3                   Air Quality Impact

3.1              introduction

3.1.1           This section presents the air quality assessment for the construction and operational phases of the proposed Project. Dust impact from the construction activities of the STW and the terrestrial components of the outfalls was identified as the key issue of concern during construction phase. The operational phase air quality impact would be the odour emitted from the operation of the STW upgrade.

3.2              Relevant Legislation, Policies, Plans, Standards and Criteria

Air Pollution Control Ordinance

3.2.1           The Air Pollution Control Ordinance (APCO) (CAP. 311) is the principal legislation for the management of air quality. The maximum acceptable Total Suspended Particulates (TSP) concentration, as defined in the Air Quality Objectives (AQOs) encompass by the APCO, is listed in Table 3-1.

Table 3-1       Hong Kong Air Quality Objectives

Pollutant (mg/m3)

Average Time

24-hour (2)

Annual (3)

TSP (1)

260

80

Notes:       (1)    Measured at 298 K (25 oC) and 101.325 KPa (one atmosphere).

                                       (2)    Not to be exceeded more than once per year.

                                       (3)    Arithmetic means. 

 

Technical Memorandum on EIA Process

3.2.2           The criteria for assessing air quality impacts are specified in Annex 4 of the TMEIAP, of which the following criteria are relevant to this study:

(a)       meet the Air Quality Objectives and other standards established under the Air Pollution Control Ordinance;

(b)       meet hourly TSP concentration of 500 microgrammes per cubic metre measured at 298 K (25 ºC) and 101.325 KPa (one atmosphere) for construction dust impact assessment; and

(c)       meet 5 odour units based on an averaging time of 5 seconds for odour prediction assessment.

Other Legislation

3.2.3           Another air pollution regulation related to this Study is the Air Pollution Control (Construction Dust) Regulation 1997 which stipulates the dust reduction measures required to be carried out in construction phase.

3.3              Description of Environment

Existing and Planned Land Use

3.3.1           The vicinity of the Peng Chau STW study area is mostly rural in nature.  Major air pollution source is not identified in the vicinity of the assessment area.  Existing land use includes residential use, recreational use, green belt and ancillary (or site) office of the Peng Chau Refuse Transfer Station (PCRTS).  The open land areas between Sea Crest Villa and Kam Peng Estate has been zoned for open space and comprehensive residential development including commercial complex, and residential development. Another two areas in the north of Kam Peng Estate were reserved for comprehensive development and educational use.  The implementation programme of these zoned lands are, however, not definite.

Background Air Quality

3.3.2           EPD has performed routine air quality monitoring over the territories. However, there is no air quality monitoring station (AQMS) in the immediate vicinity of the Project area.  The nearest AQMS is at Tung Chung located on the northern Lantau Island. The air quality data obtained from Tung Chung AQMS station is selected to represent the background air quality of Peng Chau as both places are located at the southwest of the territories. The meteorological conditions of these two locations are similar.  The ambient air quality data from the Tung Chung AQMS for the year 2002 are summarized in Table 3-2.

Table 3-2       Background Air Quality at Tung Chung AQMS for 2002

Pollutant

Annual Average (mg/m3)

Total Suspended Particulates (TSP)

62

Respirable Suspended Particulates (RSP)

46

Sulphur Dioxide (SO2)

18

Nitrogen Dioxide (NO2)

43

Carbon Monoxide (CO)

612

Source:             (EPD, 2003)

 

3.3.3           The Hong Kong Electric Co. Ltd (HEC) and CLP Power Hong Kong (CLP) have also performed territory wide air quality monitoring on sulphur dioxide and nitrogen dioxide. The closest monitoring station to the Project area is located in Penny’s Bay by CLP. The monitoring activities of Penny’s Bay station have been discontinued since May 2000, and no monitoring data are available since then. The monitoring results are presented in Table 3-3.

Table 3-3       Air Quality Monitoring at Penny’s Bay (2000)

Pollutant

Annual Average (mg/m3)

Range of Monthly Mean Concentration (mg/m3)

Sulphur Dioxide (SO2)

9

6-14

Nitrogen Dioxide (NO2)

48

34-59

Source: (EPD, 2001a)

3.3.4           The monitoring data from both Tung Chung and Penny’s Bay suggest that the air quality in Lantau and its close proximity is generally good.

