3.1
introduction
3.1.1
This section presents the air quality
assessment for the construction and operational phases of the proposed Project.
Dust impact from the construction activities of the STW and the terrestrial
components of the outfalls was identified as the key issue of concern during
construction phase. The operational phase air quality impact would be the odour
emitted from the operation of the STW upgrade.
3.2
Relevant Legislation, Policies, Plans,
Standards and Criteria
Air Pollution Control
Ordinance
3.2.1
The Air Pollution Control Ordinance
(APCO) (CAP. 311) is the principal legislation for the management of air
quality. The maximum acceptable Total Suspended Particulates (TSP)
concentration, as defined in the Air Quality Objectives (AQOs) encompass by the
APCO, is listed in Table 3-1.
Table 3-1 Hong Kong
Air Quality Objectives
Pollutant (mg/m3)
|
Average Time
|
24-hour (2)
|
Annual (3)
|
TSP (1)
|
260
|
80
|
Notes: (1) Measured at 298 K (25 oC)
and 101.325 KPa (one atmosphere).
(2) Not to be exceeded more than
once per year.
(3) Arithmetic means.
Technical Memorandum on
EIA Process
3.2.2
The criteria for assessing air quality
impacts are specified in Annex 4 of the TMEIAP, of which the following criteria
are relevant to this study:
(a) meet the Air Quality Objectives and other standards established
under the Air Pollution Control Ordinance;
(b) meet hourly TSP concentration of 500 microgrammes per cubic metre
measured at 298 K (25 ºC) and 101.325 KPa (one atmosphere) for construction
dust impact assessment; and
(c) meet 5 odour units based on an averaging time of 5 seconds for odour
prediction assessment.
Other Legislation
3.2.3
Another air pollution regulation
related to this Study is the Air Pollution Control (Construction Dust)
Regulation 1997 which stipulates the dust reduction measures required to be
carried out in construction phase.
3.3
Description of Environment
Existing and Planned Land
Use
3.3.1
The vicinity of the Peng Chau STW
study area is mostly rural in nature.
Major air pollution source is not identified in the vicinity of the
assessment area. Existing land use
includes residential use, recreational use, green belt and ancillary (or site)
office of the Peng Chau Refuse Transfer Station (PCRTS). The open land areas between Sea Crest
Villa and Kam Peng Estate has been zoned for open space and comprehensive residential
development including commercial complex, and residential development. Another
two areas in the north of Kam Peng Estate were reserved for comprehensive
development and educational use.
The implementation programme of these zoned lands are, however, not
definite.
Background Air Quality
3.3.2
EPD has performed routine air quality
monitoring over the territories. However, there is no air quality monitoring
station (AQMS) in the immediate vicinity of the Project area. The nearest AQMS is at Tung Chung located
on the northern Lantau Island. The air quality data obtained from Tung Chung
AQMS station is selected to represent the background air quality of Peng Chau
as both places are located at the southwest of the territories. The
meteorological conditions of these two locations are similar. The ambient air quality data from the
Tung Chung AQMS for the year 2002 are summarized in Table 3-2.
Table 3-2 Background
Air Quality at Tung Chung AQMS for 2002
Pollutant
|
Annual Average (mg/m3)
|
Total Suspended Particulates (TSP)
|
62
|
Respirable Suspended Particulates
(RSP)
|
46
|
Sulphur Dioxide (SO2)
|
18
|
Nitrogen Dioxide (NO2)
|
43
|
Carbon Monoxide (CO)
|
612
|
Source: (EPD,
2003)
3.3.3
The Hong Kong Electric Co. Ltd (HEC) and
CLP Power Hong Kong (CLP) have also performed territory wide air quality
monitoring on sulphur dioxide and nitrogen dioxide. The closest monitoring
station to the Project area is located in Penny’s Bay by CLP. The monitoring
activities of Penny’s Bay station have been discontinued since May 2000, and no
monitoring data are available since then. The monitoring results are presented
in Table 3-3.
Table 3-3 Air Quality
Monitoring at Penny’s Bay (2000)
Pollutant
|
Annual Average
(mg/m3)
|
Range of Monthly
Mean Concentration (mg/m3)
|
Sulphur
Dioxide (SO2)
|
9
|
6-14
|
Nitrogen
Dioxide (NO2)
|
48
|
34-59
|
Source: (EPD, 2001a)
3.3.4
The monitoring data from both Tung
Chung and Penny’s Bay suggest that the air quality in Lantau and its close
proximity is generally good.
