3.1 This section presents the potential air quality impacts associated with the construction and operation of the Project. Air sensitive receivers (ASRs) were identified for this study and the potential construction dust impact and odour impact on these receivers were assessed. Mitigation measures required to protect the ASRs were recommended, where necessary, to meet the air quality criteria.
Environmental Legislation, Policies, Plans, Standards and Criteria
3.2 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.
3.3 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
|
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 |
|
|
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
3.4
The EIAO-TM also stipulates
that the hourly total suspended particulates (TSP) level at sensitive receivers
should not exceed 500 µgm-3 (measured at
3.5 The Air Pollution Control (Construction Dust) Regulation specifies site formation is one of the controlled processes. Contractors and site agents are required to inform EPD and adopt dust reduction measures to minimize dust emission, while carrying out construction works, to the acceptable level.
Description of the Environment
3.6 The Project area is in Tuen Mun. The air quality in the study area is primary affected by traffic emissions and industrial emissions. The nearest EPD air monitoring station is in Yuen Long. According to EPD’s “Guideline on Assessing the ‘TOTAL’ Air Quality Impacts”, the latest five years averages monitoring data should be adopted as the background concentration. Table 3.2 summarizes the annual average concentrations of the pollutants (NO2, RSP and TSP).
Table 3.2 Annual Average Concentrations of Pollutants in the Latest Five Years (Year 2002 - 2006) at Yuen Long Air Quality Monitoring Station
|
Pollutant |
Annual Average Concentration in the Latest Five Years (mgm-3) |
|
NO2 |
60 |
|
RSP |
62 |
|
TSP |
100 |
3.7
Representative
air sensitive receivers (ASRs), which are in the vicinity of the sites, were
identified for assessment, in accordance with the criteria stipulated in the
EIAO-TM. The details of the
representative ASRs for the air quality assessment are listed in Table 3.3 and its locations are shown
in Figure 3.1.
Table 3.3 Summary of Representative Air Sensitive Receivers
|
ASR No. |
Description |
Distance to the nearest Site Boundary (m) |
|
A1 |
Planned
Secondary School |
10 |
|
A2 |
Planned
Primary School |
70 |
|
A3 |
Planned
Secondary School |
42 |
|
A4 |
Planned
Primary School |
30 |
|
A5 |
|
95 |
3.8
The
planned schools in the vicinity of the TM54SPS are
tentatively in operation at end 2017.
The construction dust impact would be focused on the existing ASRs.
Identification of Environmental Impacts
3.9
The
construction works for the Project are scheduled to commence in early 2011 for
completion in early 2014. The major
construction activities for the Project including excavation, steel fixing,
concreting, brick work and finishing, mechanical equipment installation,
pipeworks, roadworks and landscape works.
3.10
Referring to the construction
programme, the bulk excavation activities to be completed within 7 months and
the amount of soil materials to be excavated would be about 18,200 m3. Assuming the construction period is 10
hours per day and 26 working days per month, the average volume of excavated
materials to be handled per hour would be 10 m3. The average number of trucks (carrying 5
m3 excavated materials) required on-site would be 2 trucks per
hour. The area of the proposed site
is about 2000 m2. The
work site is limited area, all the trucks would be well covered and dust
suppression measures stipulated in the Air
Pollution Control (Construction Dust) Regulation would be applied and the
nearest existing ASRs are located at least 95m away from the site, no adverse
dust impact at the existing ASRs would be expected.
3.11
There
would have two other projects to be undertaken concurrently within 500m of the
study area. The concurrent
activities includes (i) Site 2 site formation and superstructure works under
the project “Formation, Roads and Drains in Tuen Mun Area 54 – Phase 1 and 2,
Private and Public Rental Housing Development in Tuen Mun Area 54”; and (ii)
Village sewerage works under the project “Sewerage
Upgrading Works in Tune Mun under Agreement No. CE 38/2006 (DS)”.
3.12
Referring
to the approved EIA Report of “Formation, Roads and Drains in Tuen Mun Area 54
– Phase 1 and 2, Private and Public Rental Housing Development in Tuen Mun Area
3.13
Taking into account the
insignificant dust impacts arising from the project “Sewerage Upgrading Works
in Tune Mun under Agreement No. CE 38/2006 (DS)” and the project “Formation, Roads and Drains in Tuen
Mun Area 54 – Phase 1 and 2, Private and Public Rental Housing Development in
Tuen Mun Area
3.14
The
main potential air quality impact during the operational phase of the project
would be odour including hydrogen sulphide gas (H2S) arising from
TM54SPS. There is no other odour emission source within 500m from the project
boundary that should be considered for cumulative odour impact assessment. The
potential source of odours of TM54SPS would be the wet wells and screen
chambers. Deodourizing unit is
therefore considered in the engineering design. The wet wells and screening chambers
would be enclosed and the vented air would be treated in the deodourizing unit
before discharging into the atmosphere.
