3 Air
Quality Impact Assessment
3.2 Environmental Legislation, Standards and Guidelines
3.3 Baseline Environmental Condition
3.5 Identification of Pollution Sources
3.7 Prediction of Odour Impact
3.10 Environmental Monitoring and Audit Requirements
3
AIR QUALITY IMPACT ASSESSMENT
3.1.1
This
Chapter presents an assessment of the potential air quality impact arising from
the construction and operation of the Project. Mitigation measures have been
identified to alleviate the impact and their effectiveness has been
evaluated.
3.2
Environmental Legislation, Standards
and Guidelines
Environmental
Impact Assessment Ordinance
3.2.1
For construction dust, Annex 4
of EIAO-TM
specifies a TSP limit concentration averaged over a 1-hour period to be 500
µg/m3.
3.2.2
For odour impact assessment, Annex 4 of EIAO-TM
stipulates an odour limit for odour prediction assessment of 5 odour units based
on an average time of 5 seconds at an air sensitive receiver.
3.2.3
Annex
12 of EIA-TM provides the guidelines for conducting air quality assessments
under the EIA process, including determination of air sensitive receivers,
assessment methodology and impact prediction and assessment.
Air Pollution
Control Ordinance
3.2.4
The Air Pollution
Control Ordinance (CAP 311) provides for the
control of air pollutants from a variety of stationary and mobile sources
through the establishment of a set of Air Quality Objectives
(AQOs). As of
1st January 2014, a new set of air quality objectives which stipulates maximum
concentrations for a range of pollutants, namely nitrogen dioxide (NO2),
sulphur dioxide (SO2), respirable suspended particulates (RSP), fine suspended particulates (FSP), carbon
monoxide (CO), photochemical oxidants (O3) and lead (Pb) has been in force. The AQOs are listed
in Table 3-1 below.
Table
3-1: Hong Kong Air Quality Objectives
Pollutant |
Averaging time |
Concentration limit [i] (μg/m3) |
Number of exceedances allowed |
Sulphur
dioxide |
10-minute |
500 |
3 |
24-hour |
125 |
3 |
|
Respirable
suspended particulates (PM10)
[ii] |
24-hour |
100 |
9 |
Annual |
50 |
Not applicable |
|
Fine
suspended particulates
(PM2.5) [iii] |
24-hour |
75 |
9 |
Annual |
35 |
Not applicable |
|
Nitrogen
dioxide |
1-hour |
200 |
18 |
Annual |
40 |
Not applicable |
|
Ozone |
8-hour |
160 |
9 |
Carbon
monoxide |
1-hour |
30,000 |
0 |
8-hour |
10,000 |
0 |
|
Lead |
Annual |
0.5 |
Not applicable |
Note:
[i]
All measurements of the concentration of gaseous air pollutants, i.e., sulphur
dioxide, nitrogen dioxide, ozone and carbon monoxide, are to be adjusted to a
reference temperature of 293Kelvin and a reference pressure of 101.325
kilopascal.
[ii]
Respirable suspended particulates means suspended particles in air with a
nominal aerodynamic diameter of 10 μm or less.
[iii]
Fine suspended particulates means suspended particles in air with a nominal
aerodynamic diameter of 2.5 μm or less.
Air Pollution
Control (Construction Dust) Regulation
3.2.5
Mitigation measures for
construction sites have been specified in the Air Pollution Control
(Construction Dust) Regulation. It also requires Contractors and site agents to inform EPD and adopt dust reduction
measures while carrying out “Notifiable Works” or “Regulatory Works” as defined
under the regulation. Works relevant to
this Project include both “Notifiable Works” (road construction) and
“Regulatory Works” (dusty materials handling, excavation).
3.3
Baseline Environmental Condition
3.3.1
Po Toi O is located in a rural area to the south of
Clear Water Bay Country Park. Its openness to the South China Sea and the
absence of industrial undertakings within 1 km imply that good air quality is
anticipated. Po Toi O is home to some village houses and two seafood
restaurants.
