As the majority of the Extension site will
be covered with an impermeable liner and LFG will be extracted via a comprehensive
LFG collection system during the operation phase, it is anticipated that the
fugitive LFG emission from the Extension due to waste tipping activities will
be significantly reduced relative to the existing SENT Landfill. Taking account of the ambient VOC
monitoring results at the existing SENT Landfill, it is expected that the
ambient VOC concentrations at the Extension Site boundary will be well below
the trigger levels for individual compounds. Further dilution of the VOC
concentration is expected due to dispersion off-site. The anticipated VOC concentrations at
the identified ASRs will be minimal and will not cause adverse impacts.
Odour Emissions from Waste Filling
Activities and Operation of LTP
The restoration of the landfill will take place
progressively, whilst operations are ongoing on other parts of the site,
therefore, these two phases have been considered together in the assessment.
Potential sources of odour impact during
operation/restoration phase included:
·
Waste
filling area; and
·
Operation
of the LTP and the LFG treatment facility.
In order to minimize the potential odour
emissions during the operational phase of the Extension, a number of odour
management and control measures have been incorporated into the outline
design. These measures are
summarized in Table 4.8a.
Odour Emissions from
Waste Filling Area
The Extension is scheduled to commence
operation in 2013 and will be designed to receive MSW, special waste ([8])
and construction waste. By that time, the Sludge Treatment
Facilities (STF) are scheduled to be in operation ([9])
and sludge from sewage
treatment works (STWs) will be diverted to the STF
for treatment and disposal.
The operational life of the Extension is
expected to be about 6 years. The
Extension will be developed in 6 phases (Phases 1 to 6) and each phase will be
in operation for approximately 1 year.
The ground level of the first phase will be at about +6mPD and the
highest level will be at +150 mPD. The Extension will open to receive
wastes from 8 am to 11 pm every day.
Waste
Reception Area: All incoming and outgoing refuse
collection vehicles (RCVs) will be weighed at the
enclosed weighbridge office at the waste reception area. All RCVs
visiting the Extension are of enclosed-type and expected to comply with
relevant regulations and to be properly maintained, therefore, the potential
odour emission from RCVs and at the waste reception
area are assumed to be minimal.
Active
Tipping Face: After weighing, the RCVs
will be directed to the active tipping face for unloading. The operation at the active tipping face
will be similar to that of the existing SENT Landfill. Two platforms (ie
lower and upper platforms) will be used for separate unloading of MSW (at the
lower platform) and construction waste (at the upper platform). The construction waste will overlay the
MSW. The wastes will be promptly
spread by bulldozer and compacted by a landfill compactor to minimize the
exposure time of MSW thus minimise the opportunity of odour emission to the
atmosphere. The tipping face area
will be 30m x 40m ([10]).
After 11 pm, the Extension will be closed and the compacted waste will
be covered with 300mm of cover soil immediately. Therefore, odour emissions from the
active tipping face are expected during the operating hours; however, the
emissions will be much reduced thereafter.
Special
Waste Trench: A trench will be excavated into the
landfill mass for the disposal of waste that needs special handling. The trench will be located at least 50 m
from the active tipping face and the waste boundary. The trench will only operate when the
waste depth is at least 10m above the base to avoid damage to the leachate collection system. The size of the trench will vary in
accordance with the volume of special waste that has been pre-registered for
disposal by the special waste producers.
With reference to the operational experience at the existing SENT
Landfill and the quantity of special waste received, it is expected that the maximum
size of the trench will be about 6m x 2.5m. The trench will be open to receive
special waste from 9 am to 5 pm everyday.
After 5 pm, the trench will be backfilled with inert waste and covered
by 600 mm of soil and an impermeable liner to minimise odour emissions. Special waste for trench disposal is
normally required to be delivered in sealed bags and no odour will be generated
from the bagged waste. However,
odour will potentially be emitted from the side walls and the base of the
trench itself during operating hours.
In order to reduce the odour emission from the trench, the trench will
be covered by a movable cover with retractable or suitable opening so that the
trench is covered at all times except during waste deposition. The air trapped inside the trench will
be extracted and scrubbed by a mobile odour removal
unit prior to discharge to the atmosphere.
Therefore, the odour emitted from the trench
will be minimal. However, for the
worst-case assessment in this Study, it is assumed that the trench is open to
atmosphere without any odour removal.
Main
Haul Road to Active Tipping Face: The MSW will be delivered
in RCVs with enclosed compactor body. It is therefore anticipated that the
potential odour emission from the RCVs along the haul
road of the Extension will be minimal.
