3. AIR QUALITY IMPACT

3.1 Legislation, Policies, Plans, Standards, and Criteria

3.1.1 The air quality impact assessment criteria make reference to the Air Pollution Control Ordinance (APCO) (Cap.311), and Annex 4 of the Technical Memorandum on Environmental Impact Assessment Process (EIAO-TM).

3.1.2 For construction dust, Annex 4 of EIAO-TM specifies a total suspended particulates (TSP) limit in air over an 1-hour period of 500 mgm-3. The maximum acceptable TSP concentration averaged over 24-hour and annual periods is 260mgm-3 and 80 mgm-3 respectively as defined in the Air Quality Objectives (AQOs) encompass by the APCO.

3.1.3 The Air Pollution Control (Construction Dust) Regulation specifies processes that require special control. Contractors and site agents are required to inform EPD before commence "notifiable work" and adopt dust reduction measures while carrying out "notifiable work" or "regulatory work" as defined under the regulation. "Notifiable work" includes site formation, reclamation, demolition of a building, construction of the foundation or superstructure of a building, road construction work, etc. "Regulatory work" includes road opening or resurfacing work, slope stabilization work, handling or transfer of dusty materials, etc

3.1.4 Amendment to the APCO (1993) has included objectionable odour as an air pollutant, but with no quantitative criteria. The EIAO-TM stipulates an odour nuisance limit of 5 odour units (OU) based on an averaging time of 5 seconds. An OU is defined as the dilution factor required for samples of odourous gases to be diluted with clean odour-free air to the detection threshold.

3.2 Baseline Conditions

3.2.1 The vicinity of the study area is mostly rural in nature. Existing land uses in the vicinity of the proposed sewage treatment works (STW) site and sewerage system at Ngong Ping include mainly the Po Lin Monastery, a youth hostel, and some scattered village houses. The elevation of the area is about 450mPD. Other than fugitive air emissions associated with the daily activities of Po Lin Monastery, major air pollution source was not identified in the area.

3.2.2 The alignment of the proposed effluent export pipeline follows along the existing Ngong Ping Road, Sham Wat Road, Keung Shan Road, and Tai O Road. There are village houses and monasteries scattered in the vicinity of the proposed alignment of the effluent export pipeline. In the Tai O area, there are various community uses including public housing, schools, stores, village houses, and also some agricultural uses. Other than traffic emissions from vehicles travelling on the existing roads, major air pollution source was not identified in the area.

3.2.3 There is currently no EPD-operated air quality monitoring station located within the study area. However, in view of the rural nature of the area and that no major air pollution source is identified in the area, the existing air quality condition in the area is expected to be in good condition.

3.3 Air Sensitive Receivers

3.3.1 In accordance with the definition stipulated in Annex 12 of EIAO-TM, air sensitive receiver (ASR) is defined as:
· Any domestic premises, hotel, hostel, hospital, clinic, nursery, temporary housing accommodation, school, educational institution, office, factory, shop, shopping centre, place of public worship, library, court of law, sports stadium or performing arts centre; and
· 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 above listed premises and places.

3.3.2 The study area for air quality impact assessment includes an area of 500m from the boundary of all works areas. Drawing No. 23400/EN/004 shows the study area around the proposed STW and sewerage system at Ngong Ping. Drawing No. 23400/EN/005 shows the study area around the proposed effluent export pipeline.

3.3.3 Representative existing ASRs within the study area likely to be affected by the works related to the construction and/or operation of the proposed Ngong Ping STW, Ngong Ping Sewerage System, and effluent export pipeline are initially identified and summarised in Table 3.1 and shown in Drawing Nos. 23400/EN/009 to 015(010, 011,012, 013, 014).

3.3.4 In addition to the existing ASRs listed above, future ASRs include the proposed Ngong Ping Cable Car Terminal development adjacent to the proposed Ngong Ping STW.

3.4 Assessment Methodology

Construction Phase Assessment

3.4.1 Construction dust impacts were assessed by determining dust-generating activities and recommending corresponding dust control and suppression measures. Details of dust-generating construction activities, including the site area and the construction program were reviewed.

3.4.2 The construction works involved in this Project include works associated with the construction of the proposed STW and the pipe laying works for the proposed Ngong Ping sewerage system and the effluent export pipeline.