3.3.2
There is currently no EPD-operated air quality
monitoring station located in Po Toi O area.
The nearest air quality monitoring station operated by EPD is located at
Kwun Tong, but the air quality data is considered inappropriate as the station
is located in a mix of industrial/commercial/domestic premises, mildly affected
by the open sea. The more appropriate station is the one at Tap Mun where the
data will be used to describe the existing air quality in Po Toi O. Table 3-2 summarizes the annual average
concentrations of the air pollutants recorded at the Tap Mun Air Quality
Monitoring Station from Year 2010 to Year 2014.
Table
3-2: Annual Average Concentration of
Pollutants from Year 2010 to 2014 at EPD’s Air Quality Monitoring Station (Tap
Mun)
Pollutant |
Annual Average Concentration (mg/m3) |
5-year average |
||||
Year 2010 |
Year 2011 |
Year 2012 |
Year 2013 |
Year 2014 |
||
RSP |
41 |
47 |
38 |
49 |
44 |
44 |
SO2 |
10 |
10 |
11 |
13 |
9 |
11 |
NO2 |
13 |
12 |
11 |
11 |
10 |
11 |
O3 |
67 |
71 |
71 |
75 |
72 |
71 |
Reference: Air Quality in
Hong Kong 2010-2013 & Air Quality Statistical Summary in Hong Kong 2014
3.3.3
Based on the on-site survey, although Po Toi O is
not serviced by public sewers, no significant odour was noted in the area.
3.4.1
The assessment area for air quality impact is
defined by a distance of 500 m from the proposed works boundary
as shown
in Figure 3-1. Within the assessment area, representative
Air Sensitive Receivers (ASRs) have been identified for this assessment, in
accordance with the Annex 12 of 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 recreational activity areas are
classified as ASRs.
3.4.2
ASRs in the vicinity of the proposed works boundary
are identified for the assessment and the locations of these ASRs and their
nearest horizontal separation from the proposed sewer and rising mains pipes
and odour emission point of sewage treatment plant (STP) are tabulated in Table 3-3. Locations of the
representative ASRs and the assessment points are shown in Figure 3-1.
Table
3-3: Representative Air Sensitive Receivers
ASR |
Description |
Land Use |
Ground mPD |
Nearest Horizontal Separation(m) From Proposed |
No. of Storey |
|
Pipes |
Odour Emission Point |
|||||
PTO_A1 |
Village House in PTOCR 28 |
Village |
6 |
3 |
80 |
2 |
PTO_A2 |
Village House in PTOCR 23 |
Village |
3.7 |
2.1 |
97 |
2 |
PTO_A3 |
Village House in PTOCR 7 |
Village |
4 |
1 |
147 |
3 |
PTO_A4 |
Village House in PTOCR 7A |
Village |
4.1 |
0.95 |
208 |
3 |
PTO_A5 |
Village House in PTOCR 3A |
Village |
4.5 |
3.9 |
257 |
2 |
FV_A1 |
Fairway Vista House 3 |
Village |
23.1 |
1.5 |
284 |
3 |
FV_A2 |
Fairway Vista House 12 |
Village |
7.5 |
1.6 |
350 |
3 |
REC_A1 |
Grassland |
Recreation |
23.1 |
38 |
194 |
1 |
TEM_A1 |
Hung Shing Temple |
Place of Worship |
4.1 |
5.2 |
180 |
1 |
* PTOCR = Po Toi O Chuen Road
3.5
Identification of Pollution Sources
3.5.1
Air Pollutant Control Ordinance (APCO) (Cap311) and EIAO-TM
stipulate statutory Air Quality Objective (AQO) for 7 criteria air pollutants
including NO2, SO2, FSP, RSP, CO, O3 and
lead.
Construction Phase
3.5.2
The Project
involves provision of village sewerage to the unsewered areas of Po Toi O with
800m of gravity sewers and 400m of rising
mains, construction of local sewage treatment plant with reprovision of road facilities (about 893m2) and construction of a submarine outfall.