Daily
Covered Area: At the end of each working day (ie after 11 pm), the active tipping face will be covered
with 300 mm of soil and compacted.
Intermediate
Cover Area: Except for the active tipping face and
the final cover area (see below for details), all other areas of the Extension
will be covered with 600mm of soil ([11])
and an impermeable liner in
order to minimize rainwater infiltration into the waste and odour emission as
well as to enhance LFG extraction.
It is therefore anticipated that no odour will be emitted from this area
([12]).
Final
Cover Areas: After waste tipping reaches the final
levels, a capping system will be installed. The capping system will comprise (from
bottom to top) a soil layer, a non-woven geotextile,
an HDPE liner (impermeable layer), a sub-soil drainage layer and a final cover
soil layer. Permanent gas
extraction system will be installed to extract LFG from the waste mass. Planting will also be provided for the
final covered area. It is therefore
anticipated that no odour will be emitted from this area ([13]).
Operation of Leachate
Treatment Plant (LTP)
Leachate collected from the Extension and the
existing SENT Landfill will be pumped to the LTP in the new infrastructure
area. The LTP will consist of four
buffer storage tanks, two ammonia stripping towers, two thermal oxidisers
(i.e., one in operation and one standby), a stripped leachate
storage tank, two SBR tanks and a sludge holding tank. Except for the SBR tanks, all tanks will
be enclosed and the air exhaust from the tanks will be diverted to the thermal
oxidiser. The operation temperature
of the thermal oxidizer is about 850°C.
Odorous gas in the exhaust air (such as ammonia) will be oxidised and
destroyed at such high temperature ([14])
in the thermal oxidiser prior
to discharge to the atmosphere. The
SBR tanks will therefore be the only odour emission source in the LTP.
The dimension of each of the SBR tanks is
20m (width) x 35m (length). The
tank height is about 5m. The leachate temperature in the SBR will be maintained at about
40°C throughout
the year. The LTP will operate on a
24-hours per day basis.
Operation of LFG Treatment Facility
The LFG treatment facility will be
operated on a 24-hours per day basis.
The LFG collected from the LFG extraction system will be either diverted
to other utilization scheme for beneficial use or flared at the treatment
facility. The flaring temperature
is about 850°C and odorous compounds such as VOCs
or H2S in the LFG will be oxidised and destroyed at such
temperatures. Therefore, no odour
emission is expected from the LFG treatment facility.
Summary of Potential Odour Emission
Sources
As discussed above, the major potential
odour sources will include waste tipping activities at the active tipping face
and at the special waste trench as well as the operation of the LTP.
The odour emission sources during the
operation/ restoration phase are summarized in Table 4.5g.
Table 4.5g Summary
of Odour Emission Sources
|
Odour Emission Source |
Area |
Remarks |
|
During Operation Hour (8am – 12 midnight) |
||
|
Active tipping face for MSW +
construction waste |
30m x 20m |
·
From
8am to 11pm. Covering the active
tipping face after operation at 11pm to 12 midnight |
|
Active tipping face for construction
waste |
30m x 20m |
·
From
8am to 11pm. Covering the active
tipping face after operation at 11pm to 12 midnight |
|
Special waste trench |
6m x 2.5m (plan area exposed to air) (a) |
·
From
9am to 5pm. Covering the trench
at 5pm – 6pm |
|
After Operation Hour (12 midnight – 8am on the next day) |
||
|
Daily cover area |
30m x 40m |
12 midnight – 8am (on the next day) |
|
24-hour Operation |
||
|
SBR tanks of the LTP |
20m x 35m |
24 hours |
|
Note: (a)
Longer
side : 6m (l) x 2m (H); shorter side: 2.5m (l) x 2m (H); bottom: 6m x 2.5m |
||
4.5.3
Aftercare Phase
Upon completion of final filling and
capping, the aftercare phase will commence and is estimated to last for up to
30 years. The LFG and leachate management systems as well as the LFG generator
will continue to operate during the aftercare phase.
Operation of LTP
It should be
noted that once the landfill is restored, the leachate
generation rate from the Extension will be significantly reduced and hence the
average daily volume of leachate to be treated will
be reduced from about 350 m3 d-1 to 23 m3 d-1,
ie approximately an 93% reduction). With respect to the small volume of leachate generated, it will be able to reduce the nitrogen
levels in the leachate using biological treatment (ie, nitrification and denitrification)
so that the effluent will comply with the discharge standards. The operation of the ammonia strippers
and thermal oxidisers will not be necessary.
