their respective storage tanks” can only occur if:

·           Both the ferric chloride and sodium hypochlorite solutions are somehow released, at the same period of time, into unintended areas of SCISTW from their respective storage; and

·           Then, the two substances are allowed to migrate to a location(s) where they come into contact with each other

 

14C.107            Unintended release of either chemical from their respective storage tanks could be caused by failure of the storage tank because of cracks, punctures, rupture, etc.  These in turn could be caused by intrinsic factors (e.g., weathering of tank material) or external impacts (e.g., a crashing airplane, earthquakes, malicious attacks, etc).

14C.108            The following paragraphs outline the existing/proposed engineering control measures in place/to be implemented to prevent the ferric chloride and sodium hypochlorite solutions from coming into contact with each other, in the event of simultaneous failure of the storage tanks leading to unplanned release of the chemicals.

14C.109            Firstly, it should be noted that ferric chloride and sodium hypochlorite are stored under ambient atmospheric pressure conditions, which are not pressurised.  Therefore, in the event of leakage of chemical from the storage tank, the content will be released from a hole in the tank discharged as a liquid stream under the force of gravity.

14C.110            In the event of storage tank rupture, the stored chemical will be released in a manner similar to a cylindrical column of liquid free to collapse under gravity (i.e. released liquid would fall vertically into the bund).  Owing to the vertical momentum of the released liquid, some liquid may splash over the bund wall and spread to other areas of SCISTW.

Existing Ferric Chloride Storage Facility

 

14C.111            There are six existing ferric chloride solution storage tanks (each about 8m diameter, 13m tall).  These are located in a tank farm on the northeastern corner of the SCISTW, as shown in Figure 14C.7.  The figure presents a proposed layout plan of SCISTW after HATS Stage 2A construction works are completed, which shows the layout of the existing facilities, the proposed sodium hypochlorite storage tanks (located at the southern side of the site) and other structures for HATS Stage 2A.

14C.112            To the north of the ferric chloride tank farm is the sea.  At its farthest point, the tank farm is approximately 30m from the seawall cope line.  The waterfront pavement is graded towards the sea to facilitate surface drainage.  Stormwater drains are also present to intercept surface runoff.

14C.113            Immediately behind the tank farm on the south (landward) side is the Chemical Dosing Building, which is approximately 64m long by 9m wide, with a height greater than the ferric chloride tanks and the filled ferric chloride level by 1 to 7m and 2.5 to 8.5m respectively.  Photos showing such feature are presented in Figure 14C.11.  A bund wall of more than 1m above adjacent ground level surrounds the ferric chloride tank farm; this is to contain any chemicals spilled or leaked from tanks in the event of failure of the tanks.  Per design requirements overseen by Fire Services Department, the volume of the containment is 110% of the capacity of a single tank, acknowledging that it is unlikely that all tanks will fail at the same time.

14C.114            Any leaked or spilled ferric chloride solution will be caught initially inside the bund wall.  Depending on the rate of release, two scenarios may arise:

·           Scenario 1 - If the rate of the unintended release is low, the ferric chloride solution can be pumped out before it overtops the containment.  The pumped-out liquid can then be disposed of as chemical waste safely and transported out of the SCISTW.  This way, the released ferric chloride will not enter into the vicinity of the chlorination plant located on the other (southern) side of the site.

·           Scenario 2 - If the rate of release is high or violent (e.g., due to rupture of several or all storage tanks at the same time), the ferric chloride solution could overtop the bunded containment before it can be pumped away or it somehow could escape the confines of the bunded containment.  In this case, there is a potential for the escaped solution to migrate farther than the immediate vicinity of the tank farm.

 

14C.115            However, even under Scenario 2, given the substantial size and form of the Chemical Dosing Building relative to the storage tanks, it is providing a solid, effective barrier to prevent any spilled or leaked chemical from flowing towards the south side of the SCISTW site (where the chlorination plant is located).  Moreover, the new pumping station and associated works will serve as additional barrier to stop spilled ferric chloride flowing towards southward.

14C.116            Further, the site surface gradient will direct the spilled ferric chloride solution towards the sea before it can reach the road separating the sedimentation tank and the sludge treatment facilities.  In addition, the surface drains located southwest of the ferric chloride tank farm will intercept any spilled ferric chloride that find its way in the uphill direction and will discharge the ferric chloride solution into the sea through the stormwater outfall.

14C.117            In view of the above mitigating factors or engineering controls, it is concluded that any ferric chloride solution released unintentionally from the tank farm would not flow to the southeastern part of SCISTW, where the sodium hypochlorite tank farm will be located.

Sodium Hypochlorite Tanks (to be constructed in HATS ADF)

 

14C.118            The sodium hypochlorite tank farm will be located at the southeastern part of SCISTW, as shown in Figure 14C.7.  In the tank farm, there will be six sodium hypochlorite storage tanks (each about 8m in diameter, 12.5m tall) and these tanks are surrounded by bund walls with 2m high.

14C.119            As mentioned previously, located between the two tank farms will be the existing Chemical Dosing Building, a new pumping station, rapid mixing tanks as well as flocculation tanks to be constructed under HATS Stage 2A.  These permanent structures would serve as obstacles to prevent the pools of the two spilled chemicals from coming into contact with each other.

Multi-barrier Approach to Avoid Mixing of Ferric Chloride and Sodium Hypochlorite Solutions

 

14C.120            Since the spilled chemicals will spread by flowing along the ground surface, the scenario of ferric chloride mixes with sodium hypochlorite (in the event of simultaneous tank failure in both tank farms) can be avoided by the following design features, which serve as a “multi-barrier system”, along the separation area between the tank farms:

·           Surface gradient to direct the spilled chemicals to flow in opposite direction

·           The HATS Stage 2A structures as shown in Figure 14C.7 to block the spilled sodium hypochlorite from flowing towards the ferric chloride tank

 

14C.121            The objective of the above mitigation measures is to prevent the spilled sodium hypochlorite from flowing to the northeastern corner of SCISTW, to avoid the pools of the two spilled chemicals from contacting with each other.

14C.122            At the same time, the areas where the two pools of spilled chemicals will be contained would be served by separate drainage systems with different discharge points to prevent mixing of spilled chemicals in the drainage system.

14C.123            Specific design measures will be included in HATS ADF:

·           Design Feature – Containment Structure

-           A bund wall of 2m tall will be built around the sodium hypochlorite tank farm to contain spillage of the chemical (up to 110% of the capacity of a single tank), which will retain the spilled sodium hypochlorite within the bund wall in the event of storage tank leakage.

·           Design Feature – Surface Gradient

-           To enhance further the effectiveness of the screening structure, the surface for the land reserved for the future flocculation tanks and rapid mixing tanks will be designed to grade towards the sodium hypochlorite tank farm.  This arrangement could reduce some momentum of the spilled chemical flowing northeast when it travels uphill along the 70m long surface.

·           Design Feature – Separate Drainage System

-           To avoid mixing of spilled chemicals within the drainage systems, the drainage channels within the containment made of screening structure will be connected to drainage system different from those serving the area around the ferric chloride tank farm.  The two drainage systems will discharge via different outfall to different sea area.

 

14C.124            Figure 14C.12 presents the cross section from the ferric chloride tank to the sodium hypochlorite tank farm in 2013/14 (completion of HATS Stage 2A works).  The cross sections show a series of structures and engineering measures that avoid mixing of spilled sodium hypochlorite and ferric chloride.

14C.125            In view of the above mitigating factors or engineering controls, it is concluded that any sodium hypochlorite solution released unintentionally from the tank farm will not flow to the northern part of SCISTW, where the ferric chloride tank farm is located.

14C.126            A sodium hypochlorite day tank with capacity of about 100m3 will be installed at the southwestern corner of SCISTW, at about 320m from the ferric chloride tank farm.  Due to the small capacity of the sodium hypochlorite day tank, even assuming the spilled liquid pool is allowed to spread freely in an undiked area, the largest possible size of the liquid pool will be limited, with a radius of 56m [5].  In reality, spillage from the day tank flowing in the direction towards the ferric chloride tank farm will be blocked by the surrounding structures (the existing main pumping station, switchgear building, and sedimentation tanks).  Therefore, spilled sodium hypochlorite from the day tank would not be in contact with the spilled ferric chloride.

Conclusion

 

14C.127            The above discussion has demonstrated that in the events of low chemical release, such as tank leakages and single-tank rupture, the provision of the bund wall, the large separation distance and also the physical barriers between the two chemical farms will be more than adequate to prevent the spilled incompatible chemicals from mixing.

14C.128            Even in the extreme scenario when all storage tanks in the two tank farms collapse simultaneously (which by the way is considered an extremely remote scenario), the design features can prevent the spilled ferric chloride solution from flowing to the south-eastern part of SCISTW and prevent the spilled sodium hypochlorite from flowing to the north-eastern corner of SCISTW.

14C.129            Therefore, ferric chloride and sodium hypochlorite will not mix with each other, under any situations at SCISTW involving unintended release of the two chemicals from their storage tanks.  Therefore, the occurrence frequency of incompatible chemicals mixing due to the cause “simultaneous release incompatible chemicals from storage tanks” is zero.

Cause 3 - Simultaneous Release Incompatible Chemicals from Conveyance Pipelines

Release Mode

 

14C.130            The incompatible chemical mixing scenario under the cause “simultaneous release of incompatible chemicals from conveyance pipelines” can only occur if:

·           Both the ferric chloride and sodium hypochlorite solutions are somehow released, at the same period of time, into unintended areas of SCISTW from their respective conveyance pipelines; and

Then, the two substances are allowed to migrate to a location(s) where they come into contact with each other

 

14C.131            Unintended release of either chemical from their respective pipelines could be caused by failure of the pipeline because of cracks, punctures, rupture, etc.  These in turn could be caused by intrinsic factors (e.g., weathering or degradation of pipe material) or external impacts (e.g., construction of the new plant facilities near to the pipelines, earthquakes, malicious attacks, etc).

