3. AIR QUALITY IMPACT

3.1 Introduction

3.1.1.1 Air quality is considered as one of the key environmental issues of concern during both construction and operational phases of the proposed project. During the construction phase, there will be potential dust impacts on existing and future sensitive receivers from the construction activities undertaken at the project sites. During the operational phase, potential odour emissions from the proposed expanded San Wai STW and Ha Tsuen Pumping Station on nearby sensitive receivers would be a major environmental issue of concern.

3.1.1.2 This section presents the assessments on construction phase dust impacts and the operational phase odour emission impacts.

3.2 Environmental Legislation, Policies, Plans, Standards and Criteria

3.2.1.1 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).

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

3.2.1.3 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 stabilisation work, handling or transfer of dusty materials, etc

3.2.1.4 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 odorous gases to be diluted with clean odour-free air to the detection threshold.

3.3 Description of the Environment

3.3.1.1 The vicinity of the western part of the study area is mostly rural in nature (and with some container storage areas) and become more urbanised in the eastern part. Existing air quality in the area is influenced by emissions from:
· Road network within and around the study area; and
· Construction activities within and around the study area.

3.3.1.2 There is currently no EPD-operated air quality monitoring station located within the study area. Historical air quality monitoring data from the nearest station, namely the rooftop Yuen Long station operated by EPD are taken to examine the historical trend of the air quality condition in the vicinity of the study area.

3.3.1.3 The last three published years of TSP monitoring data, namely 1998, 1999 and 2000 at Yuen Long station are tabulated in Table 3.1.

3.3.1.4 As shown by the monitoring results in Table 3.1 for the rooftop Yuen Long station, it is observed that the TSP concentrations in the Yuen Long area remained high during the last few years and exceedance of the annual average AQO for TSP was recorded for the last three years. The reasons may be largely due to dust emissions from the construction activities and traffic emissions from road network.

3.3.1.5 As shown in Figure 1.8, the proposed Deep Bay Link will encroach upon the southwestern corner of the existing San Wai STW in the form of a viaduct. With reference to the Environmental Impact Assessment Report for Agreement No. CE109/98 Deep Bay Link Investigation and Preliminary Design (July 2002), the modelling results showed exceedance of the 24-hour average AQO for nitrogen dioxide at 10m above ground at the existing San Wai STW under the worst-case scenario. The predicted exceedance is at 10m above ground level and covers an area within about 50m from the southwestern corner of the existing San Wai STW. Since there is no sensitive use located at the area of exceedance within the existing and the future San Wai STW, adverse air quality impact is therefore not expected.

3.4 Air Sensitive Receivers

3.4.1.1 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.

3.4.1.2 The Assessment Area for air quality impact assessment includes an area 500m from the boundary of the works areas. Figures 3.1 and 3.2 show respectively the Assessment Area around the proposed expanded San Wai STW and Ha Tsuen Pumping Station. Existing land uses within the Assessment Area of San Wai STW include mainly agricultural land, container storage areas, and scattered village houses. Whereas the existing land uses around the Ha Tsuen Pumping Station are mainly public and private residential housing estates with a number of schools.

3.4.1.3 Future ASRs around the proposed expanded San Wai STW and Ha Tsuen Pumping Station include the planned sensitive land uses of the HSKNDA and HSK North NDA as shown in Figure 1.9. Immediately to the southwest of the proposed San Wai STW is the proposed Deep Bay Link. The road users of the proposed Deep Bay Link might be susceptible to the future odour impacts from the operation of San Wai STW. For reference purpose, the closest point of San Wai STW to Deep Bay Link is taken as an air quality assessment point (A41) of the operational phase odour impact assessment for San Wai STW.

3.4.1.4 Existing and future representative ASRs within the Assessment Area likely to be affected by the construction and/or operation of the San Wai STW, Ha Tsuen Pumping Station, and emergency bypass culvert are identified as assessment points for this assessment. The selected assessment points are summarised in Tables 3.2 to 3.4 and are shown in Figures 3.1 to 3.3. With reference to Section 1.2 of this report, the proposed emergency bypass will be constructed in the form of box culvert and will only be used for conveying treated effluent from San Wai STW. The major air quality impact of the proposed emergency bypass culvert would be associated with the construction activities.

3.5 Enhancement Measures to Control Emergency Discharge

3.5.1 General

3.5.1.1 With reference to Section 1.2 of this report, in order to reduce the chance of discharging raw sewage into Tin Shui Wai Drainage Channel and hence Deep Bay, an emergency bypass culvert will be constructed in the form of box culvert from San Wai STW to nearby drainage channel as part of this project.

3.5.1.2 Four alternative alignments of the proposed emergency bypass culvert were considered and are shown in Figure 3.4. Figure 3.4 also indicates the extent of the construction works areas associated with the four alternative alignments.