Odour Climate at Tai Lei Island

3.3.5           The existing Peng Chau STW at Tai Lei Island, and the PCRTS to the east of the Island are potential odour sources. Both facilities have been operated for some time and no odour complaints had been received in the vicinity of the Tai Lei Island (DSD, 2003). The background odour nuisance in the study area is considered not significant.

3.4              Air Sensitive Receivers

3.4.1           Air Sensitive Receivers (ASRs) are defined in Annex 12 of TMEIAP. ASRs include any domestic premises, hotel, hostel, hospital, clinic, nursery, temporary housing accommodation, school, education institution, office, factory, shop, shopping centre, place of public worship, library, court of law, sports stadium or performing arts.  Any other premises or place with which, in terms of duration or number of people affected, has a similar sensitivity to the air pollutants as the aforelisted premises and places are also considered as an ASR.

3.4.2           The study area of the air quality impact assessment is defined as 500 m from the pollution source.  The construction areas mainly include the area of Peng Chau STW, Tai Lei Island and Tai Lei Bridge. However, the magnitude of dust emission from pipe laying on Tai Lei Island section outside the STW is localised and insignificant, the study area is thus defined as 500m from the boundary of the STW (Figure 3-1). The ASRs identified in the vicinity of the assessment area during construction phase and operational phase are listed in Table 3-4. The sensitive use and approximate height of ASRs as well as the approximate horizontal distances from the boundary of the pollution sources are also provided in Table 3-4.  The locations of the identified ASRs are also shown in Figure 3-1

Table 3-4        Identified Air Sensitive Receivers

ASR

Location

Sensitive Use

Approx. Distance from Construction Site boundary  (m)

Approx. Distance from Operational Site boundary (m)

Approx. Height (m)

Approx. Grade Level  (mPD)

ASR 1

Peng Chau Refuse Transfer Station

Ancillary/site Office

10

10

10

5.2

ASR 2

Sea Crest Villa

Block D

Residential

170

170

16

4.5

ASR 3

Sea Crest Villa

Block C

Residential

180

180

16

4.5

ASR 4

Sea Crest Villa

Block B

Residential

200

200

16

4.5

ASR 5

Sea Crest Villa

Block A

Residential

190

190

16

4.5

ASR 6

Temporary Structure in the North of Kam Peng Estate

Residential

340

340

4

3.8

ASR 7

Kam Peng Estate

Residential

370

370

30

4.7

ASR 8

Peng Lai Court

Residential

410

410

30

4.3

ASR 9*

Area for future educational use in the north of Kam Peng Estate

Educational (under planning)

--

440

--

8.0

ASR 10*

Area for future residential use in the north of Kam Peng Estate

Residential (Group B) (under planning)

--

370

24

4.3

ASR 11*

Newly reclaimed land in the south of Sea Crest Villa

Residential and Commercial (under planning)

--

290

8

3.4

Note: * No implementation programme is available.  Not anticipated to be an ASR during construction stage.

 

3.4.3           The planned ASRs 9, 10 and 11 are not considered as construction phase ASRs as no implementation schedule is available at the time of this Study. However, they are considered as operational phase ASRs.

3.5              impacts identification

Construction Phase

3.5.1           Fugitive dust is likely to be associated with construction activities such as site excavation and backfilling, foundation works, civil construction, demolition of building structures and on-site concrete batching. They are also contributed by spoil handling, construction and demolition materials handling and transportation, wind erosion of stockpiles and open site erosion. The replacement of inlet pumping main would be carried out on the service trough on the Tai Lei Bridge and the magnitude of pipe laying for Tai Lei Island section outside the STW is not considered to be significant. The layout of the dust emission sources is depicted in Figure 3-2.

3.5.2           The potential dust impacts from submarine outfall would likely be the construction of land components, which would involve destruction of seawall for the pipelaying.  For the worst case assessment, it is assumed that similar activities as section 3.5.1 would be carried out in the vicinity of seawall.

3.5.3           The construction works will be divided into two phase: Phase 1 works include all major construction activities, while some minor remaining works and demolition works will be carried out in Phase 2 (Figure 2-6).  Both phases of works cover the same portion of site area.  From dust impact assessment point of view, the nature of works during both phases is similar and considered as general construction activities.  Therefore, the dust impact assessment presented in this section represents the predicted condition for both phases.

3.5.4           Construction of Peng Chau Helipad would be carried out from March 2005 to February 2006, which fall within the construction schedule of most STW Upgrade activities. However, construction dust from Peng Chau Helipad Project was not considered as a key issue and therefore dust impact from the helipad project is considered insignificant.  In addition, no other dust emitting activities has been identified to be carried out concurrently with the proposed Project.