14C.132            The following paragraphs outline the existing/proposed engineering control measures in place/to be implemented to prevent the ferric chloride and sodium hypochlorite solutions from coming into contact with each other, in the event of simultaneous failure of the pipelines leading to unplanned release of the chemicals.

Existing Ferric Chloride Pipeline

 

14C.133            As shown in Figure 14C.8, the alignment of the existing ferric chloride delivery pipeline runs from the ferric chloride tank farm to a reception point at the upstream end of the CEPT plant.  As shown in Figure 14C.9A, the ferric chloride pipeline is protected by an external sleeve pipeline in a pipe-in-pipe configuration, inside a buried watertight reinforced-concrete service-duct.  Alongside the duty pipeline is a duplicate (standby) pipeline that is kept empty.

14C.134            The service-duct cross-section has internal height and width of 2.2m and 3.0m, respectively, and provides for easy access by personnel for regular maintenance and inspections.  The service-duct is buried with a minimum cover of 0.55m below ground surface.  The thickness of both the wall and the roof slab is 0.3m, and the ground slab is 0.6m thick.  The service-duct structure is supported on piles.

I.            Release Mitigation Measures

14C.135            The ferric chloride solution is pumped from the tank farm to the CEPT plant.  Two measures are in place to minimise the amount of chemical release from the pipeline in case of pipe leaks or bursts.  These are:

·           While there are six storage tanks (five-duty and one-standby) in the tank farm, only one tank will be connected to the delivery pipeline at any one time.  This is achieved by controlling the valves at the bottom of each storage tank.

·           In addition, differentiate pipe pressure is continuously monitored by plant operators, who will shut down the pumps immediately if irregular pressure drops occur (indicating faults such as pipe leaks or bursts).

 

14C.136            In addition, it was proposed to install a vibration sensing system[6] at the ferric chloride tank farm to enable shut down of the chemical pumping system whenever excessive vibrations are detected.  The objective of the vibration sensing system is to isolate the ferric chloride delivery pipeline from the upstream storage tanks in case of excessive vibrations, so that the amount of chemical that can escape in the event of failure of the pipeline due to excessive vibrations is minimised.

I.            Release during Normal Operations

14C.137            Owing to the above multi-barrier arrangement, release of ferric chloride solution into the environment is impossible under normal operating conditions.  In this scenario, the concern is on intrinsic factors such as weakening or failure of pipe material due to aging, leading to leakage.  Alternatively, pipe bursting could occur at undetected weak points of the pressurised pipeline.  For this to happen, however, the condition of the pipeline will have to deteriorate significantly without being noticed, which is unlikely given regular inspection and maintenance.  In any case, there are two further barriers that will prevent release of the chemical into the environment:

·           Firstly, any ferric chloride solution released from the duty pipeline due to leakage or a pipe burst will be contained by the sleeve pipeline.

·           Secondly, even if the sleeve pipe fails, the pressure monitoring procedure would have worked to isolate the pipeline from the upstream storage tank.  If the isolation could be effected immediately, then the amount of chemical release would be limited to the volume of the pipeline (i.e., about 7.8m3).  In practice, a short period would have elapsed before all the concerned pumps and valves can be completely closed.  Therefore, it is possible that more than one pipe volume of chemical would be released.  Given that the pumping rate for ferric chloride is 0.4L/s and assuming that it would take 10 minutes to completely isolate the pipeline, then up to 1.03 (1 pipe-volume of chemical originally filled in the pipeline plus the additional 0.03 pipe-volume leak before the pipeline isolation) (i.e. 8.03m3) could be released into the service-duct.  However, the void space available in the service-duct[7] is about 2,464m3, which is more than adequate to contain the released chemical.  This also means that, even in the unlikely scenario of failure of the pressure monitoring procedure, and a whole tank of ferric chloride solution (about 400m3) is released, the entire tank load will still be able to be contained in the service-duct.  No release into the environment is possible.

II.          Release during Extreme Scenario

14C.138            In the extreme scenario, a catastrophic external impact on the chemical pipeline is assumed to occur at SCISTW.  In this context, as the pipeline is buried underground, the external impact may be represented by large vibrations caused by an earthquake affecting Hong Kong.

14C.139            In this case, the duty pipeline could burst and sleeve pipeline could crack simultaneously, but the pressure and vibration monitoring procedures would have worked to contain the chemical release to 8.03m3.  Even if the both the pressure and vibration monitoring procedures were to fail, the reinforced-concrete service duct would still act as a barrier to release of chemical into the environment, as elaborated below.

14C.140            The reinforced concrete service-duct is a strong piled-structure.  Therefore, it could only fail in the sense that its watertightness might be compromised due to cracks induced by excessive vibrations, and that it may suffer some minor structural damages, but the service-duct should physically remain intact.  This means that the service-duct would still act a barrier to the immediate release of the chemical solution to the environment.

14C.141            While the cracks so opened up in the reinforced concrete slab by the vibrations would allow liquid to seep through under the force of gravity, several mitigating factors would stop the spread of the chemical beyond the immediate vicinity:

·           Firstly, the liquid chemical in the large confines of the service-duct would be spread along the length (several hundred metres) of the service-duct.  This means that the liquid chemical will not create a sizable hydraulic head that can rapidly drive the liquid through the cracks under the force of gravity only.

·           Secondly, the concrete slabs are extremely thick (0.3m sidewalls and 0.6m bottom slab), presenting significant resistance to the flow of liquid even through the cracks.

·           Thirdly, unsaturated concrete could absorb some of the liquid into its own material matrix, thereby reducing the amount of ferric chloride that could pass through.

·           Fourthly, any liquid chemical that eventually migrates through the reinforced-concrete box duct, which will be relatively small in quantity, will be quickly soaked up by the vast underlying soil matrix.

 

14C.142            Overall, it is concluded that no ferric chloride solution can migrate beyond the immediate alignment of the box duct even in the extreme scenario.

Sodium Hypochlorite Pipelines (to be constructed under HATS ADF)

 

14C.143            The alignments of the sodium hypochlorite pipelines to be constructed under HATS ADF are shown in Figure 14C.8.  The first section (or feed section) starts at the barge unloading point and travels in a buried reinforced-concrete pipe trench to the sodium hypochlorite tank farm.  This section of pipeline (0.25m diameter) is pressurised in that the liquid has to be pumped to the top of the tanks for discharge into storage tanks.  As barge deliveries will only be made twice a week, the feed section will not be in constant use, but only once every three days and for four hours on each occasion to convey the sodium hypochlorite solution to the storage tanks.  After each use, the sodium hypochlorite feed pipeline will be completely drained to avoid accumulation of any residual chemical solution in the pipeline.

14C.144            From the tank farm, the second section (or dosing section) of the pipeline (0.1m diameter) starts at the bottom of the tanks and convey the hypochlorite solution by pumping to the day tank adjacent to the flow distribution chamber.  The dosing section will convey hypochlorite solution to the day tank on a daily or bi-daily basis for about three hours each time.

14C.145            The hypochlorite pipe trench is of watertight reinforced-concrete construction.  Alongside the duty pipeline in the same trench will be a standby pipeline.  Figures 14C.9A and 14C.9B show the typical cross-sections of the feed and dosing sections, respectively.

14C.146            Similarly, a multi-barrier approach to containment of any leaks or pipe bursts was proposed in HATS ADF:

·           The duty pipeline (for both feed and dosing sections) will be protected by a sleeve pipeline, and both will be contained in a covered concrete trench

·           The pipeline sections will be regularly inspected and maintained, such that any deterioration in pipe material condition will be repaired or rectified before it becomes critical

·           A leak (pressure) monitoring procedure will be implemented during unloading of the hypochlorite solution from the barge

·           A vibration sensing system will be installed at the hypochlorite tank farm to raise alarm for shut down of the pumping system in the event of excessive vibrations

·           Further, a section of the hypochlorite feed pipe-trench will be wrapped by heavy-duty impervious membrane[8] to provide an added barrier to migration of hypochlorite solution in the unlikely event of failure of all the other barriers.  As shown in Figure 14C.8, this section has a length of about 170m and runs alongside the existing ferric chloride service duct.  As shown in Figure 14C.9A, the impervious membrane will be applied to the three external surfaces (two sidewalls and the bottom slab) of the hypochlorite pipe trench to prevent passage of liquid from inside to the outside.  To isolate the impervious membrane from the concrete surface, such that the impervious membrane will not be stressed by any movement of the concrete, a geo-textile[9] sheet (i.e., a de-bonding layer) will be installed between the impervious membrane and the concrete surface.

·           Also shown in Figure 14C.8, the hypochlorite pipe trench will cross over the ferric chloride service-duct at a point near the middle of the southern site boundary.  At this cross over location, the impervious membrane will be protected by a geo-textile sheet (de-bonding layer) on both sides to isolate it from the concrete surface.  Details of the arrangement are shown in Figure 14C.9C.

Feed Section (from Barge Unloading Point to Storage Tank Farm)

14C.147            With respect to the sodium hypochlorite feeding pipe trench (section between the barge unloading facility and hypochlorite storage tank), its average cross-section area is 1.0m x 1.7m = 1.7m2.  With a pipe trench length of about 357m, the effective void space available in the pipe trench to contain leaked sodium hypochlorite[10] would be about 548m3.

14C.148            The crossover section is the point where the hypochlorite pipeline will be closest to the ferric chloride pipeline.  Therefore, in addition to the multi-barriers (i.e., sleeve pipe, watertight reinforced-concrete pipe trench, impervious membrane, feed pressure monitoring, vibration monitoring, etc), it is proposed to grade (drain) the pipe trench to prevent accumulation of any spilled or leaked hypochlorite solution inside the feed pipe trench.  The intention is to eliminate the possibility of hypochlorite migrating to the ferric chloride pipeline located below under any circumstances.  The proposed cross-over arrangement is illustrated in Figure 14C.9C, and described below:

·           The hypochlorite pipe trench normally runs flush with ground surface except at where it crosses over the ferric chloride pipeline service-duct.  At this point, the former will rise above the ground surface to cross over the latter.  Thereafter, the hypochlorite pipe trench will sink back to below ground surface and runs towards the hypochlorite tank farm

·           To prevent accumulation of any spilled or leaked hypochlorite solution at this point (which is the highest point of the feed pipe trench), the feed pipe trench will be drained in two directions from this point:

-           Moving upstream from this point, the pipe trench will be graded to fall towards the north and northeast so that any liquid inside this section of the pipe trench will be discharged into the sea via an opening at the upstream end of the pipe trench through the existing seawall near the Barge Unloading Facility.