3.5.2 Construction Phase Impact

3.5.2.1 In general, the construction works areas for the emergency bypass culverts would be a 10m wide strip along the culvert alignment from the expanded San Wai STW up to the connection point with the existing Ting Shui Wai Drainage Channel for Alternatives 1, 2, and 3, and with the existing Lo Uk Tsuen Drainage Channel for Alternative 4. A summary of the length and the total construction works areas for the four alternatives is shown in Table 3.5.

* The length of Alternative 4 requiring construction (600m) is from San Wai STW to the existing Lo Uk Tsuen drainage channel only.

3.5.2.2 It is expected that the works related to the construction of the emergency bypass culvert would be carried out section by section. The dust generated from each section of the culvert construction works would be of small scale, localised, and short-term. By comparing the length and works area associated with the four alternatives, Alternative 4 would involve the shortest length and the smallest works area. The dust emissions associated with the construction of Alternative 4 would thus be the lowest among the four alternatives.

3.5.2.3 With the implementation of proper dust control and suppression measures stipulated in the Air Pollution Control (Construction Dust) Regulation and described in Section 3.9 below, adverse dust impact from the construction activities of the proposed emergency bypass culvert is not expected.

3.5.3 Operational Phase Impact

3.5.3.1 All the four alternatives of the emergency bypass culvert will be constructed in the form of box culvert up to the connection point with the existing Ting Shui Wai Drainage Channel for Alternatives 1, 2, and 3, and with the existing Lo Uk Tsuen Drainage Channel for Alternative 4. The existing drainage channels will then convey the effluent to the Deep Bay.

3.5.3.2 With reference to the discussion in Section 1.2, the proposed emergency bypass culvert will only be used for conveying treated effluent from San Wai STW under the emergency event when the NWNT effluent tunnel or the Urmston Road outfall is out of operation. Since the residual sulphide content in the treated effluent would be minimal if any. Hydrogen sulphide emission from treated effluent flowing along the proposed emergency bypass culvert and the existing open drainage channels would thus not be expected. Odour impact at the nearby existing and future ASRs (including Tin Shui Wai New Town, HKSNDA and HSK Norht NDA) from the operation of the proposed emergency bypass culvert is therefore not anticipated.

3.5.3.3 Other than the scenario described above with the break down of the NWNT effluent tunnel or the Urmston Road outfall, the other emergency discharge scenarios for the existing and the future arrangements are the same and would result in the discharge of raw sewage. Nonetheless, the chance of discharging raw sewage into the Tin Shui Wai Drainage Channel is in fact very remote and has not happened since the commissioning of the existing San Wai STW, the Ha Tsuen Pumping Station, and the NWNT effluent tunnel and Urmston Road outfall for more than 10 years. Besides routine monitoring, inspection, and maintenance to ensure satisfactory working condition of the system, the following have been and will be applied to the Ha Tsuen Pumping Station to further reduce the chance of system failure:

3.5.3.4 A contingency plan would also be developed at the detailed design stage to deal with the emergency discharges that may occur during the operational stage of the project. It is recommended to include the following items in the contingency plan:

3.6 Assessment Methodology

3.6.1 Construction Phase Assessment

3.6.1.1 Construction dust impacts are 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.

3.6.1.2 The construction works involved in this project include works associated with the construction of the proposed expanded San Wai STW, the expanded Ha Tsuen Pumping Station, and the emergency bypass culvert.

Emission Inventories

3.6.1.3 The major potential air quality impacts during the construction phase of the proposed expanded San Wai STW and the expanded Ha Tsuen Pumping Station would result from dust arising from construction activities including:
· Site clearance and preparation;
· Excavation and filling;
· Open site erosion;
· Construction of foundation and superstructure; and
· Handling and transportation of construction and demolition material.

3.6.1.4 With reference to Section 1.3 of this report, apart from the construction of this project, there are a number of major projects planned in the vicinity of the study area which might cause cumulative construction phase impacts to the environment. These projects include YLKTSSD Stage 1, YLKTSSD Stage 2, Deep Bay Link, and the HSKNDA. Concurrent construction activities identified within 500m from the San Wai STW and the Ha Tsuen Pumping Station are shown in Figures 3A.1 and 3A.2 in Appendix 3A. For the purpose of this assessment, it was assumed that all these construction activities, together with the construction of preferred Alternative 4 of the proposed emergency bypass culvert, would occur at the same time. All these construction activities were included in the dust modelling to predict the worst-case cumulative dust impacts at the assessment points. Details of the construction dust emissions calculation are presented in Appendix 3A.

3.6.1.5 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 3.6.

3.6.1.6 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. Dust generated from vehicle traffic on unpaved site roads would be reduced by lowering the vehicle travelling speed. The percentage dust reduction is estimated in accordance with Section 13.2.2.2 of AP-42 5th Edition.
Dispersion Modelling

3.6.1.7 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.