3.4.3 It is expected that the works related to the pipe laying works for the proposed Ngong Ping sewerage system and the effluent export pipeline would be carried out section by section. The dust generated from each section of the pipe laying works would be of small scale, localised, and short-term. It would not be useful to perform dust dispersion modelling for this type of transient dust generating activities. This also applies to the pipe laying works for the proposed fresh water main in Ngong Ping area (see Drawing No. 23400/EN/018). In this assessment, dust mitigation measures and protection measures stipulated in the Air Pollution Control (Construction Dust) Regulation that are relevant to the identified dust-generating construction activities were identified. With the implementation of proper dust control and suppression measures, adverse dust impact from these construction activities is not expected.

3.4.4 Besides, a monitoring and audit program during the construction phase of the proposed project is formulated and included in the Environmental Monitoring and Audit (EM&A) Manual. Relevant dust control and suppression measures in accordance with the Air Pollution Control (Construction Dust) Regulation are included in the EM&A Manual.

3.4.5 For the construction of the proposed STW at Ngong Ping, a quantitative dust impact assessment was carried out. Potential cumulative dust impacts would be related to the concurrent construction activities associated with the proposed Tung Chung Cable Car Terminal located immediately to the northeast of the proposed STW at Ngong Ping (see Drawing No. 23400/EN/018).

Emission Inventories

3.4.6 The major potential air quality impacts during the construction phase of the proposed STW at Ngong Ping would result from dust arising from construction activities including:
· Site clearance and preparation;
· Drilling and rock breaking using penetrating cone fracture (PCF) or similar;
· Open site erosion;
· Construction of foundation and superstructure; and
· Handling and transportation of construction and demolition material.

3.4.7 The prediction of dust emissions was based on typical values and emission factors from United States Environmental Protection Agency (USEPA) Compilation of Air Pollution Emission Factors (AP-42), 5th Edition. A ten-hour working day was assumed during construction phase of the Project. References of the calculations of dust emission factors for different dust generating activities are listed in Table 3.2.

3.4.8 In this assessment, dust suppression measures and estimated mitigation efficiencies were incorporated into the dust emission calculations. With reference to Section 11.2.4.4 of AP-42 4th Edition, dust emissions from construction areas could be reduced by 50% by twice daily watering with complete coverage of active construction areas. If necessary, the watering frequency could be increased to increase the dust control efficiency. With reference to USEPA's Control of Open Fugitive Dust Sources, EPA-450/3-88-008, 1988, the dust control efficiency for watering of unpaved roads is inversely proportional to the duration between each watering. Dust emissions could therefore be reduced by up to 75% with four times daily watering with complete coverage of active construction areas. Details of the construction dust emissions calculation is presented in Appendix 3B.

3.4.9 For the purpose of this assessment, it was assumed that the construction of both the Ngong Ping STW and the adjacent Cable Car Terminal would occur at the same time. Since details of the construction activities for the proposed Ngong Ping Cable Car Terminal are currently not available, it was assumed that the intensity of the construction activities for the proposed Cable Car Terminal would be similar to that for the Ngong Ping STW. The construction dust emission rates estimated for the Ngong Ping STW were thus taken for the proposed Cable Car Terminal in this assessment.

Dispersion Modelling

3.4.10 The USEPA approved Industrial Source Complex Short-Term 3 (ISCST3) model was used to model dust dispersion. The model assumed the algorithm for the "rural" mode, with the dry depletion and gradual plume rise options. The minimum Monin-Obukhov length of 1 metre suggested in ISC3 Model User's Guide for rural area was taken in the ISCST3 model for the dry deposition algorithm.

3.4.11 For the purpose of this assessment, it is considered that dust emissions from vehicles moving on unpaved site areas would constitute the major dust source for the general works areas. Since no site specific information is available relating to particle size distribution, and the unpaved road emission equation from AP-42 5th Edition is applicable for different geographical conditions, the particle size distribution used in the ISCST3 model was estimated based on the particle size multipliers for the unpaved road emission equation (Equation (1) of Section 13.2.2 of AP-42 5th Edition). With particle size classes of 0-2.5 µm, 2.5-10 µm and 10-30 µm, the percentage in each class was estimated to be 3.8%, 22.2% and 74% respectively.

3.4.12 During daytime working hours (8am to 6pm), it was assumed that dust emissions would be generated from all dust generating activities and site erosion. Worst-case meteorological condition was taken as atmospheric stability class D with wind speed of 1ms-1. During nighttime non-working hours (6pm to 8am of the next day), it was assumed that dust emissions would only be generated from site erosion. Worst-case meteorological condition was taken as atmospheric stability class F with wind speed of 1ms-1. Worst-case wind direction based on a wind angle resolution of 10º was taken in this assessment.