The vent gas from the enclosed leachate storage and treatment tanks will be diverted to an
air scrubber or the flares prior to discharge to the atmosphere. The designed odour (including ammonia
gas) removal efficiency of the air scrubber will be at least 95%. Therefore, majority of the odorous gas
in the vent gas from enclosed tanks will be removed. The scrubbed vent gas will be used as
part of the air intake for the aeration system of the SBR tank. If the vent is diverted to the flare(s)
as part of the air intake, the odorous gas will be destroyed at high combustion
temperature (at 850oC).
The potential source of odour emission
during the aftercare phase will only be the open SBR tanks (please refer to Table 4.6e for the odour emission rate
of the SBR tanks).
Operation of LFG Treatment Facility
Together with the final capping system,
the permanent LFG extraction system will prevent fugitive emission of LFG from
the restored landfill. The LFG
abstracted will be utilised or flared.
Under a high combustion temperature (850oC) at the flare, the
odorous VOCs in the LFG will be completely oxidised
and destroyed.
Conversely, the
total LFG generated from the restored SENT Landfill and the Extension will
increase (maximum yield of about 17,000 m3 hr-1). A worst case scenario has been assumed
where the two flares will be operated at full load (20,000 m3 hr-1). The emission inventory of flares is
summarized in Table 4.5h. The detailed calculation is summarized
in Annex A1.
LFG Generator
LFG generator will continue to provide
power supply for the operation of LFG Treatment Facility, LTP and other
facilities at the infrastructure area.
The emission inventory of the LFG generator is summarized in Table 4.5h and detailed calculations are
presented in Annex A1.
Table 4.5h Summary
of Gaseous Emission Inventory for the Flares and Generator During
Aftercare Phase (a) (b)
|
Parameter |
Flare |
LFG Generator |
|
No. of emission points |
2 |
1 (one duty and
one standby) |
|
Stack height (m) |
25 |
28 |
|
Stack diameter (m) |
3.8 |
0.305 |
|
Exhaust gas velocity (m s-1) |
12.24 |
48.6 |
|
Exhaust gas flowrate (m3
s-1) |
499,582 |
12,780 |
|
Exit temperature (°C) |
850 |
454 |
|
Emission limit for NOx
(c) |
11.28 mg m-3 |
0.14 lb mmBTU-1 |
|
Emission limit for CO (c) |
28.19 mg m-3 |
0.44 lb mmBTU-1 |
|
Emission limit for SO2 (c) |
1.55 mg m-3 |
0.045 lb mmBTU-1 |
|
Emission limit for benzene (c) |
2.98x10-3 mg
m-3 |
2.1x10-5
lb mmBTU-1 |
|
Emission limit for vinyl chloride (c) |
1.88x10-3 mg
m-3 |
1.6x10-6
lb mmBTU-1 |
|
Emission rate for NO2 (g s-1) (d) |
0.31 |
0.11 |
|
Emission rate for CO (g s-1) |
3.91 |
1.721 |
|
Emission rate for SO2 (g s-1) |
0.22 |
0.176 |
|
Emission rate for vinyl chloride (g s-1) |
4.14x10-4 |
8.22x10-5 |
|
Emission rate for benzene (g s-1) |
2.61x10-4 |
6.26x10-6 |
|
Notes: (a)
Detailed calculations are summarized in Annex A1. (b)
Reference to Table
4.5d (c)
All emission limits are under exhaust gas condition.
(d)
Assuming 20% of NOx
is NO2 |
||
4.5.4
Cumulative Impacts
According to the EIA Study Brief requirement,
major emission sources in the vicinity should be included to assess the
cumulative air quality impact.
Construction Phase
The operation of the existing SENT
Landfill (last year of operation) and the C&DM Handling Facility in TKO
Area 137 are identified as potential concurrent projects during the
construction phase of the Extension.
During the last year of the operation of
the existing SENT Landfill, most of the landfill area will be capped and
restored. Dust will be emitted from
the placement of cover materials, traffic movements on the unpaved haul roads
and traffic movements at the waste reception area (please refer to Figure
4.5a). As discussed in Section 4.5.3, due generation will be
minimised by implementation of dust control measures, including immediate
compaction of the fill area; regular damping down of the surface of the haul
road; provision of vehicle washing facility for RCVs
at the exit of the existing SENT Landfill (to ensure no significant dust will
be brought onto the public road); and regular cleaning of the main access road
and waste reception area by road sweeper.