-           Moving downstream from this high point, the pipe trench will be graded to fall towards the southwest so that any liquid inside this section of the pipe trench will be collected in a sump inside the hypochlorite tank farm bunded enclosure.

 

14C.149            Under normal chemical unloading conditions, any leakage or pipe burst (due to undetected weak points in the pipe material) causing a pressure drop will be identified by the operators, who will quickly shut down the pumps to isolate the pipeline from the delivery barge.  Assuming that the pump rate is 0.035m3/s, and it would take 10 minutes to completely isolate the pipeline, up to about 2.2 (1 pipe-volume of chemical originally filled in the pipeline plus the additional 1.2 pipe-volume leak before the pipeline isolation) pipe-volumes (38.5m3) may be released.  In such case, the pipe trench will be able to contain totally the released sodium hypochlorite solution.  It will be impossible for any chemical to escape into the environment.

14C.150            It should be noted that as the chemical is fed from the top into the storage tanks, no hypochlorite solution could escape back into the feed pipe under any circumstances.

14C.151            Again, for the purposes of this assessment, the extreme scenario would be for excessive vibrations from an earthquake affecting Hong Kong to damage the feed pipeline and sleeve pipeline simultaneously, and to jeopardise the watertightness of the reinforced-concrete pipe trench.  For this to happen, the catastrophic impact must occur at the same time as chemical unloading is conducted (which only occurs once every two days for about four hours each time).  The likelihood of this is extremely remote.

14C.152            However, in any case, the unloading operation will cease within a short period after the catastrophic impact strikes, as the vibration detection system will raise an alarm to cause the operators to stop the pump at the barge.  Allowing for 10 minutes for reaction time, the maximum quantity of chemical release into the pipe trench would be equivalent to perhaps no more than 2.2 pipe-volumes of chemical (38.5m3), which would be comfortably contained by the pipe trench (capacity 548m3).  Even if the alarm system was to fail or the operators were unable to shut down the system for whatever reasons, and the whole barge load (500m3) of hypochlorite is drained, the hypochlorite solution would still be contained by the feed pipe trench (capacity 548m3). Then, the collected liquid will be drained to the sea or a sump inside the hypochlorite tank farm.

14C.153            As mentioned previously, while the cracks so opened up in the reinforced concrete slab by the vibrations would allow liquid to seep through under the force of gravity, several mitigating factors would stop the spread of the chemical beyond the immediate vicinity:

·           Firstly, as mentioned above, the gradient in the pipe trench will quickly drain the liquid rather than allowing it to accumulate at any point in the pipe trench.  This means that the condition for formation of a sizable hydraulic head that can rapidly drive the liquid through the cracks will not occur.

·           Secondly, the reinforced concrete slab of 0.15m thick, would present much resistance to the flow of liquid even through the cracks.  As the liquid will flow through the path of least resistance, it will drain preferentially to the sea or the sump inside the hypochlorite tank farm, rather than vertically through the cracks.

·           Thirdly, unsaturated concrete could absorb some of the liquid into its own material matrix, thereby reducing the amount of ferric chloride that could pass through.

·           Fourthly, any liquid chemical that eventually migrates through the reinforced-concrete pipe trench, which will be relatively small in quantity, will be stopped by the heavy-duty impervious HDPE membrane.  The membrane will probably deform under stress, but owing to its high tensile strength, it will not tear, break, or puncture in the circumstance.

 

14C.154            Overall, it is concluded that no sodium hypochlorite solution can migrate beyond the immediate alignment of the concrete pipe trench even in the extreme scenario. An assessment of the potential for the sodium hypochlorite solution released from the feed pipeline to mix with the ferric chloride solution simultaneously released from the ferric chloride is summarised in Section 14C.157.

Dosing Section (from Hypochlorite Tank Farm to Day Tank)

14C.155            The cross sectional area of the sodium hypochlorite dosing pipe trench (from storage tank to day tank) is about 0.5m x 1.3m = 0.65m2.  The cross-section area taken up by the duty and stand-by dosing pipes[11] would be = 2 x p x 0.1352 = 0.115m2.  With a length of about 332m, the void space available in the dosing pipe trench would be about 177.6m3.

14C.156            Under normal conditions, any leakage or pipe burst (due to undetected weak points in the pipe material) can be quickly identified by the pressure monitoring procedure and rectified by shut down of the upstream valve connecting it to the storage tank.  As mentioned previously, if instantaneous isolation is achieved, the volume of chemical that can be released cannot be more than the capacity of the duty dosing pipeline, which is about p x 0.0752 x 332 = 5.9m3.  Assuming that the pump rate is 0.01m3/s, and it would take 10 minutes to completely isolate the pipeline, up to about two (1 pipe-volume of chemical originally filled in the pipeline plus the additional 1 pipe-volume leak before the pipeline isolation) pipe-volumes (12m3) may be released.  Therefore, the pipe trench (capacity of 177.6m3) will be able to contain totally the released sodium hypochlorite solution.

14C.157            In case the alarm system was to fail or the operators were unable to shut down the system for whatever reasons, a whole tank load of sodium hypochlorite could be drained.  However, even in this case, mixing of the two chemicals will not be possible, as there is significant separation (160m) with a substantial barrier (e.g. the existing sedimentation tanks) located in between the service duct accommodating the ferric chloride dosing pipelines and the sodium hypochlorite dosing pipe trench.

14C.158            In conclusion, in the extreme scenario of an earthquake affecting Hong Kong, the vibration sensing system, the reinforced concrete pipe trench, and the underlying soil matrix will work together to stop chemical migration beyond the immediate vicinity of the pipe trench alignment.  In any case, as mentioned earlier, the presence of a massive barrier (in the form of the existing sedimentation tanks) will prevent any released hypochlorite solution from reaching the ferric chloride pipeline located some 160m away.

Assessment of Possibility of Mixing of Incompatible Chemicals

 

14C.159            For mixing of sodium hypochlorite and ferric chlorides solutions to occur, they must be released from their respective pipelines at the same time due to a rare catastrophic impact (e.g., a significant earthquake), and then be allowed to migrate to a point(s) where they meet.

14C.160            The above assessments have demonstrated that any chemical releases from the pipelines in the event of either normal or extreme conditions will not be able to migrate beyond the immediate vicinity of the service-duct/ pipe trench alignments:

·           For the feed section of the hypochlorite pipeline, the most critical section for mixing is where the two pipelines run parallel to each other (as shown in Figure 14C.8).  Here, the extreme case will be an earthquake (occurring at the same time as unloading of the hypochlorite solution) leading to excessive vibrations that simultaneously damage the two pipelines and associated ducting/trenches.  However, as discussed above, mixing of the two chemicals will still not be possible due to the multi-barriers, in particular the functioning of the heavy-duty impervious membrane (which might deform but will not tear or fail) which will totally contain the hypochlorite solution in this extreme circumstance.

·           For the dosing section of the hypochlorite pipeline, it is both remote and separated from the ferric chloride pipeline.  As shown in Figure 14C.8, the two pipelines are located on opposite sides of the existing sedimentation tanks (which may be regarded as a massive physical barrier) with a separation of about 160m.  It is therefore impossible for any released chemicals from the dosing section of the hypochlorite pipeline and the ferric chloride pipeline to reach each other, even without any leakage containment or mitigation measures for the dosing section of hypochlorite pipeline.

·           Therefore, mixing of the two chemicals is not possible in any circumstances.

 

14C.161            In conclusion, ferric chloride and sodium hypochlorite will not mix with each other, under any situations at SCISTW involving unintended release of the two chemicals from their respective pipelines.  A range of mitigation measures has been proposed, including measures to drain the released hypochlorite solution rapidly to the sea or an underground sump pit even under the extreme scenario.  Therefore, the occurrence frequency of incompatible chemicals mixing due to the cause “simultaneous release incompatible chemicals from conveyance pipelines” is zero.

Cause 4 - Simultaneous Failure of Storage Tanks and Pipelines

14C.162            This scenario is a combination of Causes 2 and 3, and refers to mixing due to the simultaneous failure of pipeline and storage tank.  It  can only occur if:

·           Both the ferric chloride and sodium hypochlorite solutions are somehow released, at the same period of time, into unintended areas of SCISTW from their respective conveyance pipeline and storage tank; and

·           Then, the two substances are allowed to migrate to a location(s) where they come into contact with each other

 

14C.163            For this scenario, two cases are considered:

·           Case 1 – Simultaneous Failure of a Ferric Chloride Tank and a section of the Sodium Hypochlorite Pipeline

·           Case 2 – Simultaneous Failure of a Hypochlorite Tank and a section of the Ferric Chloride Pipeline

Case 1

 

14C.164            The Ferric Chloride Tank Farm is located on the eastern edge of the SCISTW site adjacent to the sea, as shown in Figure 14C.8.  As mentioned earlier, failure of any ferric chloride storage tank will lead to a release of the chemical.  As the tank farm is surrounded by a 1.2m high bund wall all round, any leaks from the storage tanks will be contained within the bunded enclosure.

14C.165            In the event of tank rupture, however, some of the released chemical solution could overtop the bund wall.  As the tank farm is shielded on the western side by the massive Chemical Building, the ferric chloride solution that manages to overtop the bund wall would spread to the other three directions, where the spilled ferric chloride spreading to the eastern direction will flow into the sea.  This means that, if not controlled, some ferric chloride solution could reach the sodium hypochlorite pipeline near the Barge Unloading Facility, which is located about 35 metres away to the north of the Ferric Chloride Tank Farm.

