3.1.1.1
This
section evaluates
the potential air quality and odour
impacts that are likely to be generated during the construction and operation
phases of the proposed Project. Appropriate mitigation measures were
identified, where necessary, to mitigate the potential air quality and odour
impacts to acceptable levels.
3.2.1.1
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 EIAO-TM, respectively.
3.2.1.2
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)
should be satisfied at the Hong Kong Special Administrative Region (HKSAR). The
AQOs stipulate the maximum allowable concentrations over specific period for
typical pollutants. 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)
|
|
TSP
|
-
|
-
|
260
|
80
|
|
RSP (5)
|
-
|
-
|
180
|
55
|
|
SO2
|
800
|
-
|
350
|
80
|
|
NO2
|
300
|
-
|
150
|
80
|
|
CO
|
30,000
|
10,000
|
-
|
-
|
Note: (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 mm or smaller.
3.2.1.3
The
EIAO-TM also stipulates that the hourly TSP level should not exceed 500 µgm-3 (measured at 25°C and one atmosphere) for construction dust
impact assessment. Mitigation measures
for construction sites have been specified in the Air Pollution Control
(Construction Dust) Regulations.
3.2.1.4
In
accordance with the EIAO-TM, an odour prediction limit of 5 odour units based
on an averaging time of 5 seconds at an air sensitive receiver should be met.
3.3.1.1
The
existing PPSTW is located at the southern coast of the western New Territories. To the south of the site is the River Trade
Terminal at Tuen Mun Area 38. To the west of the site are Green Island Cement
Plant and Castle Peak Power Station.
Most of the area surrounding Pillar Point STW is unpopulated. The proposed site of the upgrading facilities
is located within a reserved area next to the existing STW compound, and the
surrounding area comprises various government agencies and industrial
buildings.
3.3.1.2
The
ambient air quality of the site is mainly affected by the emissions from Castle
Peak Power Station. Vehicle exhaust
emissions from the local road network also contribute to the ambient air
quality of the assessment area.
3.3.1.3
As
there are no EPD air quality monitoring stations located in the Tuen Mun area,
reference has been made to representative data from other monitoring
stations. According to EPD's Guidelines
on Assessing the 'Total' Air Quality Impacts, Tuen Mun has an air quality
category of "Rural / New Development", which is the same as Yuen Long
area. The background air quality for the
assessment area has thus been estimated with reference to five years of
monitoring data at EPD’s Yuen Long ambient air quality monitoring station as
stated in Air Quality in Hong Kong” (2002 – 2006) as published by EPD. The background levels of the major air
pollutants are summarised in Table 3.2.
Table 3.2 Background Air Quality
|
Pollutants
|
5-year
Annual Average (mg m-3)
|
|
Total Suspended Particulates
|
100
|
|
Respirable Suspended Particulates
|
62
|
|
Sulphur Dioxide
|
24
|
|
Nitrogen Dioxide
|
60
|
3.4.1.1
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 assessment 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. The nearest ASRs in
the vicinity of the proposed site within 500m from the site boundary are
identified and the locations
of these ASRs and their horizontal separation from the site boundary are listed
in Table 3.3. Figure 3.1 shows the locations of the
ASRs.
3.4.1.2
The
air quality impact at 1.5m above local ground level of representative ASRs,
which is the average height of the human breathing zone, will be assessed in
the study. Higher assessment levels are
also selected for elevated ASRs to show the vertical variation of the pollutant
concentrations. The maximum building heights of the ASRs
within 500m from the site boundary are 25m above ground level and therefore it
is the maximum assessment height for this study.
Table 3.3 Details of Representative Air
Sensitive Receivers
|
ASR I.D.
|
Description
|
No. of Floor
|
Uses
|
Assessment Height (above ground)
|
Distance between ASR and plant boundary (1)
|
|
A1
|
River Trade Terminal Office
|
5
|
Commercial
|
1.5m, 5m, 10m, 15m, 20m & 25m
|
80m
|
|
A2
|
Chu Kong Warehouse 1
|
5
|
Factory
|
1.5m, 5m, 10m, 15m, 20m & 25m
|
180m
|
|
A3
|
Chu Kong Warehouse 2
|
3
|
Factory
|
1.5m, 5m, 10m, 15m, & 20m
|
150m
|
|
A4
|
Sawmill
|
1
|
Factory
|
1.5m, 5m & 10m
|
250m
|
|
A5
|
Pillar Point Fire Station
|
3
|
GIC
|
1.5m, 5m, 10m, 15m, & 20m
|
250m
|
|
A6
|
Sunhing Hung Kai Tuen Mun Godown
|
3
|
Factory
|
1.5m, 5m, 10m, 15m, & 20m
|
370m
|
|
A7
|
Tuen Mun EMSD Servicing Vehicle Station
|
1
|
GIC
|
1.5m, 5m & 10m
|
120m
|
Note : 1. Distances are
measured between ASRs and the nearest boundary of Pillar Point STW.
2. The assessment height of each ASR is based
on its maximum height.
3.5.1
Construction Phase
3.5.1.1
Potential dust impacts
would be generated from site clearance, excavation, materials handling,
concreting operation and wind erosion.
All the construction works would be limited to within the boundary of
PPSTW. The construction works are mainly
carried out in the open area (about 32,800 m2) adjacent to the
existing PPSTW. There are also new buildings to be constructed inside the
existing PPSTW and the total area required for the new buildings would be about
3,710 m2. Based on the
preliminary design, the construction works would create about 61,489 m3
excavated material in total, and the number of trucks hauling on-site depends
on the schedule of construction activities.
In accordance with the tentative construction programme, the excavation
activities are to be completed within 213 days.