Odour Climate at Tai Lei
Island
3.3.5
The existing Peng Chau STW at Tai Lei
Island, and the PCRTS to the east of the Island are potential odour sources.
Both facilities have been operated for some time and no odour complaints had
been received in the vicinity of the Tai Lei Island (DSD, 2003). The background odour nuisance in the
study area is considered not significant.
3.4
Air Sensitive Receivers
3.4.1
Air Sensitive Receivers (ASRs) are
defined in Annex 12 of TMEIAP. ASRs include any domestic premises, hotel,
hostel, hospital, clinic, nursery, temporary housing accommodation, school,
education institution, office, factory, shop, shopping centre, place of public
worship, library, court of law, sports stadium or performing arts. Any other premises or place with which,
in terms of duration or number of people affected, has a similar sensitivity to
the air pollutants as the aforelisted premises and places are also considered
as an ASR.
3.4.2
The study area of the air quality
impact assessment is defined as 500 m from the pollution source. The construction areas mainly include
the area of Peng Chau STW, Tai Lei Island and Tai Lei Bridge. However, the
magnitude of dust emission from pipe laying on Tai Lei Island section outside
the STW is localised and insignificant, the study area is thus defined as 500m
from the boundary of the STW (Figure 3-1). The ASRs identified in the
vicinity of the assessment area during construction phase and operational phase
are listed in Table 3-4. The
sensitive use and approximate height of ASRs as well as the approximate
horizontal distances from the boundary of the pollution sources are also
provided in Table 3-4. The
locations of the identified ASRs are also shown in Figure 3-1.
Table 3-4 Identified
Air Sensitive Receivers
ASR
|
Location
|
Sensitive
Use
|
Approx.
Distance from Construction Site boundary (m)
|
Approx.
Distance from Operational Site boundary (m)
|
Approx.
Height (m)
|
Approx.
Grade Level (mPD)
|
ASR 1
|
Peng
Chau Refuse Transfer Station
|
Ancillary/site
Office
|
10
|
10
|
10
|
5.2
|
ASR 2
|
Sea
Crest Villa
Block
D
|
Residential
|
170
|
170
|
16
|
4.5
|
ASR 3
|
Sea
Crest Villa
Block
C
|
Residential
|
180
|
180
|
16
|
4.5
|
ASR 4
|
Sea
Crest Villa
Block
B
|
Residential
|
200
|
200
|
16
|
4.5
|
ASR 5
|
Sea
Crest Villa
Block
A
|
Residential
|
190
|
190
|
16
|
4.5
|
ASR 6
|
Temporary Structure in the
North of Kam Peng Estate
|
Residential
|
340
|
340
|
4
|
3.8
|
ASR 7
|
Kam Peng Estate
|
Residential
|
370
|
370
|
30
|
4.7
|
ASR 8
|
Peng Lai Court
|
Residential
|
410
|
410
|
30
|
4.3
|
ASR 9*
|
Area for future educational
use in the north of Kam Peng Estate
|
Educational
(under planning)
|
--
|
440
|
--
|
8.0
|
ASR 10*
|
Area for future residential
use in the north of Kam Peng Estate
|
Residential
(Group B) (under planning)
|
--
|
370
|
24
|
4.3
|
ASR 11*
|
Newly reclaimed land in the
south of Sea Crest Villa
|
Residential and
Commercial (under planning)
|
--
|
290
|
8
|
3.4
|
Note: * No implementation
programme is available. Not
anticipated to be an ASR during construction stage.
3.4.3
The planned ASRs 9, 10 and 11 are not
considered as construction phase ASRs as no implementation schedule is available
at the time of this Study. However, they are considered as operational phase
ASRs.
3.5
impacts identification
Construction Phase
3.5.1
Fugitive dust is likely to be
associated with construction activities such as site excavation and backfilling,
foundation works, civil construction, demolition of building structures and
on-site concrete batching. They are also contributed by spoil handling,
construction and demolition materials handling and transportation, wind erosion
of stockpiles and open site erosion. The replacement of inlet pumping main
would be carried out on the service trough on the Tai Lei Bridge and the
magnitude of pipe laying for Tai Lei Island section outside the STW is not
considered to be significant. The layout of the dust emission sources is
depicted in Figure 3-2.