Operational Phase

3.5.5           The key potential air quality impact during the operational phase of the STW Upgrade would be odour arising from sewage treatment facilities, as well as  sludge treatment and handling facilities.  Major odour sources identified in the proposed STW are shown in Figure 3-3 and include:

(a)    inlet works;

(b)    grit chamber;

(c)     equalisation tank;

(d)    Sequencing Batch Reactor (SBR);

(e)     sludge thickener;

(f)      sludge digester;

(g)    screening and grit storage room and return liquor pumping station (referring to engineering design, these will be enclosed by air-tight covers and deodourisation units will be provided), and

(h)    sludge drying bed (to be used as standby unit at initial stage and will serve as duty unit in later stage.  Details refer to Appendix 2C

3.5.6           The civil component of the total five SBR units will be constructed.  However, the E&M equipment for the SBR No. 5, which is for future expansion, will not be installed.  Therefore, only four SBR units can be put into operation during the commissioning of the Project.  In spite of this, for the planning worst scenario, it will be assumed in the odour impact assessment that the SBR No. 5 will also be operated.   

3.5.7           As the digested sludge will be carried by enclosed container during transportation which is the common practice of DSD, the odour impact from sludge export activities is considered insignificant.

3.5.8           As the sludge pumping system is confined in pipes, the odour emission from it is considered insignificant.

3.5.9           Odour emission from effluent pumping station and chlorine contact tank is considered insignificant as the sewage would have undergone secondary treatment.

3.5.10       There will be no concurrent operation of the existing and upgraded STW.  The upgraded plant will be commissioned and will then replace the existing plant when the construction works are completed. 

3.5.11       The capacity of the proposed duty units will be able to achieve the designed capacity of the STW Upgrade.  Therefore, no concurrent operation of duty and standby units is anticipated.

3.5.12       Another potential operational phase impact in the study area would be the odour emitted from the Peng Chau Sewage Pumping Station (PCSPS) and PCRTS (Figure 3-4).  The odour source from PCSPS will be taken into account in the study.  The PCRTS is not considered as significant odour source as stated in Section 3.3.5 and it would not be included in the modeling assessment.

3.6              Assessment Methodology

Construction Phase

Dispersion Model

3.6.1           To determine the extent of impacts from the construction of the Project, the Industrial Source Complex (ISC) short-term dispersion model (Version 4.0), which has been developed and validated by the United States Environmental Protection Agency (USEPA), and accepted by EPD for regulatory applications, was used to predict the potential impacts arising from the construction activities. 

Dust Emission Factor/Rate

3.6.2           The type and magnitude of the dust-generating activities were reviewed to estimate the dust emission rate during construction phase.  The estimate of dust emission was based on typical values and emission factors from USEPA’s Compilation of Air Pollution Emission Factors (AP-42), 5th Edition (USEPA, 1995). As a conservative simulation, general construction activities, on-site concrete batching and open sites wind erosion were considered as the major dust emission sources in construction phase.  The calculation of construction dust emission factor/rate is detailed in Appendix 3A.

Assumption

3.6.3           In assessing the dust impacts during construction phase, the following assumptions were made:

(a)       The study area is defined as rural.

(b)       For a more conservative (higher) dust concentration prediction, no dry or wet depletion is considered in the model run.

(c)       The daily working hours would be from 08:00 to 18:00.  The TSP emission period of the general construction activities was assumed for the same period (10 hours in total).  However, 24 hours TSP emission period for open site wind erosion is assumed.

(d)      The concrete batching plant is assumed to be operated from 1300 to 1700 (4 hours a day).

(e)       There will be no on-site construction traffic.  The transportation of construction waste/material will be carried out by small size and hand-operated equipment. Dust generated from site vehicle movement is thus considered negligible.

Background TSP

3.6.4           A value of 87 µg/m3 as suggested in Guideline on Accessing the “Total” Air Quality Impacts (EPD, 2000) for rural areas is adopted as the background TSP concentration for total predicted TSP concentration.  

Meteorological Data

3.6.5           Meteorological data from Cheung Chau Automatic Weather Station provided by the Hong Kong Observatory for the year 2001 was used as one of the input parameters dispersion model.  The following data were provided:

(a)    flow vector of wind;

(b)    wind speed;

(c)     air temperature;

(d)    atmospheric Pasquill stability class; and

(e)     morning mixing height and maximum mixing height recorded at King’s Park.