The separation distance between the active
tipping area of the existing SENT Landfill and the dusty construction work area
of the Extension site is about 850m (refer to Figure 4.5a). As the worse wind angles which carry the
dust from dusty activity area of Extension and that for dust generated from the
active tipping area of the existing SENT Landfill are different, no cumulative
dust impacts are anticipated due to the operation of the existing SENT Landfill
and the construction of the Extension.
TKO Area 137 is currently planned for Deep
Water Front Industrial uses. A Construction
and Demolition Material (C&DM) Handling Facility is scheduled to be
commissioned in phases in TKO Area 137 (see Figure 3.9b) in 2009. The capacity of the C&DM Handling
Facility is about 20,000 tonnes per day.
However, the detailed design information is not available at this stage
but it is understood that the potential dust impacts associated with the
operation of the C&DM Handling Facility will be assessed as part of the
feasibility and engineering design of the facility. It is recommended that the cumulative
dust impact in the vicinity should be addressed in the environmental study
under that study. It is anticipated
that the facility will incorporate necessary dust control measures (as stipulated
in the Air Pollution Control
(Construction Dust) Regulations) in the design of the facility (which may
include enclosure of the dusty operations) and good site practices to control
dust emissions from the facility. It is expected that no adverse dust impact
will result from the operation of the facility.
As of the existing fill bank at TKO Area
137 will be decommissioned by the end of 2008, no cumulative dust impact will
be anticipated.
In summary, no cumulative dust impact is
anticipated during the construction of the Extension.
Operation/Restoration and Aftercare Phases
Odour Impact
When the Extension commences operation,
the existing SENT Landfill will be closed and will not generate odour. No other similar concurrent type of
odour source is identified within 500m of the Extension site boundary during
the operation/restoration and aftercare phases. Hence, no cumulative landfill odour
impact is expected.
Gaseous Emissions from the existing TKO
Industrial Estate
Within 500m from the Extension site
boundary, emissions from TVB City and HAESL may cause cumulative air quality
impact. On-site chimney survey
within the 500m area from the Extension site boundary was conducted in January
2008. Interviews were also conducted
to validate the stack operation and its emission inventory.
According to the information provided by
TVB City and the public information obtained from the EPD Regional Office
(East), the major gaseous emission sources identified at TVB City are the emergency
generators. As the emergency
generators will only operate when CLP’s grid is
suspended, the operating time of these generators is very limited and it will
not expected to cause cumulative air quality impact within the Study Area.
With reference to
the EIA Report of HAECO Aircraft Engine
Test Cell Facility at TKO, NO2, CO and SO2 are the
key air pollutants to be emitted during engine testing. These emission rates and stack
characteristics are summarized in Table
4.5i.
Table 4.5i Stack
and Emission Characteristics in Study Area (a)
|
Stack ID |
No. of Stacks |
Efflux Velocity (m s-1) |
Stack Diameter (m) |
Stack Height Above Ground
(m) |
Exit Temp. (°C) |
Emission Rate (g s-1) |
||
|
|
|
|
|
|
|
NO2 |
CO |
SO2 |
|
HAECO
/ HAESL (c) |
1 |
16.4
for NO2 and SO2; |
14.7 |
40 |
52 |
21.2 |
23.9 |
1.92 |
|
Notes: (a)
Reference to the EIA Report of HAECO Aircraft Engine Test Cell Facility at TKO. (b)
It is the equivalent diameter. The stack is in square shape with an
area of 13m x 13m. |
||||||||
The above stack characteristic, emission
inventory and engine type being tested at HAESL have been confirmed by HAESL.
The emissions of NO2, CO and SO2
from HAESL are included to assess the cumulative air quality impact during both
the operation/restoration and aftercare phases.
4.6
Assessment Methodology
4.6.1
Construction Phase
Dust will be generated from blasting,
materials handling, wind erosion, rock crushing and truck movements on paved
haul roads within the site. It
should be noted that no construction works will be carried out during blasting
due to the site constraint and safety reason. The dust impact from blasting will be
assessed individually.
TSP levels at
the identified ASRs were predicted using the Fugitive Dust Model (FDM). The 2006 meteorological data obtained
from the existing SENT Landfill weather station and TKO weather station
operated by the Hong Kong Observatory (HKO) were used for the model runs. Dust emission rates and associated
particle size distributions for the assessment were determined in accordance
with the Compilation of Air Pollutant
Emission Factors, AP-42, 5th Edition. One blast will be made each day and the
construction works would be carried out for 12 hours (from 7am to 7pm) per day
and 24 days per month. During
night-time (7pm to 7am on next day), only wind erosion of open fill area was
considered. Mitigation measures
recommended in Section 4.8.1 have
been considered in the dust emission rate estimation.