3.15
As
mentioned in Section 3.9 – 3.13, insignificant dust impact would be expected
during construction phase of the Project, therefore, only qualitative
assessment would be undertaken in the study.
3.16
The
odour emission rates of raw sewage at sewage pumping station including wet well
and screen chamber 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 of the sewage pumping station. 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. The equation has been employed
under various approved EIA Studies such as odour assessment presented in North Lantau Development Topic Report TR23,
Environmental Impact Assessment of the
Tung Chung Main Sewage Pumping Station, Final Report and the Final Assessment Report of Outlying Islands
Sewerage Stage 1 Phase 1 – EIA Study.
Thus emission rates were calculated based on the following two
equations:
DF = 1.6 x (T/10)4.9
x (ORP + 200)-0.59
E = DF x V
where DF = Odour
concentration expressed as dilution factor, OUm-3
T = Temperature of
sewage, Fehrenheit F
ORP = Oxidation-reduction
potential of sewage, mV
E = Emission rate,
OUs-1
V = Ventilation rate,
m3/sec
= Displayed air
volume in the wet well and screen chamber, m3/Retention time, sec
3.17
The
sewage characteristics were obtained by the Engineer of the consultant team and
agreed with the Client. The sewage temperature
was assumed to be 30oC in the assessment as a worst-case
scenario. The sewage was considered
as fresh as the sewage comes from Tuen Mun
3.18
The
rural model dispersion option of the ISCST3 Model was used to predict 1-hour
average odour concentrations at selected sensitive receivers, based on the
topographical nature of the vicinity of TM54SPS.
3.19
To
convert the 1-hour averages to 3 minute averages, the power-law relationship
was applied for different stability classes. For stability class A to F, multiplying
factors of 2.23, 2.23, 1.7, 1.38, 1.31 and 1.31 respectively were applied. To further convert 3-minutes averages to
5-second averages, 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. This was in accordance with EIA Study Report, Sha Tin Sewage Treatment
Work Stage III Extension, 1999.
3.20
To
simulate the worst-case scenario, 360 predefined separate meteorological
conditions were used in the dispersion modeling. The resolution on the wind direction was
set to 1 degree increment and a wind speed conditions of 1ms-1. The models were tested with Pasquil
stability classes B and D for daytime hours and class F for nighttime hours.
3.21
The
assessment points at the residential
receivers and schools were considered to the height of
Prediction and Evaluation of Environmental Impacts
3.22
Due
to limited work areas and implementation of dust control and suppression
measures stipulated in the Air Pollution
Control (Construction Dust) Regulation, adverse dust impact at the ASRs
would not be expected from the construction activities of this Project. No unacceptable cumulative dust impact
would be anticipated as mentioned in Section 3.11 – 3.13.
3.23
Maximum
5-second average odour levels at the closest ASRs were predicted and are shown
in Table 3.4 below. Results indicated that the
representative ASRs would all exceed the EIAO-TM odour criterion. Mitigation measures are required. Detailed results are presented in Appendix 3.2. The predicted unmitigated odour contour
at worst hit level (10m above ground) is presented in Figure 3.2.
Table 3.4 Predicted maximum odour levels at ASRs (Unmitigated)
|
ASR |
Description |
Maximum Odour level (5 seconds
average) (ou) at different heights (above ground level) |
||||
|
1.5m |
5m |
10m |
15m |
20m |
||
|
A1 |
Planned
Secondary School |
2.7 |
5.8 |
46.0 |
7.3 |
1.9 |
|
A2 |
Planned
Primary School |
2.6 |
2.6 |
11.5 |
2.0 |
1.5 |
|
A3 |
Planned
Secondary School |
3.6 |
3.9 |
21.7 |
3.6 |
2.1 |
|
A4 |
Planned
Primary School |
3.6 |
4.5 |
28.4 |
4.7 |
2.3 |
|
A5 |
|
2.7 |
2.6 |
11.9 |
3.0 |
1.5 |
Note:
- the odour criterion is 5 odour units over 5-second average.
Mitigation of Adverse Environmental Impact
3.24
It
is anticipated no adverse construction dust impacts at nearby ASRs. The requirements of the dust suppression
measures stipulated in the Air Pollution
Control (Construction Dust) Regulation shall be adhered to during the
construction period. In addition,
good site practices and a comprehensive dust monitoring and audit programme are
recommended to minimise dust impacts.
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;
•
Use of regular watering, with complete coverage, to reduce
dust emissions from exposed site surfaces and unpaved roads, particularly
during dry weather;
•
Side enclosure and covering of any aggregate or dusty
material storage piles to reduce emissions. Where this is not practicable owing to
frequent usage, watering shall be applied to aggregate fines;
•
Open stockpiles shall be avoided or covered. Where possible, prevent placing dusty
material storage piles near ASRs;
•
Tarpaulin covering of all dusty vehicle loads transported
to, from and between site locations;
•
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;
•
Every vehicle should be washed to remove any dusty materials
from its body and wheels before leaving the construction sites; and
•
Instigation of an environmental monitoring and auditing
program to monitor the construction process in order to enforce controls and
modify method of work if dusty conditions arise.