3.5.2
The potential dust impacts from
submarine outfall would likely be the construction of land components, which
would involve destruction of seawall for the pipelaying. For the worst case assessment, it is
assumed that similar activities as section 3.5.1 would be carried out in the
vicinity of seawall.
3.5.3
The construction works will be divided
into two phase: Phase 1 works include all major construction activities, while
some minor remaining works and demolition works will be carried out in Phase 2
(Figure
2-6). Both phases of works
cover the same portion of site area.
From dust impact assessment point of view, the nature of works during
both phases is similar and considered as general construction activities. Therefore, the dust impact assessment
presented in this section represents the predicted condition for both phases.
3.5.4
Construction of Peng Chau Helipad
would be carried out from March 2005 to February 2006, which fall within the
construction schedule of most STW Upgrade activities. However, construction
dust from Peng Chau Helipad Project was not considered as a key issue and
therefore dust impact from the helipad project is considered
insignificant. In addition, no
other dust emitting activities has been identified to be carried out
concurrently with the proposed Project.
Operational Phase
3.5.5
The key potential air quality impact
during the operational phase of the STW Upgrade would be odour arising from
sewage treatment facilities, as well as
sludge treatment and handling facilities. Major odour sources identified in the proposed STW are shown
in Figure
3-3 and include:
(a) inlet works;
(b) grit chamber;
(c) equalisation tank;
(d) Sequencing Batch Reactor (SBR);
(e) sludge thickener;
(f) sludge digester;
(g) screening and grit storage room and return liquor pumping station
(referring to engineering design, these will be enclosed by air-tight covers
and deodourisation units will be provided), and
(h) sludge drying bed (to be used as standby unit at initial stage and
will serve as duty unit in later stage.
Details refer to Appendix 2C)
3.5.6
The civil component of the total five
SBR units will be constructed.
However, the E&M equipment for the SBR No. 5,
which is for future expansion, will not be installed. Therefore, only four SBR units can be
put into operation during the commissioning of the Project. In spite of this, for the planning
worst scenario, it will be assumed in the odour impact assessment that the SBR
No. 5 will also be operated.
3.5.7
As the digested sludge will be carried
by enclosed container during transportation which is the common practice of
DSD, the odour impact from sludge export activities is considered
insignificant.
3.5.8
As the sludge pumping system is
confined in pipes, the odour emission from it is considered insignificant.
3.5.9
Odour emission from effluent pumping
station and chlorine contact tank is considered insignificant as the sewage
would have undergone secondary treatment.
3.5.10 There will be no concurrent operation of the existing and upgraded
STW. The upgraded plant will be
commissioned and will then replace the existing plant when the construction
works are completed.
3.5.11 The capacity of the proposed duty units will be able to achieve the
designed capacity of the STW Upgrade.
Therefore, no concurrent operation of duty and standby units is
anticipated.
3.5.12 Another potential operational phase impact in the study area would
be the odour emitted from the Peng Chau Sewage Pumping Station (PCSPS) and
PCRTS (Figure
3-4). The odour source
from PCSPS will be taken into account in the study. The PCRTS is not considered as significant odour source as
stated in Section 3.3.5 and it would not be included in the modeling
assessment.
3.6
Assessment Methodology
Construction Phase
Dispersion Model
3.6.1
To determine the extent of impacts
from the construction of the Project, the Industrial Source Complex (ISC)
short-term dispersion model (Version 4.0), which has been developed and
validated by the United States Environmental Protection Agency (USEPA), and
accepted by EPD for regulatory applications, was used to predict the potential
impacts arising from the construction activities.
Dust Emission Factor/Rate
3.6.2
The type and magnitude of the
dust-generating activities were reviewed to estimate the dust emission rate
during construction phase. The estimate
of dust emission was based on typical values and emission factors from USEPA’s
Compilation of Air Pollution Emission Factors (AP-42), 5th Edition (USEPA,
1995). As a conservative simulation, general construction activities, on-site
concrete batching and open sites wind erosion were considered as the major dust
emission sources in construction phase.
The calculation of construction dust emission factor/rate is detailed in
Appendix 3A.
Assumption
3.6.3
In assessing the dust impacts during
construction phase, the following assumptions were made:
(a) The study area is defined as rural.