The works are planned to commence in
year 2017 for completion by year 2021. The major
potential air quality impact during construction phase of the Project would be
dust impact arising
from:
l
Excavation and slope cutting for site formation
work for the proposed sewage treatment plant
l
Materials handling
l
Excavation for pipeline installation for the
village sewerage
l
Dredging and disposal of
excavated/dredged materials,
l
Backfilling
l
Building construction, and
installation of electrical and mechanical equipment.
l
Wind erosion of open sites and stockpiling areas.
3.5.3
The major dust impact arising from the construction
of the proposed sewage treatment plant would be the excavation and slope
cutting for site formation work. Based on the site condition, the excavation
rate is limited to about 11.9 m3 per day only. Appropriate dust
control measures should be implemented during the construction stage in
accordance with the requirements in the Air Pollution Control (Construction
Dust) Regulation. Given the small scale works involved with proposed mitigation
measures as stated in Section 3.8,
adverse dust impact is not expected.
3.5.4
The potential dust generating construction
activities associated with the sewers works will mainly be excavation of a
localised scale. It is expected that the sewers works will be carried out
sections by sections. Under normal
practice, the sewers would be constructed in section of about 10m to 20m at any
one time and each work front would be separated by a clearance distance. For such typical arrangement, construction
works for each section would normally be completed in about 29 working days.
Given each section of the works would be of a small scale, localised and
short-term, it would not reveal useful information by performing dust
dispersion modelling for this type of transient dust generation activities.
Moreover, based on this small scale of works and the limited excavation rate,
1-2 dump truck(s) would be allowable on site for unloading
materials. Therefore, extensive excavation and transportation of dusty material
are highly unlikely. With the
implementation of practicable dust suppression measures stipulated in the Air
Pollution Control (Construction Dust) Regulation, adverse construction dust
impact at the ASR is not expected during construction of the Project.
3.5.5
Potential
odour impact may arise from excavation of potential odorous sediments under
soil or concrete surface. Temporary stockpiles of odorous excavated material
and the waste disposal are also identified as odour sources. However, due to
the limited quantity of excavated soil for the Project, odour impact during the
construction phase of Project is anticipated to be acceptable if control
measures as stated in Section 3.8.4 are implemented. Potential odour
impact during construction stage shall not be assessed quantitatively.
3.5.6
Based on the best available information at time of
preparation of this EIA, there is no concurrent project in Po Toi O area in the
construction phase of the Project. Given the small scale works involved for the
Project, potential dust impact arising from the Project should be limited. With
the mitigation measures and continual monitoring and review of dust impact in
the area, adverse cumulative dust impact would not be anticipated.
Operational Phase
3.5.7
Based
on the nature of the Project, no air pollutants set out in AQOs shall be
generated during the operational phase of the Project. The only identifiable
potential impact would be odour impact arising from the operation of the
proposed sewage treatment plant.
3.5.8
Hydrogen
sulphide produced during sewage treatment is the main odour source. The vent
gas will be de-odourized prior to discharge into the atmosphere. The primary
idea on the de-odourizing system (DO system) is to adopt biotrickling filters
and/or activated carbon filters. While the exact type or combination of the DO
system has not been confirmed, the ultimate goal is to achieve removal
efficiency of at least 99.5%. All equipment will be stored underground with an
exhaust emission point at +17.75mPD for dispersion.
3.6.1
Under
the APCO, dust suppression measures stipulated in the Air Pollution Control
(Construction Dust) Regulation should be implemented and detailed in Section 3.8. With effective
implementation of these mitigation measures, adverse construction dust impacts
are not expected at the ASRs.
Quantitative assessment is therefore considered not necessary.
3.6.2
Audit
and monitoring program during the construction phase of this Project has been
formulated and is presented in the Environmental Monitoring and Audit Manual
prepared under this study.