The mitigated TSP emission rates during
blasting, rock crushing, materials handling, wind erosion and truck movement on
unpaved haul road within the construction site are estimated and summarized in Table 4.6a and detailed calculations are
presented in Annex A2.
Table 4.6a Mitigated
Dust Emission Rates (a) (b) (c)
|
Construction Works |
Dust Generating Activities |
Dust Emission Rate |
Remarks |
|
Slope Cutting |
Blasting |
1.93 gs-1 |
· Blasting area = 1,000 m2
(estimated by the engineer) · 1 blast per day during daytime · Total no. of day = 107 days · Emission height = 0,5m |
|
Excavation |
Materials
Handling |
0.0103 gs-1 |
· Excavation period = 1.5 year · Total volume of soil excavated = 770,000
m3 · Hourly soil generation rate = 148.5 m3/hr · 50% dust removal efficiency by watering · Working time: between 7am and 7pm · Emission height: 0.5m |
|
|
Rock crushing |
· Crushing = 0.0098 gs-1 · Screening = 0.018 gs-1 |
· Rock to be crushed per day = 400m3
per day (max.) · Working time: between 7am and 7pm · Emission height = 5m |
|
|
Truck movement on
unpaved haul road |
0.00435 gm-2s-1 |
· Total no. of vehicle trip per hour = 70
(including return trip) · Average truck weight = 21.5 tonnes · 90% dust removal efficiency by watering
of main haul road, limiting vehicle speed and paving with aggregate/gravel · Working time: between 7am and 7pm · Emission height: 0.5m |
|
Filling |
Materials
Handling |
0.0054 gs-1 |
· Filling period = 1.5 year · Total volume of fill materials = 407,200
m3 · Hourly filling rate = 78.5 m3/hr · 50% dust removal efficiency by watering · Working time: between 7am and 7pm · Emission height: 0.5m |
|
|
Truck movement
unpaved haul road |
0.00156 gm-1s-1 |
· Total no. of vehicle trip per hour = 26
(including return trip) · Average truck weight = 20 tonnes · 90% dust removal efficiency by watering of
main haul road, limiting vehicle speed and paving with aggregate/gravel · Only carried out during daytime between
7am and 7pm |
|
|
Wind erosion |
§ Daytime : 1.35x10-6 gm-2s-1 § Night-time : 2.7x10-6 gm-2s-1 |
· Total area = about 15 hectare · 50% dust removal efficiency by watering
during daytime and no dust reduction at night-time · 24-hour |
|
Notes: (a)
Detailed
calculations and location of the dust emission sources are presented in Annex A2. (b)
Dust
emission factors in Compilation of Air Pollutant
Emission factors, (AP-42), 5th Edition by USEPA is used. (c)
Dust
control measures recommended in Section
4.8.2 have been adopted. |
|||
Hourly and daily TSP concentrations were
predicted at 1.5m and 10m above ground of the representative ASRs A1 to A4, A7
and A8 which are located within 500m of the Extension site boundary as the dust
impact is localized. Daily TSP
concentrations predicted from blasting, construction works and night-time wind
erosion activities will be directly added to obtain an overall daily TSP
concentration at the ASRs. The
background TSP concentration, as presented in Table 4.3a, was also used to assess the cumulative TSP
concentrations.
4.6.2
Cumulative Gaseous Emissions During
Operation/Restoration and Aftercare Phases
An EPD approved air dispersion model,
ISCST3, was employed for the assessment.
The 2006 meteorological data obtained from the existing SENT Landfill
weather station and TKO weather station operated by the Hong Kong Observatory
(HKO) were used for the model runs.
The “rural” mode was used. Terrain effects within 500m of the
Extension site boundary have been included.
The emission
rates of NO2, CO, SO2, benzene and vinyl chloride from the
operation of the LFG treatment facility, LTP and LFG generator during
operation/restoration and aftercare phases, presented in Tables 4.5d and 4.5h,
respectively, were used for the prediction. The thermal oxidiser, LFG flares and LFG
generator will be operated 24 hours per day. It is conservatively assumed that the
engine testing at HASEL will be carried on a 24-hour basis. The locations of the LFG treatment facility,
LTP, LFG generator and the emission points at HASEL are shown in Figure
4.5b.