Since the Cheung Chau meteorological station is on the peak of a small steep hill and Cheung Chau itself is small, the wind measured at that station may not be strongly affected by the land surface.  As a result, the anemometer height is referred to 95m above ground level for the dispersion modelling.

Assessment Height

3.6.6           Since the construction activities would be undertaken at ground level and underground level, the worst dust impact on the ASRs would be at ground floor of the ASRs.  The height of 1.5m above ground, which is the breathing level of human, was adopted for construction dust impact assessment.

Operational Phase

Dispersion Model

3.6.7           Similar to the dust dispersion modelling, the ISC was also used to predict the potential odour impacts arising from the operation of the STW.

Emission Rate

3.6.8           The odour emission rate from STW can be estimated by the equation adopted for the odour assessment presented in the Final Assessment Report of Outlying Islands Sewerage Stage 1 Phase 1 – EIA Study (OISEIA).  This approach was also adopted for the estimation of odour emission rates from STW in some previous EIA studies in Hong Kong, such as Sha Tin Sewage Treatment Work Stage III Extension –EIA Report, 1999 and Sok Kwu Wan Sewage Collection, Treatment and Disposal Facilities – EIA Report, 2002.  The emission rates were calculated based on two relationships: the established relationship between odour concentration and physical factors and the volumetric emission flow rate compared with the rate of ventilation.

DF                     =              1.6 x (T/10)4.9 x (ORP + 200)-0.59          Equation 1

E                        =              DF x A x (V/3600) x Cf                       Equation 2

 

Where

DF

=

Odour concentration expressed as dilution factor, OU/m3

 

T

=

Temperature of sewage, Fehrenheit (F)

 

ORP

=

Oxidation-reduction potential of sewage, mV

 

E

=

Emission rate, OUs-1

 

A

=

Air volume of the emission source, m3

 

V

=

Ventilation rate, air changes per hour

 

Cf

=

Correction factor to adjust emission rates in the ratio of design ventilation rate to that used in the derivation of Equation 1 (for 10 air changes per hour, Cf=0.26; for 5 air changes per hour, Cf=0.52)

 

 

 

 

The following sewage characteristics were adopted for a worst case analysis:

T

=

30 oC

ORP

=

50 mV for septic sewage (recommended in OISEIA)

air volume

=

0.5m x sewage surface area

 

3.6.9           Based on the above equations and assumptions, the emission rates for the proposed STW are calculated and provided in Appendix 3B.

Emissions from Peng Chau Sewage Pumping Station

3.6.10       As advised by DSD, the average inlet hydrogen sulphide concentration of PCSPS is 5 ppm and the removal efficiency of the deodourisers is 99.5% (Appendix 3B).

Conversion of 1-Hour Average to 5-Second Average

3.6.11       Due to the short exposure period tolerable by human, odour impact assessment is based on a 5-second average level.  However, the shortest averaging period of ISC is 1-hour.  Conversion of model output from 1-hour average to the required 5-second average is needed.  The 1-hour average odour concentration is first converted to 3-minute average by the power law relationship which is related to the stability.  To further convert 3- minute average to 5-second average, a multiplying factor of 10 was applied for those hours with atmospheric stability classes A to B, and a factor of 5 for those hours with stability classes C to F (Table 3-5) (Maunsell, 1999). 

Table 3-5       Odour Level Conversion Factors

Stability Class

Conversion Factors

1-hr to 3-min average

(A)

3-min to 5-sec average

(B)

Overall

(A X B)

A

2.23

10

22.3

B

2.23

10

22.3

C

1.70

5

8.50

D

1.38

5

6.90

E

1.31

5

6.55

F

1.31

5

6.55

 

Assumption

3.6.12       In assessing the odour impacts during operational phase, the following assumptions were made:

(a)           The study area is defined as rural.

(b)           For a more conservative (higher) odour concentration prediction, no dry or wet depletion is considered in the model run.

Meteorological Data

3.6.13       The same set of meteorological data for the dust dispersion modelling (described in Section 3.6.5) was adopted. 

Sensitive Receivers

3.6.14       The highest air sensitive receiver identified is about 30m above ground level. Therefore, the assessment height are taken as 1.5m, 5m, 10m, 15m, 20m, 25m, 30m above ground.