3.25
In
accordance with the unmitigated results, the odour levels at all representative
ASRs would exceed the odour criterion.
It is recommended that the air ventilated from the wet wells and screen
chambers should be treated by deodorizers (granular activated carbon) with 90%
odour removal efficiency before discharge to the atmosphere. All the treated air would be emitted
from the exhaust vent of the ventilation fan. A standby deodourizer would be provided
to cater for breakdown and maintenance of the duty deodourizer.
3.26
With
the installation of deodourizer, the worst-case 5-second average odour levels
at the representative ASRs in the vicinity of TM54SPS would comply with the
odour criterion and results are summarized in Table 3.5. The result
details are presented in Appendix 3.2. The predicted mitigated odour contour at
worst hit level (10m above ground) is presented in Figure 3.3. Results
indicated that all representative ASRs in the vicinity of the SPS would comply
with the odour criterion of 5 odour units.
It is noted that there is exceedance of the 5 odour units out of the
project boundary, however, there is no ASR located within the exceedance zone.
Table 3.5 Predicted maximum odour levels at ASRs (Mitigated)
|
ASR |
Description |
Maximum Odour level (5 seconds average)
(ou) at different heights (above ground level) |
||||
|
1.5m |
5m |
10m |
15m |
20m |
||
|
A1 |
Planned
Secondary School |
0.3 |
0.6 |
4.6 |
0.7 |
0.2 |
|
A2 |
Planned
Primary School |
0.3 |
0.3 |
1.2 |
0.3 |
0.2 |
|
A3 |
Planned
Secondary School |
0.4 |
0.4 |
2.2 |
0.4 |
0.2 |
|
A4 |
Planned
Primary School |
0.4 |
0.5 |
2.8 |
0.5 |
0.2 |
|
A5 |
|
0.3 |
0.3 |
1.2 |
0.3 |
0.2 |
Note:
- the odour criterion is 5 odour units over 5-second average.
3.27
The
following odour precautionary measures should be implemented during operation
of TM54SPS:
•
The entire TM54SPS; especially wet wells and screening
collection areas should be enclosed in a building structure;
•
Discharge point of the odour removal system should be
directed away from the adjacent sensitive uses as shown in Figure 3.1 and the
exhaust air should be at a vertical upward flow direction;
•
Screened material from the TM54SPS should be stored in a
covered container;
•
The transportation of screened material during maintenance
should be transported in an enclosed type vehicle and disposed off on the same
working day;
•
Checking and maintenance of the odour removal system should
be implemented at least once every half year to maintain the removal
efficiency;
•
Odour removal systems, e.g. activated carbon filters, should
be provided to reduce the odour emissions; and
•
A standby deodourizer would be provided to cater for
breakdown and maintenance of the duty deodourizer.
3.28
As
indicated in the above paragraphs, the air quality at the ASRs has already
satisfied the odour requirement stipulated in the EIAO-TM with the installation
of the deodourization unit for the proposed development at Site 4A as shown on
Figure 3.1. With a view to further improving the air quality, the Project
Proponent would liaise with the relevant government departments to see if the
school layout could be designed with a larger buffer distance from the TM54SPS
such as the tentative layout as shown in Appendix
3.3.
Evaluation of Residual Impacts
3.29
With the incorporation of Air Pollution Control (Construction Dust)
Regulation and EM&A programme, adverse residual impact would not be
expected.
3.30
It
is predicted that air quality at the ASRs would satisfy with
the odour criteria with the installation of deodourizing unit, and adverse
residual impact is not expected. In
order to ensure compliance of the odour criterion stipulated in the EIAO-TM,
commissioning tests for all deodorization system should be included in the
Contract Document.
Environmental Monitoring and Audit
3.31
With
the implementation of mitigation measures stipulated in the Air Pollution
Control (Construction Dust) Regulation, no adverse dust levels at ASRs would be
expected. Environmental monitoring and audit is recommended to be conducted
during construction phase of the Project so as to check whether it is in
compliance with legislative requirement. Details of the monitoring and audit
programme are presented in EM&A Manual.
3.32
The predicted odour levels at the
ASRs would comply with the EIAO-TM criterion. It is recommended that odour monitoring and audit should be
carried out during the first year of the operation phase by the Environmental
Team and Independent Environmental Checker employed by the Drainage Services Department (DSD) to
ensure the compliance of odour criteria at the ASRs. Details of monitoring programme are
presented in the EM&A Manual.
3.33
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 was
recommended for the construction phase to ensure that the dust criteria would
be satisfied at the ASRs.
3.34
No
adverse odour impact would be expected at nearby ASRs during operation of
TM54SPS with the installation of deodourization unit. Odour monitoring and audit is proposed
during the first year of the operation phase to ensure the compliance of odour
criteria at the ASRs.