The hourly, daily and annual average
concentrations of the key air pollutants were predicted at 1.5m to 30m above
ground at the representative ASRs A1 to A4, A7 and A8 as the maximum height of
these ASRs is 30 m above ground.
The worst affected height was identified and isopleths showing the
levels of these key air pollutants at 1.5m above ground and the worst affected
height were plotted.
Background concentrations presented in Table 4.3a were included in the assessment
of the cumulative air quality impact.
4.6.3
Odour Emissions from Waste Filling
Activities and Operation of LTP During Operation/Restoration Phase
Selection of Emission Source Locations for
Worst Cases
The operation/restoration of the Extension
will be divided into six phases starting from the south and filling
progressively to the north (see Figures 4.6a-1 and 4.6a-2) in general. Three worst-cases (Cases 1 – 3) in each
phase (except Phase 6) have been identified for the odour impact assessment,
which have taken into account the worst case odour impacts to existing ASRs in
TKOIE (eg TVB City), planned ASRs in the TKO Area
137, and ASRs at higher elevations (eg LOHAS Park
(ASR A20)). Odour emission
inventory including type of source, source area, source height, duration and
the temperature of each worst-case are summarized in Table 4.6b.
Table 4.6b Odour
Emission Inventory in Each Worst Case
|
Source Height |
Worst-case Scenario (b) |
Odour Source |
Area |
Air Temperature of Odour
Emission (a) |
|
Phase 1 · 10m above ground Phase 2 · 30m above ground Phase 3 · 50m above ground Phase 4 · 70m above ground Phase 5 · 100m above ground Phase 6 · 130m above ground |
In each phase, there will
be 3 worst cases (b): ·
Case
1 ·
Case
2 ·
Case
3 |
During Extension
Opening Hours (8am – 12 midnight) |
||
|
Active tipping face for MSW + construction waste |
· 30m x 20m |
30°C |
||
|
Active tipping face for construction waste |
· 30m x 20m |
30°C |
||
|
Special waste trench |
· 6m x 2.5m (plan area exposed to air) |
30°C |
||
|
After Extension Opening
Hours (12 midnight – 8am on the next day) |
||||
|
Daily cover area |
· 30m x 40m |
30°C |
||
|
24-hour Operation |
||||
|
SBR tanks |
· 35m x 20m |
30°C |
||
|
Notes: (a)
Reference
to the sensitivity analysis summarized in Annex
A3. (b)
For
Phase 6, since the waste tipping area is small, therefore, 2 worst cases are
assumed. |
||||
Reasonable Worst-case Odour Modelling
Parameter
Odour Sampling at SENT Landfill
Odour generated from landfill operation varies
from landfill site to landfill site; no general odour emission rates for landfilling activities are available. Odour samples were taken from the
existing SENT Landfill for olfactometry analysis by
the Odour Research Laboratory of the Hong Kong Polytechnic University to
establish a set of odour emission rates for this study.
Sampling
Time and Locations: Measurements
were taken between 9:00 am and 9:30 pm at four locations (see Table 4.6c). The sampling locations and the ambient
temperature during sampling are presented in Table 4.6c.
Table 4.6c Odour
Sampling Regime
|
Location |
Sampling ID |
Ambient Temperature (°C) |
|
MSW + Construction Waste
(S1) |
S1-1 |
30.83 |
|
|
S1-2 |
31.45 |
|
|
S1-3 |
26.01 |
|
|
S1-4 |
23.03 |
|
|
S1-5 |
20.85 |
|
|
S1-6 |
30.05 |
|
MSW + Construction Waste +
300mm Soil Cover (S2) |
S2-1 |
30.97 |
|
|
S2-2 |
31.58 |
|
|
S2-3 |
26.16 |
|
|
S2-4 |
29.55 |
|
Special Waste Trench (S3) |
S3-1 |
27.00 |
|
|
S3-2 |
26.47 |
|
SBR of LTP (S4) |
S4-1 |
26.90 |
As the existing SENT Landfill receives
MSW, construction waste, special wastes as well as dewatered sludge from sewage
treatment works (STWs), the sampling locations for S1
and S2 were therefore selected away the existing active tipping face and at the
upwind location to avoid potential odour contamination. A new tipping platform was formed at the
sampling location and MSW and construction waste were disposed of using the
normal practices. The ratio of MSW
to construction waste disposed was the same as that predicted for the Extension
(ie the ratio of MSW to construction waste is about 1
: 2). For S2, the compacted MSW and
construction waste was covered with 300mm of cover soil. The odour samples for S3 and S4 were
taken at the base of the special waste trench and at the water surface of the
SBR tank of the SENT Bioplant, respectively.
