3 air quality impact

Agreement No CE 43/2001 (EP) Tai Po Sewage Treatment Works – Stage V Drainage Services Department Final EIA Report

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3 AIR QUALITY IMPACT

3.1 Introduction

3.1.1 This Section presents an assessment of the potential air quality impacts associated with the construction and operation of the Project.

3.1.2 During the construction phase, demolition of the existing structures and construction of new treatment facilities would be dusty. Fugitive dust emissions arising from materials handling and site excavation would be the main sources of air pollution. Dust impacts associated with the construction works have been assessed and mitigation measures recommended, where necessary.

3.1.3 Emission of odour from the TPSTW after the commission of the Project is a key issue during the operation phase. Odorous compounds would be generated during the treatment of sewage and sludge of the TPSTW. Potential odour impacts on air sensitive receivers have been assessed. Odour control measures were also recommended to minimize the odour emission from the plant.

3.2 Environmental Legislation, Policies, Plans, Standards and Criteria

3.2.1 The criteria for evaluating air quality impacts and the guidelines for air quality assessment are laid out in Annex 4 and Annex 12 of the Technical Memorandum on Environmental Impact Assessment Process (EIAO-TM), respectively.

3.2.2 The Air Pollution Control Ordinance (APCO) provides the statutory authority for controlling air pollutants from a variety of sources. The Hong Kong Air Quality Objectives (AQOs), which stipulate the maximum allowable concentrations over specific periods for typical pollutants, should be met. The relevant AQOs are listed in Table 3.1.

Table 3.1 Hong Kong Air Quality Objectives

Pollutant Maximum Average Concentration (µg m-3) (1) 1-Hour (2) 8-Hour (3) 24-Hour (3) Annual (4)

Total Suspended Particulates - - 260 80 Respirable Suspended Particulates (5) - - 180 55

Sulphur Dioxide 800 - 350 80 Nitrogen Dioxide 300 - 150 80 Carbon Monoxide 30,000 10,000 - -

(1) Measured at 298 K and 101.325 kPa. (2) Not to be exceeded more than three times per year. (3) Not to be exceeded more than once per year. (4) Arithmetic mean. (5) Suspended particulates in air with a nominal aerodynamic diameter of 10 µm or smaller.

3.2.3 The EIAO-TM also stipulates that the hourly total suspended particulates (TSP) level at sensitive receivers should not exceed 500 µg m-3 (measured at 25°C and one atmosphere) for construction dust impact assessment. Mitigation measures from construction sites have been specified in the Air Pollution Control (Construction Dust) Regulations.


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3.2.4 In accordance with the EIAO-TM, odour at an air sensitive receiver should not exceed 5 odour units based on an averaging time of 5 seconds for odour prediction assessment.

3.3 Description of the Environment

Baseline Conditions

3.3.1 The TPSTW is located within the Tai Po Industrial Estate, and a number of industrial premises are located adjacent to the plant. The Tai Po Shuen Wan golf practice range is located to the east of the plant.

3.3.2 The ambient air quality of the site is mainly affected by the industrial emissions of the Tai Po Industrial Estate. Vehicle exhaust emissions from the local road network also contribute to the ambient air quality of the site.

3.3.3 Ambient air quality of Tai Po has been monitoring at the Tai Po Monitoring Station, operated by the Environmental Protection Department, located at the Tai Po Government Offices Building, 1 Ting Kok Road, Tai Po. The annual averages of pollutants monitored at the Tai Po station for the year 2001 are summarized in Table 3.2.

Table 3.2 Background Air Quality

Pollutants Annual Average (µµg m-3) Total Suspended Particulates 68

Respirable Suspended Particulates 50 Sulphur Dioxide 13 Nitrogen Dioxide 50

3.3.4 A leachate pre-treatment works is located at the north of the TPSTW but there is no operation for treatment of leachate now. The leachate from the Shuen Wan Landfill just passes through the leachate pre-treatment works and then enters into the TPSTW. The average volume of leachate from the landfill site is about 40 – 50 m3/ day in accordance with regular monitoring data provided by DSD. Referring to these data, the measured sulphate concentration in leachate is generally less than 50mg/l, which is a low value. The measured average pH value of the leachate is 8. This alkaline condition is not favourable for forming hydrogen sulphide from sulphate in the leachate.

3.3.5 During site visits, there was no unpleasant odour detected from the leachate works. The leachate may be temporarily stored in the lagoon of the leachate pre-treatment works and then discharged into the TPSTW every one or two days. In view of the short storage period of leachate at the plant and unfavourable condition for the formation of hydrogen sulphide in the leachate, the leachate pre-treatment works is not expected to be a potential odour source.

3.3.6 Ambient H2S level around the TPSTW were measured at six monitoring stations at the perimeter of the plant for five typical working days between 20th January 2003 and 8th February 2003. Sampling was conducted at the height of 1.5m and 4 m above ground at the monitoring locations. Locations of ambient H2S monitoring station and details of ambient survey are shown in Appendix 3.1.

3.3.7 Average H2S concentrations ranged between 4 ppb and 24 ppb were recorded during the monitoring period.


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Air Sensitive Receivers

3.3.8 The assessment area for air quality impact is defined by a distance of 500 m from the boundary of the project as shown in Figure 3.1. Within the study area, representative worst affected Air Sensitive Receivers (ASRs) have been identified for this assessment, in accordance with the criteria stipulated in the EIAO-TM. Domestic premises, hotel, hostel, hospital, clinic, nursery, temporary housing accommodation, school, educational institution, office, factory, shop, shopping centre, home for the aged and active recreational activity areas are classified as ASRs. Locations of ASRs and their horizontal separation from the site boundary are listed in Table 3.3. Figure 3.2 shows the locations of the ASRs.