The average volume of excavated materials per day during construction period
would be 288 m3. The average
number of trucks (carrying 5m3 excavated materials) required on-site
would be about 6 trucks per hour. As all
the trucks would be well covered and the dust suppression measures stipulated
in the Air Pollution Control (Construction Dust) Regulation would be applied,
and the nearest ASRs is located at least 80m away from the site boundary, it is
expected there would be no adverse construction dust impacts on the
nearby ASRs. Qualitative assessment was
therefore conducted for the construction of the Project and mitigation measures
are recommended to protect ASRs.
3.5.1.2
According to the latest programme and
preliminary design, the upgraded PPSTW is scheduled to start commissioning in
May 2012. Phased construction of PPSTW
would not be required. During construction of the Project, the existing sewage
treatment facilities would not be affected and provision of temporary sewage
treatment units would be unnecessary.
Therefore, no additional odour sources would be expected during the
construction phase of the Project.
3.5.1.3
It
is noted that there are planned developments in the Tuen Mun Area 38, which
include Permanent Aviation Fuel Facility and EcoPark. However, all of the developments are located
outside 500m from the
PPSTW site boundary. No
other major project is
identified to be carried out concurrently in the vicinity of the Project site
and within the 500m
assessment area. Cumulative construction dust
impacts would not be expected.
3.5.2
Operational Phase
Potential Source of Pollutants
Existing
Pillar Point Sewage Treatment Facilities
3.5.2.1
The existing PPSTW was built in the early
1980’s as part of the development planning for the new town population
increasing in the Tuen Mun area. The existing works comprises plant provided under
two stages of construction – Stage I in 1980 and Stage II (Phase I) in 1988.
The works was originally designed to treat dry weather flows to 192,000 m3/day
in Stage I. In the late 1980’s the works was subsequently designed and extended
to treat dry weather flows of 246,000 m3/day under the Stage II
(Phase I) Programme.
3.5.2.2
The existing PPSTW is a
preliminary treatment plant. Existing major facilities comprise inlet screw
pumps, coarse screens, aerated grit channels, fine screens, and outfall screw
pumps. The preliminary treated effluent is discharged into the Urmston Road tidal
stream through a 2,000m long twin 1,500mm diameter outfall with an approximate
capacity of 6.66 m3/s. There is also a 700m long 2,100mm diameter
emergency outfall next to the twin outfall. Figures 2.3 and 2.4
show the preliminary layout plan of the upgraded PPSTW and the process flow
diagram. In accordance with the
preliminary design, the design sewage flow of the upgraded PPSTW is 6.04m3/s.
The individual treatment facilities are described as follows.
Inlet Pumping Station
3.5.2.3
A 2,100 mm diameter pipe
delivers sewage flows to inlet / bypass chamber. The flows then divert into the
Stage I and Stage II screw pumping station (or the emergency bypass
outfall). Three 2.9 m diameter screw
pumps (2 duty + 1 standby), each with a maximum capacity of 143,856 m3/d
were installed and commissioned under the Stage I Works in 1982. Two with
identical capacity to those installed under Stage II Works. The existing capacity should be able to
handle the ultimate flow of 6.04 m3/s. In total, there are five 2.9 m diameter screw
pumps (4 duty + 1 standby), which provide a total capacity of 575,425 m3/d
(6.66 m3/s). The existing capacity should be able to handle the
ultimate flow of 6.04 m3/s.
3.5.2.4
According to the site
observation, all five existing screw pumps have been covered. The odour emission sources include the exposed areas with
sewage or sludge of the inlet chambers / influent channel to coarse screens. All the exposed area with sewage or sludge of the inlet chambers / influent channel would be covered in the
preliminary design. The vented air from
the inlet chambers / influent channel, screw pumps and wet wells of the inlet
pumping station would be discharged into the atmosphere via two deodourizing
units (OD1 and OD2).
Coarse Screens
3.5.2.5
Outlet channels from both
Stage I and Stage II pumping stations are connected such that flow can be directed
to both Stage I Screens and Stage II Screens. Stage I and Stage II screens are
installed mechanical raked 2.5m wide coarse screens with 18mm bar spaces. At
present, all four mechanical screens are configured to duty unit with one
screen acting as standby. The design capacity of 6.66 m3/s is
adequate to handle the ultimate flow of 6.04 m3/s. All screenings
from all five screens are currently directed to the solids handling building.
3.5.2.6
The existing coarse screens
are partially enclosed. Potential odour
emissions would be expected from some exposed area with
sewage or screens of coarse screen. Based on the preliminary
design, all the coarse screens would be completely enclosed and the vented air
from the coarse screens would be discharged into the atmosphere via two
deodourizing units (OD1 and OD2).
Aerated Grit Channels
3.5.2.7
There are four aerated grit
channels (3 duty + 1 standby) at the existing PPSTW. Two units were installed under the Stage I
while two units were installed under the Stage II. A common flow channel connects the coarse
screens and the two pairs of aerated grit channels. The size of each grit
channel is 14m x 5.7m wide x 3.8m depth.
A common main transfers grit from each of the air lift pumps to the solids
handling building.
3.5.2.8
Each aerated grit channel
has capacity of 1.94 m3/s. They are installed and operated as 3 duty
and 1 standby which provide a total capacity of 502,848 m3/d (5.82 m3/s).
As such, additional one unit of aerated grit channel would be constructed to
cater for the projected flow of 6.04m3/s.
3.5.2.9
Two air lift pumps are
installed in each aerated grit channel nos. 1 and 2, and three air lift pumps
are installed in each aerated grit channel nos. 3 and 4. Air lift pumps, with
each design capacity of 24 L/s, are used to remove grits from the grit
channels. All air lift pumps are
enclosed.
3.5.2.10
As all aerated grit
channels as well as the common flow channel are not covered, they would be
potential odour sources in the STW. Therefore, all the exposed area with sewage or grits of the aerated grit
channels and the common flow channel would be covered in the preliminary
design. The vented air from all aerated grit channels and the common flow
channel would be discharged into the atmosphere via two deodourizing units (OD1
and OD2).