14C.166            Therefore, in the extreme scenario of a seismic event occurring during unloading of sodium hypochlorite solution and simultaneous failure of the feed hypochlorite pipeline/pipe-trench and one or more ferric chloride storage tanks, there is a potential for the two chemicals to mix with each other.

14C.167            To prevent the mixing of incompatible chemicals from occurring in the extreme scenario, the following precautionary or mitigation measures are proposed (Figure 14C.8 and 14C.9D refers):

·           Increase the height to 3.0m of a (12m + 10m =) 22m long section of the bund wall around the northernmost storage tank (which is the tank closest to the hypochlorite pipeline to the west).  This tank is hereinafter referred to as Tank A.

·           Construct a raised road kerb with height of 0.1m near the barge unloading facility, which is located about 22m from the nearest ferric chloride tank farm bund wall.  The kerb is to run perpendicular to the seawall, and has a length of about 20m.

·           Construct a drain at the foot of the kerb to collect and drain any spilled ferric chloride solution to the sea.

 

14C.168            The following calculations explain how the abovementioned precautionary mitigation measures could avoid released ferric chloride solution from storage tank spreading to the barge unloading facility.

14C.169            Thyer et al. (2002) give the following correlation for the bund overtopping fraction Q over vertical bund walls:

·           Q = 0.044 – 0.229 ln(h/H) – 0.116 ln(r/H)

·           Where Q = bund overtopping fraction; h = bund wall height (which is 3.0m in this case); H = tank liquid height (which is 8.61m), and r = distance from the centre of the Tank A to the bund wall (which is 5m)

 

14C.170            This gives Q = 0.044 – 0.229 ln(3.0/8.61) – 0.116 ln(5/8.61), that is Q = 0.348.  In other words, 34.8% of the liquid in the tank could overtop the bund wall.  Therefore, the volume of ferric chloride solution that could overtop the bund wall is = 433m3 x 0.348 = 150.7m3.

14C.171            Considering a circular liquid pool is formed by the 150.7m3 of ferric chloride solution with radius of 22m (the distance between the road kerb and the bund wall), the depth of the circular liquid pool would be 0.099m.  This calculation shows that the proposed 0.1m high kerb can prevent the spilled ferric chloride solution from reaching the hypochlorite pipeline at the barge unloading facility.  The proposed drain at the foot of the road kerb will intercept and drain the some of spilled ferric chloride to the sea before it reaches the road kerb.

14C.172            In conclusion, with the above proposed precautionary mitigation measure, it will be impossible for the two chemicals to mix in any circumstances at this location.

Case 2

 

14C.173            As shown in Figure 14C.8, the Hypochlorite Tank farm is located on the western side of the SCISTW site.  The nearest section of the ferric chloride pipeline is located about 80m away to the east.  The ferric chloride pipeline is located in a reinforced-concrete service duct buried below ground.  Also, additional primary sedimentation tanks will be built in the intervening space under HATS Stage 2A.  These two features (pipeline in buried underground service-duct and presence of a massive barrier in the form of sedimentation tanks) will serve as effective barriers to prevent mixing of the two chemicals in the extreme scenario of a seismic event.

Conclusion

 

14C.174            In conclusion, with the proposed precautionary design features (i.e., For Case 1, measures listed in Section 14C.162 and, for Case 2, additional primary sedimentation tanks between the hypochlorite tank farm and the ferric chloride pipeline as discussed in Section 14C.168), mixing of the two chemicals is not possible under any circumstances.  Therefore, the occurrence frequency of incompatible chemicals mixing due to simultaneous release incompatible chemicals from pipeline and storage tank is zero.

Operation and Construction of the New Plant Facilities

14C.175            A comprehensive systematic brainstorming HAZOP workshop was conducted in January 2008 for the operation and construction of the new plant facilities.  Hazards associated with the operation of the new plant facilities were discussed.  Further, detailed procedures for the construction and associated hazards, as well as the implementation of safety and precautionary measures for construction activities has been discussed and agreed in the HAZOP.  Details of hazards identified are given in the hazard register and meeting minutes attached in Appendix 14C.8.

Operation of New Plant Facilities

 

14C.176            The proposed influent pumping station, CEPT tanks and sludge treatment facilities are the extensions of the existing SCISTW facilities.  Also, there are no hazardous chemicals or dangerous goods involved in the proposed odour control facilities and the proposed effluent tunnel.  As such, there is no risk due to the new plant facilities that would cause hazardous interaction with the disinfection chemicals (i.e. sodium hypochlorite and sodium bisulphite).

14C.177            The proposed electrical substation (132kV / 11kV) is located next to the proposed dechlorination facilities (Figure 14C.7).  A typical substation of CLP consists of four 132kV / 11kV transformers, relevant switchboxes and panels.  It would not impose any hazards to the adjacent facilities under normal operation but there would be risk of explosion escalated from short circuit fire.   However, it should be noted that even if explosion of the electrical substation does occur, incompatible mixing is impossible, as the substation would be located more than 200m away from the sodium hypochlorite or ferric chloride pipelines or storage tank. Only spillage of sodium bisulphite is anticipated.  No toxic gas generation and offsite fatality associated with the disinfection facilities due to the operation of the new plant facilities would result.  Therefore, the occurrence frequency of incompatible chemicals mixing caused by the electrical substation operation is zero.

Construction of New Plant Facilities

 

14C.178            Construction method of the new plant facilities, associated hazards and precautionary measures have been specified in the HAZOP held in January 2008.  Details are given in the hazard register and meeting minutes attached in Appendix 14C.8.

14C.179            The deep effluent tunnel will be constructed using Tunnel Boring Machine or drill and blast method.  The location of the proposed effluent tunnel is near to the hypochlorite dosing pipelines but remote and far away from the ferric chloride pipelines (Figure 14C.8).  The ferric chloride pipelines is located at the opposite side of the existing sedimentation tanks (which is regarded as a massive physical barrier) with a separation of about 160m.  It is therefore impossible for ferric chloride and sodium hypochlorite released from their respective pipelines to reach each other even if there were simultaneous failures of these pipelines caused by vibration or settlement arising from the drill and blast construction of the deep effluent tunnel. 

14C.180            Other new plant facilities will be constructed using conventional methods including excavation and piling.  Given that the proposed influent pumping station and proposed sludge holding tanks are located close to the sodium hypochlorite and ferric chloride pipelines, more thorough discussion were given in the HAZOP.  Excavation with diaphragm wall techniques would be applied for influent pumping stations construction while bore piling instead of percussion piling would be used for the sludge treatment facilities construction so to keep the vibration to a minimum.  It was also proposed to maximise the distance between the excavation works and the pipelines.  Since the effect of diaphragm wall techniques and bore pile are anticipated to be no worse than the effect of the external events like earthquake and aircraft crash, incompatible substance come into contact with each other to cause incompatible mixing is impossible with the existing / proposed engineering control measures applied as stated in Section 14C.120 to 14C.126.  Nevertheless, other general precautionary measures would further be implemented to reduce the probability of damaging the storage tanks and pipelines.  The proposed mitigation measures during construction have been categorized and tabulated in Table 14C.5.

Table 14C.5      Mitigation Measures during construction associated with SCISTW

Ref No.

Major Construction Activity

Anticipated Mitigation Measures[12]

-

General

Ø  Set up monitoring system for vibration and settlement control 

Ÿ             Employ vibration detectors and settlement markers

Ÿ             Develop action plan(s) for situations where vibration or settlement level is found to exceed the set limits (vibration limit would be in the order of 10 mm/s; settlement limit: would be in the order of 25mm)

Ø  Strict Traffic Management

Ÿ             Designated delivery route and off-loading area for delivery trucks

Ø  Close supervision and monitoring by safety officers

Ÿ             If there is any construction work within 2m of the pipelines, an immediate inspection to the pipeline section and the impervious membrane wrapping    should be conducted to ensure no damage to the integrity of the pipeline and the membrane

Ÿ              Report any damage of the disinfection facilities to operators for remedial actions

Ø  Provide indication / signs for sodium hypochlorite pipeline and associated impervious membrane wrapping, as well as the ferric chloride pipelines

Ø  Any damage during construction work to the sodium hypochlorite/ ferric chloride pipelines/tanks would be immediately repaired to its original safety function

Ø  Regular checking of chemical delivery pipelines

Ø  Provide a physical barrier between the sodium hypochlorite tanks and the ferric chloride tanks during the construction stage before the new above ground structures for HATS Stage 2A are erected

1

Excavation works

Damage of chemical storage / delivery facilities

Ø   General precautionary measures to be implemented

Ø   Close liaison with plant operators should be maintained at all times to minimize impacts during chemical deliveries and blasting operations 

Ø   Excavation running close or parallel to sodium hypochlorite delivery pipelines and associated impervious membrane wrapping under road / pavement shall be avoided as far as possible

2

Piling works

Ø   General precautionary measures to be implemented

Ø   Use bore piles instead of percussion piles in order to keep vibration to a minimum

Ø   Maximise the distance between piling and delivery pipelines, as well as the associated impervious membrane wrapping. Monitor vibration resulted from construction works to ensure the velocity and amplitude of vibration limit will not be exceeded

3

Installation of electrical and mechanical equipments

 

 

Ø   General precautionary measures to be implemented

Ø   Designated routes for delivery trucks

Ø   Designation of off-loading area

Ø   Effective traffic management

Ø   Conduct hazard assessment and obtain Hot Work Permit before starting welding / hot works

4

Mobilising and usage of construction equipment (e.g. drill rig, backhoe, bulldozer, dump truck, site vehicle etc)

Construction plant overturns / crashes

Ø   Control of truck transportation route

Ø   Stability of the equipment should be checked and certified

Ø   Location of any large-scale or high-elevated equipment should be agreed with SCISTW operator before delivery

Mitigation measures associated with crane operation

Ø   Close inspection and supervision to ensure proper operation of construction plants

5

Welding / Hot works

Ø   To confine hot works in designated areas.