Table 3.3 Location of Air Sensitive Receiver

ASR Description Uses No. of Storeys

Distance between ASR and plant boundary (m)

AS1 Watson’s Water Centre Factory 5 39 AS2 HK Yakult Factory 4 39 AS3 Rainbow Latex Factory 4 56 AS4 Interpac Containers Ltd Factory 1 38 AS5 Cabot Plastics Factory 4 62 AS6 Nukabe Factory 2 87 AS7 Pan Asia Factory 4 87 AS8 Talcon Industrial Ltd Factory 3 8 AS9 Hung Hing Printing Centre Factory 2 9 AS10 Tai Po Waterfront Park Recreational - 340 AS11 Tai Po Shuen Wan golf practice

range (1) Recreational - 176

AS12 Government Staff Quarters Residential 2 6 AS13 Planned commercial buildings Commercial 12 100

(1) Operated between 0700 hours and 2200 hours

3.4 Identification of Environmental Impacts

Construction Phase

3.4.1 The modification work for TPSTW was described in Section 2. Potential dust impacts would be generated from site clearance, excavation, materials handling, and wind erosion. As the construction area would be small and limited construction activities would be operated to prevent affecting normal operation of the STW, significant dust impacts would not be expected at the nearby ASRs.

Operational Phase

3.4.2 In the inlet pumping station, mechanical screens are employed to remove large clumpy items. Currently only part of the inlet wet wells and influent channels have been covered and the exposed part have the potential to release odour. Downstream of the pumping stations is the grit removal where dense and inorganic solids in wastewater are removed. Potential emission from odorous materials is also expected from the grit removal.

3.4.3 In the primary sedimentation tanks, the turbulence resulting from the flow over weirs and evaporative losses from quiescent liquid surfaces of the wastewater treatment tanks would be


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the main source of odour emissions. In the bioreactor, air is supplied to support the metabolic reaction of micro-organisms suspended in the flow. The scum and sludge removal system are the major sources of odours from final sedimentation tanks.

3.4.4 During the sludge handling process, potential odorous emissions could come from feed sludge and storage tank.

3.4.5 The anaerobic digestion occurs in an enclosed vessel equipped with valves and piping to remove the digester gas. Occasional excursion of digester gas via the digested sludge outlet chamber is the cause of odour problem associated with digestion tank.

3.4.6 At the dewatering house, solid materials are fed into the filter press in enclosed pipes, and sludge cake is stored temporarily in the solids handling area. Ferrous chloride is added to suppress the release of hydrogen sulphide and the sludge cake is temporarily stored within the dewatering house and is disposed at landfill site everyday. The odour generated from digested sludge cake in the solids handling area would therefore be negligible. Based on the site visit, odour would be generated from a small screening unit, which is located in open space next to the dewatering house.

3.4.7 Potential sources of odour for different treatment facilities for TPSTW were identified during site visits. Some existing treatment facilities have already been enclosed/covered and odour emission to the atmosphere is not expected. Referring to the construction programme, there are two phases of works proposed, Phase 1 and 2. The identified potential odour sources for Phase 1 and 2 and existing odour mitigation measures are summarized in Table 3.4 and Table 3.5 below. Layout of the treatment facilities for Phase 1 and 2 and the odour sources considered in the model are shown in Figure 3.3 and 3.4. The odour sources considered in the model represented the worst-case scenario in view of the operation process of the STW.

Table 3.4 Potential Odour Sources of Different Treatment Facilities (Phase 1)

Process Potential Odour Source Source ID Existing Odour Control Measures

Odour Source considered in the assessment

Stage I/II Inlet Works • Inlet pumping station

• Grit removal • Flume channel

IN201 GR201 GR202

Most of the areas including screw pumps have been covered

The remaining exposed area

Wastewater Treatment

• Primary sedimentation tank (6 nos. operated + 2 standby) • Bioreactor (5 nos. operated + 1 standby) • Sequence batch reactor • Final sedimentation tank (10 nos. operated)

PRI201-203, 205-207, 209-211 AER201-202 SBR2 FST201-206

None Open area of the operated tanks

Sludge digestion tank (anaerobic)

N/A Fully enclosed, no foul air release to atmosphere

None Sludge Treatment

Sludge digestion tank outlet chamber

DI201 None Open area of the chamber


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Sludge gravity thickener (4 nos. operated)

TH201

None Open area of thickener

Return activated sludge pumping station

RAS2 Most of the area has been covered


Sludge consolidation tank (2 nos. operated)

SOL201-202 None Open area of the tank

Sludge pumping station (wet well)

PUMP2 None Area of wet well

Screening unit (next to Dewatering House)

DEW202 None Open area of the unit

Dewatering House (solid handling area)

N/A Part of area enclosed, Ferrous chloride is added to suppress the release of H2S; sludge cake is covered & stored temporarily

None

Stage IV • Inlet pumping station • Grit removal

- Grit channel - 2 detritors (1 existing + 1

new)

IN401 GR402 GR401

Most of the areas have been covered, including the new detritor


Inlet Work

Screen House SRC401-403 Enclosed; three ventilation fans for air exchange. The foul air from exposed area of sewage (about 20% area of Screen House)

The foul air via the three ventilation fans


• Primary sedimentation tank (3 nos. operated + 1 standby) • Bioreactor (4 nos. operated) • Final sedimentation tank (10

nos. operated)

PRI401-402 AER401 FST401-405




None



Sludge gravity thickener N/A Fully enclosed None

Sludge Treatment



Table 3.5 Potential Odour Sources of Different Treatment Facilities (Phase 2)


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Stage I/II Inlet Works • Inlet pumping station

• Grit removal • Flume channel

IN201 GR201 GR202

Most of the areas including screw pumps have been covered



• Primary sedimentation tank (6 nos. operated + 2 standby) • Bioreactor (5 nos. operated + 1 standby) • Sequence Batch Reactor • Final sedimentation tank (10 nos. operated)