3.7              Impact Assessment

Construction Phase

Unmitigated Scenario

3.7.1           The worst unmitigated TSP concentration at nearby ASRs is presented in Table 3-6. Figures 3-5 and 3-6 show the TSP concentration contour for 1-hour and 24-hour average respectively.  A typical TSP modeling output file is shown in Appendix 3C.

Table 3-6       Worst TSP Concentrations (Unmitigated Scenario)

ASR

TSP Concentration (µg/m3)

1-hour average

24-hour average

ASR 1

3734

431

ASR 2

1230

143

ASR 3

1184

140

ASR 4

1256

143

ASR 5

1331

147

ASR 6

536

119

ASR 7

444

102

ASR 8

303

97

 

3.7.2           The predicted results show that the 5 closest ASRs would be impacted by elevated dust generated from the construction site. Exceedance of 1-hour average TSP concentration (500 µg/m3) at ASRs of the PCRTS and Sea Crest Villa are observed.  At the PCRTS, the predicted dust concentration also exceeds the 24-hour average TSP concentration of 260 µg/m3. Mitigation measures are required to be implemented to reduce the potential dust impacts.

Mitigated Scenario

3.7.3           Typical dust control methods include ground watering, equipment and vehicle watering, proper handling of material and stockpile, and fencing of construction site. In the mitigated scenario, it is assumed that the construction site area will be watered to reduce the fugitive dust.  In accordance with Control Techniques for Particulate Emissions from Stationary Sources -Volume 2  (USEPA-1982), reduction of dust generation by about 90% can be achieved when the construction site is kept in wet condition.

3.7.4           The TSP concentrations are predicted with the proposed mitigation measures at the identified ASRs. Dust levels are reduced significantly and the predicted results are shown in Table 3-7Figures 3-7 and 3-8 show the TSP concentration contours for 1-hour and 24-hour average respectively.

Table 3-7       Worst TSP Concentrations (Mitigated Scenario)

ASR

TSP Concentration (µg/m3)

1-hour average

24-hour average

ASR 1

487

125

ASR 2

212

93

ASR 3

207

93

ASR 4

215

93

ASR 5

224

93

ASR 6

128

90

ASR 7

126

89

ASR 8

111

88

 

3.7.5           With the implementation of fugitive dust control measures, the worst 1-hour TSP concentrations at identified ASRs range from 111 to 487 µg/m3, which comply with the recommended construction dust standards stipulated in the TMEIAP.  Identified ASR at the adjacent refuse transfer station is also in compliance with AQO standard.

Operational Phase

Unmitigated Scenario

3.7.6           To assess the potential odour impact, the conversion factors from 1-hour average to 5-second average concentration is incorporated into the model input.  The predicted odour levels at the identified ASRs are summarised in Table 3-8. A typical odour modeling output file is shown in Appendix 3D. The worst odour levels are predicted to be occurred at 1.5m and the odour contours are shown in Figure 3-9.

Table 3-8       Worst 5-second Average Odour Levels (Unmitigated Scenario)

Assessment Point

Assessment Height above Local Ground Level

1.5m

5m

10m

15m

20m

25m

30m

ASR 1

29

18

12

6

3

1

1

ASR 2

9

6

3

3

3

2

2

ASR 3

7

5

3

3

2

2

2

ASR 4

8

6

3

2

2

2

2

ASR 5

13

9

4

2

2

2

1

ASR 6

2

2

1

1

1

1

1

ASR 7

5

4

3

1

1

1

1

ASR 8

5

4

4

2

1

1

1

ASR 9

7

6

4

2

1

1

1

ASR 10

9

7

4

2

1

1

1

ASR 11

11

8

3

2

2

2

2

 

3.7.7           Elevated odour levels are predicted at some identified ASRs.  As the odour level exceed the 5 OU criteria stipulated in TMEIAP, mitigation measures are necessary.

Mitigated Scenario

3.7.8           To mitigate the potential odour impact, major odour sources, including inlet works, grit chamber, equalization tank, SBR, sludge thickener, sludge digester, screening and grits storage area and return liquor pumping station would be enclosed by air-tight covers. All the odour emission from the enclosed sources would be ventilated to a deodorization unit.  The deodourisation unit would be able to achieve an odour removal efficiency of 99.5%.  The treated gas would then be emitted vertically via a 0.4m diameter duct and at 4m above ground level (at the roof level of the odour control room).  The predicted maximum odour level is shown in Table 3-9.  The odour level contours at 10m where worst odour levels are predicted, are presented in Figure 3-10.