Ø   Area within the boundaries of chemical storage facilities are subject to hazardous area control

Ø   Conduct hazard assessment and obtain Hot Work Permit before starting welding / hot works

6

Shafts construction

Ø   General precautionary measures to be implemented (e.g. probing)

Ø   Provide lateral support to excavation

Ø   No explosives to be stored on-site or used close to the chemical storage compounds

Ø   Maximise the distance between the tunnel shaft and delivery pipelines

Ø   To adopt a blasting vibration limit  in order to minimise the impact upon storage tanks and pipelines during blasting operations

 

14C.181            As stated above, the proposed precautionary measures would eliminate the risk of damage to the tanks,, pipelines and the impervious membrane wrapping. Furthermore, the existing/proposed engineering control measures stated in Section 14C.120 to 14C.126 would also prevent two incompatible substances come into contact with each other to cause incompatible mixing.  No toxic gas generation and offsite fatality associated with the construction of the new plant facilities would occur.  Therefore, the occurrence frequency of incompatible chemicals mixing caused by the construction of new plant facilities is zero.

Frequency Estimation

Frequency Estimation based on Incident Review Data

14C.182            As mentioned above, the chemical spillage scenarios will not cause off-site fatality under the circumstances of the current Project.  Therefore, the frequency of such scenarios to cause off-site fatality is zero.

14C.183            The occurrence frequency of “mixing of incompatible chemicals on-site” may be determined based on an incident review.  By using the incident review results, the occurrence frequency of the scenario of ‘mixing of incompatible chemicals on-site” for the Project can be determined by the following equation:

 

Occurrence frequency = A / B                                         Equation 1

 

Where,

A = No. of incompatible chemicals mixing incidents in wastewater disinfection facilities (or similar facilities, e.g. wastewater recycling plant) identified in incident review

B = The population of wastewater disinfection facilities (in terms of facility-year) located in the region during the period that the incident review has covered

 

14C.184            A comprehensive review of recorded international incidents involving mixing of either sodium hypochlorite or sodium bisulphite with an incompatible chemical has been conducted, as summarized in Appendix 14C.7.  In summary,

·           This showed that mixing did occur at leisure centres (swimming pools) and some industrial facilities, because of unloading errors or spillage/leakage from storage tanks, but none of these incidents caused fatality.

·           Further, the review did not find any incompatible chemicals mixing incidents in wastewater disinfection facilities (i.e. A = 0).  This may be explained by the generally stringent safety measures (particularly in handling chemicals) adopted in wastewater disinfection facilities.

 

14C.185            For the estimation of parameter “B”, since the incident review has covered the incidents occurring in Europe and the USA for the period from 1970 to 2005, the population of wastewater disinfection facilities (that use hypochlorite) located in Europe and the USA for the above period was determined.  Based on the results of survey on disinfection practice in coastal cities conducted under this Study, 57 wastewater treatment works (that use sodium hypochlorite for disinfection) were identified in 10 cities with a total population of 37,355,000.  That is, a wastewater disinfection facility using sodium hypochlorite would serve a population of about 655,351 people.

14C.186            The human population at 1990 was taken to represent the population for the period from 1970 to 2005.  The population of Europe and the USA at 1990 was 721,390,000 and 248,000,000 respectively; giving a total population of 969,390,000.

14C.187            From the above data, the population (in terms of facility-year) of the wastewater disinfection facility in Europe and the USA for the period from 1970 to 2005 can be estimated by the equation: [(969,390,000 / 655,351)] x 35 = 1,479 facilities x 35 operation- year = 51,765 facility-year.  Since no identified incompatible chemicals mixing incident occurred in wastewater disinfection facilities, the number of such incident occurring in wastewater disinfection facilities is <1, hence the incompatible chemicals mixing scenario occurrence frequency at wastewater disinfection facilities is estimated to be less than 1.9x10-5 (i.e. < 1 / 51,765) per year.

14C.188            None of the identified incompatible chemicals mixing incidents occurred in wastewater disinfection facilities.  This shows that such mixing incident has not occurred in the experience history of wastewater disinfection facilities.  Where an incident that could occur (but has not occurred) it is normal to base the expected number of incidents on a 50% chance of it having (or not having) occurred, leading to an estimate of 0.7 incidents based on a Poisson distribution.  The probability of not having seen an incident as a function of the expected number of incidents based on a Poisson distribution is shown below.

 

14C.189            Generally, wastewater disinfection facilities are operated by experienced and trained personnel in an organized and careful manner.  Design and operational measures for wastewater disinfection facilities are mature and well developed.  Therefore, assuming a 50% chance of occurrence of mixing of incompatible chemicals, per the general Poisson model, leading to an estimate of 0.7 incidents, may be overly pessimistic for wastewater disinfection facilities.  For a less pessimistic but yet cautious estimate, we propose to apply a factor of “2” to the expected number of incidents (making it 0.35 instead of 0.7), which is equivalent to a 30% chance that an incompatible chemicals mixing incident should have occurred in the experience history of wastewater disinfection facilities.

14C.190            Therefore, the generic occurrence frequency of incompatible chemicals mixing incident in wastewater disinfection facilities is estimated to be = 0.35 / 51,765 = 6.76 x 10-6 per year.

Adjustment of Generic Occurrence Frequency based on Project-specific Circumstances

14C.191            Next, the generic occurrence frequency of 6.76 x 10-6 per year for incompatible chemicals mixing incident in wastewater disinfection facilities needs to be adjusted to take into account the specific circumstances of the Stage 2A in SCISTW.

14C.192            In considering what might be a reasonable adjustment factor, the following aspects may be relevant:

·           Precautionary measures to avoid incompatible chemicals mixing in Stage 2A

·           Number of transportation modes for chemical delivery to Stage 2A

·           Number of incompatible chemicals to be handled at Stage 2A

·           Number of chemicals delivery to be made to Stage 2A

Precautionary Measures to Avoid Incompatible Chemicals Mixing

 

14C.193            As mentioned above, a comprehensive package of precautionary measures, including special chemical supply contract arrangement, design measures as well as operation procedures and safety measures, has been developed for HATS ADF and Stage 2A to avoid the occurrence of incompatible chemical mixing incidents.  These measures represent latest generation of state-of-the-art hazard/risk control measures for chemical management at wastewater disinfection plants.

14C.194            Referring to the surveyed wastewater disinfection plants, these included facilities that have been operating for a long period.  For example, some of the wastewater disinfection facilities using sodium hypochlorite were commissioned in the 1970s or earlier years.

14C.195            Therefore, while chemical hazard control measures are routinely adopted in wastewater disinfection facilities, it is likely that the level and sophistication of the precautionary measures adopted in the proposed Stage 2A would be somewhat higher (better) than the “average” conditions of wastewater disinfection plants in general.  This is because the latter group would include modern as well as older disinfection plants, where the hazard control measures could be less stringent than the Stage 2A.

14C.196            This suggests that, with respect to precautionary measures for avoiding unloading error, the proposed Stage 2A would have a lower frequency of unloading error incidents when compared to wastewater disinfection facilities in general.

Number of Transportation Mode for Chemical Delivery

 

14C.197            In HATS Stage 2A, there will be technically two chemical transportation modes because sodium hypochlorite can be delivered either by barge or road tankers.  However, sodium bisulphite and ferric chloride will only be delivered by road tankers.  As mentioned above, barge-unloading facility would be provided only for sodium hypochlorite tank farm via a dedicated pipeline in a separate pipe trench.  It is therefore impossible for sodium hypochlorite delivered by barge to be unloaded into sodium bisulphite or ferric chloride tanks.  Therefore, it may be argued that, for the purposes of the present discussion on frequency of incompatible chemical mixing, the proposed Stage 2A will involve a single mode of delivery.

14C.198            For wastewater disinfection plants in general, they could involve one or more modes of chemical delivery.  Obviously, the minimum would be “one”, either by land or by sea.  On this basis, and considering the unique chemical delivery arrangement for the Stage 2A, it is considered that the number of delivery mode for wastewater disinfection plants in general and the Stage 2A are the same (i.e., one).  No adjustment to the frequency of occurrence of incompatible mixing due to number of delivery mode is therefore proposed for the Stage 2A.

Number of Incompatible Chemicals to be Handled

 

14C.199            Three incompatible chemicals (sodium hypochlorite, sodium bisulphite, and ferric chloride) will be handled in SCISTW in Stage 2A.  There are two key concerns, i.e., mixing of sodium hypochlorite with ferric chloride solutions, and mixing of sodium bisulphite with ferric chloride solutions.  However, as discussed earlier, the sodium bisulphide tank farm is located remote from the ferric chloride tank farm at a different area of SCI.  Tanker delivery of sodium bisulphide solution to the sodium bisulphite tank farm will be made following a dedicated route that via an entrance/exit gate that is isolated from the ferric chloride tankers.  In essence, the sodium bisulphite tank farm is isolated from the other incompatible chemical.  Therefore, it may be argued that, for the purposes of the present discussion on frequency of incompatible chemical mixing, the proposed Stage 2A will involve only two incompatible chemicals.

14C.200            It is not clear from the incident review or survey the average number of incompatible chemicals that are handled in wastewater disinfection plants in general.  Obviously, the minimum would be “two”, but for those chlorination plants employing dechlorination, the number could be “three”, as in the case of the Stage 2A.

14C.201            On this basis, and considering the unique chemical delivery arrangement for the Stage 2A, it is considered that the number of incompatible chemicals handled at the Stage 2A would not be dissimilar to the average number of incompatible chemicals handled at wastewater disinfection in general.  Therefore, no adjustment to the frequency of occurrence of incompatible mixing due to number incompatible chemicals is therefore proposed for the Stage 2A.

Number of Chemical Delivery to be Made

 

14C.202            The Stage 2A in SCISTW is a facility of large scale (serving a population of about 5,230,000) when compared to wastewater disinfection facilities in general (each facility serves on average 655,351 people).  On a population basis, the Stage 2A is about eight times larger than an average wastewater disinfection plant.

14C.203            Owing to the larger scale of the Stage 2A operation, the amount of chemicals used in the Stage 2A and in turn, the number of chemical delivery operations would be larger than other wastewater disinfection facilities in general.  Therefore, in principle, an adjustment to the generic frequency of occurrence of incompatible mixing would be warranted.