(b) For a more conservative (higher) dust concentration prediction, no
dry or wet depletion is considered in the model run.
(c)
The daily working hours would be from
08:00 to 18:00. The TSP emission
period of the general construction activities was assumed for the same period
(10 hours in total). However, 24
hours TSP emission period for open site wind erosion is assumed.
(d) The concrete batching plant is assumed to be operated from 1300 to
1700 (4 hours a day).
(e) There will be no on-site construction traffic. The transportation of construction
waste/material will be carried out by small size and hand-operated equipment.
Dust generated from site vehicle movement is thus considered negligible.
Background TSP
3.6.4
A value of 87 µg/m3 as
suggested in Guideline on Accessing the “Total” Air Quality Impacts (EPD, 2000)
for rural areas is adopted as the background TSP concentration for total
predicted TSP concentration.
Meteorological Data
3.6.5
Meteorological data from Cheung Chau
Automatic Weather Station provided by the Hong Kong Observatory for the year
2001 was used as one of the input parameters dispersion model. The following data were provided:
(a) flow vector of wind;
(b) wind speed;
(c) air temperature;
(d) atmospheric Pasquill stability class; and
(e) morning mixing height and maximum mixing height recorded at King’s
Park.
Since the
Cheung Chau meteorological station is on the peak of a small steep hill and
Cheung Chau itself is small, the wind measured at that station may not be
strongly affected by the land surface.
As a result, the anemometer height is referred to 95m above ground level
for the dispersion modelling.
Assessment Height
3.6.6
Since the construction activities
would be undertaken at ground level and underground level, the worst dust
impact on the ASRs would be at ground floor of the ASRs. The height of 1.5m above ground, which
is the breathing level of human, was adopted for construction dust impact
assessment.
Operational Phase
Dispersion Model
3.6.7
Similar to the dust dispersion
modelling, the ISC was also used to predict the potential odour impacts arising
from the operation of the STW.
Emission
Rate
3.6.8
The odour emission rate from STW can
be estimated by the equation adopted for the odour assessment presented in the
Final Assessment Report of Outlying Islands Sewerage Stage 1 Phase 1 – EIA
Study (OISEIA). This approach was
also adopted for the estimation of odour emission rates from STW in some
previous EIA studies in Hong Kong, such as Sha Tin Sewage Treatment Work Stage
III Extension –EIA Report, 1999 and Sok Kwu Wan Sewage Collection, Treatment
and Disposal Facilities – EIA Report, 2002. The emission rates were calculated based on two
relationships: the established relationship between odour concentration and
physical factors and the volumetric emission flow rate compared with the rate
of ventilation.
DF = 1.6
x (T/10)4.9 x (ORP + 200)-0.59 Equation 1
E = DF
x A x (V/3600) x Cf Equation
2
Where
|
DF
|
=
|
Odour concentration expressed as dilution factor,
OU/m3
|
|
T
|
=
|
Temperature of sewage, Fehrenheit (F)
|
|
ORP
|
=
|
Oxidation-reduction
potential of sewage, mV
|
|
E
|
=
|
Emission
rate, OUs-1
|
|
A
|
=
|
Air volume
of the emission source, m3
|
|
V
|
=
|
Ventilation
rate, air changes per hour
|
|
Cf
|
=
|
Correction
factor to adjust emission rates in the ratio of design ventilation rate to
that used in the derivation of Equation 1 (for 10 air changes per hour,
Cf=0.26; for 5 air changes per hour, Cf=0.52)
|
|
|
|
|
The
following sewage characteristics were adopted for a worst case analysis:
|
T
|
=
|
30 oC
|
ORP
|
=
|
50 mV for
septic sewage (recommended in OISEIA)
|
air volume
|
=
|
0.5m x sewage
surface area
|
3.6.9
Based on the above equations and
assumptions, the emission rates for the proposed STW are calculated and
provided in Appendix
3B.
Emissions from Peng Chau Sewage
Pumping Station
3.6.10 As advised by DSD, the average inlet hydrogen sulphide concentration
of PCSPS is 5 ppm and the removal efficiency of the deodourisers is 99.5% (Appendix 3B).
Conversion
of 1-Hour Average to 5-Second Average
3.6.11
Due to the short exposure period
tolerable by human, odour impact assessment is based on a 5-second average
level. However, the shortest
averaging period of ISC is 1-hour.