PRI201-203, 205-207, 209-211 AER201-202 SBR2 FST201-206




None



Sludge gravity thickener (4 nos. operated)

TH201 None Open area of thickener

Return activated sludge pumping station

RAS2 Most of the area has been covered





PUMP2 None Area of wet well

Sludge Treatment

Screening unit (next to Dewatering House)

DEW202 None Open area of the unit

Dewatering House (solid handling area)

N/A Part of area enclosed, Ferrous chloride is added to suppress the release of H2S; sludge cake is covered and stored temporarily

None

Stage IV Inlet Work • Inlet pumping station

• Grit removal - Grit channel

- 2 detritors (1 existing + 1 new)

IN401 GR402 GR401

Most of the areas have been covered, including the new detritor


Screen House SRC401-403 Enclosed; three ventilation fans for air exchange. The foul air from exposed area of sewage (about 20% area of Screen House)

The foul air via the three ventilation fans (3 point sources)


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• Primary sedimentation tank (4 nos. operated + 1 standby) • Bioreactor (7 nos. operated) • Final sedimentation tank (11

nos. operated + 1 standby)

PRI401-402 AER401-402 FST401-406




None



Sludge gravity thickener N/A Fully enclosed None

Sludge Treatment



3.5 Assessment Methodology

Inlet Pumping Station, Grit Removal and Flume Channel

3.5.1 The odour emission rates of raw sewage were derived from the empirical equation developed by Shahalem(1). The equations had been employed to estimate the treatment plant in Hong Kong under different approved EIA Studies such as EIA of the Tung Chung Main Sewage Pumping Station, Outlying Islands Sewerage Stage 1 Phase I - EIA Study, and Sha Tin Sewage Treatment Works, Stage III Extension, EIA Study. The equations below were adopted for calculation of odour emission rates for exposed area of the inlet pumping station, grit removal and flume channel in the study.

DF = 1.6 x (T/10) 4.9 x (ORP + 200) –0.59

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

where DF = odour concentration, ou m-3 T = temperature of sewage, oF ORP = oxidation-reduction potential of sewage, mV E = odour emission rate, ou s-1 A = air volume of the emission source, m3 V = ventilation rate, air changes per hour Cf = Correction factor = 0.52 for 5 air changes per hour

3.5.2 The ORP depends on the strength and retention time of sewage, and varies from 50 mV for septic sewage to 200 mV for fresh sewage. The sewage leading to the TPSTW is close to the pumping station with short retention time (less than 2 hours). Anaerobic conditions were not expected, and ORP of semi-fresh sewage 150 mV, was assumed in the model. The temperature of sewage was assumed to be 30 oC (86 oF). In addition, the following parameters, which were adopted in the Outlying Islands Sewerage Stage 1 Phase 1 EIA Study for similar facilities, were employed for this odour assessment.

(1) Shahalam, A. (1982), Scrubbing Odors from Wastewater Treatment, Journal of the Environmental Engineering

Division, Proceedings of the ASME, Vol. 108, No. EE4.


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A = 1.0 m x surface area of tanks, (inlet works, grit removal & flume channels) V = 5 air changes per hour for open area

Primary Sedimentation Tanks

3.5.3 The emission of odorous gas from the primary sedimentation tank is highly dependent on the turbulence at the water’s surface. For a circular primary sedimentation tank, wastewater enters the tank in the centre ring and moves radially from the centre ring to the outer weirs. A turbulence zone is expected when the wastewater moves over the outer weirs; odour is therefore released predominantly from the overflow weirs. Odour release at rectangular sedimentation tank occurs mostly at the settled sewage overflow weir and channels. Odour emission from the top-water horizontal surface, quiescent zone, is also expected but with a lesser odour strength. The equations below were adopted for calculation of odour emission rates for the weir area and quiescent zone developed by Stuetz & Frechen(2) and J Hobson(3) .

E weir = 7.16 x 10-4 x OP x Fweir x h x KpH

where E weir= odour emission rate per unit length of weir, ou/s/m OP = odour potential of the liquid flowing over the weir, ou/m3 Fweir = weir loading rate, m2/h h = height of drop of liquid flow at weir, m KpH = pH correction coefficient, take a value of 1.17 at pH7.

Eton = 4 x 10-3 (0.0103 V wind1.42 +2.93 Vliquid) OP

where Eton = surface emissions of quiescent zone, ou m-2 s-1 OP = odour potential, ou m-3 Vwind = wind speed on surface of tank, m s-1 Vliquid = velocity of liquid across the tank, m s-1

3.5.4 The odour emission rate from quiescent zone is dependent on the wind velocity at the wastewater surface; the wind velocity of different stability classes (Class A to F) was calculated(4) based on different Mon-Obukov length(5) and wind velocity recorded at Tai Mei Tuk Weather Station in 2001.

Secondary Treatment Unit, Sequencing Batch Reactor & Final Sedimentation Tank

3.5.5 The odour emission of treated sewage of TPSTW (that is, bioreactor, final sedimentation tank and return activated sludge pumping station) was calculated based on the empirical formula developed by J Hobson. The equation was used in the approved Sha Tin Sewage Treatment Works (STSTW), Stage III Extension, EIA Study.

Eton = 4 x 10-3 (0.0103 V wind1.42 +2.93 Vliquid) OP

where Eton = surface emissions (ou m-2s-1) OP = odour potential (ou m-3)

(2) Richard Stuetz and Franz-Bernd Frechen (2001), Odours in Wastewater Treatment Measurement, Modelling and

Control, IWA Publishing. (3) Hobson, J. (1995), Emission Rate from Sewage Works, WRc Report PT 1048, Water Research Centre, (unpublished

report). (4) A.P. Van Ulden & A.A.M. Holtslag (1985), Estimation of Atmospheric Boundary Layer Parameters for Diffusion

Application, Volume 24, 1196-1207, Journal of Climate and Applied Meteorology. (5) Paslo Zannett (1990), Air Pollution Modelling Theories, Computational Method and Available Software, MIT Press.