14C.204            However, for the Stage 2A, owing to the large amount of chemical to be used, bulk transport (i.e. barge) has been chosen to be the main transportation mode for sodium hypochlorite.  As mentioned above, it is impossible for sodium hypochlorite delivered by barge to be unloaded to the sodium bisulphite or ferric chloride tanks (i.e. unloading error).

14C.205            In view of this barge delivery arrangement, the number of land delivery of sodium hypochlorite solution to the Stage 2A is greatly reduced (as this is restricted to situations when sea delivery is not possible due to adverse weather or other unforeseen conditions).  This means that while the number of chemical delivery to Stage 2A could be higher than a wastewater disinfection plant of average capacity or size, the magnitude of adjustment to the generic frequency should be moderate.

Conclusion

 

14C.206            The above assessment suggests that:

·           The Stage 2A would employ better than average chemical hazard/risk control measures, which would serve to lower the generic frequency of incompatible chemical mixing.

·           On the number of transportation mode and number of incompatible chemicals, no adjustment to the generic frequency is proposed.

·           The larger scale/capacity of the Stage 2A compared to wastewater disinfection plants in general would suggest that an upward adjustment to the generic frequency of incompatible mixing would be warranted.

 

The first and last point would act to counter-balance the adjustment factor to be applied to the Stage 2A.  In our opinion, for the case of the Stage 2A, the enhanced control measures would greatly reduce the frequency of occurrence of incompatible mixing, while the larger than average scale of the Stage 2A would only have limited impact on the generic frequency.

 

14C.207            Nevertheless, as a conservative approach, we propose to apply an upward adjustment factor of 8 to the generic frequency.  This means that the occurrence frequency of incompatible chemicals mixing incident of Stage 2A operations should be:

= 6.76 x 10-6 per year x 8

= 5.408 x 10-5 per year.

Reference to Estimated Frequency in other Relevant Project

14C.208            Reference is drawn to the approved EIA Study Report for the Hong Kong International Theme Park (HKITP), which provides relevant information on the occurrence frequency of incompatible chemical mixing in its on-site attractions water disinfection facility (also using sodium hypochlorite solution).  In this case, the frequency of occurrence of mixing sodium hypochlorite with an incompatible chemical has been estimated at 3 x 10-6 per year.

14C.209            In the HKITP operation, it was estimated that the operation of the attractions water disinfection facility would involve about 312 chemical deliveries per year.  For the Stage 2A operation, there will be a total of less than 1,602 road tanker deliveries of sodium hypochlorite (510), sodium bisulphite (312) and ferric chloride (780) per year to the SCISTW after the commencement of the Stage 2A Project, which is about 5 times of the deliveries made for the HKITP attractions water disinfection facility.

14C.210            In comparing the occurrence frequency and conditions of both HATS Stage 2A and the HKITP project, it can be seen that the adopted occurrence frequency of 5.408 x 10-5 per year for incompatible chemicals mixing scenario at HATS Stage 2A, which is over 10 times higher than the HKITP disinfection plant, would likely be a very conservative estimate.

Consequence Analysis and Risk Summation

Determination of Toxic Gas Generation Rate

 

14C.211            In HATS Stage 2A operation, the scenario “mixing of incompatible chemicals on-site” (due to error in chemicals unloading operation) could lead to one of the following events:

·           Sodium hypochlorite is unloaded into ferric chloride tank, generates chlorine gas – event 1

·           Ferric chloride is unloaded into sodium hypochlorite tank, generates chlorine gas – event 2

·           Sodium bisulphite is unloaded into ferric chloride tank, generates sulphur dioxide gas – event 3

·           Ferric chloride is unloaded into sodium bisulphite tank, generates sulphur dioxide gas – event 4

·           Sodium hypochlorite is unloaded into sodium bisulphite tank, generates heat (but no toxic gas would be generated) – event 5

·           Sodium bisulphite is unloaded into sodium hypochlorite tank, generates heat (but no toxic gas would be generated) – event 6

 

14C.212            The rate of toxic gas generation in events 1 to 4 is estimated by the following steps:

Determination of Toxic Gas Generation Reaction

 

14C.213            When ferric chloride solution is accidentally mixed with sodium hypochlorite / sodium bisulphite, the hydrochloric acid in the ferric chloride solution will react with sodium hypochlorite / sodium bisulphite to form chlorine / sulphur dioxide gas.  The chemical reactions and the molecular weight of the reactants are written as follows:

·           Sodium hypochlorite reacts with hydrochloric acid to give chlorine gas

NaOCl  +    HCl       à         NaOH +  Cl2

74.45         36.45                40.00      70.91

·           Sodium bisulphite reacts with hydrochloric acid to give sulphur dioxide gas

NaHSO3  +  HCl  à  H2O  +  NaCl  +  SO2

104.06         36.45    18.02    35.00      64.1

 

Determination of Density and Concentration of Involved Chemicals

 

14C.214            According to the Material Safety Data Sheet, the density and concentration of the involved chemicals are as follows:

·           Sodium hypochlorite – density: 1.2kg/L, concentration: 12% w/w

·           Sodium bisulphite – density: 1.33kg/L, concentration: 38% w/w

·           Ferric chloride – density: 1.3kg/L, concentration of HCl: 1% w/w

 

14C.215            Combining the unloading rate of the chemical with the abovementioned data, the toxic gas generation rate of the incompatible chemicals mixing events can be calculated.  The toxic gas generation rate is estimated based on the conservative assumption that there would be perfect mixing and reaction between the incompatible chemicals.  In the real situation, chemical is loaded from high level of storage tank (i.e. above the liquid level of chemical present in the tank) and therefore the loaded chemical and the chemical present in the tank will not perfectly mix with each other.  Hence, the toxic gas generation rates estimated below are at the conservative side.

14C.216            The calculation of toxic gas generation rate for events 1 to 4 is shown below:

I.            Event 1

14C.217            The sodium hypochlorite unloading rate by the road tanker is assumed to be 10L/s.  Therefore, there will be 10L x 1.2kg/L x 0.12 = 1.44kg of NaOCl unloaded to the ferric chloride tank per second.  Assuming there is sufficient HCl in the ferric chloride tank for the reaction, 1.44kg of NaOCl reacts with 0.71kg of HCl to give 1.37kg of Cl2.  Therefore, the Cl2 generation rate of 1.37kg/s is estimated for the event “sodium hypochlorite unloaded to ferric chloride tank”.

II.          Event 2

14C.218            The ferric chloride unloading rate by the road tanker is assumed to be 10L/s.  Therefore, there will be 10L x 1.3kg/L x 0.01 = 0.13kg of HCl unloaded to the sodium hypochlorite tank per second.  Assuming there is sufficient sodium hypochlorite in the sodium hypochlorite tank for the reaction, 0.27kg of NaOCl reacts with 0.13kg of HCl to give 0.25kg of Cl2.  Therefore, the Cl2 generation rate of 0.25kg/s is estimated for the event “ferric chloride unloaded to sodium hypochlorite tank”.

III.        Event 3

14C.219            The sodium bisulphite unloading rate by the road tanker is assumed to be 10L/s.  Therefore, there will be 10L x 1.33kg/L x 0.38 = 5.05kg of NaHSO3 unloaded to the ferric chloride tank per second.  Assuming there is sufficient HCl in the ferric chloride tank for the reaction, 5.05kg of NaHSO3 reacts with 1.77kg of HCl to give 3.11kg of SO2.  Therefore, the SO2 generation rate of 3.11kg/s is estimated for the event “sodium bisulphite unloaded to ferric chloride tank”.

IV.        Event 4

14C.220            The ferric chloride unloading rate by the road tanker is assumed to be 10L/s.  Therefore, there will be 10L x 1.3kg/L x 0.01 = 0.13kg of HCl unloaded to the sodium bisulphite tank per second.  Assuming there is sufficient sodium bisulphite in the sodium bisulphite tank for the reaction, 0.37kg of NaHSO3 reacts with 0.13kg of HCl to give 0.23kg of SO2.  Therefore, the SO2 generation rate of 0.23kg/s is estimated for the event “ferric chloride unloaded to sodium bisulphite tank”.

14C.221            The sodium hypochlorite, sodium bisulphite and ferric chloride storage tanks are atmospheric tanks equipped with vent pipe for venting of built up air / gas inside the tanks.  When the air / gas pressure inside the tanks is higher than the atmospheric pressure outside, the built up air / gas will be released to the outside atmosphere through the vent pipe.  Since the toxic gas will be continuously generated during the wrong chemical unloading operation, it is reasonable to assume that the toxic gas release rate (from storage tank to the atmosphere) is equal to the estimated toxic gas generation rate.

14C.222            The toxic gas generated will be released through the vent pipe at the top of the storage tank.  Therefore, the toxic gas will be released at an elevation level similar to the top of the storage tank.

V.          Events 5 and 6

14C.223            No toxic gas will be generated in Events 5 and 6.

14C.224            Table 14C.6 presents the summary of the estimated toxic gas generation rates in various incompatible chemicals mixing events.

Table 14C.6      Estimated Toxic Gas Generation Rates in Various Incompatible Chemicals Mixing Events

Event No.

Incompatible Chemicals Mixing Event

Toxic Gas Generation Rate

Toxic Gas Release Location

Toxic Gas Release Height

1

Sodium hypochlorite unloaded into ferric chloride tank

Chlorine, 1.37kg/s

Ferric chloride tank farm

13m above ground

2

Ferric chloride unloaded into sodium hypochlorite tank

Chlorine, 0.25kg/s

Sodium hypochlorite tank farm

12.5m above ground

3

Sodium bisulphite unloaded into ferric chloride tank

Sulphur dioxide, 3.11kg/s

Ferric chloride tank farm

13m above ground

4

Ferric chloride unloaded into sodium bisulphite tank

Sulphur dioxide, 0.23kg/s

Sodium bisulphite tank farm

6m above ground

5

Sodium hypochlorite unloaded into sodium bisulphite tank

No toxic gas generated

6

Sodium bisulphite unloaded into sodium hypochlorite tank

No toxic gas generated

Consequence and Risk Summation Modelling

14C.225            Risk Software SAFETI micro was used as the tool to execute the consequence analysis and risk summation.  SAFETI micro is a consequence and risk summation model, which can handle heavy gas (such as chlorine) dispersion and has been used in previous risk assessment studies in Hong Kong.  Description of the modelling works is presented below.