Conversion of model output from 1-hour average to the required 5-second
average is needed. The 1-hour
average odour concentration is first converted to 3-minute average by the power
law relationship which is related to the stability. To further convert 3- minute average to 5-second average, a
multiplying factor of 10 was applied for those hours with atmospheric stability
classes A to B, and a factor of 5 for those hours with stability classes C to F
(Table 3-5) (Maunsell, 1999).
Table 3-5 Odour Level
Conversion Factors
Stability
Class
|
Conversion
Factors
|
1-hr to 3-min average
(A)
|
3-min to 5-sec average
(B)
|
Overall
(A X B)
|
A
|
2.23
|
10
|
22.3
|
B
|
2.23
|
10
|
22.3
|
C
|
1.70
|
5
|
8.50
|
D
|
1.38
|
5
|
6.90
|
E
|
1.31
|
5
|
6.55
|
F
|
1.31
|
5
|
6.55
|
Assumption
3.6.12 In assessing the odour impacts during operational phase, the
following assumptions were made:
(a)
The study area is defined as rural.
(b)
For a more conservative (higher) odour
concentration prediction, no dry or wet depletion is considered in the model
run.
Meteorological Data
3.6.13 The same set of meteorological data for the dust dispersion
modelling (described in Section 3.6.5) was adopted.
Sensitive Receivers
3.6.14 The highest air sensitive receiver identified is about 30m above
ground level. Therefore, the assessment height are taken as 1.5m, 5m, 10m, 15m,
20m, 25m, 30m above ground.
3.7
Impact Assessment
Construction
Phase
Unmitigated
Scenario
3.7.1
The worst unmitigated TSP
concentration at nearby ASRs is presented in Table 3-6. Figures 3-5 and 3-6 show the TSP concentration
contour for 1-hour and 24-hour average respectively. A typical TSP modeling output file is shown in Appendix 3C.
Table 3-6 Worst TSP Concentrations
(Unmitigated Scenario)
ASR
|
TSP
Concentration (µg/m3)
|
1-hour average
|
24-hour average
|
ASR 1
|
3734
|
431
|
ASR 2
|
1230
|
143
|
ASR 3
|
1184
|
140
|
ASR 4
|
1256
|
143
|
ASR 5
|
1331
|
147
|
ASR 6
|
536
|
119
|
ASR 7
|
444
|
102
|
ASR 8
|
303
|
97
|
3.7.2
The predicted results show that the 5
closest ASRs would be impacted by elevated dust generated from the construction
site. Exceedance of 1-hour average TSP concentration (500 µg/m3) at
ASRs of the PCRTS and Sea Crest Villa are observed. At the PCRTS, the predicted dust concentration also exceeds
the 24-hour average TSP concentration of 260 µg/m3. Mitigation
measures are required to be implemented to reduce the potential dust impacts.
Mitigated Scenario
3.7.3
Typical dust control methods include
ground watering, equipment and vehicle watering, proper handling of material
and stockpile, and fencing of construction site. In the mitigated scenario, it
is assumed that the construction site area will be watered to reduce the
fugitive dust. In accordance with
Control Techniques for Particulate Emissions from Stationary Sources -Volume
2 (USEPA-1982), reduction of dust
generation by about 90% can be achieved when the construction site is kept in
wet condition.
3.7.4
The TSP concentrations are predicted
with the proposed mitigation measures at the identified ASRs. Dust levels are
reduced significantly and the predicted results are shown in Table 3-7. Figures 3-7 and 3-8 show the TSP concentration
contours for 1-hour and 24-hour average respectively.
Table 3-7 Worst TSP
Concentrations (Mitigated Scenario)
ASR
|
TSP Concentration (µg/m3)
|
1-hour average
|
24-hour average
|
ASR 1
|
487
|
125
|
ASR 2
|
212
|
93
|
ASR 3
|
207
|
93
|
ASR 4
|
215
|
93
|
ASR 5
|
224
|
93
|
ASR 6
|
128
|
90
|
ASR 7
|
126
|
89
|
ASR 8
|
111
|
88
|
3.7.5
With the implementation of fugitive
dust control measures, the worst 1-hour TSP concentrations at identified ASRs
range from 111 to 487 µg/m3, which comply with the recommended
construction dust standards stipulated in the TMEIAP. Identified ASR at the adjacent refuse transfer station is
also in compliance with AQO standard.