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Vwind = wind speed on surface of tank (m s-1) Vliquid = velocity of liquid across the tank (m s-1)

3.5.6 In the odour assessment, the wind speed on the surface of the tank, Vwind, of each stability class was calculated as mentioned in Section 3.5.4; while the odour potential was assumed to be 710 ou m-3, as suggested by G Yang & J Hobson (6).

3.5.7 It was assumed the odour emission rate applied for the whole bioreactor / final sedimentation tank as a worst-case scenario.

Sludge Digestion Tank Outlet Chamber, Sludge Gravity Thickeners, Sludge Consolidation Tanks

3.5.8 The odour characteristics of sludge holding facilities would be in a similar order to that at the inlet works(7). The calculation method for emission rates for the sludge facilities were therefore assumed to be equal to that of the inlet pumping station in this assessment. However, the ORP was assumed to be 50mV due to anaerobic condition.

3.5.9 The odour emission parameters for the TPSTW for Phase 1 & 2 to be considered in the model, which would be the worst-case scenario, are summarised in Table 3.6 and 3.7. Detailed calculations of the emission rates are shown in Appendix 3.2.

Table 3.6 Odour Emission Rate of TPSTW (Phase 1)

Emission Source Source ID No. of unit operated

Dimension of Surface

(m)

Exposed Surface

Area (m2)

Height above

ground (m)

Odour Emission

Rate (ou m-2 s-1)

Stage I/II Inlet pumping station IN201 - - 16 0 1.383 Grit removal GR201 - - 84 4 Flume channel GR202 - - 15 4

1.383

PRI201 20.5 dia.(b) 330 0 PRI202 20.5 dia.(b) 330 0 PRI203 20.5 dia.(b) 330 0 PRI205 20.5 dia.(b) 330 0 PRI206 20.5 dia.(b) 330 0 PRI207 20.5 dia.(b) 330 0

9.9651 x 10-3 –1.0454x 10-2 (c)

Primary sedimentation Tank “Quiescent” Zone “Weir” Zone

PRI 209-211

6 + 2*

Weir width:0.65m

43.2 x 6 =259.2

0 1.6258

AER202 2 + 1* 36 x 40 (a) 1,440 0.2 Bioreactor AER201 3 36 x 60 (a) 2,160 0.2

3.3633 x 10-2 – 3.3738 x 10-2 (c)

FST201 2 21.8 dia.(b) 373 x 2 = 746 0 FST202 2 21.8 dia.(b) 373 x 2 = 746 0 FST203 2 21.8 dia.(b) 373 x 2 = 746 0 FST204 2 21.8 dia.(b) 373 x 2 = 746 0 FST205 1 21.8 dia.(b) 373 0

Final sedimentation tank

FST206 1 21.8 dia.(b) 373 0

1.1363x 10-3 – 1.2414x 10-3 (c)

(6) Yang, G. and Hobson, J. (1999), Validation of the Sewage Treatment Odour Production (STOP) Model, J. Inst. Water &

Environmental Management, 13(2); 115-120. (7) Shanahan, I. (1993), Odour Control in Wastewater Treatment Processes, Institution of Water Officers Journal.


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(m)

Exposed Surface

Area (m2)

Height above

ground (m)

Odour Emission

Rate (ou m-2 s-1)

Sequence batch reactor

SBR2 1 20 x 15 300 0.2 9.4539 x 10-7 – 1.0604 x 10-4 (c)

RAS pumping station RAS2 - - 66 3 1.1674 –1.2764 (c)


DI201 - 3 x 1.7 (a) 5.1 8 1.6867

Sludge gravity thickener

TH201 4 11 dia.(b) per unit

Total area : 380

4 1.6867

SOL201 20 dia.(b) 314 8 Sludge consolidation tank SOL202

2 20 dia.(b) 314 8

1.6867

Screening Unit (next to Dewatering House)

DEW202 - 3.3 x 2.7 (a) 9.0 0 1.6867

Sludge pumping station (Wet well)

PUMP2 1 5.4 x 2.5 (a) 14 0 1.6867

Stage IV Inlet pumping station IN401 - 10 x 8.4 84 0 1.383

SCR401 SCR402

Screen House

SCR403

3 Fans Exposed sewage area = 20% of screen house = 21m2

8.7 9.1907 ou/s per source (each

fan) GR401 - 11 dia.(b) 95 5 Grit removal (2

detritors (1 existing + 1 new (enclosed) and grit channel

GR402 - 2 x 10 20 5

1.383

Primary sedimentation tank

PRI402 (Weir)

PRI401

(Quiescent zone)

3 + 1* 1 x 45 (a)

45 x 45 (a)

45

2,025

2 Weir: 28.102 Quiescent zone: 1.8886 x 10-1 – 5.2395x 10-1 (c)

Bioreactor AER401 4 70 x 60 (a) 4,200 1 4.1105 x 10-2 – 9.0121x 10-2 (c)

FST404 -FST405

4 26.5 dia.(b) 2,206 1 1.3895 x 10-3 – 4.058 x 10-2 (c)


FST401 - FST403

6 30 dia (b) 4,241 1 1.1746 x 10-3 – 4.0365x 10-2 (c)


DI401 - 3 x 1.7 (a) 5 5.9 1.6867

SOL401 15 dia.(b) SOL402 15 dia. (b)

Sludge consolidation tank

SOL403

3

15 dia. (b)

530 (a) 4 1.6867

(a) Exposed area of all units

(b) Diameter of each tank (c) Odour emission rate depends on different stability classes, details refer to Appendix 3.2 * standby unit

Table 3.7 Odour Emission Rate of TPSTW (Phase 2)


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(m)

Exposed Surface

Area (m2)

Height above

ground (m)