Fine Screens
3.5.2.11
There are four bar screens
(3 duty + 1 standby) with a total capacity up to 5.79 m3/s (1.93 m3/s
each) at the existing PPSTW. The
extension of existing fine screens is found to be not necessary by increasing
the velocity across the screens from 1.0m/s to 1.04m/s. The bars are
mechanically raked and the screenings are fed into a hopper and then into a
screening press for dewatering. The screenings are then transferred to the
sludge holding building through a series of screw conveyors.
3.5.2.12
All existing fine screens,
the hopper and screening press have been enclosed except minor exposed area with sewage or screens at fine screens. The possible emission sources would be the
exposed areas with sewage or screens at fine screens and also the common flow channel after the fine
screens. Therefore, all the exposed area with sewage or
screens of fine screens and common flow channel would be
covered in the preliminary design. The vented air from the covered areas of
these facilities would be discharged into the atmosphere via two deodourizing
units (OD1 and OD2).
Solids Handling
Building
3.5.2.13
The solids handling
building (SHB) houses the main control panel for sewage solids removal
processes. There are 4 nos. (3 duty and 1 standby) grit classifiers inside the
SHB for dewatering grit removed from the aerated grit channels. Grit is fed to
each classifier through a ductile iron main. A series of screw conveyors are
also located within the SHB. All the
screw conveyors are fed separately from the coarse and fine screenings removal
processes. The screenings and grit are fed into a skip for disposal.
3.5.2.14
The storage area of
grits/screening and dewatering facilities including grit classifiers are
located inside the SHB. Deodourizing
chemical is currently used in the SHB as a odour control measure, which is
found to be effective. Nevertheless,
this existing odour control measure would not be taken into account in the air
quality modelling in order to assess the need of odour control measures for SHB
and/or to provide flexibility for the future DBO Contractor to consider their
own odour control measures. In this assessment, the odour emission from the
screenings and grits and dewatering facilities inside the SHB would be assumed
to be released into the atmosphere via two deodourizing units (OD1 and
OD2).
Outfall Pumping Station
3.5.2.15
In the existing operation,
the treated sewage from the fine screen is diverted into the outfall pumping
station. After upgrading, the treated
sewage from UV contact tank would be diverted into the outfall pumping
station. The outfall pumping station is
equipped with four fully covered Archimedean screw pumps actuated penstocks,
level control, control panel and switchgear together with a separate power
supply to that of the PPSTW. The Archimedean
screw pumps are operated at 3 duty and 1 standby mode with the maximum capacity
of 6.66 m3/s. All screw pumps have been covered. Based on the
preliminary design, the inlet chamber before outfall pumping station would be
covered and the vented air from the inlet chamber, screw pumps and wet wells of
outfall pumping stations would be discharged to the atmosphere via two
deodourizing units (OD1 and OD2).
Septic Waste Reception Facility
3.5.2.16
Currently, there is septic
waste reception facility at the inlet chamber of the Inlet Pumping Station of
the PPSTW. The septic waste is delivered by road tankers and discharged into
the inlet chamber. The existing septic
waste reception facility will be decommissioned and another new septic waste
reception facility would be provided and located next to the UV contact tanks
in accordance with the preliminary design.
During the operation, the septic waste is to be unloaded via a duct
connecting the road tanker and the septic waste receiving equipment, which is
installed inside a confined area. Air
inside the confined area would be extracted to centralized odour control
plants. The tank for containing septic
waste is well isolated from the environment even in the unloading process, as
such, the potential odour impact arising from the septic waste facility as well
as the road tankers would be remote.
New
Sewage Treatment Facilities after Upgrading Pillar Point STW
3.5.2.17
Based on the preliminary
design, a chemical enhanced primary treatment (CEPT) process and disinfection
would be provided for sewage treatment in the upgraded PPSTW.
3.5.2.18
The individual new
treatment facilities proposed in the upgraded STW are described as follows.
Low Lift & Returns Pumping
Station, Flash Mixing Tanks and Flocculation Tanks
3.5.2.19
After the upstream existing
preliminary treatment (screening processes), the flow would be distributed to
the flash mixing tanks after the addition of coagulant and then the flow would
be distributed to the flocculation tanks after the addition of polymer. There are four flash mixing tanks and two
flocculation tanks to be provided and operated in the upgraded PPSTW. The turbulence created by the coagulation and
flocculation systems would lead to odour emissions. Therefore, all the tanks would be covered in
the preliminary design. The vented air
from these tanks as well as low lift and returns pumping station would be
discharged into the atmosphere via two deodourizing units (OD1 and OD2).
Sedimentation Tanks
3.5.2.20
After flocculation process,
the sewage would divert into the sedimentation tanks. There would be nine sedimentation tanks. In
the primary sedimentation tanks, the turbulence resulting from the flow over
weirs and evaporative losses from quiescent liquid surfaces of the tanks would
be the main source of odour emission.
Based on the preliminary design, the weir areas and effluent channel of
sedimentation tanks would be enclosed and the vented air would be treated in
the two deodourizing units (OD1 & OD2).
UV Contact Tanks
3.5.2.21
After the CEPT process, the
effluent would flow to the UV disinfection facilities for disinfection. The UV disinfection facilities would include
one fine screen influent channel, four fine screens, two UV contact tank
influent channels, ten UV contact tanks and one UV contact tank effluent
channel. Based on the preliminary
design, the exposed area with sewage of UV contact tanks and fine screens (including all influent &
effluent channels) of UV disinfection facilities would be enclosed and the
vented air would be treated in two deodourizing units (OD3 & OD4).