Operational Phase
Unmitigated Scenario
3.7.6
To assess the potential odour impact,
the conversion factors from 1-hour average to 5-second average concentration is
incorporated into the model input.
The predicted odour levels at the identified ASRs are summarised in Table 3-8. A typical odour modeling
output file is shown in Appendix 3D. The worst odour levels are
predicted to be occurred at 1.5m and the odour contours are shown in Figure 3-9.
Table 3-8 Worst
5-second Average Odour Levels (Unmitigated Scenario)
Assessment Point
|
Assessment Height above
Local Ground Level
|
1.5m
|
5m
|
10m
|
15m
|
20m
|
25m
|
30m
|
ASR 1
|
29
|
18
|
12
|
6
|
3
|
1
|
1
|
ASR 2
|
9
|
6
|
3
|
3
|
3
|
2
|
2
|
ASR 3
|
7
|
5
|
3
|
3
|
2
|
2
|
2
|
ASR 4
|
8
|
6
|
3
|
2
|
2
|
2
|
2
|
ASR 5
|
13
|
9
|
4
|
2
|
2
|
2
|
1
|
ASR 6
|
2
|
2
|
1
|
1
|
1
|
1
|
1
|
ASR 7
|
5
|
4
|
3
|
1
|
1
|
1
|
1
|
ASR 8
|
5
|
4
|
4
|
2
|
1
|
1
|
1
|
ASR 9
|
7
|
6
|
4
|
2
|
1
|
1
|
1
|
ASR 10
|
9
|
7
|
4
|
2
|
1
|
1
|
1
|
ASR 11
|
11
|
8
|
3
|
2
|
2
|
2
|
2
|
3.7.7
Elevated odour levels are predicted at
some identified ASRs. As the odour
level exceed the 5 OU criteria stipulated in TMEIAP, mitigation measures are
necessary.
Mitigated Scenario
3.7.8
To mitigate the potential odour
impact, major odour sources, including inlet works, grit chamber, equalization
tank, SBR, sludge thickener, sludge digester, screening and grits storage area
and return liquor pumping station would be enclosed by air-tight covers. All
the odour emission from the enclosed sources would be ventilated to a
deodorization unit. The
deodourisation unit would be able to achieve an odour removal efficiency of
99.5%. The treated gas would then
be emitted vertically via a 0.4m diameter duct and at 4m above ground level (at
the roof level of the odour control room). The predicted maximum odour level is shown in Table 3-9. The odour level contours at 10m where worst odour levels are
predicted, are presented in Figure 3-10.
Table 3-9 Worst
5-second Average Odour level (Mitigated Scenario)
Assessment Point
|
Assessment Height above
Local Ground Level
|
1.5m
|
5m
|
10m
|
15m
|
20m
|
25m
|
30m
|
ASR 1
|
0.1
|
0.1
|
0.1
|
0.1
|
0.1
|
0.0
|
0.0
|
ASR 2
|
0.2
|
0.3
|
0.3
|
0.3
|
0.1
|
0.1
|
0.1
|
ASR 3
|
0.2
|
0.2
|
0.3
|
0.2
|
0.1
|
0.1
|
0.1
|
ASR 4
|
0.1
|
0.1
|
0.1
|
0.1
|
0.1
|
0.1
|
0.1
|
ASR 5
|
0.2
|
0.3
|
0.4
|
0.3
|
0.1
|
0.1
|
0.1
|
ASR 6
|
0.3
|
0.2
|
0.2
|
0.2
|
0.1
|
0.1
|
0.1
|
ASR 7
|
1.1
|
1.0
|
1.6
|
0.6
|
0.1
|
0.0
|
0.0
|
ASR 8
|
2.4
|
3.3
|
3.9
|
1.7
|
0.0
|
0.0
|
0.0
|
ASR 9
|
0.1
|
0.1
|
0.1
|
0.1
|
0.1
|
0.1
|
0.1
|
ASR 10
|
0.6
|
0.6
|
0.5
|
0.4
|
0.1
|
0.1
|
0.1
|
ASR 11
|
0.4
|
0.6
|
0.9
|
0.6
|
0.2
|
0.1
|
0.1
|
3.7.9
With the implementation of mitigation
measures, it can be seen the predicted odour level would comply with the 5 OU
criterion stipulated in TMEIAP.