Odour Emission

Rate (ou m-2 s-1)

Stage I/II Inlet pumping station IN201 - - 16 0 1.383 Grit removal GR201 - - 84 4 Flume channel GR202 - - 15 4

1.383

PRI201 20.5 dia.(b) 330 0 PRI202 20.5 dia.(b) 330 0 PRI203 20.5 dia.(b) 330 0 PRI205 20.5 dia.(b) 330 0 PRI206 20.5 dia.(b) 330 0 PRI207 20.5 dia.(b) 330 0

9.9651 x 10-3 – 1.0454 x 10-2 (c)

Primary sedimentation Tank “Quiescent” Zone “Weir” Zone

PRI209 -PRI211

6 + 2*

Weir width:0.65m

43.2 x 6 =259.2

0 1.6258

AER202 2 + 1* 36 x 40 1,440 0.2 Bioreactor AER201 3 36 x 60 2,160 0.2

3.3633 x 10-2 – 3.3738 x 10-2 (c)

FST201 2 21.8 dia.(b) 373 x 2 = 746 0 FST202 2 21.8 dia.(b) 373 x 2 = 746 0 FST203 2 21.8 dia.(b) 373 x 2 = 746 0 FST204 2 21.8 dia.(b) 373 x 2 = 746 0 FST205 1 21.8 dia.(b) 373 0


FST206 1 21.8 dia.(b) 373 0

1.1363 x 10-3 – 1.2414x 10-3 (c)

Sequence batch reactor

SBR2 1 20 x 15 300 0.2 9.4539 x 10-7 – 1.0604 x 10-4(c)

RAS pumping station RAS2 - - 66 3 1.1674 – 1.2764(c)


DI201 - 3 x 1.7 (a) 5.1 8 1.6867


TH201 4 11 dia.(b) per unit

Total area : 380

4 1.6867

SOL201 20 dia.(b) 314 8 Sludge consolidation tank SOL202

2 20 dia.(b) 314 8

1.6867

Screening Unit (next to Dewatering House)

DEW202 - 3.3 x 2.7 (a) 9.0 0 1.6867

Sludge pumping station (Wet well)

PUMP2 1 5.4 x 2.5 (a) 14 0 1.6867

Stage IV Inlet pumping station IN401 - 10 x 8.4 84 0 1.383

SCR401 SCR402

Screen House

SCR403

3 Fans Exposed sewage area = 20% of screen house = 21m2

8.7 9.1907 ou/s per source (each

fan) GR401 - 11 dia.(b) 95 5 Grit removal (2

detritor (1 existing + 1 new (enclosed) and grit channel

GR402 - 2 x 10 20 5

1.383


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(m)

Exposed Surface

Area (m2)

Height above

ground (m)

Odour Emission

Rate (ou m-2 s-1)


PRI402 (Weir)

PRI401

(Quiescent zone)

4 + 1* 1 x 60 (a)

45 x 60 (a)

60

2,700

2 Weir: 28.102 Quiescent zone: 2.1441 x 10-1 – 5.495x 10-1 (c)

Bioreactor AER401 AER402

6 1

70 x 90 (a)

50.9x21.2 (a) 6,300 1,079

1 3.6239 x 10-2 – 8.5255 x 10-2 (c)


FST401- FST406

11 + 1* 30 dia.(b) 7,775 1 1.4769 x 10-3 – 4.0667 x 10-2 (c)


DI401 - 3 x 1.7 (a) 5 5.9 1.6867

SOL401 15 dia.(b) SOL402 15 dia.(b)


SOL403

3

15 dia.(b)

530 (a) 4 1.6867

(a) Exposed area of all units (b) Diameter of each tank (c) Odour emission rate depends on different stability classes, details refer to Appendix 3.2 * standby unit

Dispersion Model

3.5.10 Air quality impacts of odour on ASRs were modelled with the air dispersion model, AUSPLUME. Hourly meteorological data for the year 2001 (including wind speed, wind direction, air temperature, Pasquill stability class and mixing height) of the Tai Mei Tuk Weather Station were employed for the model run.

3.5.11 The modelled hourly odour concentrations at the ASRs were converted into the 5-second odour concentration by the methodology proposed by Duffee et al.(8) and Keddie(9). In addition, Turner(10) has identified that the Pasquill-Gifford vertical dispersion parameter used in the ISC3 model is around 3 to 10 minutes. As a conservative assumption, the hourly average estimated by AUSPLUME model was assumed as 15 minutes, and the conversion factors for the predicted 1-hour averaged concentration of odour at the receivers was adjusted to 5-second averaging time by the values shown in Table 3.8.

Table 3.8 Conversion Factors to 5-second Mean Concentration

Conversion Factor Pasquill Stability Class 15 min to 3 min 3 min to 5 sec Overall

A 2.23 10 22.3 B 2.23 10 22.3 C 1.7 5 8.5 D 1.38 5 6.9 E 1.31 5 6.55

8 Richard A. Duffee, Martha A. O”Brien and Ned Ostojic (1991). Odor Modeling – Why and How, Recent

Developments and Current Practices in Odor Regulation, Controls and Technology, Air & Waste Management Association.

9 Keddie, A. W. C (1980). Dispersion of Odours, Odour Control – A Concise Guide, Warren Spring Laboratory. 10 Turner, D. (1994). Workbook of Atmosphere Dispersion Estimates, 2nd Edition, Lewis Publishers.


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Conversion Factor Pasquill Stability Class 15 min to 3 min 3 min to 5 sec Overall

F 1.31 5 6.55

3.5.12 The Tai Po town centre is located to the west of the TPSTW at about 1 km from the site. High-rise developments over 100m have been developed. Higher topographical relief (50m – 100m above ground) and some mid-rise residential developments are found on the north and north-east of the TPSTW. Surface roughness of Tai Po is estimated to be 100 cm. However, at the south-east direction of the TPSTW, the wind is directly coming from the sea and passing across the low rise structures (less than 10m in height). The surface roughness for the wind angle between 90 to 130 degrees adopted in the model and emission calculation was 50cm.