Operation Phase
Meteorology
3.6.3
The
wind speed and wind direction are the two major features affecting the odour
dispersion. Low wind speeds can allow for accumulation of odour which may be
swept off site when the wind speed increases. At high wind speeds, odour
emissions can become significant. Seasonal wind data were reviewed in order to
design the most appropriate exhaust location of the proposed sewage treatment
plant.
3.6.4
PATH MM5
data was utilized as the meteorological data during modelling for four PATH
grids (37, 25), (38, 25), (37, 24) and (38, 24). The hourly input data include
wind direction, wind speed, atmospheric stability class, mixing height,
temperature and others. Minimum mixing height of 121m recorded in King’s Park
weather station in year 2010 was adopted.
Model Description
3.6.5
Significant
odour impact is anticipated if the STP is not properly designed and/or
operated. Gaussian model ISCST3 was used for modelling potential effects from
odour within the 500m assessment area of the Project (see Section 3.4.1).
3.6.6
The odour criterion is based on an averaging
time of 5 seconds. The model output corresponds more closely to a maximum
15-minute average concentration. This
matter relates to the Pasquill-Gifford vertical dispersion parameter used in
the ISCST3 model which is fully documented in the Workbook on Atmospheric
Dispersion Estimates. With reference to the “Odour Modelling - Why and How,
Richard A. Duffee et al” and the “Odour Control - A Concise Guide, Warren
Spring Laboratory”, the conversion of maximum 15-minute average odour to
5-second average odour are shown in Table
3-4. The overall conversion factors
under different stability classes are applied to the model so that the
predicted outputs are in 5-second average concentration.
Table
3-4: Summary of Conversion Factors from
1-hour to 5-second
Stability Class |
Conversion Factor |
A, B |
22.3 |
C |
8.50 |
D |
6.90 |
E, F |
6.55 |
Emission Strength
3.6.7
At-source
odour concentration inside the sewage treatment plant was assessed based on the
inflow quality, flow rate, and preliminary design of the plant and the data was
used as input to the ISCST3 model as provided in Table 3-5 below.
Table
3-5: Design Parameter of the Sewage
Treatment Plant
Design
Parameter |
Strength |
De-odourizing
treatment capacity |
1,500 m3/hr |
Air velocity at
exhaust |
3.32 m/s |
Exhaust diameter |
400mm |
Exhaust location |
+17.75 mPD |
Exhaust
temperature |
Ambient
temperature |
H2S
concentration at the inlet |
20 ppm |
H2S
concentration at the exhaust |
<0.1 ppm (99.5% removal efficiency) |
Exhaust Odour
Concentration [1] |
200 OU/m3 |
* All information in the table was provided by
DSD unless
otherwise stated.
[1]
Refer to EIA for Tuen Mun Sewerage - Eastern Coastal Sewerage Extension (2000),
H2S was converted to odour units using 0.0005ppm odour threshold
value.
3.7
Prediction
of Odour Impact
3.7.1
Standard equipment will be installed in the
deodourizing units of the proposed sewage treatment plant. For the purpose of
this assessment, an odour removal efficiency of 99.5% as mentioned in Table 3-5 above has been assumed for
the predicted results.
3.7.2
In general, the village houses are three-storey high. Assuming that the height of each floor is 3 m, all ASRs were modelled for three heights (i.e. 1.5m, 4.5m and 7.5m above ground).
The modelled results are shown in Table
3-6.