Unmitigated Scenario (when Drying Bed
is used)
3.7.10
As mentioned in Appendix 2C, the sludge dewatering operation will be
reversed (i.e. drying bed as duty and export as standby) when the sludge export
option turns out to be expensive due to the build up of sludge amount in the
future. In this scenario, drying bed
is considered to be the duty unit and the associated odour impact will be
assessed in the assessment.
3.7.11
In this unmitigated scenario, drying
bed will be left open, while other major odour sources mentioned in Section
3.7.8 will still be covered. The
predicted maximum odour level is shown in Table
3-10. The odour level contour
at the worst height (i.e. 1.5m) is presented in Figure 3-11.
Table 3-10 Worst
5-second Average Odour level (Unmitigated Scenario) (when Drying Bed is used)
Assessment
Point
|
Assessment
Height above Local Ground Level
|
1.5m
|
5m
|
10m
|
15m
|
20m
|
25m
|
30m
|
ASR 1
|
21
|
15
|
6
|
1
|
0
|
0
|
0
|
ASR 2
|
3
|
2
|
2
|
1
|
1
|
1
|
1
|
ASR 3
|
3
|
2
|
1
|
1
|
1
|
1
|
1
|
ASR 4
|
6
|
4
|
2
|
1
|
1
|
1
|
1
|
ASR 5
|
5
|
3
|
2
|
1
|
1
|
1
|
1
|
ASR 6
|
2
|
2
|
1
|
1
|
1
|
1
|
0
|
ASR 7
|
5
|
4
|
2
|
1
|
0
|
0
|
0
|
ASR 8
|
4
|
4
|
4
|
2
|
1
|
0
|
0
|
ASR 9
|
4
|
3
|
2
|
1
|
1
|
1
|
0
|
ASR 10
|
4
|
3
|
2
|
1
|
1
|
0
|
0
|
ASR 11
|
2
|
1
|
1
|
1
|
1
|
1
|
1
|
3.7.12
Elevated odour levels are predicted at
the ASR 1 and 4, namely the PCRTS and Sea Crest Villa Block B. As the odour level exceed the 5 OU
criteria stipulated in TMEIAP, mitigation measures are necessary.
Mitigated Scenario (when Drying Bed is
used)
3.7.13
To mitigate the potential odour
impact, drying bed, like other odour sources, will be enclosed. The collected gas will be diverted to
the odour control unit. The modeling
results are summarised in Table 3-11
and the odour level contours at 10m where worst odour levels are predicted, are
presented in Figure 3-12.
Table 3-11 Worst
5-second Average Odour level (Mitigated Scenario) (when Drying Bed is used)
Assessment Point
|
Assessment Height above
Local Ground Level
|
1.5m
|
5m
|
10m
|
15m
|
20m
|
25m
|
30m
|
ASR 1
|
0.1
|
0.1
|
0.1
|
0.1
|
0.1
|
0.1
|
0.0
|
ASR 2
|
0.2
|
0.3
|
0.3
|
0.3
|
0.1
|
0.1
|
0.1
|
ASR 3
|
0.2
|
0.2
|
0.3
|
0.2
|
0.1
|
0.1
|
0.1
|
ASR 4
|
0.1
|
0.1
|
0.1
|
0.1
|
0.1
|
0.1
|
0.1
|
ASR 5
|
0.2
|
0.3
|
0.4
|
0.3
|
0.1
|
0.1
|
0.1
|
ASR 6
|
0.3
|
0.2
|
0.2
|
0.2
|
0.1
|
0.1
|
0.1
|
ASR 7
|
1.1
|
1.0
|
1.6
|
0.6
|
0.1
|
0.0
|
0.0
|
ASR 8
|
2.4
|
3.3
|
3.9
|
1.7
|
0.0
|
0.0
|
0.0
|
ASR 9
|
0.1
|
0.1
|
0.1
|
0.1
|
0.1
|
0.1
|
0.1
|
ASR 10
|
0.6
|
0.6
|
0.5
|
0.4
|
0.1
|
0.1
|
0.1
|
ASR 11
|
0.4
|
0.6
|
0.9
|
0.6
|
0.2
|
0.1
|
0.1
|
3.7.14
With the implementation of mitigation
measures on drying bed, it can be seen the predicted odour level would also
comply with the 5 OU criterion when the drying bed is used.