3.6 Prediction and Evaluation of Environmental Impacts

Construction Phase

3.6.1 Given the small scale of construction works, dust impacts at the ASRs would be low. Construction works are controlled by the Air Pollution Control (Construction Dust) Regulation, and mitigation measures such as watering are required under the regulation to limit its dust emission. Typical dust control measures are presented in Section 3.7. With the implementation of recommended dust suppression measures in the Air Pollution Control (Construction Dust) Regulation , the dust impacts would be controlled within the relevant standards as stipulated in the EIAO-TM.

3.6.2 It is recommended that an Environmental Monitoring and Audit (EM&A) programme should be implemented during the construction phase to ensure that the dust criteria would be satisfied at the ASRs. Key EM&A requirements are presented in Section 8.

Operational Phase

3.6.3 Odour levels at the ASRs have been predicted for two scenarios, i.e. after completion of Phase 1 of Stage V and after completion of Phase 2 of Stage V. The unmitigated results are shown in Table 3.9 and 3.10. The odour contours at 1.5m after completion of Phase 1 and Phase 2 are illustrated in Figures 3.5 and 3.6.

Table 3.9 Predicted Unmitigated Odour levels at ASRs (Completion of Phase 1)

Odour level (5 seconds average) (ou) at different height ASR Description 1.5 m 5m 10 m 15m 20 m

AS1 Watson’s 42.5 36.3 14.9 8.0 6.8 AS2 HK Yakult 31.7 27.1 14.6 5.5 4.9 AS3 Rainbow Latex 20.4 18.1 11.7 5.6 4.4 AS4 Interpac Containers Ltd 17.3 18.6 18.8 7.6 6.3 AS5 Cabot Plastics 20.8 26.0 22.8 8.1 6.9 AS6 Nukabe 18.7 16.3 12.8 6.6 5.8 AS7 Pan Asia 17.5 15.2 12.8 6.5 5.7 AS8 Talcon Industrial Ltd 16.1 14.6 10.1 5.3 4.0 AS9 Hung Hing Printing Centre 10.1 9.3 7.1 4.6 4.2 AS10 Tai Po Waterfront Park 6.9 6.5 5.4 4.0 2.6 AS11 Tai Po Shuen Wan golf

practice range 2.4 2.3 2.1 2.0 1.8


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AS12 Government staff quarters 24.0 20.3 11.4 5.3 4.4 AS13 Planned ASR 18.8 15.9 9.7 5.4 4.8

(1) Italic and underlined figures exceed the 5 ou odour criterion.

Table 3.10 Predicted Unmitigated Odour levels at ASRs (Completion of Phase 2)



practice range 2.9 2.8 2.5 2.4 2.1 AS12 Government staff quarters 23.9 21.1 13.5 6.4 4.8 AS13 Planned ASR 21.5 18.3 11.6 6.7 5.0

(1) Italic and underlined figures exceed the 5 ou odour criterion.

3.6.4 It has been predicted that unmitigated odour levels at most ASRs would exceed the odour criterion of 5 ou. Highest odour concentration 46 ou was predicted at AS1 under the worst- case scenario. Mitigation measures would be required to protect receivers in the vicinity of the TPSTW.

3.7 Mitigation of Adverse Environmental Impacts

Construction Phase

3.7.1 Dust mitigation measures stipulated in the Air Pollution Control (Construction Dust) Regulation should be incorporated to control dust emission from the site. Control measures relevant to this Project are listed below:

• skip hoist for material transport should be totally enclosed by impervious sheeting;

• vehicle washing facilities should be provided at every vehicle exit point;

• the area where vehicle washing takes place and the section of the road between the washing facilities and the exit point should be paved with concrete, bituminous materials or hardcore;

• where a site boundary adjoins a road, streets or other areas accessible to the public, hoarding of not less than 2.4 m high from ground level should be provided along the entire length except for a site entrance or exit;

• every main haul road should be paved with concrete and kept clear of dusty materials or sprayed with water so as to maintain the entire road surface wet;


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• the portion of road leading only to a construction site that is within 30 m of a designated vehicle entrance or exit should be kept clear of dusty materials;

• every stock of more than 20 bags of cement should be covered entirely by impervious sheeting placed in an area sheltered on the top and the 3 sides;

• all dusty materials should be sprayed with water prior to any loading, unloading or transfer operation so as to maintain the dusty materials wet;

• every vehicle should be washed to remove any dusty materials from its body and wheels before leaving the construction sites;

• the load of dusty materials carried by vehicles leaving a construction site should be covered entirely by clean impervious sheeting to ensure dusty materials do not leak from the vehicle.

Operation Phase

3.7.2 The predicted results indicate that all ASRs would exceed the odour criterion under the worst- case scenario and suppression measures are required to protect the ASRs against odour nuisance. Odorous gases from a sewage treatment works may be controlled by either treating the odorous gas or by reducing the generation of odorous gases at source. Common methods that could be used to treat odorous gases are atmospheric discharge and dilution, masking, containment, collection and treatment.

3.7.3 The use of atmospheric dilution requires enclosing the system, collecting odorous gas and discharging via a high chimney, to allow good dispersion of pollutants. A high chimney may impose visual impact to the environment. In addition, building a high chimney at TPSTW is not feasible due to the site constraint. The option of high chimney is, therefore, not considered further.

3.7.4 Odour masking, on the other hand, attempts to disguise an unpleasant odour with a strong, pleasant odour. However, the masking effect is unpredictable in the long run, attributed to ever-changing odour concentration and atmospheric condition. Also, someone may consider the “pleasant” smell an objectionable odour. The use of masking is considered not applicable to TPSTW.