The odour emission from construction waste
is very low and on a conservative basis, it is assumed that the odour emission
rate from construction waste tipping is the same as that for S2.
It should be noted that the existing SENT
Landfill also receives sewage sludge; therefore, the odour emission rate
measured at the trench will be much higher than that expected for the
Extension. Adopting the measured
odour emission rates measured at the existing special waste trench in the
assessment is a conservative approach.
Odour
Sampling and Analysis Methods: Odour samples were taken using the flux
chamber method which is the method recommended by the USEPA ([15])
and is also the most commonly
used odour sampling method for large surface emission source such as landfill
sites. The flux chamber used is a
circular chamber with a diameter of 0.41m and an area of 0.13 m2. It was tightly placed on the surface of
the odour source and the air inside the chamber was purged with nitrogen gas at
a sweeping rate of 5 litres per minute.
The odour sample was collected in a Tedlar bag
at a rate of 3 litres per minute.
Before taking the next sample, the flux chamber was cleaned with
distilled water and then flushed with nitrogen for about 10 minutes to remove
residual odour in the chamber. The
sampling system and the flux chamber are shown in Figure 4.6b.
The odour
samples were analysed within 24 hours of the sampling using the olfactometry method by the Odour Research Laboratory of the
Hong Kong Polytechnic University.
The odour concentration of the samples, measured in Odour Units (OU) per
m3, was determined by a Forced-choice Dynamic Olfactometer
in accordance with the European Standard Method EN 13725.
Odour emission rate was then calculated
using the following equation:
Odour
Sampling Results: The measured odour concentrations and
calculated odour emission rates of each odour source are summarized in Table 4.6d.
Table 4.6d Odour
Sampling Results
|
Location |
Sampling ID |
Onsite Ambient
Temperature During Sampling (°C) |
Measured Odour
Concentration (OU/m3) |
Odour Emission Rate (OU/m2/s) |
|
MSW + Construction Waste (S1) |
S1-1 |
30.83 |
1,092 |
0.70 |
|
S1-2 |
31.45 |
1,738 |
1.11 |
|
|
S1-3 |
26.01 |
1,521 |
0.98 |
|
|
S1-4 |
23.03 |
1,296 |
0.83 |
|
|
S1-5 |
20.85 |
264 |
0.17 |
|
|
S1-6 |
30.05 |
1,579 |
1.01 |
|
|
MSW + Construction Waste + 300mm Soil Cover (S2) |
S2-1 |
30.97 |
80 |
0.051 |
|
S2-2 |
31.58 |
160 |
0.10 |
|
|
S2-3 |
26.16 |
169 |
0.11 |
|
|
S2-4 |
29.55 |
193 |
0.12 |
|
|
Special Waste Trench (S3) |
S3-1 |
27.00 |
10,768 |
6.90 |
|
S3-2 |
26.47 |
16,830 |
10.79 |
|
|
SBR of LTP (S4) |
S4-1 |
26.90 |
76 |
0.049 |
Definition of a Reasonable Worst-case
Odour Modelling Parameters
A sensitivity analysis was undertaken to determine
a reasonable worst-case scenario for the odour assessment. Details of the sensitivity analysis can
be found in Annex A3. The analysis shows that the reasonable
worst-case ambient temperature for estimating the odour emission rate is 30°C.
Therefore, the reasonable worst-case odour emission rates at this
temperature are summarized in Table 4.6e.
Table 4.6e Reasonable
Worst-case Odour Emission Rates Adopted in Odour Impact Assessment
|
Odour Source |
Source Area |
Odour Emission Rates at 30°C (OU/m2/s) (a) |
Total Odour Emission (OU/s) |
|
During Extension Opening Hours (8am – 12
midnight) |
|||
|
Active tipping face for MSW +
Construction Waste |
30m x 20m |
0.94 |
564 |
|
Active tipping face for construction
waste |
30m x 20m |
0.12 |
72 |
|
Special waste trench |
6m x 2.5m (plan area exposed to air) |
31.74 (c) |
476 |
|
After Extension Opening Hours (12
midnight – 8am on the next day) |
|||
|
Daily cover area (b) |
30m x 40m |
0.12 |
144 |
|
24-hour Operation |
|
|
|
|
SBR tanks |
2 number of |
0.049 |
69 |
|
Notes: (a)
Reference
to Annex A3. (b)
Total
area of active tipping face. (c)
Reference
to Annex A3 for the adjustment of
the odour emission rate at 30°C. |
|||
Air Dispersion Model and Worst-case Odour Modelling
Parameters
The AUSPLUME model, developed by the
Australian Government (Environmental Protection Agency, Victoria), was employed
for the odour impact assessment.