Sludge
Pumping Station and Sludge Holding Tanks
3.5.2.22
The sludge and scrum
removed from the sedimentation tanks would be pumped into the sludge holding
tanks via sludge pumping station. There are three sludge holding tanks to be
operated in the upgraded PPSTW. Based on
the preliminary design, the sludge pumping station and all the sludge holding
tanks would be enclosed and the ventilated air would be treated in the two
deodourizing units (OD3 & OD4) before releasing into the atmosphere.
Sludge
Dewatering Building and Return Liquor Pumping Station
3.5.2.23
At the sludge dewatering
building, sludge materials are fed into the centrifuges in enclosed pipes and
then fed into the hopper after dewatering.
The extracted liquor would be pumped into the intermediate low lift and
returns pumping station via the return liquor pumping station. The dewatered sludge would be stored
temporarily in the sludge containers and would be disposed at landfill site
everyday. Both the centrifuge feed pumps and the sludge
containers would be enclosed with sealed covers. The odour generated from the hopper would be
a potential odour source. The vented air
would release to the atmosphere via two deodourizing units (OD3 and OD4) in accordance with preliminary design.
Sludge/Septic Waste Reception
Area
3.5.2.24
In the preliminary design
of the upgraded PPSTW, pre-treatment of septic waste is recommended. The
function of the pre-treatment is mainly for suspended solids and grits removal.
In the preliminary design, there would be some automatic, self-operating
septage receiving station for receiving septic waste. After the grit removal,
the liquid fraction of septic waste would be discharged into the sludge pumping
station for well mixing prior to discharging into the sludge holding tanks.
Basically, the septic waste receiving station is an enclosed system. The vented air would release to the
atmosphere via two deodourizing units (OD3 and OD4).
Odour Control Units
3.5.2.25
In the preliminary design, the
two deodourizing units (OD1 and OD2) provided for the solids handling building
(SHB) would also be used to treat the vented air from the covered odour source areas of the inlet
pumping station, coarse screens, aerated grit channels, fine screens, common
flow channels, outfall pumping station, flash mixing tanks, flocculation tanks,
low lift and returns pumping station, weir area and effluent channel of
sedimentation tanks. Another
deodourizing units would be provided for treatment of vented air from the
sludge pumping station, raw sludge holding tanks, sludge dewatering building,
return liquor pumping station, centrifuge, septic waste reception facility, UV
contact tanks and fine screens (including all influent & effluent channels)
of UV disinfection facilities. The
treated air will be discharged into the atmosphere via two deodourizing units
(OD3 & OD4). The odour removal
efficiency of all the deodourizing units is 90%, which is considered common and
practically achievable.
3.5.2.26
Potential sources of odour for different
sewage treatment facilities at the PPSTW were identified. With reference to Sections 3.5.2.3 to
3.5.2.25 above, apart from the exposed quiescent zone of the sedimentation
tanks, all the exposed areas ()
with sewage or sludge of the upgraded PPSTW would be covered with
providing deodourizing units in the preliminary design. The identified potential odour sources for
existing and planned sewage treatment facilities are summarized in Tables 3.4 and 3.5, respectively. Layouts
of the existing and upgraded treatment facilities are shown in Figures
2.3 and 3.2, respectively. The odour sources considered in the model
would represent the worst-case scenario in view of the operation process of the
STW.
Table 3.4 Potential
Odour Sources of Existing Treatment Facilities
|
Process
|
Potential Odour Source
|
Source ID
|
Odour Control Measures in the Design
|
Odour Source considered in the assessment
|
|
Inlet Pumping Station
|
·
Inlet Chambers/Influent Channel
·
Screw Pumps
·
Wet Wells
|
-
|
Fully enclosed, foul air would release to atmosphere
via the deodourizing
units
|
Outlet of vent
pipes from the deodourizing units (OD1&OD2)
|
|
Coarse Screens
|
·
Coarse Screens
·
Common Flow Channel
|
-
|
|
Aerated Grit Channels
|
·
Aerated Grit Channels
·
Common Flow Channel
|
-
|
|
Fine Screens
|
·
Fine Screens
·
Common Flow Channel after Fine Screens
|
-
|
|
Solid Handling
Building (a)
|
·
Solid Handling Building
|
-
|
|
Outfall Pumping Station
|
·
Inlet Chambers
·
Screw Pumps
·
Wet Wells
|
-
|
Remarks
(a) : The existing odour control measure
by adding chemicals would not be taken into account in the air quality
modeling.
Table 3.5 Potential Odour Sources of New
Sewage Treatment Facilities after Upgrading
|
Process
|
Potential Odour Source
|
Source ID
|
Odour Control Measures in the Design
|
Odour Source considered in the assessment
|
|
Coagulation & Flocculation
|
4
nos. Flash Mixing Tanks and 2 nos. Flocculation Tanks
|
-
|
Fully
enclosed, foul air would release to atmosphere via the deodourizing units
|
Outlet
of vent pipes from the deodourizing units (OD1&OD2)
|
|
Low Lift and Returns Pumping Station
|
Wet
well
|
-
|
|
Sedimentation
|
9 nos.
Sedimentation tanks (Weir Area) and effluent channel for sedimentation tank
|
-
|
|
Sedimentation
|
9
nos. Sedimentation tanks (Quiescent Zone)
|
PR02
|
None
|
Open
area of the operated tanks
|
|
Disinfection Facilities
|
1 no.
Fine Screen Influent Channel
|
-
|
Fully
enclosed, and foul air via deodourizing units release to atmosphere
|
Emission
source from the outlet of vent pipes of the deodourizing units (OD3 &
OD4)
|
|
4
nos. Fine Screens
|
-
|
|
2 nos.