3.7.5 Odour would be released at the inlet works as a result of discharges of sewage. The turbulence induced by drops would release the odorous gas from sewage. The immediate area of turbulence: mechanical screen, grit removal and flume channel could be covered to reduce odour emission from site. Foul air underneath the covers would be withdrawn by pumps to create a slight negative pressure, which eliminate odour leakage from seals. It is recommended that foul air be drawn through a deodorizer and be discharged after treatment. Vincent and Hobson(11) reported that 99% and even 99.9% odour reduction could be achieved with deodourizing units. Deodourizing unit with control efficiency of 99% was assumed in this Study.

3.7.6 Odour emission from the Stage I/II and Stage IV primary sedimentation tanks could be controlled by covering the weir launders. Chemical addition (e.g. addition of calcium nitrate) to sewage at Tai Yuen Sewage Pumping Station No. 4 (convey sewage to Stage IV works) before sewage flowing into TPSTW Stage IV could effectively reduce the odour strength at the inlet work, grit removal, screen house and primary sedimentation tanks. According to

11 Vincent, A. and Hobson J. (1998), Odour Control, Chartered Institution of Water and Environmental Management.


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prediction of odour model, which represents a worst-case scenario, i.e. maximum flow coupled with most adverse meteorological conditions, the required odour removal efficiency by chemical dosage for Stage IV Primary Sedimentation Tank after completion of Phase 1 works would be up to a maximum of 45%. After completion of Phase 2 works, the odour removal efficiency by chemical dosage for Stage IV should be up to a maximum of 65%.

3.7.7 Odour emissions from the sludge facilities, including: sludge gravity thickener, sludge consolidation tanks, screening unit (next to dewatering house) and wet well of sludge pumping stations should also be enclosed, to ensure no leakage of odorous gas. The foul air from the sludge gravity thickener and sludge consolidation tanks would be discharged into atmosphere after passing through deodorizers.

3.7.8 It is recommended that the sludge digestion tank outlet chamber should be covered, with the foul air treated by deodorisation unit before discharging into atmosphere.

3.7.9 In the final sedimentation tank, effluent and waste sludge should be well oxidised in the clarifier, and adverse odour impact would not be generated unless there are operational problems. The two major sources of odours are scum and sludge removal system. Without proper, daily housekeeping, accumulations of scum on clarifier water surface, sludge accumulation on walls, and organic matter on effluent weir troughs can produce odours.

3.7.10 The following measures are recommended for the final sedimentation tanks.

• Scum scrapers, troughs, weirs and interior walls of final clarifiers should be cleaned and flushed regularly; and

• Settled solids should not be allowed to accumulate long enough for development of anaerobic conditions. Residence times not exceeding 2 hours is recommended12 to avoid septic conditions in the settled sludge.

3.7.11 In addition, the practices of good housekeeping for a STW shown in Table 3.11 should be followed to ameliorate the odour impact from the plant and these standard practices should be included in the plant operator manual.

Table 3.11 Practices for Good House Keeping

Location Good Practices Preliminary treatment processes

• Bar screens should be cleaned regularly to remove any accumulated organic debris

• Grit and screening transfer systems should be flushed regularly with water to remove organic debris and grit

• Grit and screened materials should be transferred to closed containers

Primary sedimentation tank, bioreactor and final sedimentation tank

• Scum and grease collection wells and troughs should be emptied and flushed regularly to prevent putrefaction of accumulated organics

• Skim and remove floating solids and grease from primary clarifiers regularly

• Frequent sludge withdrawal from tanks is necessary to prevent the production of gases

Sludge Transfer Pumping Station

• Screened material should be transferred to closed containers to minimise odour escape

Sludge Dewatering Area

• Sludge cake should be transferred to closed containers • Sludge containers should be flushed with water regularly

12 WEF Manual of Practice No 22 (1995), Odour Control in Wastewater Treatment Plants.


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3.7.12 Table 3.12 below summarizes the proposed control measures for the odorous facilities, outlined in this air quality assessment. The DSD’s design team may propose alternative control measures, taking into account of constraints of the facilities, so long as it can be demonstrated that equivalent mitigated odour emission rates, listed in Table 3.12 could be achieved.

3.7.13 Detailed calculation of mitigated emission rates for the proposed control scheme for Phase 1 & 2 of Stage V works is shown in Appendix 3.2 and the recommended covered/enclosed facilities/area are presented in Figures 3.7 and 3.8. Except 65% of odour removal efficiency by chemical dosage for Stage IV Primary Sedimentation Tank to be implemented after completion of Phase 2 of Stage V of TPSTW, all proposed odour mitigation measures would be implemented after completion of Phase 1 of Stage V of TPSTW.

Table 3.12 Proposed Control Method for Odorous Facilities

Facility Control Method Deodorizer Requirement

Odour Emission Rate

Inlet pumping station

Cover exposed area (IN201) (1) + deodorizer(2) (IN2)

Height = 3m Dia. = 0.3m

Flow rate = 0.16 m3/s

0.22129 ou/s

Flume channel & grit removal

Cover exposed area (GR201 & GR202)(1)

- -


Cover all weir launders (PRI209 – PRI215) (1)

- Refer to Appendix 3.2

Return Activated Sludge Pumping Station

Cover exposed area (RAS2) (1)

- -

Sludge digestion tank

Cover outlet chamber (DI201)(1) + deodorizer(2) (DI2)



8.6024 x 10-2 ou/s


Cover (TH201) (1)

+ deodorizer(2) (TH2) Height = 3m Dia. = 0.3m


6.4119 ou/s


Cover (SOL201 & SOL202)(1) + deodorizer(2) (SOL2)



10.598 ou/s

Screening unit outside Dewatering house

Cover (DEW202) (1) - -

Stage I/II


Cover exposed area (PUMP2)(1)