3.6.1.8 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 is 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 is estimated to be 3.8%, 22.2% and 74% respectively.

3.6.1.9 During daytime working hours (8am to 6pm), it was assumed that dust emissions would be generated from all dust generating activities and site erosion. 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.

Concentration Calculations

3.6.1.10 The worst-case 1-hour average TSP concentrations and the worst-case 24-hour average TSP concentrations are calculated at the assessment points at height of 1.5m above ground, 1.5m is the average height of human breathing zone. Since all the dust generating sources are at ground level, this assessment height would represent the height with the worst-case impact.

3.6.1.11 Historical wind data from Lau Fau Shan meteorological station for year 1996 to 2000 are used in the calculation of the worst-case 1-hour and worst-case 24-hour average concentrations. For each hour of a year, the wind direction from the historical data was taken at 10-degree resolution. Calm hours (hours with wind speed less than 1ms-1) were disregarded in accordance with Section 9.3.4 of the Guideline on Air Quality Models, USEPA. For wind speed less than 2ms-1, the wind speed was conservatively converted to 1ms-1 in the model. For wind speed equal to or larger than 2ms-1, the wind speed was conservatively converted to 2ms-1 in the model.

3.6.1.12 The hourly average concentration for each assessment point is then calculated as described above based on the converted wind direction and wind speed. The maximum 1-hour average and 24-hour average concentrations over the five years are then taken as the worst-case concentrations for the assessment points.

3.6.1.13 After all, background TSP concentration of 87 µgm-3 is 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.

3.6.2 Operational Phase Assessment

3.6.2.1 Odour would be one of the key environmental concerns during the operational phase of the project. Potential odour sources within the proposed expanded San Wai STW include the fine screen, the detritors, the sludge holding tanks, the solid handling house, the sludge dewatering house, the rapid mix and flocculation tank, and the primary sedimentation tanks. Whereas the major odour sources for the expanded Ha Tsuen Pumping Station would be the existing and the future wet wells of the pumping station.

3.6.2.2 The odour is mainly due to the presence of hydrogen sulphide (H2S) that is a major odorous 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 is therefore adopted as the main parameter for assessing odour impact in this study.

3.6.2.3 The odour impact assessment for this study is carried out by first estimating the H2S emission rates using well-recognised equations and then predicting the worst-case H2S concentrations at the assessment points using computer dispersion modelling. As discussed in Section 3.2 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.

3.6.2.4 Therefore, in order to convert the predicted H2S concentration at the assessment points 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 is adopted (Source: Woodfield M. and Hall D. (1994). Odour Measurement and Control - An Update. AEA Technology and National Environmental Technology Centre).

Odour Emission Estimation

3.6.2.5 The generation of sulphide in sewerage systems is predominantly an anaerobic microbiological reaction involving sulphate and sulphate-reducing bacteria. The bacteria are concentrated in slimes that form on the walls of sewers and associated facilities. Although sulphide is also produced in the wastewater, these slimes are generally responsible for the majority of sulphide generated in sewerage systems. Apart from the need to have anaerobic conditions, the following factors may also influence the rate of sulphide generation:

Ø Liquid velocity
Ø Soluble electron donor concentration (which effectively is the soluble biochemical oxygen demand concentration)
Ø Sulphate concentration
Ø Temperature

3.6.2.6 Several predictive equations (e.g. Pomeroy, Thistlethwayte) are available for estimating the build-up of sulphide in sewerage systems. These equations were derived empirically from measurements taken on a large number of operating systems. Although different equations and different coefficients adopted in these equations may result in different predictions, in most cases, these equations should provide a reasonable basis for developing sulphide control strategies.

3.6.2.7 Having said that, Thistlethwayte equation has been reported to overestimate the sulphide flux when excess sulphate was available in sewage (USEPA, 1985). The use of the Thistlethwayte equation is also not recommended for low sewage velocity (say less than 0.5m/s) (Technological Standing Committee on Hydrogen Sulphide Corrosion in Sewage Works, 1989a). It is recommended in the Hydrogen Sulphide Control Manual (Technological Standing Committee on Hydrogen Sulphide Corrosion in Sewage Works, 1989a), one of the most comprehensive guidelines available today, that the Pomeroy equation be used because of its simplicity and reliability in predicting sulphide flux over a wider range of velocities.

Prediction of Sulphide Build-up in Pumping Mains

3.6.2.8 The equation suggested by Pomeroy to predict the sulphide flux from wall slimes (Technological Standing Committee on Hydrogen Sulphide Corrosion in Sewage Works, 1989a) is:

G = M[BOD₅]1.07^T-20

where G sulphide flux from wall slimes, g/m²h

[BOD₅] 5-day biochemical oxygen demand

T temperature, ^oC

M coefficient, m/h

3.7 Identification of Environmental Impacts

3.7.1.1 The major potential air quality impacts during the construction phase of this project would result from dust arising from construction activities including:
· Site clearance and preparation;
· Excavation and filling;
· Open site erosion;
· Construction of foundation and superstructure; and
· Handling and transportation of construction and demolition material.