Frequency of the Incompatible Chemicals Mixing Events

 

14C.226            As presented above, the occurrence frequency of incompatible chemicals mixing incident of Stage 2A operations is estimated to be 5.408 x 10-5 per year and there are 6 possible incompatible chemicals mixing events in the Stage 2A operation.  The estimation of the distribution of the occurrence frequency of the mixing events is presented in the paragraphs below.

14C.227            The occurrence frequency of incompatible chemicals mixing incident, which is due to the chemical unloading error, is estimated to be 5.408 x 10-5 per year.  There would be 1,602 chemical unloading operations made per year and therefore each unloading operation would contribute to an occurrence frequency of 3.38 x 10-8 per year (= 5.408 x 10-5 / 1,602).

14C.228            As an example, if chemical unloading operation error occurs in a sodium hypochlorite delivery operation, the delivered chemical is not unloaded into the sodium hypochlorite tank (the intended storage tank) and would be unloaded into either sodium bisulphite or ferric chloride storage tank.  Since dedicated road tanker transport route will be assigned to each chemical, the probable cause of the unloading operation error would be the road tanker driver enters the SCISTW site at the wrong entrance.  It is considered that the probability of the road tanker driver enters the wrong entrance for ferric chloride tank farm would be the same to that for entering the wrong entrance for sodium bisulphite tank farm.  Hence, in case of chemical unloading operation error, the probability of delivered sodium hypochlorite being mistakenly unloaded into the ferric chloride tank would be equal to that being mistakenly unloaded into the sodium bisulphite tank, which is equal to 0.5.

14C.229            The situation is similar to chemical unloading error in sodium bisulphite and ferric chloride delivery operation.   Based on the above, the occurrence frequency of each incompatible chemicals mixing event can be estimated as shown in Table 14C.7:

Table 14C.7      Estimated Frequency of Various Incompatible Chemicals Mixing Events

Event No.

Event Description

Estimated Frequency

1

Sodium hypochlorite unloaded into ferric chloride tank

= 3.38 x 10-8 per year (freq. of error of an unloading operation) x 510 (no. of NaOCl unloading operation) x 0.5 (prob. of NaOCl being unloaded into ferric chloride tank, given error occurs) = 8.619 x 10-6 per year

2

Ferric chloride unloaded into sodium hypochlorite tank

= 3.38 x 10-8 per year x 780 (no. of FeCl3 unloading operation) x 0.5 (prob. of FeCl3 being unloaded into sodium hypochlorite tank, given error occurs) = 1.318 x 10-5 per year

3

Sodium bisulphite unloaded into ferric chloride tank

= 3.38 x 10-8 per year x 312 (no. of NaHSO3 unloading operation) x 0.5 (prob. of NaHSO3 being unloaded into ferric chloride tank, given error occurs) = 5.273 x 10-6 per year

4

Ferric chloride unloaded into sodium bisulphite tank

= 3.38 x 10-8 per year x 780 (no. of FeCl3 unloading operation) x 0.5 (prob. of FeCl3 being unloaded into sodium bisulphite tank, given error occurs) = 1.318 x 10-5 per year

5

Sodium hypochlorite unloaded into sodium bisulphite tank

= 3.38 x 10-8 per year x 510 (no. of NaOCl unloading operation) x 0.5 (prob. of NaOCl being unloaded into sodium bisulphite tank, given error occurs) = 8.619 x 10-6 per year

6

Sodium bisulphite unloaded into sodium hypochlorite tank

= 3.38 x 10-8 per year x 312 (no. of NaHSO3 unloading operation) x 0.5 (prob. of NaHSO3 being unloaded into sodium hypochlorite tank, given error occurs) = 5.273 x 10-6 per year

 

Total

5.408 x 10-5 per year

 

14C.230            Wrong chemical unloading operation would be detected by the installed toxic gas detectors (and alarm will be annunciated) and stopped rapidly by automatic closure of emergency shutdown valve upon alarm activation.  Also, as chemical supplier staff and SCISTW operator will be present throughout the chemical unloading operation, they can stop the chemical unloading operation by turning off the pump for pumping the chemical to the storage tank when they notice the activation of the alarm or other abnormal conditions.  With the gas detector activated automatic stoppage system, toxic gas emission could be stopped almost immediately and the staff on-site provides a backup mechanism to stop the wrong chemical unloading operation.  The time needed for the rapid stoppage of wrong chemical unloading operation is assumed to be 3 minutes.  This assumption of the time needed for isolation by such stoppage system (gas detector automatic stoppage system, with manual stoppage as backup) is consistent with previous risk assessment study (Meinhardt Infrastructure and Environment Limited, 2007) considering similar emergency shutdown systems.  Events 1b, 2b, 3b and 4b are developed to represent the above scenario.

14C.231            It is specified that the gas detector activated automatic stoppage system shall meet the Safety Integrity Level (SIL) 2.  According to International Electrotechnical Commission (1997), a system meeting SIL 2 shall have a failure probability (on demand) of 0.001 to 0.01.  Taking into account the extreme case of the automatic stoppage system failure, a failure probability of 0.01 (higher bound failure probability for SIL 2) is taken for the rapid stoppage of unloading operation.

14C.232            In the case of automatic unloading operation rapid stoppage failure, the chemical unloading operation can be stopped by the SCISTW operator by turning off the pump for pumping the chemical to the storage tank.  Nevertheless, it is conservatively assumed that the wrong chemical unloading operation does not stop until all chemical in the road tanker is unloaded.  Such scenario is represented by Events 1a, 2a, 3a and 4a.  Table 14C.8 presents the properties for each incompatible chemicals mixing event.

Table 14C.8      Properties of Various Incompatible Chemicals Mixing Events

Event No.

Release Material

Release Rate

Occurrence Frequency

Release Location

Toxic gas Release Duration

Rapid Stoppage of Wrong Unloading Operation?

1a

Cl2

1.37kg/s

8.619 x 10-8

Per year

FeCl­3 tank farm

1700sa

No

1b

Cl2

1.37kg/s

8.533 x 10-6

Per year

FeCl­3 tank farm

180sb

Yes

2a

Cl2

0.25kg/s

1.318 x 10-7

Per year

NaOCl tank farm

1700sa

No

2b

Cl2

0.25kg/s

1.305 x 10-5

per year

NaOCl tank farm

180sb

Yes

3a

SO2

3.31kg/s

5.273 x 10-8

per year

FeCl­3 tank farm

1700sa

No

3b

SO2

3.31kg/s

5.220 x 10-6

per year

FeCl­3 tank farm

180sb

Yes

4a

SO2

0.23kg/s

1.318 x 10-7

per year

NaHSO3 tank farm

1700sa

No

4b

SO2

0.23kg/s

1.305 x 10-5

per year

NaHSO3 tank farm

180sb

Yes

5

Event not modelled as no toxic gas is generated

6

Note: a The release duration is the time for the chemical delivery road tanker to unload all the chemical delivered, where the duration is = capacity of road tanker / chemical unloading rate = 17000L / 10L/s = 1700s

b The wrong unloading operation is rapidly stopped within 3 minutes (180s)

 

Evaluation of Toxic Impact

 

14C.233            In SAFETI micro, the probability of death due to toxic gas impact at a point is calculated by the “probit equation”, Pr = a + ln L, where “Pr” is the probit value, “a” is probit equation constant and “L” is the toxic load.  The Probit value can be transformed to probability of fatality.  For example, probit value of 6.28, 5.00, 3.12 and 2.67 corresponds to fatality probability of 90%, 50%, 3% and 1% respectively.

14C.234            Toxic load “L” is calculated by the following equation, where the toxic load at a particular point is dependent to the toxic gas concentration-time history over the point:

       T

L =  ò Cn dt,

       0

 

where “T” = duration of toxic gas exposure

           “C” = concentration of toxic gas

           “n” = probit equation constant

 

14C.235            The probit equation constants applied for chlorine and sulphur dioxide gas are listed as follows, which are consistent with the probit constants recommended by the Dutch Government (TNO, 1992):

 

a

n

Chlorine

-14.3

2.3

Sulphur Dioxide

-19.2

2.4

 

14C.236            The maximum duration of toxic gas exposure by population for each event is taken as 10 minutes.  Even in the cases of failure to stop the wrong chemical unloading operation rapidly (toxic release duration = 1700s), the population exposed to the released toxic gas shall be able to take action to escape from the toxic gas cloud rather than remains at the original location and exposes to the toxic gas for the whole toxic gas release period.  Therefore, the maximum duration of toxic gas exposure in these cases is limited to 10 minutes.  This assumption is consistent with the previous risk assessment studies for local water treatment works (ERM, 2001).

14C.237            According to the User Manual of SAFETI micro, in the case of toxic gas continuous release, the duration of the toxic cloud’s passage at various locations is equal to the duration of the toxic gas release.  Therefore, for Events 1b, 2b, 3b and 4b that rapid stoppage of wrong chemical unloading operation is successful, the toxic gas exposure duration is same as the toxic gas release duration (3 minutes).

Model Parameters

 

14C.238            The relevant SAFETI parameters are presented in Appendix 14.5.  By adopting the parameters, probability of fatality for a person indoor is 10% of that for a person remains at outdoor environment.

14C.239            By extracting the modelling results from SAFETI micro, the estimated fatality probability at various distances (at 25m interval) from the toxic gas release location for various events is presented in Figure 14C.13.  Table 14C.9 summarizes the distance from the toxic gas release location that receives a fatality probability of 3% and 1% under the weather class 1F (the weather class with worst dilution).