Table 3-9       Worst 5-second Average Odour level (Mitigated Scenario)

Assessment Point

Assessment Height above Local Ground Level

1.5m

5m

10m

15m

20m

25m

30m

ASR 1

0.1

0.1

0.1

0.1

0.1

0.0

0.0

ASR 2

0.2

0.3

0.3

0.3

0.1

0.1

0.1

ASR 3

0.2

0.2

0.3

0.2

0.1

0.1

0.1

ASR 4

0.1

0.1

0.1

0.1

0.1

0.1

0.1

ASR 5

0.2

0.3

0.4

0.3

0.1

0.1

0.1

ASR 6

0.3

0.2

0.2

0.2

0.1

0.1

0.1

ASR 7

1.1

1.0

1.6

0.6

0.1

0.0

0.0

ASR 8

2.4

3.3

3.9

1.7

0.0

0.0

0.0

ASR 9

0.1

0.1

0.1

0.1

0.1

0.1

0.1

ASR 10

0.6

0.6

0.5

0.4

0.1

0.1

0.1

ASR 11

0.4

0.6

0.9

0.6

0.2

0.1

0.1

 

3.7.9           With the implementation of mitigation measures, it can be seen the predicted odour level would comply with the 5 OU criterion stipulated in TMEIAP.

Unmitigated Scenario (when Drying Bed is used)

3.7.10    As mentioned in Appendix 2C, the sludge dewatering operation will be reversed (i.e. drying bed as duty and export as standby) when the sludge export option turns out to be expensive due to the build up of sludge amount in the future.  In this scenario, drying bed is considered to be the duty unit and the associated odour impact will be assessed in the assessment. 

3.7.11    In this unmitigated scenario, drying bed will be left open, while other major odour sources mentioned in Section 3.7.8 will still be covered.  The predicted maximum odour level is shown in Table 3-10.  The odour level contour at the worst height (i.e. 1.5m) is presented in Figure 3-11.


Table 3-10        Worst 5-second Average Odour level (Unmitigated Scenario) (when Drying Bed is used)

Assessment Point

Assessment Height above Local Ground Level

1.5m

5m

10m

15m

20m

25m

30m

ASR 1

21

15

6

1

0

0

0

ASR 2

3

2

2

1

1

1

1

ASR 3

3

2

1

1

1

1

1

ASR 4

6

4

2

1

1

1

1

ASR 5

5

3

2

1

1

1

1

ASR 6

2

2

1

1

1

1

0

ASR 7

5

4

2

1

0

0

0

ASR 8

4

4

4

2

1

0

0

ASR 9

4

3

2

1

1

1

0

ASR 10

4

3

2

1

1

0

0

ASR 11

2

1

1

1

1

1

1

 

3.7.12    Elevated odour levels are predicted at the ASR 1 and 4, namely the PCRTS and Sea Crest Villa Block B.  As the odour level exceed the 5 OU criteria stipulated in TMEIAP, mitigation measures are necessary.

Mitigated Scenario (when Drying Bed is used)

3.7.13    To mitigate the potential odour impact, drying bed, like other odour sources, will be enclosed.  The collected gas will be diverted to the odour control unit.  The modeling results are summarised in Table 3-11 and the odour level contours at 10m where worst odour levels are predicted, are presented in Figure 3-12.

Table 3-11     Worst 5-second Average Odour level (Mitigated Scenario) (when Drying Bed is used)

Assessment Point

Assessment Height above Local Ground Level

1.5m

5m

10m

15m

20m

25m

30m

ASR 1

0.1

0.1

0.1

0.1

0.1

0.1

0.0

ASR 2

0.2

0.3

0.3

0.3

0.1

0.1

0.1

ASR 3

0.2

0.2

0.3

0.2

0.1

0.1

0.1

ASR 4

0.1

0.1

0.1

0.1

0.1

0.1

0.1

ASR 5

0.2

0.3

0.4

0.3

0.1

0.1

0.1

ASR 6

0.3

0.2

0.2

0.2

0.1

0.1

0.1

ASR 7

1.1

1.0

1.6

0.6

0.1

0.0

0.0

ASR 8

2.4

3.3

3.9

1.7

0.0

0.0

0.0

ASR 9

0.1

0.1

0.1

0.1

0.1

0.1

0.1

ASR 10

0.6

0.6

0.5

0.4

0.1

0.1

0.1

ASR 11

0.4

0.6

0.9

0.6

0.2

0.1

0.1

 

3.7.14    With the implementation of mitigation measures on drying bed, it can be seen the predicted odour level would also comply with the 5 OU criterion when the drying bed is used.           