Table
3-6: Maximum 5-second Odour Levels for
ASRs
ASR |
Description |
Predicted
Maximum 5-second Odour Level (Odour Unit) |
|||
G/F |
1/F |
2/F |
Maximum |
||
PTO_A1 |
Village House in
PTOCR 28 |
0.57 |
0.67 |
- |
0.67 |
PTO_A2 |
Village House in
PTOCR 23 |
0.58 |
0.64 |
- |
0.64 |
PTO_A3 |
Village House in
PTOCR 7 |
0.35 |
0.35 |
0.35 |
0.35 |
PTO_A4 |
Village House in
PTOCR 7A |
0.26 |
0.26 |
- |
0.26 |
PTO_A5 |
Village House in
PTOCR 3A |
0.22 |
- |
- |
0.22 |
FV_A1 |
Fairway |
0.19 |
0.22 |
0.32 |
0.32 |
FV_A2 |
Fairway Vista
House 12 |
0.21 |
0.35 |
0.58 |
0.58 |
REC_A1 |
Grassland |
0.28 |
- |
- |
0.28 |
TEM_A1 |
Hung |
0.25 |
- |
- |
0.25 |
5-second Odour Criterion : |
5 |
3.7.3
The
predicted 5-second average odour levels at various heights at the identified
ASRs are well within the odour criterion (i.e. 5 OU in 5-second averaging
time).
3.7.4
Figures 3-2 to 3-4 present the odour concentration contour plots of the predicted maximum
5-second odour levels at 1.5 m, 4.5 m and 7.5 m above ground, respectively. The
predicted results demonstrate that the odour concentrations at the selected
ASRs are all below the EIAO TM assessment criterion and the contours confirm
that the odour emitted from the sewage treatment plant will not cause any
exceedance at the reach of the ASRs.
3.8
Mitigation Measures
Construction Phase
Construction Dust Impact
3.8.1.
Due
to the scale of the works of the Project, the construction works are unlikely
to cause unacceptable dust impact on surrounding sensitive receivers if
standard control measures are implemented. Dust control measures as part of
good construction practice should be implemented to minimize dust nuisance to
within the acceptable levels:
l
Hoarding of not less than 2.4 m high shall be
erected from ground level to surround the construction site for STP along Po
Toi O Chuen Road except for a construction site entrance or exit
l
As there is limited space in Po Toi O, stockpiling
should be avoided. However, if found necessary, the materials should be covered
by impervious materials such as tarpaulin
l
Good housekeeping to minimize dust generation,
e.g. by properly handling and storing
dusty materials
l
Store cement bags in shelter with 3 sides and the
top covered by impervious materials if the stack exceeds 20 bags
l
Minimize excavation area as far as possible
l
Maintain a reasonable height when dropping
excavated materials to limit dust generation
l
Cover materials on trucks before leaving the
construction site to prevent debris from dropping during traffic movement or
being blown away by wind
l
Minimize exposed earth after completion of work in
a certain area by hydroseeding, vegetating, soil compacting or covering with
bitumen
l
Carry out dust suppression without excessive
wastewater generation, e.g. use mist spray
l
Limit vehicle speed within construction site and in
Po Toi O to 10km/hr and confine vehicle movement in haul road
l
Provide wheel washing at construction site exit to
clean the vehicle body and wheel
l
Regular maintenance of plant equipment to prevent
black smoke emission
l
Throttle down or switch off unused machines or
machine in intermittent use
3.8.2.
According
to USEPA AP-42, a dust suppression of 50% can be achieved by watering twice a
day. Thus, a dust removal efficiency of 75% is expected by regular watering
four times a day. With regular watering for dust suppression, the mitigated
dust levels during construction phase should comply with the EIAO-TM guideline
limit for all the air sensitive receivers.
3.8.3.
Dust
impact could be effectively mitigated by inclusion of appropriate contracts
clauses (e.g. regular and sufficient watering, enclosure of potential dust
sources, etc.) for dust minimisation in the works contract. A control programme can be instigated to
monitor the construction process in order to enforce dust controls and modify
methods of works to reduce the dust emission down to acceptable levels.
Odour Impact
3.8.4.
To
minimize odour problem during construction phase, excavation works should be
conducted in the shortest time span possible. In the event that on-site storage
cannot be avoided, odourous excavated materials should be temporarily stored in
covered containers. They should be removed off-site as soon as practically
possible within 24 hours to avoid odour nuisance arising. Any leachate from
storage skips should be stored in covered buckets and tanks.
Operational Phase
Odour Impact
3.8.5.