UV Contact Tank Influent Channel
|
-
|
|
10
nos. UV Contact Tanks
|
-
|
|
1
nos. UV Contact Tank Effluent Channel
|
-
|
|
Sludge Treatment
|
Sludge
Pumping Station
|
-
|
|
3
nos. Sludge Holding Tanks
|
-
|
|
Sludge
Dewatering Building
|
|
|
Return
Liquor Pumping Station
|
-
|
|
Sludge/Septic Waste Reception Area
|
Sludge/Septic
Waste Reception Area
|
-
|
Deviation of Odour Emission Rates
Inlet
Chambers/Influent Channel, Screw Pumps & Wet Wells of Inlet Pumping Station
3.5.2.27
The odour emission rates of
raw sewage at inlet works including inlet chamber/inlet channel, screw pumps
and wet wells of inlet pumping station in the study were derived from empirical
equation developed by Shahalem([2]). This equation was established from the
laboratory results which indicated that the odour intensity of raw sewage at
wet well was found to be a function of sewage temperature and
oxidation-reduction potential (ORP). Therefore,
it is appropriate to use this equation to determine the odour emission rates of
the facilities at inlet works. The
equation has been employed under various approved EIA Studies of STW in Hong Kong, 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.
DF = 1.6 x
(T/10) 4.9 x (ORP + 200) –0.59
E
= DF x A x (V/3600)
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
3.5.2.28
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 PPSTW is located close to the pumping station with short
retention time (less than 2 hours).
Anaerobic conditions would not be expected, and ORP of semi-fresh sewage
150 mV, was assumed in the model. Referring
to the measurement data at the STWs in summer season, the temperature of sewage
was not higher than 30 oC, therefore, the temperature of sewage was
assumed to be 30 oC (86 oF) as a worst case scenario in
the assessment. In addition, the following parameters, which were adopted in
the Outlying Islands Sewerage Stage 1 Phase 1 EIA Study for similar facilities,
was employed for this odour assessment.
A
= 1.0 m x surface area of tanks, (for sources confined in
cover areas/buildings)
V =
5 air changes per hour
Coarse Screens & Fine Screens
3.5.2.29
The large particles in
sewage would be removed in the coarse screen and fine screen facilities. It is expected that the strength of the sewage
at coarse screen and fine screen facilities would be similar to or less than
that at inlet works. The equation
(Section 3.5.2.27) developed by Shahalem is still applicable for determination of odour
emission rates of sewage at coarse screens and fine screens.
Aerated Grit Channel, Wet
Well of Low Lift and Returns Pumping Station, Flash Mixing and Flocculation
Tanks
3.5.2.30
It is expected that the
sewage after passing the screening facilities would have a lower odour strength
comparing with that at inlet works. As a
conservative approach, the Shahalem’s equation (Section 3.5.2.27) was still
adopted for the determination of the odour emission rates of sewage in Aeration
Grit Channel, Wet Well of Low Lift and Returns Pumping Station, Flash Mixing
Tanks and Flocculation Tanks. As the time for screening processes is short, the
ORP of sewage at these facilities are assumed to be similar to the condition of
sewage at inlet works.
Solid Handling
Building
3.5.2.31
The potential odour sources
in Solid Handling Building
are temporary storage area of the grits/screens. There is no empirical formula appropriate
to estimate odour concentration of grits/screens. To determine the odour emission of
grits/screens, measured odour concentration of grits/screens produced at
similar process of STWs was adopted in the assessment. Regarding the adoption of appropriate odour concentration of
grits/screens in the assessment, the characteristics of influent, the treatment
method and the availability of measured odour concentration have been taken
into consideration. It is considered
that the influent to both the PPSTW and the SCISTW is mainly domestic, and the
flow process and treatment method to be adopted for the proposed upgraded PPSTW
and the SCISTW are identical. The odour
characteristics of the screens/grits are also considered similar. As such, the odour concentration of the
grits/screens measured at SCISTW, which is 6,595 oum-3 (This is the
maximum value measured in a four-day grits/screens odour measurement exercise
at SCISTW.
3.5.2.32
The average daytime temperature in the odour surveys at SCISTW was 29°C. In accordance with Year 2005 to 2007
meteorological data from the Hong Kong Observatory, the mean daily maximum
ambient temperature in the summer (June – September) were in the range of 29 to
33°C. With
reference to the Hydrogen Sulphide Control Manual (Technological Standing
Committee on Hydrogen Sulphide Corrosion in Sewage Works, 1989), the equation
below presented by Pomeroy and Parkurst was taken to estimate the variation of
odour emissions due to temperature changes:
G =
M[BOD5]1.07T-20
where G = sulphide
flux
[BOD5] = 5-day
biochemical oxygen demand
T = Temperature °C
M = coefficient, m/h
3.5.2.33
Based on the above equation, .the sulphide
flux increased by about 31% when temperature increased from 29 to 33°C. Therefore, 31% increase or a 1.31 correction
factor was applied in the odour emission rate which was calculated based on the
measurement data from the SCISTW to estimate the worst case odour
emission. In accordance with the
detailed design, the air exchange rate of solid handling house is 15 air
change/hr.
Primary
Sedimentation Tanks
3.5.2.34
The emission of odorous gas
from the primary sedimentation tank is highly dependent on the turbulence at
the water’s surface. 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([3]) and J Hobson([4]). The OP value for primary sedimentation tank
is 3305 ou/m3 based on the Sewage Treatment Odour Production (STOP)
Model.
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
Sedimentation
Tank Effluent Channel, Fine Screen of UV Disinfection Facilities, UV Contact
Tank Influent Channel, UV Contact Tanks, UV Contact Tank Effluent Channel,
Inlet Chamber & Screw Pumps & Wet Wells of Outfall Pumping Station
3.5.2.35
In view of the top-water
surface of the above facilities is quiescent condition, the odour emission from
the above sewage treatment facilities of PPSTW were therefore calculated based
on the empirical formula developed by J Hobson.