3.8
Cumulative Impacts
Construction
Phase
3.8.1
Air quality is not considered as a key
issue for construction of Peng Chau Helipad Project. It is not anticipated there would be any cumulative impacts
during construction phase.
Operational
Phase
3.8.2
The PCSPS would have odour impact to
the surrounding ASRs during the operation of Peng Chau STW Upgrade. However, the proposed pumping station
is enclosed and equipped with odour control equipment with over 99.5% odour
removal efficiency. As confirmed
by the modelling results, the overall odour impacts at the all identified ASRs
are acceptable.
3.9
Mitigation Measures
Construction
Phase
3.9.1
The suitable requirements stipulated
in the Air Pollution Control (Construction Dust) Regulation shall be
implemented during the construction activities to minimise the dust impact. It
is recommended that typical dust control methods including the following good
site practices should also be incorporated during construction phase:
(a) Stockpiles of imported material kept on site shall be contained
within hoarding, dampened and/or covered during dry and windy weather.
(b) Material stockpiled alongside trenches should be covered with
tarpaulins.
(c) Stockpile of cement should be covered entirely by impermeable
sheeting.
(d) All dusty materials shall be sprayed with water prior to any
loading, unloading or transfer operation so as to keep the dusty materials wet.
(e) Water sprays shall be used during the delivery and handling of sands
aggregates and the like.
(f)
All demolished items that may emit
dust particles should be covered entirely by impervious sheeting or placed in
an area sheltered on the top and the 3 sides within a day of demolition.
Operational Phase
3.9.2
The recommended mitigation measure to
minimize potential odour impact is to enclose all the major odour sources,
including the drying bed. The SBR
No. 5, which is for future expansion, will be enclosed when it is put into
operation. The sludge should be
carried by enclosed container during sludge transportation. The odorous gas should then be
collected and treated by deodorization unit with odour removal efficiency of
more than 99.5%. With the
implementation of these mitigation measures, the odour impacts to the
identified ASRs would become insignificant.
3.10
residual impacts
3.10.1 With the implementation of the recommended mitigation measures for
both the construction and operational phases, no residual adverse air quality
impacts are anticipated.
3.11
Environmental Monitoring and Audit
Construction Phase
3.11.1 The predicted TSP results show that exceedance of recommended TSP
levels would occur during most of the construction period. Mitigation Measures
are therefore needed to reduce the dust impact to an acceptable level. EM&A
is recommended to be carried out during construction phase to ensure the
effectiveness of the mitigation measures implemented by the Contractor. Detailed EM&A requirement for
construction dust is provided in the separate EM&A Manual prepared under
the Project.
Operational
Phase
3.11.2 As all the odorous gas would be collected and properly treated by
deodorization unit with 99.5% odour removal efficiency and no odour impact
would be anticipated, EM&A during operational phase of Peng Chau STW
Upgrade would not be required.
3.12
conclusions and recommendations
Construction
Phase
3.12.1 Dust generating activities were identified and evaluated. If
un-mitigated, construction of the Peng Chau STW upgrade would have short term
adverse impact on air quality, in the forms of fugitive dust emission at the
identified ASRs. The predicted construction dust impact exceeds the 1-hour and
24-hour TSP criteria at some ASRs. Mitigation measures including watering of
on-site construction area are expected to limit fugitive dust levels to
acceptable levels. Model simulation results show that the 1-hour and 24-hour
TSP criterion could be met after implementing the recommended mitigated
measures.
3.12.2 The implementation of the Air Pollution Control (Construction Dust)
Regulation and good site practice during construction phase are recommended.
Operational
Phase
3.12.3 During the operational phase of the Project, all the potential odour
generating facilities such as inlet works, grit chamber, equalization tank,
SBR, sludge thickener, sludge digester, screening and grits storage area, return
liquor pumping station and sludge drying bed would be enclosed by air-tight
covers. Odourous gas would be ventilated to the deodourisation facility for
further treatment before discharge. The deodourisation facility would be
capable of removing 99.5% of odour.
During sludge transportation, it is recommended that the sludge should
be carried by enclosed container to avoid unacceptable odour nuisance.
3.12.4 With the above mitigation measures, no unacceptable odour impact
would be envisaged.