3.4.13 The worst-case 1-hour average TSP concentrations and the worst-case 24-hour average TSP concentrations were calculated at representative ASRs at height of 1.5m above ground, 1.5m is the average height of human breathing zone. Locations of the representative ASRs are shown in Drawing No. 23400/EN/035. Since the construction phase impact assessment assumed concurrent construction of both the Ngong Ping STW and the proposed Ngong Ping Cable Car Terminal, assessment point numbers A17 to A20 that represent the boundary of the proposed Ngong Ping Cable Car Terminal were thus not included as representative ASRs in the construction dust impact assessment.

3.4.14 The maximum concentration among the worst-case daytime and worst-case nighttime 1-hour average concentrations was taken as the worst-case hourly average TSP concentration for each ASR. The worst-case 24-hour average TSP concentration was taken as the average of 10 hours with worst-case daytime 1-hour average concentration and 14 hours with worst-case nighttime 1-hour average concentration.

3.4.15 After all, background TSP concentration of 87µmg-3 was added to the results calculated above to produce the worst-case concentrations. The background concentration is derived from EPD's monitoring data for rural area.

3.4.16 Odour would be one of the key environmental concerns during the operational phase of the Project. Potential odour sources within the proposed Ngong Ping STW include the inlet work, the activated sludge treatment system namely Sequencing Batch Reactor (SBR) system, the sludge thickener, the aerobic digester building, the sludge dewatering and storage building, and the emergency storage tank for temporary storage of treated or raw sewage under emergency and maintenance situations.

3.4.17 The odour is mainly due to the presence of hydrogen sulphide (H2S) that is a major odourous gas in sanitary sewer systems, particularly in places with relatively warm climate such as Hong Kong. Sulphide generation would be promoted under anaerobic conditions in sewage conveyance system. H2S is often detectable when all odour components have been diluted to below their detection thresholds. H2S was therefore adopted as the main parameter for assessing odour impact in this study.

3.4.18 The odour impact assessment for this study was carried out by first estimating the H2S emission rates using well-recognised equations and then predicting the worst-case H2S concentrations at representative ASRs using computer dispersion modelling. As discussed in Section 3.1 above, the EIAO-TM stipulates an odour nuisance limit of 5 odour units (OU) based on an averaging time of 5 seconds. An OU is defined as the dilution factor required for samples of odorous gases to be diluted with clean odour-free air to the detection threshold.

3.4.19 Therefore, in order to convert the predicted H2S concentration at ASRs to OU for compliance checking with the EIAO-TM odour nuisance limit, it is required to correlate the H2S concentration with OU, or in simple term, to determine the detection threshold (i.e. one OU) of H2S for those potential odour sources of concern in this assessment. For the purpose of this assessment, a conservative detection threshold for H2S of 0.0005ppm or 0.00076 mgm-3 was adopted (Source: Woodfield M. and Hall D. (1994). Odour Measurement and Control - An Update. AEA Technology and National Environmental Technology Centre).

3.4.20 The odour assessment is based on a worst scenario of possible location of the proposed Ngong Ping cable car terminal which is situated immediately to the northeast of the proposed Ngong Ping STW (see Drawing No. 23400/EN/019). It was assumed that a buffer distance of 50m is provided between the odour emission sources at the proposed STW and the nearest boundary of the cable car terminal.

Odour Emission Estimation

3.4.22 With reference to the Hydrogen Sulphide Control Manual (Technological Standing Committee on Hydrogen Sulphide Corrosion in Sewage Works, 1989), one of the most comprehensive guidelines available today, the equation below presented by Pomeroy and Parkurst was taken to estimate the sulphide concentration for the gravity sewerage system proposed at Ngong Ping:

S₂ = {(aS₁ – b)e^-at + b} / a

where S₁ sulphide concentration at start of section, mg/L

S₂ sulphide concentration at end of section, mg/L

a = N(SV)^3/8 / dm

b = M’[BOD₅]1.07^T-20

t flow time through the section of sewer, h

S total energy head gradient

V sewage velocity, m/s

dm mean hydraulic depth, m

[BOD₅] 5-day biochemical oxygen demand

T temperature, ^oC

M’ specific sulphide flux for partially filled pipes, m/h

N constant used in sulphide build-up equation

3.4.27 The H2S emission flux from the inlet work (including the coarse/fine screens and the grit removal) was estimated as a free surface sewage flow with the following equation (Design Manual on Odor and Corrosion Control in Sanitary Sewerage Systems and Treatment Plants, 1985):

f = 0.69(SV)^3/8 j [DS]

where f hydrogen sulphide surface flux, g/m²h

S total energy head gradient

V sewage velocity, m/s

j proportion of dissolved sulphide present in molecular form (see Figure 3.1)

[DS] dissolved sulphide concentration in the wastewater, mg/L (see equation in Section 3.4.21)

3.5 Prediction and Evaluation of Potential Impacts

Construction Phase Impacts

3.5.1 The modelling results at the representative ASRs for the mitigated and unmitigated scenarios are presented in Table 3.4. The assessment height is 1.5m above local ground level. All the results presented in the table included the background concentration. For the mitigated scenario, 75% dust reduction by four times daily watering with complete coverage was assumed for the active construction areas. It was also assumed that the construction vehicle travel speed on unpaved site areas would be limited to not more than 10 km per hour. Locations of the assessment points are shown in Drawing No. 23400/EN/035.

Note: Modelling results that exceeded the guideline level or AQO are shown in bolded characters.

** Points A14, A6 and A16 represent the three ASRs nearest to the proposed STW site, namely Tin Tan Buddha Statue, village house along Ngong Ping Road near bus terminal and Lin Chi Monastery respectively. Points A1 to A5, A7 to A13 and A15 are selected air assessment points around the proposed STW site.

3.5.2 As shown by the modelling results, without any dust suppression measures, exceedance of the TSP 1-hour average guideline level of 500 µgm-3 and/or the 24-hour average AQO of 260 µgm-3 would be expected at most of the identified ASRs in close proximity to the works areas in Ngong Ping.

3.5.3 With the implementation of dust suppression measures, exceedance of the TSP guideline level and AQO would not be expected. The modelling results showed that the worst-case dust impacts at the ASRs would be on average reduced by about 70% and 55% respectively for the 1-hour average TSP level and the 24-hour average TSP level.

3.5.4 The predicted worst-case hourly and daily average TSP concentration contours at 1.5m above ground for the unmitigated and mitigated scenarios are shown in Drawing No. 23400/EN/036 to 039 (037, 038).

3.5.5 With the implementation of proper dust control and suppression measures stipulated in the Air Pollution Control (Construction Dust) Regulation and described in Section 3.6 below, adverse dust impact from the construction activities of the Project would not be expected.

3.5.6 Two operation scenarios namely the unmitigated and the mitigated scenarios were considered and assessed with the odour dispersion model. In the unmitigated scenario, it was assumed that all major odour sources within the proposed Ngong Ping STW namely the inlet work, the activated sludge treatment system, the sludge thickener, the aerobic digester building, the sludge dewatering and storage building, and the emergency storage tank would be open to the atmosphere. All H2S emissions would be dispersed directly from the sources at ground level and impacted onto the ASRs in the vicinity. Besides, it was assumed that the emergency storage tank would be storing raw sewage under the worst-case situation.

3.5.7 In the mitigated scenario, it was assumed that all the major odour sources would either be contained by building structures or constructed as underground tanks. All the odour emissions from the odour sources would be ventilated to a centralised deodorisation unit located at the centre of the STW. In this assessment, it is estimated that an H2S removal efficiency of up to 99.5% would be required for the deodorisation unit so as to achieve the 5 OU criteria at all the ASRs in the area. All the treated air would be emitted from the exhaust vent shaft of the deodorisation unit with an exhaust height of 5m above ground and an exit velocity of 10m/s.

3.5.8 The worst-case 5-second average odour levels were calculated at the representative ASRs in the vicinity of the proposed Ngong Ping STW as shown in Drawing No. 23400/EN/010. The assessment heights were taken as 1.5m to 30m above local ground level. The modelling results for the unmitigated and mitigated scenarios at the selected air assessment points (Drawing No. 23400/EN/035) are shown in Tables 3.5 and 3.6 respectively.

Note: Modelling results that exceeded the 5OU criteria level are shown in bolded characters.