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)
Vwind = wind speed on
surface of tank (m s-1)
Vliquid = velocity of liquid across the tank (m s-1)
3.5.2.36
After the sedimentation
process, the odour potential of the effluent in the sedimentation tank
effluent channel, fine screen of UV disinfection
facilities, UV contact tank influent channel, UV contact tank, UV contact tank
effluent channel and inlet chamber, screw pumps and wet wells
of outfall pumping station would decrease. However, the exact odour potential values of these treatment facilities
are not available, as a conservative approach, the odour potential
of sewage of these
facilities was still assumed to be 3305 ou m-3, similar to the odour potential of sewage at
primary sedimentation tank.
Sludge Pumping Station, Raw
Sludge Holding Tanks, Return Liquor Pumping Station, Sludge Dewatering
Building, Centrifuge & Sludge/Septic Waste Reception Area
3.5.2.37
There is no empirical formula appropriate
to estimate odour concentration of fresh dewatered sludge. To determine the odour emission of sludge
handling facilities, including sludge pumping station, raw sludge holding tanks, return liquor pumping station, sludge dewatering building, and centrifuge and sludge/septic waste reception area,
it considered appropriate to adopt measured odour concentration of fresh
dewatered sludge produced at sewage treatment works using CEPT process. Currently, there are
several existing sewage treatment works using CEPT process, including Siu Ho
Wan STW, Sham Tseng STW, Cyperport STW and SCISTW. Regarding the adoption of appropriate odour
concentration of fresh dewatered sludge in the assessment, the characteristics
of influent, the treatment method and the availability of measured odour concentration
have been taken into consideration. It
is considered that the influent to both the PPSTW and the SCISTW is mainly
domestic, and the operation of the SCISTW is in full swing. In addition, the sludge treatment method to
be adopted for the proposed upgraded PPSTW and the SCISTW is identical, i.e.
temporary stored in sludge holding tanks and then dewatered by centrifuge, the
odour emitted during the sludge treatment process and the odour characteristics
of the dewatered sludge are also considered similar. As such, the odour concentration of the fresh
dewatered sludge measured at SCISTW, which is 7,603 ou m-3 (This is
the maximum value measured in a four-day sludge odour measurement exercise at
SCISTW. The ambient temperature with this measured value is 31oC.),
is considered appropriate to be assumed as the odour concentration of the fresh
dewatered sludge at the upgraded PPSTW.
For the odour emission rate of the Sludge
Pumping Station, Raw Sludge Holding Tanks, Return Liquor Pumping Station, Sludge Dewatering
Building, Centrifuge and
Sludge/Septic Waste Reception Area of the upgraded PPSTW, the following
theoretical formula is used in the assessment:
E = DF x A x (V/3600)
where DF
= measured odour concentration, ou m-3
of fresh dewatered sludge (i.e. 7,603 ou
m-3)
E =
odour emission rate, ou s-1
A
= air volume of the emission source, m3
V
= ventilation rate, air changes per hour
3.5.2.38
As the odour
emission rate is calculated based on the measurement data from SCISTW, a
correction factor for temperature change as mentioned in Section 3.5.2.32 -
3.5.2.33 was applied to estimate the worst case odour emission rate.
3.5.2.39
The odour emission parameters for the
upgraded PPSTW to be considered in the model, which would be the worst-case
scenario, are summarised in Table 3.6. Detailed calculations of the emission rates
are shown in Appendix 3.1. The locations of emission sources at upgraded
PPSTW are indicated in Figure 3.2.
Table 3.6 Odour Emission Rate of Upgraded PPSTW (Unmitigated)
|
Emission
Source
|
Source ID
|
No. of unit operated
|
Exposed Surface
Area (m2) (a)
|
Stack Height
above ground (m)
|
Odour Emission
Rate
|
|
Two deodourizing units
with two vent pipes next to low lift and returns pumping
station (The
vented air from the inlet
chambers/influent channel & screw pumps & wet wells of inlet
pumping station, coarse screens, aerated grit channels, fine screens, common
flow channels, inlet chamber & screw pumps & wet wells of outfall
pumping station, solid handling building, wet well of the low lift and returns pumping station, flash mixing tanks, flocculation tanks, weir zone of
sedimentation tank and sedimentation tank effluent channel would be treated before
release to the atmosphere. )
|
OD1
& OD2 (b)
|
-
|
-
|
6.58
|
1787.95ou s-1
(total emission from both vent pipes)
|
|
Sedimentation
Tanks -
(Quiescent Zone)
|
PR02
|
9
|
6210
|
1.8
|
2.065x10-2 ou m-2 s-1 (c)
|
|
Two deodourizing units
with two vent pipes next to septic waste reception area (The vented air from sludge/septic waste reception area, sludge holding
tanks, sludge pumping station, return liquor pumping station, sludge
dewatering building, centrifuge, UV disinfection facilities would be treated before
release to the atmosphere)
|
OD3
& OD4 (b)
|
-
|
-
|
6.81
|
1879.05 ou s-1
(total emission from both vent pipes)
|
(a)
Exposed
area with sewage or sludge of all units
(b)
Point
source
(c)
Maximum
odour emission rate is selected for assessment, details refer to Appendix 3.1
Dispersion
Model
3.5.2.40
Air quality impacts of
odour on ASRs would be modelled with the air dispersion model, AUSPLUME. Hourly meteorological data for the year 2005
(including wind speed, wind direction, air temperature, Pasquill stability
class and mixing height) of the Tuen Mun Weather Station would be employed for
the model run.
3.5.2.41
The modelled hourly odour
concentrations at the ASRs would be converted into the 5-second odour concentration
by the methodology proposed by Duffee et al.([5])
and Keddie([6]). In addition, Turner([7])
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 is assumed as 15 minutes, and the conversion factors for the predicted
1-hour averaged concentration of odour at the receivers would be adjusted to
5-second averaging time by the values shown in Table 3.7.