The use of the AUSPLUME model has been approved by the EPD.
As discussed in Annex A3, the modelling parameters are summarized in Table 4.6f.
Table 4.6f Worst-case
Odour Modelling Parameters
|
Modelling Parameter |
Setting |
|
Surface roughness |
·
120
cm |
|
Meteorological data |
·
2006
hourly SENT landfill weather data : wind speed, wind direction and air
temperature ·
2006
HKO TKO weather data : stability class ·
2006
HKO King’s Park weather data : mixing height ·
90%
of data are valid |
|
Terrain effect |
·
Terrain
data within 500m from the Extension site boundary have been included in model ·
“Egan
half height” option is selected |
|
Type of odour source in
model |
·
Area
source : active tipping faces for MSW and construction waste, daily cover area
at night-time and special waste trench ·
Point
source : SBR tanks (with very low exit velocity of 0.001 m s-1) as
the leachate temperature is slightly higher than
ambient |
Also, odour management and control
measures summarized in Table 4.8a have
been considered in the worst-case assessment.
Assessment Height and Presentation of
Predicted Results
5-second odour concentrations were
modelled at 1.5m, 10m, 20m, 30m, 50m, 70m and 90m ([16])
above ground level at the identified
ASRs and the worst affected heights under different worst-case scenarios are
also identified in the assessment.
Contours of the predicted odour concentrations at the worst affected
height within the Study Area (500m from the Extension site boundary) under
different scenarios were plotted.
Conversion of Modelled Results From
3-minute Averaging Time to 5-second Averaging Time
Under the EIAO-TM, the odour assessment criterion is defined as 5 OU under a
5-second averaging time. To convert
the AUSPLUME output (presented as the maximum 3-minute mean concentration) to a
maximum 5-second mean concentration, the approach suggested by the Warren
Spring Laboratory (WSL) ([17])
was adopted:
“Typical
maximum or peak 5-second average concentrations within any 3-minute period
appear to be of the order of 5 times the 3-minute average. During very unstable conditions larger
ratios, perhaps 10:1, are more appropriate…..”
It should be
noted that the ratios provided in the WSL report refer to peak to mean
concentrations for emissions from stacks.
Emissions from low-level area sources will fluctuate less and therefore
the peak to mean ratios will be lower.
The use of the peak to mean ratios provided in the WSL report therefore
provides a conservative estimate for the 5-second mean concentrations for area
sources.
For stable conditions (stability classes C
to F), a factor of 5 was applied whilst for unstable conditions (stability
classes A and B) a factor of 10 was applied to the emission rates input in the
model run. The modelled results
will be the 5-second odour concentrations.
The factored odour emission rates are
presented in Table 4.6g. These odour emission rates applied to
the three worst-cases described in Table
4.6b. An example showing hourly
emission rate file adopted in AUSPLUME model is presented in Annex A4.
Table 4.6g Odour
Emission Rates for AUSPLUME Model Run
|
Modelling
Period |
Odour
Emission Source |
Area Size in Model |
Air Temperature of Odour Emission |
Factored Odour Emission Rate to be used in Model Run to
obtain 5-second Results |
|
|
|
|
|
Stability Class A & B (c) |
Stability Class C to F (d) |
|
|
During Operation (8am – 12 midnight) |
Active tipping face for MSW + Construction Waste |
30m x 20m |
30°C |
9.4 |
4.7 |
|
Active tipping face for construction Waste (a) |
30m x 20m |
30°C |
1.2 |
0.6 |
|
|
Special Waste Trench |
6m x 2.5m (plan area exposed to
air) |
30°C |
317.41 |
158.7 |
|
|
Night-time (Midnight to 8am on the next day) |
Daily Cover Area (a) |
30m x 40m |
30°C |
1.2 |
0.6 |
|
24-hour Operation |
2 numbers of SBR tanks |
20m x 35m (each) |
30°C |
343 (OU s-1) |
171.5 (OU s-1) |
|
Notes: (a) The
odour emission rates of daily cover area at night and the active tipping face
for construction waste during operation are similar due to the odour nature
of the ground is the same. (b) Reference
to Tables 4.6b and 4.6e for original odour emission at 30°C. (c) | |||||