A3
Definition
of a Reasonable Worst-case Odour Scenario
A3.1
Introduction
A sensitivity test and further analysis were
performed to identify a set of assumptions which together form a reasonable
worst case scenario for the odour impact assessment. The following sections present the
assumption, modelling parameters and test results.
A3.2
Assumptions in Sensitivity Test
A3.2.1
Odour
Emission Rates
Temperature is one of the factors which
are thought to affect the strength of the odour emission. In the odour impact assessment (see Section 4.6.3), the odour emission rates
of a reasonable high temperature were used in order to represent a reasonable
worst case scenario. Our starting
assumption was that 30°C was a reasonable worst case and
sensitivity analyses were performed to test this.
By reviewing the SENT landfill meteorological
data recorded in 2006, the number of hours having the ambient temperature lower
than 30°C was about 90% of the year. Odour samples were taken at around 30°C in many instances. The odour emission rates of each odour
source at 30°C are summarized in Table A3.1.
Table A3.1 Odour
Emission Rates at 30°C
|
Odour
Source |
Odour
Emission Rate at 30°C
(OUm-2s-1) |
Source
Area in Model |
Operation
Time |
Remarks |
|
Tipping face for MSW + construction
waste |
0.94 |
30m x 20m |
08:00 – 12midnight |
Average of measured odour
emission rates (0.7 + 1.01+1.11)/3 = 0.94 |
|
Tipping face
for construction waste / daily cover area |
0.12 |
30m x 20m |
08:00 –
12midnight |
Measured odour
emission rate at 29.55°C |
|
Special waste
trench (trench itself) |
31.74 |
6m x 2.5m (plan
area exposed to air) |
09:00 – 18:00 |
Adjusted from
26.7°C to 30°C (a) |
|
SBR tank |
0.049 |
20m x 35m |
24 hours |
Measured odour
emission rate (b) |
|
Notes: (a)
Similar approach for adjusting the emission rate
adopted in the approved EIA Report for Agreement
No. CE 20/2004 (EP) (b)
As the temperature of the leachate
in the SBR tanks is kept at 40°C at any time,
therefore, it will not be varied by the ambient temperature. |
||||
Of note, the measured odour emission rate
of special waste trench at the existing SENT Landfill is high as sludge is
being disposed in the existing SENT Landfill. The Extension will not accept sludge and
hence the odour emission of special waste trench during the Extension should be
much lower. Also, in the outline
design, the trench will be covered by a movable cover with retractable or
suitable opening for unloading of special waste. The air trapped inside the trench will
be extracted and scrubbed prior to discharge to the atmosphere. Therefore, the odour emission rate of
special waste trench adopted in the assessment is conservative. The odour emission from special waste
trench of the Extension is expected to be much lower than that adopted in the
assessment.
The source height of each odour source
except SBR tanks was assumed to be 10mPD.
The source height of the SBR is 5m above ground level where the ground
level is 6mPD.
Three emission locations (Cases 1 to 3)
are selected, as shown in Figure A3.1,
for the prediction of the odour impacts at TKO Area 137 and
Figure A3.1 Location
of Odour Emission Sources
A3.2.2
Modelling
Parameters
The following assumptions are adopted for
the prediction of odour impact:
·
AUSPLUME
model used;
·
Surface
roughness is assumed to be 120cm (assuming
TKO Area 137 is occupied by either fill bank or industrial premises)
·
Hourly
meteorological data in 2006
-
SENT landfill data : wind speed, wind direction and air
temperature
-
Hong Kong Observatory data: stability class recorded at TKO station and
mixing height recorded at King’s Park station (as it is the only station to
record mixing height)
·
Terrain
data included and “Egan Half Height” option selected;
·
Conversion
factor to convert the predicted 3-minute results to 5-second results
-
Factors
of 10 and 5 applied to stability classes A-B and C-F, respectively
A3.2.3
Assessment
Points
ASRs, A1-1, A1-2 (1) (TKO Area 137 at
south of Extension), A1-3 (1) (south of TVB) and A2 (
A3.3
Sensitivity Test Results
The highest predicted 5-second odour
levels at the selected four assessment points are presented in Tables A3.2. The worst hours of the highest predicted
results and the air temperature at that worst hours record at SENT Landfill
onsite weather station are also presented.