Table 3.7 Conversion Factors to 5-second
Mean Concentration
|
Pasquill Stability Class
|
Conversion Factor
|
|
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
|
|
F
|
1.31
|
5
|
6.55
|
3.5.2.42
Higher topographical relief
(50m – 100m above ground) is found to the north of the Pillar Point STW. Mid-rise developments are found to the east, south
and west of the STW. Surface roughness of the study is estimated to be 100
cm.
3.6.1
Construction Phase
3.6.1.1
Given that 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 dust emission, no adverse construction dust impacts at the
ASRs is expected.
Typical dust control measures are presented in Section 3.7.
3.6.2
Operational Phase
3.6.2.1
Odour levels at the ASRs
have been predicted based on the preliminary upgrading design of PPSTW. The unmitigated results are shown in Table 3.8.
Table 3.8 Predicted Unmitigated Odour
Levels at ASRs after PPSTW Upgrading
|
ASR
|
Description
|
Odour level (5 seconds average) (ou) at
different height
|
|
1.5 m
|
5m
|
10 m
|
15m
|
20 m
|
25 m
|
|
A1
|
River Trade Terminal Office
|
2.7
|
2.7
|
2.7
|
2.9
|
5.2
|
8.5
|
|
A2
|
Chu Kong Warehouse 1
|
1.8
|
1.8
|
1.8
|
1.9
|
3.5
|
5.4
|
|
A3
|
Chu Kong Warehouse 2
|
1.9
|
1.9
|
1.9
|
1.9
|
3.1
|
-
|
|
A4
|
Sawmill
|
1.4
|
1.4
|
1.4
|
-
|
-
|
-
|
|
A5
|
Pillar Point Fire Station
|
1.4
|
1.3
|
1.3
|
1.7
|
2.6
|
-
|
|
A6
|
Sunhing Hung Kai Tuen Mun Godown
|
0.9
|
0.9
|
1.1
|
1.5
|
2.2
|
-
|
|
A7
|
Tuen Mun EMSD Servicing Vehicle Station
|
3.2
|
2.6
|
2.6
|
-
|
-
|
-
|
(1)
The odour
criterion is 5 ou over five second averaging.
3.6.2.2
Modelling results indicated that
unmitigated odour levels at some ASRs would
exceed the odour criterion of 5 odour units. Highest odour concentration
of 8.5 odour units would be
predicted at A1 under the worst-case scenario.
Mitigation measures would be required to protect receivers in the
vicinity of the PPSTW. No other
concurrent odour emission is identified within 500m from the project site boundary, therefore, no
cumulative odour impact is expected at the ASRs.
3.7.1
Construction Phase
3.7.1.1
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;
·
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.
3.7.2
Operation Phase
3.7.2.1
The predicted results indicated
that the nearest two ASRs would exceed the odour criterion under the worst-case
scenario and no other ASRs within the study area would exceed
the criterion. Mitigation measures are required to protect the
affected ASRs against odour nuisance.
3.7.2.2
In order to alleviate the odour
impact, it is considered to cover the exposed area with sewage or sludge of the
quiescent zone of the sedimentation tanks and the vented air would be treated
in the deodourizing units located next to the low lift and returns pumping station
before discharging into the atmosphere. In view of the increase in the air flow
rate of the deodourizing units, the number of deodourizing units increases from
two (OD1 & OD2) to three (OD1, OD2 & OD5). The odour removal efficiency
of all the deodourizing units would be 90%, which is considered common and
practically achievable.
3.7.2.3
With the implementation of the
mitigation measure as stated in Section 3.7.2.2 above, all the exposed areas() with sewage or sludge of the
upgraded PPSTW would be covered with providing deodourizing units in the
preliminary design (See Figure 3.3). The vented air from the inlet pumping station, coarse screens, aerated grit channels,
fine screens, common flow channels, outfall pumping station, flash mixing tanks, flocculation tanks, low lift and returns pumping station, weir
area and effluent channel of the sedimentation tanks, quiescent
zone of the sedimentation tanks and the solids handling
building (SHB) would be treated at the three deodourizing units (OD1, OD2 &
OD5) next to low lift and returns pumping station before discharging into the
atmosphere. While the vented air from the sludge pumping station, raw sludge holding tanks, sludge
dewatering building, return liquor pumping station, sludge/septic waste
reception area, UV contact tanks and fine screens (including all influent &
effluent channels) of UV disinfection facilities would be treated in the two
deodourizing units (OD3 & OD4). The
emission rates for mitigated scenario are summarized in Table 3.9 and the detailed calculation is presented in Appendix 3.1.
Table 3.9 Odour Emission Rate of Upgraded PPSTW (Mitigated)
|
Emission Source
|
Source ID
|
Stack Height
above ground (m)
|
Exit Velocity (m/s)
|
Diameter of the Stack (m)
|
Odour
Emission Rate
|
|
Three deodourizing unit
with three vent pipes next to low lift and returns pumping station. The vented air from the following covered
facilities(1) would be treated before release to the atmosphere.
·
inlet chambers /
influent channel, wet wells of inlet pumping station (IN01-IN04)
·
inlet screw pumps (2)
·
coarse screens (SC01 – SC04)
·
Aerated grit channel (GR01)
·
fine screens (FSC01-FSC05), common flow channels (FSC06)
·
solids handling house,
·
wet well of the low lift and returns pumping station
·
flash mixing tanks including No.1 – No. 4
·
flocculation tanks No 1 & 2
·
weir zone of sedimentation tank and effluent channel (PR01)
·
sedimentation tank (quiescent zone) (PR02)
· inlet chamber & wet
wells of outfall pumping station (OUT01) & screw pumps (2)
|
OD1, OD2 & OD5
|
6.81
|
13.82
|
1.0
|
1800.78 ous-1
(total emission from all vent pipes)
|
|
Two deodourizing unit with
two vent pipes next to septic waste reception area. The vented air from the following covered
facilities(1) would be treated before release to the atmosphere.