During
the operation phase of the sewage treatment plant, the approach is to enclose
all odour sources to minimize potential odour problems. The sewage pumps,
screen chamber, wet well, etc. will be located underground and enclosed by a
reinforced concrete structure with cover. Deodorizers with forced ventilation
will be installed to remove the odour. The deodorizer will provide an overall H2S
removal efficiency of 99.5% or higher. Moreover, the exhaust from the
deodorizer will be pointed upward to allow maximum dispersion.
3.8.6.
Sludge
should be removed regularly to prevent build-up of odourous gas. As the estimated daily
production of the sludge is 5.3m3,
it was considered not cost-effective to expand the footprint of the proposed sewage treatment plant to accommodate a sludge dewatering facility. It is proposed that the sludge will be removed for sludge cake making in other STP every 2 days
by a sludge tanker.
Sludge should be transferred to sludge tanker by coupling method to prevent
odour leakage. The tanker will be parked inside the sewage treatment plant for
sludge removal where the air is extracted for deodourization.
3.8.7.
In
addition, the following measures shall be implemented:
l
Regular inspection should be conducted to check for
leakage of odourous gas
l
Maintain the removal efficiency of screenings and
grits by flushing the screens and grit sump regularly to prevent build up of
solids
l
Screenings, grits and worn filters should be stored
in sealed containers inside the STP and during removal for disposal
l
Maintain the efficiency of MBR membrane by removing
organic and inorganic debris with sodium hypochlorite and oxalic acid
l
Replace worn activated carbon filter/biotrickling
filter to maintain the odour removal efficiency at 99.5%
l
Clean all the tanks with water regularly
3.8.8.
With
the proposed measures to be incorporated into the design, the proposed sewage
treatment plant should not cause significant odour impact on the environment.
3.9.1.
With
the implementation of mitigation measures as stated in Section 3.8, no residual impacts are anticipated for both
the construction phase and operational phase of the Project.
3.10
Environmental Monitoring and Audit
Requirements
3.10.1.
With the implementation of the proposed dust
suppression measures, good site practices and dust monitoring and audit
programme, acceptable dust level would be expected at the ASRs during construction phase. Details of the monitoring
requirements such as monitoring locations, frequency of baseline and impact
monitoring was prepared in the form of EM&A manual as part of the EIA
submission.
3.10.2.
With the implementation of mitigation measures through the design of the sewage
treatment plant as
mentioned in Section 3.8.5 – 3.8.7, air quality impact on the Project is
expected to be
acceptable during operational phase. Operational air monitoring is considered unnecessary.
3.10.3. General EM&A requirements were presented in Chapter 12.
3.11.1. Dust generating activities were identified and evaluated. Based on the best available information at time of preparation of this EIA, there is no concurrent project in Po Toi O area in the construction phase of the Project. Given the small scale of the works involved in the Project, extensive excavation and transportation of dusty material are highly unlikely. Mitigation measures including watering of on-site construction area are expected to limit fugitive dust levels to acceptable levels. With proper implementation of mitigation measures, construction dust emissions impacts are anticipated to be acceptable. An EM&A programme will be implemented to ensure construction dust impacts are controlled to acceptable level.
3.11.2. Due to the limited quantity of excavated soil for the Project, odour impact during the construction phase of Project is anticipated to be acceptable if control measures as stated in Section 3.8.4 are implemented.
3.11.3. During the operational phase of the Project, all the potential odour generating facilities would be enclosed by building structure. The deodorization facility is designed to be able to achieve an odour removal efficiency of 99.5% for the exhaust of the sewage treatment plant. Based on the odour modelling results, no exceedance of 5-OU in 5-second averaging time specified in the EIAO-TM is anticipated. During sludge transportation, it is recommended that the sludge should be carried in enclosed containers to avoid unacceptable odour nuisance. Membrane and filter in the STP should be regularly cleaned and replace to maintain the efficiency of sewage treatment and odour removal. With proper mitigation measures incorporated into the design, odour impacts arising from the proposed sewage treatment plant will be significantly reduced, and are anticipated to be acceptable.