3.8              Cumulative Impacts

Construction Phase

3.8.1           Air quality is not considered as a key issue for construction of Peng Chau Helipad Project.  It is not anticipated there would be any cumulative impacts during construction phase.

Operational Phase

3.8.2           The PCSPS would have odour impact to the surrounding ASRs during the operation of Peng Chau STW Upgrade.  However, the proposed pumping station is enclosed and equipped with odour control equipment with over 99.5% odour removal efficiency.  As confirmed by the modelling results, the overall odour impacts at the all identified ASRs are acceptable. 

3.9              Mitigation Measures

Construction Phase

3.9.1           The suitable requirements stipulated in the Air Pollution Control (Construction Dust) Regulation shall be implemented during the construction activities to minimise the dust impact. It is recommended that typical dust control methods including the following good site practices should also be incorporated during construction phase:

(a)       Stockpiles of imported material kept on site shall be contained within hoarding, dampened and/or covered during dry and windy weather.

(b)       Material stockpiled alongside trenches should be covered with tarpaulins.

(c)       Stockpile of cement should be covered entirely by impermeable sheeting.

(d)      All dusty materials shall be sprayed with water prior to any loading, unloading or transfer operation so as to keep the dusty materials wet.

(e)       Water sprays shall be used during the delivery and handling of sands aggregates and the like.

(f)        All demolished items that may emit dust particles should be covered entirely by impervious sheeting or placed in an area sheltered on the top and the 3 sides within a day of demolition.

Operational Phase

3.9.2           The recommended mitigation measure to minimize potential odour impact is to enclose all the major odour sources, including the drying bed.  The SBR No. 5, which is for future expansion, will be enclosed when it is put into operation.  The sludge should be carried by enclosed container during sludge transportation.  The odorous gas should then be collected and treated by deodorization unit with odour removal efficiency of more than 99.5%.  With the implementation of these mitigation measures, the odour impacts to the identified ASRs would become insignificant.

3.10         residual impacts

3.10.1       With the implementation of the recommended mitigation measures for both the construction and operational phases, no residual adverse air quality impacts are anticipated.

3.11         Environmental Monitoring and Audit

Construction Phase

3.11.1       The predicted TSP results show that exceedance of recommended TSP levels would occur during most of the construction period. Mitigation Measures are therefore needed to reduce the dust impact to an acceptable level. EM&A is recommended to be carried out during construction phase to ensure the effectiveness of the mitigation measures implemented by the Contractor.  Detailed EM&A requirement for construction dust is provided in the separate EM&A Manual prepared under the Project.

Operational Phase

3.11.2       As all the odorous gas would be collected and properly treated by deodorization unit with 99.5% odour removal efficiency and no odour impact would be anticipated, EM&A during operational phase of Peng Chau STW Upgrade would not be required.

3.12         conclusions and recommendations

Construction Phase

3.12.1       Dust generating activities were identified and evaluated. If un-mitigated, construction of the Peng Chau STW upgrade would have short term adverse impact on air quality, in the forms of fugitive dust emission at the identified ASRs. The predicted construction dust impact exceeds the 1-hour and 24-hour TSP criteria at some ASRs. Mitigation measures including watering of on-site construction area are expected to limit fugitive dust levels to acceptable levels. Model simulation results show that the 1-hour and 24-hour TSP criterion could be met after implementing the recommended mitigated measures.

3.12.2       The implementation of the Air Pollution Control (Construction Dust) Regulation and good site practice during construction phase are recommended.

Operational Phase

3.12.3       During the operational phase of the Project, all the potential odour generating facilities such as inlet works, grit chamber, equalization tank, SBR, sludge thickener, sludge digester, screening and grits storage area, return liquor pumping station and sludge drying bed would be enclosed by air-tight covers. Odourous gas would be ventilated to the deodourisation facility for further treatment before discharge. The deodourisation facility would be capable of removing 99.5% of odour.  During sludge transportation, it is recommended that the sludge should be carried by enclosed container to avoid unacceptable odour nuisance.

3.12.4       With the above mitigation measures, no unacceptable odour impact would be envisaged.