· sludge holding tanks No.1, No2, No3
· sludge pumping station
· return liquor pumping station
· sludge dewatering building & centrifuge
· sludge/septic waste reception area
· UV disinfection facilities (FIS01, FIS02, FIS03, UV01, UV02, UV03, UV04, UV05
& UV06)
|
OD3
& OD4
|
6.81
|
14.36
|
1.0
|
1879.05ou s-1
(total emission from both vent pipes)
|
(1) The locations of covered sewage facilities are indicated
in Figure 3.3.
(2) Existing facility has been covered.
3.7.2.4
The predicted odour level at
representative ASRs with implementing the mitigation measures as stated in
Section 3.7.2.3 above would comply with the EPD criterion
of 5 odour units. The mitigated results
are summarized in Table 3.10. Details of the emission calculation are
presented in Appendix 3.1. Contours of mitigated odour concentration at 25 m above ground (the highest
assessment height of the representative ASRs), have
been plotted and presented in Figure 3.4.
Table 3.10 Predicted Mitigated Odour levels at
ASRs after PPSTW Upgrading
|
ASR
|
Description
|
Odour level (5 seconds average) (ou) at
different height
|
|
1.5 m
|
5m
|
10 m
|
15m
|
20 m
|
25 m
|
|
A1
|
River Trade Terminal Office
|
2.0
|
2.0
|
2.0
|
2.1
|
2.4
|
4.8
|
|
A2
|
Chu Kong Warehouse 1
|
1.5
|
1.5
|
1.5
|
1.5
|
1.6
|
2.9
|
|
A3
|
Chu Kong Warehouse 2
|
1.6
|
1.6
|
1.6
|
1.6
|
1.7
|
-
|
|
A4
|
Sawmill
|
1.2
|
1.2
|
1.2
|
-
|
-
|
-
|
|
A5
|
Pillar Point Fire Station
|
1.2
|
1.2
|
1.2
|
1.2
|
1.6
|
-
|
|
A6
|
Sunhing Hung Kai Tuen Mun Godown
|
0.8
|
0.8
|
0.8
|
1.0
|
1.4
|
-
|
|
A7
|
Tuen Mun EMSD Servicing Vehicle Station
|
2.1
|
2.1
|
2.1
|
-
|
-
|
-
|
(1)
The odour
criterion is 5 ou over five second averaging.
3.7.2.5
For each odour control plants,
there will be one standby deodourizing unit to provide buffer for maintenance or breakdown of one of the duty deodourizing units. In addition, the practices of good
housekeeping for PPSTW listed below should
be followed to ameliorate any odour impact from the plant and these standard
practices should be included in the PPSTW operator manual.
·
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 to minimise
odour escape
·
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 cake should be transferred to closed
containers
·
Sludge containers should be flushed with water
regularly
3.8.1
Construction Phase
3.8.1.1
With the incorporation of Air Pollution Control (Construction Dust)
Regulation and EM&A programme, adverse residual dust impact during
construction phase is not expected.
3.8.2
Operation Phase
3.8.2.1
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. There is no ASR within the area out of the
project boundary with exceedance of EIAO-TM odour criterion.
3.9.1
Construction Phase
3.9.1.1
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. It is recommended that a
dust monitoring programme should be implemented during the construction phase
to ensure that the dust criteria would be satisfied at the ASRs. Details of the
EM&A requirements are given in the stand-alone EM&A Manual.
3.9.2
Operation Phase
3.9.2.1
The predicted air quality at
the ASRs would comply with the criteria with the implementation of proposed
mitigation measures. It is recommended
that odour patrol should be
carried out during the operation phase to ensure that the proposed mitigation
measures would be effectively implemented.
Details of monitoring programme are presented in the EM&A Manual.
3.10.1
Construction Phase
3.10.1.1
Air quality impact from the
construction 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.
3.10.2
Operation Phase
3.10.2.1
Odour emission from the
Upgraded PPSTW would be the main concern during the operation phase. Air dispersion modelling was conducted to
simulate the potential odour impacts of the proposed preliminary layout of the
Upgraded PPSTW on the ASRs. With reference to the modelling results as shown in Table 3.8,
it is noted that only providing deodourizing units for the treatment of vented
air from the covered sewage/sludge treatment facilities (as mentioned in Table
3.4 and Table 3.5) of the upgraded PPSTW except the quiescent
zone of sedimentation
tanks is not sufficient to make all ASRs below the
odour criterion of 5 odour units.
Highest odour concentration of 8.5 odour units is predicted at A1 under the worst-case scenario. With the implementing the recommended
mitigation measure, covering the quiescent zone of sedimentation tanks and
providing an additional deodourizing
unit for the treatment of vented air as stated in Section 3.7.2.3,
the predicted odour level at representative ASRs would comply with the EPD
criterion of 5 odour units.
3.10.2.2
Based on the preliminary design
with implementing the mitigation measure, all the exposed areas with sewage or
sludge of the upgraded PPSTW would be covered (See Figure 3.3). The vented air from the inlet pumping station, coarse screens, aerated grit channels,
fine screens, common flow channels, outfall pumping station, flash mixing tanks, flocculation tanks, low lift and returns pumping station, weir
area and effluent channel of the sedimentation tanks, quiescent
zone of the sedimentation tanks and the solids handling
building (SHB) would be treated at the three deodourizing units (OD1,OD2 &
OD5) next to low lift and returns pumping station before discharging into the
atmosphere. While the vented air from the sludge pumping station, raw sludge holding tanks, sludge
dewatering building, return liquor pumping station, septic waste reception
area, exposed area with sewage or sludge of UV contact tanks and fine screens
(including all influent & effluent channels) of UV disinfection facilities
would be treated in the two deodourizing units (OD3 & OD4). All the deodourizing units are with 90% odour removal
efficiency, which is considered common and practically achievable. Operational odour
monitoring is proposed to monitor the effectiveness of recommended mitigation
measures.