3.7.1.2 There would likely be cumulative dust impacts during the construction phase of this project arising from concurrent construction activities of other major projects in the vicinity of the study area. These projects include YLKTSSD Stage 1, YLKTSSD Stage 2, Deep Bay Link, and the HSKNDA.

3.7.1.3 During operational phase of the project, major air quality impacts would be related to the potential odour emissions from the activities associated with the proposed expanded San Wai STW and the expanded Ha Tsuen PS.

3.8 Prediction and Evaluation of Environmental Impacts

3.8.1 Construction Phase Impacts

Proposed Expanded San Wai STW

3.8.1.1 The modelling results at the selected assessment points for the mitigated and unmitigated scenarios are presented in Table 3.8. 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, 50% dust reduction by twice daily watering with complete coverage is assumed for the active construction areas of this project. It is also assumed that the construction vehicle travel speed on unpaved site areas of this project would be limited to not more than 10 km per hour.

3.8.1.2 As shown by the modelling results, without any dust suppression measures for the construction sites of this project, 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 some existing village houses at San Wai (assessment points A37 and A38) in close proximity to the works areas.

3.8.1.3 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 assessment points are on average reduced by about 33% and 21% respectively for the 1-hour average TSP level and the 24-hour average TSP level.

3.8.1.4 The predicted worst-case hourly and daily average TSP concentration contours at 1.5m above ground around the proposed expanded San Wai STW for the unmitigated and mitigated scenarios are shown in Figures 3.7 to 3.10. No air sensitive use is identified within the areas with predicted exceedance of the hourly and daily TSP guideline level / AQO during the construction period under the mitigated scenario.

3.8.1.5 With the implementation of proper dust control and suppression measures stipulated in the Air Pollution Control (Construction Dust) Regulation and described in Section 3.9 below, adverse dust impact from the construction activities of the proposed expanded San Wai STW is not expected.

Proposed Expanded Ha Tsuen Pumping Station

3.8.1.6 The modelling results at the selected assessment points for the mitigated and unmitigated scenarios are presented in Table 3.9. 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, 50% dust reduction by twice daily watering with complete coverage is assumed for the active construction areas of this project. It is also assumed that the construction vehicle travel speed on unpaved site area of this project would be limited to not more than 10 km per hour.

3.8.1.7 As shown by the modelling results, in view of the low intensity of the construction activities involved, exceedance of the guideline level and AQO for TSP would not be expected without any dust suppression measures.

3.8.1.8 With the implementation of dust suppression measures, the modelling results showed that the worst-case dust impacts at the assessment points are on average reduced by about 22% and 14% respectively for the 1-hour average TSP level and the 24-hour average TSP level.

3.8.1.9 The predicted worst-case hourly and daily average TSP concentration contours at 1.5m above ground around the proposed expanded Ha Tsuen Pumping Station for the unmitigated and mitigated scenarios are shown in Figures 3.11 to 3.14. No air sensitive use is identified within the areas with predicted exceedance of the hourly and daily TSP guideline level / AQO during the construction period under the mitigated scenario.

3.8.1.10 With the implementation of proper dust control and suppression measures stipulated in the Air Pollution Control (Construction Dust) Regulation and described in Section 3.9 below, adverse dust impact from the construction activities of the proposed expanded Ha Tsuen Pumping Station is not expected.

3.8.2 Operational Phase Impacts

Proposed Expanded San Wai STW

3.8.2.1 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 expanded San Wai STW would be open to the atmosphere. All H2S emissions would be dispersed directly from the sources at ground level.

3.8.2.2 In the mitigated scenario, it was assumed that all the major odour sources would be contained within either building structures or enclosures. All the odour emissions from the odour sources would be ventilated to a centralised deodorization unit located at the centre of the STW. In this assessment, it is estimated that an H2S removal efficiency of up to 96% would be required for the deodorization unit so as to achieve the 5 OU criteria at all the ASRs as well as the assessment point representing the road users of Deep Bay Link (assessment point A41). All the treated air would be emitted from the exhaust vent shaft of the deodorization unit with an exhaust height of 5m above ground and an exit velocity of 10m/s.

3.8.2.3 The worst-case 5-second average odour levels were calculated at the assessment points in the vicinity of the proposed expanded San Wai STW as shown in Figure 3.1. The ASRs in the vicinity of the proposed expanded San Wai STW are all low-rise structures. 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 assessment points are shown in Tables 3.10 and 3.11 respectively.