- -

Stage IV Inlet pumping station

Cover (IN401) (1) + deodorizer(2) (IN4) Chemical Dosage: Odour reduction up to 45% (Phase 1) Odour reduction up to 65% (Phase 2)



0.63896 ou/s (Phase 1)

0.40661 ou/s (Phase 2)


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Facility Control Method Deodorizer Requirement

Odour Emission Rate

Grit Removal Cover exposed area (GR401 & GR402) (1)

- -

Screen House Chemical Dosage: Odour reduction up to 45% (Phase 1) Odour reduction up to 65% (Phase 2)

- 5.0549 ou/s (Phase 1)

3.2168 ou/s (Phase 2)


Cover all weir launders (PRI 402) Chemical Dosage Odour reduction up to 45% (Phase 1) Odour reduction up to 65% (Phase 2)

- Refer to Appendix 3.2

Sludge digestion tank chamber

Cover (DI401) (1) + deodorizer(2) (SODI4)


Cover (SOL401, SOL402 & SOL403) (1) + deodorizer(2) (SODI4)



9.0282 ou/s

Note: (1) Cover area refers to Figure 3.7 & 3.8. (2) 99% odour removal efficiency

3.7.14 The effects of mitigation measures have been assessed and the mitigated odour prediction results are shown in Table 3.13 and 3.14 below.

Table 3.13 Predicted Mitigated Odour levels at ASRs (Completion of Phase 1)





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Table 3.14 Predicted Mitigated Odour levels at ASRs (Completion of Phase 2)




3.7.15 The above modelling results suggest that the odour criterion of 5 ou would be satisfied at all ASRs. At higher elevation the odour impacts would be lower.

3.7.16 Contours of mitigated odour concentration at the worst affected elevation, 1.5 m above ground, have been plotted and presented in Figures 3.9 and 3.10. It confirms that all areas of the air sensitive users outside the site boundary would satisfy the odour criterion of 5 ou after completion of Phase 1 of Stage V of TPSTW. After completion of Phase 2 of Stage V of TPSTW, the contour line of 5 ou would cross over the corner of the AS1 at 1.5m above ground. The affected area is the transformer plant room which should not have any person staying there. The windows at AS1 facing TPSTW are all fixed windows as confirmed with the owner of the premises. Except the intakes/louvres of the above transformer room, the air intakes/louvers at AS1 are at least 3.5m high above ground. Contours of mitigated odour concentration at 3.5m above ground has been plotted and presented in Figure 3.11. All areas of the air sensitive users outside the TPSTW boundary would satisfy the odour criterion.

3.7.17 The above recommended odour control measures in Table 3.13 are based on assessment under the worst-case scenario. The amount of chemicals to be used at Tai Yuen Pumping Station would depend on the sewage characteristics (sulphate concentration) and the flow, it is recommended to conduct a field test to determine the quantity of chemicals to achieve the recommended odour removal efficiency at Stage IV inlet pumping station, Stage IV screen house and Stage IV primary sedimentation tanks before the commissioning the Stage V of TPSTW.

3.8 Evaluation of Residual Impacts

Construction Phase

3.8.1 With the incorporation of Air Pollution Control (Construction Dust) Regulation and EM&A programme, adverse residual impact during construction phase is not expected.

Operation Phase


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3.8.2 Air quality at the ASRs has been predicted to satisfy with the odour criteria with the proposed mitigation measures, and adverse residual impact is not expected.

3.9 Environmental Monitoring and Audit

Construction Phase

3.9.1 With the implementation of mitigation measures stipulated in the Air Pollution Control (Construction Dust) Regulation, dust levels at all ASRs would comply with the dust criteria. Details of the monitoring requirements are presented in Section 8 and the stand-alone EM&A Manual.

Operation Phase

3.9.2 The predicted air quality at the ASRs complies with the criteria. It is recommended that odour monitoring should be carried out during the operation phase to ensure that the odour criteria are satisfied at the ASRs. Details of monitoring programme are presented in the EM&A Manual.

3.10 Conclusions

Construction Phase

3.10.1 Air quality impact from the construction and operation of the Project has been assessed. With the implementation of mitigation measures specified in the Air Pollution Control (Construction Dust) Regulation, dust nuisance at ASRs would not be expected. An EM&A programme is recommended for the construction phase to ensure that the dust criteria would be satisfied at the ASRs.

Operation Phase

3.10.2 Odour emission from the TPSTW would be the main concern during the operation phase. Air dispersion modelling was conducted to simulate the potential odour impacts of the TPSTW on the ASRs. It was recommended that the inlet pumping stations, sludge digestion tank outlet chambers, sludge gravity thickeners and sludge consolidation tanks should be covered, with the foul air drawn through deodorization units and discharged after treatment. Grit removal, flume channel, weir launders of Stage I/II & IV primary sedimentation tanks, exposed area of wet well of returned activated sludge pumping station, screening unit next to dewatering house and sludge pumping station (wet well) should also be covered. Good house keeping (regular cleansing schedule) should be conducted to minimise odour generation from the TPSTW. Addition of chemical in sewage at Tai Yuen Sewage Pumping Station No. 4 to reduce odour impact arising from the Stage IV inlet pumping station, Stage IV screen house and Stage IV primary sedimentation tanks is also required. It should be pointed out that the preceding odour modelling represents a worst-case scenario, i.e. maximum flow coupled with most adverse meteorological conditions, and the percentage reduction in odour to be achieved by chemical dosing in Table 3.12 corresponds to such a worst case. It is expected that for the larger part of the time the percentage reduction required to meet the odour criteria is not as high and a field test is recommended to determine the chemical dosage for different sewage flow volume, against the criterion of the odour levels at sensitive receivers not exceeding 5 OU, before the commissioning the Stage V of the TPSTW. With the proposed mitigation, the


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residual odour impacts at the ASRs were predicted to be in compliance with the odour criterion.

3 air quality impact

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