** Points A17 to A20 represent the boundary of the cable car terminal which is the future ASR nearest to the proposed STW site. Points A14, A6 and A16 represent the three existing ASRs nearest to the STW site, namely Tin Tan Buddha Statue, village house along Ngong Ping Road near bus terminal and Lin Chi Monastery respectively. Points A1 to A5, A7 to A13 and A15 are selected air assessment points around the proposed STW site.

Note: Modelling results that exceeded the 5OU criteria level are shown in bolded characters.

** Points A17 to A20 represent the boundary of the cable car terminal which is the future ASR nearest to the proposed STW site. Points A14, A6 and A16 represent the three existing ASRs nearest to the STW site, namely Tin Tan Buddha Statue, village house along Ngong Ping Road near bus terminal and Lin Chi Monastery respectively. Points A1 to A5, A7 to A13 and A15 are selected air assessment points around the proposed STW site.

3.5.9 The predicted worst-case 5-second average odour levels in the vicinity of the proposed Ngong Ping STW are shown in Drawing No. 23400/EN/021. Since the emission heights of the odour sources were taken as ground level, the odour levels presented in the drawing are at a receiver height of 1.5m above ground level, 1.5m is the average height of human breathing zone. As shown in the drawing and in Table 3.5, very high odour impacts would be expected in the area even at a large distance from the STW. Exceedances of the 5 OU criteria level would be expected at most part of Ngong Ping. More than 600 OU were predicted at the nearest boundary of the proposed Cable Car Terminal site adjacent to the proposed STW.

3.5.10 As shown in Table 3.6, the worst affected height at the boundary of the nearest ASR namely the proposed Ngong Ping Cable Car Terminal would be 25m above ground level. Drawing No. 23400/EN/022 shows the predicted worst-case 5-second average odour levels in the vicinity of the proposed Ngong Ping STW at the worst affected height of 25m above ground level. As shown in the drawing, higher odour levels were predicted around the exhaust point of the deodorisation unit. The highest odour impact at ASRs of close to 5 OU were predicted at the nearest boundary of the adjacent proposed Cable Car Terminal site. With the implementation of the proposed odour control measures, exceedance of the 5 OU criteria was not predicted at the ASRs in the area.

3.5.11 Based on the latest available information, part of the proposed Cable Car Terminal site would be developed as a public transport interchange (PTI) (see Appendix 3C). The selected assessment point numbers A17 to A20 are located at the boundary of the proposed Cable Car Terminal site nearest to the Ngong Ping STW. These assessment points represent the worst-affected locations within the proposed Cable Car Terminal site. As shown in Table 3.6 above under the mitigated scenario, exceedance of the 5 OU criteria at these assessment points is not expected. It is therefore not expected that there would be any adverse odour impact at the PTI within the proposed Cable Car Terminal site.

3.6 Mitigation of Adverse Impacts

Construction Phase Measures

3.6.1 In order to ensure that dust emission is minimised during the construction phase of the project, relevant dust control requirements set out in the Air Pollution Control (Construction Dust) Regulation should be met. The site agent of the Contractor is required to adopt dust reduction measures while carrying out construction works. In particular, the mitigation measures listed below should be adopted where applicable. With the implementation of effective dust control measures, adverse dust impacts from the construction works of the project would not be expected.

Site clearance and demolition of existing structures

3.6.2 In order to mitigate the potential odour impacts from the proposed Ngong Ping STW to an acceptable level, it is recommended that all the major odour sources within the proposed STW namely the inlet work, the sequencing batch reactors, the sludge thickeners, and the emergency storage tank should all be constructed as underground facilities to minimise direct emission of odour to the atmosphere.

3.6.3 Other components of the STW will be constructed as above ground structures. All odour emissions from the underground/above ground facilities mentioned above will be contained by building structures and ventilated to a centralised deodorisation unit. Based on the findings of the EIA study, the H2S removal efficiency of the deodorisation unit should be 99.5% or better and the H2S emission rate at the exhaust end of the deodorisation unit should not be more than 50 µg/s. The exhaust height and exit velocity of the treated air should not be less than 5m and 10m/s respectively. A backup deodorization unit should be provided to cater for the situation during maintenance or breakdown of the deodorization unit.

3.6.4 All the sludge generated from the STW will be dewatered onsite to more than 30% dry solids content before transporting to the designated landfill site for disposal and will be stored in covered container along the transporting route to avoid the possible odour impact on nearby sensitive receivers. The transportation of the sludge by sea to the disposal location is recommended, as far as practicable, in order to reduce potential air quality impacts from road transportation.