Table 14C.9      Distance from Toxic Gas Release Location Receiving Fatality Probability of  3% and 1% under Weather Class 1F

Hazardous Event

Distance to Receive  3% Fatality Probability

Distance to Receive 1% Fatality Probability

Event 1a

700m, from ferric chloride

tank farm

825m, from ferric chloride tank farm

Event 1b

425m, from ferric chloride

tank farm

500m, from ferric chloride

tank farm

Event 2a

175m, from sodium hypochlorite tank farm

225m, from sodium hypochlorite

tank farm

Event 2b

80m, from sodium hypochlorite tank farm

100m, from sodium hypochlorite

tank farm

Event 3a

110m, from ferric chloride

tank farm

150m, from ferric chloride tank farm

Event 3b

70m, from ferric chloride

tank farm

75m, from ferric chloride

tank farm

Event 4a

45m, from sodium bisulphite tank farm

50m, from sodium bisulphite tank farm

Event 4b

Fatality probability at 25m from sodium bisulphite tank farm is estimated to be lower than 1%

 

Population Input

 

14C.240            Since the outdoor population / indoor fraction of the population at locations listed in Table 14C.1 varies, adjustments were made for population input in SAFETI micro from locations to locations.  For the population at the primary school (location 14), a vulnerability factor of 3.3 is applied because the primary school population is considered as sensitive population, which is more susceptible to the toxic effect than average population.  Road population and marine population are considered as outdoor population, adjustments were also made for population input in SAFETI micro.

14C.241            After the population data, meteorological data and failure case was input, SAFETI micro combined these input data and characterized the risk levels in terms of individual risk (presented by individual risk contours) and societal risk (presented by FN curves and Potential of Loss of Life).

Evaluation of Hazard to Life Impact

Introduction

14C.242            Individual risk is a measure of the risk to a chosen individual at a particular location.  As such, this is evaluated by summing the contributions to that risk across a spectrum of incidents that could occur at a particular location.

14C.243            Societal risk is a measure of the overall impact of an activity upon the surrounding community.  As such, the likelihoods and consequences of the range of incidents postulated for that particular activity are combined to create a cumulative picture of the spectrum of the possible consequences and their frequencies.  This is usually presented as an FN curve and the acceptability of the results can be judged against the societal risk criterion under the risk guidelines.

14C.244            The hazard distance calculated by SAFETI micro was compared with the assessment results in previous hazard assessment study for local water treatment works.  It was found that modelling results by SAFETI micro are comparable to the previous assessment under weather class with neutral stability.  For weather class with stable condition, the modelling results of SAFETI micro were found to be more conservative than that of the previous hazard assessment study.  Details of the comparison of estimated hazard distance are presented in Appendix 14.6.

Assessment Results – Individual Risk

14C.245            The associated individual risk levels are shown in Figure 14C.14.  The risk levels are based on a 100%-occupancy, which can be referred from the user manual of SAFETI micro.  Two risk contours of individual risk level of 1x10-7, and 1x10-8 per year are shown.  As no off-site location would experience an individual risk level of greater than 1x10-5 per year, it can be concluded that the level of individual risk associated with the Project should be acceptable when compared to the individual risk guideline stipulated in Annex 4 of the EIAO TM.

Assessment Results – Societal Risk

14C.246            Table 14C.10 presents the FN results of each incompatible chemicals mixing event.  Fraction fatalities are conservatively rounded up to a fatality for presentation of the calculated societal risk in form of FN curve.

Table 14C.10    FN Results of Various Incompatible Chemicals Mixing Events

 

Cumulative Frequency (per year)

Event No.

Fatality of 1 or more

Fatality of 10 or more

1a

7.03E-08

1.18E-09

1b

3.40E-06

0

2a

1.27E-08

0

2b

3.64E-07

0

3a

3.74E-08

0

3b

9.10E-07

0

4a

3.74E-08

0

4b

1.70E-06

0

Note:        Cumulative frequency for fatality of 1 or more of all events = 6.53 x 10-6 per year

 

14C.247            The FN curves showing the societal risk level with fractional fatalities rounded up to 1 are shown in Figure 14C.15.  As shown in the figure, the societal risk associated with the Project was estimated to be in the “acceptable” region.  Therefore, it can be concluded that the level of societal risk associated with the Project should be acceptable when compared to the societal risk guideline stipulated in Annex 4 of the EIAO TM.

Conclusion

14C.248            Hazard to life impact associated with the proposed disinfection facilities at SCISTW was quantitatively assessed.  The risk of identified hazardous scenarios involving sodium hypochlorite and sodium bisulphite was quantitatively assessed, with consideration of identified precautionary measures / operation procedures that minimize the risks associated with the chemicals related operations.

14C.249            The individual risk and societal risk associated with the chemicals related operations were found to be acceptable in accordance with the risk guidelines stipulated in the Annex 4 of the EIAO TM.

 

References

 

1.                   Chlorine Institute (2003).  Sodium Hypochlorite Manual.

2.                   Clarence Valley Council (2005).  Yamba Sewerage Augmentation – Environmental Impact Statement.

3.                   ERM (2000).  Construction of an International Theme Park in Penny’s Bay of North Lantau together with its Essential Associated Infrastructures – Environmental Impact Assessment.

4.                   ERM (2001).  Reassessment of Chlorine Hazard for Eight Existing Water Treatment Works: Hazard Assessment for Au Tau Water Treatment Works.

5.                   Health Council of the Netherlands (2005).  Sodium hydrogen sulphite; Health-based Reassessment of Administrative Occupational Exposure Limits.

6.                   International Electrotechnical Commission (1997).  IEC 61508, “Functional Safety of Electrical / Electronic / Programmable Electronic Safety Related Systems”.

7.                   International Maritime Organization.  The International Maritime Dangerous Goods Code.

8.                   International Programme on Chemical Safety.  Poisons Information Monographs 495 – Sodium Hypochlorite.

9.                   Major Hazard Incident Data Service (MHIDAS)

10.               MCAL (2007). Harbour Area Treatment Scheme Environmental Impact Assessment Study for the Provision of Disinfection Facilities at Stonecutters Island Sewage Treatment Works – Investigation.

11.               Meinhardt Infrastructure and Environmental Limited (2007).l Environmental Assessment Services for Permanent Aviation Fuel Facility – Final EIA Report.

12.               New York City Department of Environmental Protection (2004).  Croton Water Treatment Plant – Final Supplemental Environmental Impact Statement.

13.               Ove Arup (2001).  EIA Study for Comprehensive Feasibility Study for the Revised Scheme of South East Kowloon Development

14.               TNO (1992).  Methods for the Determination of Possible Damage to People and Objects Resulting from Release of Hazardous Materials.  CPR 16E “The Green Book”.

15.               Thyer et al. (2002).  Bund Overtopping – The Consequence of Catastrophic Tank Failure.  Journal of Loss Prevention in the Process Industries 15 357-363.

16.               United Nations Population Division (2005).  2004 Revision World Population Prospects : The 2004 Revision Population Database.

17.               United States National Library of Medicine.  Hazardous Substances Data Bank.

18.               US Census Bureau.  Population Profile of the United States: 2000

 


 



[1] Sodium hydroxide is currently used for deodorization in the deodorizing facilities in the existing SCISTW operation. Such deodorizing facilities may be remained after the construction of HATS Stage 2A. Nevertheless, the usage and storage amount of sodium hydroxide during HATS Stage 2A would remain the same as the existing SCISTW operation because no further facilities involving use of sodium hydroxide would be constructed and operated. Therefore, sodium hydroxide is not considered within the scope of the Project.

[2] Ferric Chloride is not classified as DG under the Dangerous Goods Ordinance in Hong Kong.

[3] New York City Department of Environmental Protection (2004).  Croton Water Treatment Plant – Final Supplemental Environmental Impact Statement.

[4] Clarence Valley Council (2005).  Yamba Sewerage Augmentation – Environmental Impact Statement.

[5] According to USEPA (1999) “Risk Management Program Guidance for Offsite Consequence Analysis”, the spilled chemical can be assumed to spread instantaneously to a depth of 10mm in an undiked area.  Therefore, the 100m3 of sodium hypochlorite in the day tank,  if spilled and allowed to spread freely in undiked area, the radius of the spilled pool                 =Ö(100m3/p 0.01m) = 56m.

 

[6] The vibration sensing system will raise an alarm signal to the plant operators when excessive vibrations (exceeding 25mm/s peak particle velocity and 0.2mm amplitude) are detected.  Upon receiving the alarm signal, the operators will stop pumping immediately.

[7] As seen in Figure 14C.9A, the cross section area of the service duct is = 3m x 2.2m = 6.6m2.  Assume 15% of the area (i.e. 1m2) is occupied by the ferric chloride pipeline and other cables installed in the service duct. As the length of the service duct is about 440m, the volume of the service duct is about 5.6 x 440 = 2,464m3.

[8] The membrane would be made of high-density polyethylene (HDPE), which is a high-tensile strength, impermeable, and corrosion-resistant material that is typically used in landfill liners.

[9] A geotextile fabric, similar to felt in appearance, has high puncture resistance, large elongation before break, and drainage characteristics will be used to separate the impervious membrane from the concrete surface.

[10] With consideration of the space taken by the two sodium hypochlorite feed pipes (with outer diameter of 370mm for sleeve pipeline), the void space of pipe trench would be about = (1.7m2 x 357m) – 2 x (p x 0.1852 x 357) = 530m3.  Since the feed pipelines will not be filled except during the barge unloading process, the cavity of the duty feed pipeline will provide space to contain the sodium hypochlorite being unloaded.  With diameter of 250mm for the delivery pipe, the cavity of the delivery pipe is about (p x 0.1252 x 357)m3 = 17.5m3.  Therefore, the available void space for containment of the unloading sodium hypochlorite would be about 530m3 + 17.5m3 = 547.5m3.    

[11] The diameter of the dosing pipeline and sleeve pipeline is 150mm and 270mm respectively.

[12] The anticipated mitigation measures shall be further reviewed and developed in the detailed design stage