Table A3.2 Predicted
Maximum 5-second Odour Level & Corresponding Air Temperature for 2006 at
A1-1, A1-2 (1), A1-3 (1) and A2 (1.5m above ground)
|
|
A1-1 |
A1-2
(1) |
A1-3
(1) |
A2 |
|
Case 1 |
||||
|
Highest 5-sec Odour Level |
6.1
(a) |
7.9 |
2.4 |
1.9 |
|
Worst Hour |
27/11/06, hr 13 |
28/05/06, hr19 |
18/07/06, hr 23 |
11/07/06, hr 20 |
|
Air Temperature (°C) |
28 |
25 |
28 |
29 |
|
Case 2 |
||||
|
Highest 5-sec Odour Level |
6.3 |
2.0 |
5.5 |
4.1 |
|
Worst Hour |
16/11/06, hr 20 |
22/11/06, hr 19 |
10/02/06, hr18 |
18/07/06, hr23 |
|
Air Temperature (°C) |
24 |
22 |
18 |
28 |
|
Case 3 |
||||
|
Highest 5-sec Odour Level |
3.2 |
1.9 |
23.2 |
12.5 |
|
Worst Hour |
06/04/06, hr 22 |
22/11/06, hr 19 |
18/11/06, hr 19 |
12/08/06, hr 19 |
|
Air Temperature (°C) |
22 |
22 |
24 |
30 |
|
Note: (a)
Figures in bold
and underline represents the exceedance of 5-sec odour criterion (5OU). |
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The results indicate that the air
temperatures of the maximum predicted odour levels at the four selected
assessment points in each case are equal to or less than 30°C.
Meteorology is a decisive factor affecting the impacts to these ASRs.
A3.4
Effect of Odour Levels at Air Temperature at or Above
30°C
In order to further assess the basic of
the reasonable worst case scenario, a further analysis of the odour levels at
the four assessment points using odour emission rates corresponding to a
temperature range of 30°C to 38°C was performed as the highest air
temperature recorded at SENT Landfill weather station in 2006 is 38°C.
Approach for adjusting odour emission
rates up to 38°C follows the approach adopted in the approved EIA
for Agreement No. CE
20/2004 (EP) North East New Territories Landfill Extension (EIA 133/2007). A percentage increase of odour emission
established by tipping face for construction waste (2.682% per 1°C)
is used. The adjusted odour
emission rates at 30°C
to 38°C are
summarized in Table A3.3 and the
detailed calculation is shown in Table
A3.4.
Table A3.3 Adjusted
Odour Emission Rates
|
Odour
Source |
Odour
Emission Rate (OU m-2 s-1) |
||||||||
|
|
30°C |
31°C |
32°C |
33°C |
34°C |
35°C |
36°C |
37°C |
38°C |
|
MSW + construction waste
tipping |
0.94 |
0.97 |
0.99 |
1.02 |
1.04 |
1.07 |
1.10 |
1.13 |
1.16 |
|
Construction waste
tipping / Daily cover area |
0.12 |
0.123 |
0.127 |
0.13 |
0.133 |
0.137 |
0.141 |
0.144 |
0.148 |
|
Special waste
trench |
31.74 |
32.59 |
33.47 |
34.36 |
35.29 |
36.23 |
37.20 |
38.20 |
39.22 |
|
Note: (a)
The odour emission rate is estimated by liner extrapolation
of odour strength measured at tipping face for construction waste from 26.11°C to 29.55°C (2.682% of odour
emission rate increased per 1 degree increase in temperature) (Also, refer to footnote (a) of Table A1.1
for the adjustment). |
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The
model was rerun to simulate odour dispersion for these hours at which those
higher temperatures were reported. The results are summarized in Tables A3.5 below.
