8
LANDFILL GAS HAZARD
8.1
Introduction
8.1.1.1
The landfill gas hazard
assessment undertaken in the EIA Study indentified the hazards that are likely
to be generated from the WENT Landfill and WENT Landfill extension, during the
construction and operation phases of this Project and evaluate the associated
risk. The EIA Report recommended that some precautionary measures would
be required to protect the proposed STF from the landfill gas risk due to WENT
Landfill and WENT Landfill extension. Use of “semi active” or “enhanced
passive” gas control protection system referenced from the EPD’s Landfill
Gas Hazard Assessment Guidance Note were recommended. Regular
monitoring during construction and operation phases was also recommended.
8.2
Monitoring and Measurement of Landfill Gas
8.2.1.1
During construction,
routine monitoring should be carried out in all excavations, manholes,
chambers, relocation of monitoring wells and any other confined spaces that may
have been created. All measurements in excavations should be made with
the extended monitoring tube located not more than 10 mm from the exposed
ground surface. Monitoring should be performed properly to make sure that
the area is free of landfill gas before any man enters in the area.
8.2.1.2
For excavations deeper
than 1m, measurements should be carried out:
·
at the ground surface
before excavation commences;
·
immediately before any
worker enters the excavation;
·
at the beginning of each working day
for the entire period the excavation remains open; and
·
periodically through the working day whilst
workers are in the excavation.
8.2.1.3
For excavations between
300mm and 1m deep, measurements should be carried out:
·
directly after the
excavation has been completed; and
·
periodically whilst the excavation remains open.
8.2.1.4
For excavations less
than 300mm and 1m deep, monitoring may be omitted, at the discretion of the
Safety Officer or other appropriately qualified person.
8.2.1.5
It was also recommended that several landfill
gas monitoring wells be installed into the ground on the development side of
the gas barrier. These would be used to measure the concentrations of CH4
and CO2 within the ground and hence determine the effectiveness of
the measures in preventing LFG migration.
8.2.1.6
Depending on the
results of the measurements, actions required will vary and should be set down
by the Safety Officer or other appropriately qualified person. The
actions shown in Table 8.1 should be referred as the minimum
requirements to be encompassed.
Table 8.1 Actions to be Encompassed
Parameter
|
Measurement
|
Action
|
|
< 19 %
|
|
|
< 18 %
|
·
Stop works
·
Evacuate personnel/prohibit
entry
·
Increase ventilation to
restore oxygen to > 19 %
|
Methane
|
> 10 % LEL (i.e. > 0.5 % by volume)
|
·
Prohibit hot works
·
Ventilate to restore methane
to < 10% LEL
|
|
|
·
Stop works
·
Evacuate personnel/prohibit
entry
·
Increase ventilation to
restore methane to < 10 % LEL
|
Carbon Dioxide
|
> 0.5 %
|
· Ventilate to restore carbon dioxide to
< 0.5%
|
> 1.5 %
|
·
Stop works
·
Evacuate personnel/prohibit
entry
·
Increase ventilation to
restore carbon dioxide to <0.5%
|
8.2.1.7
During operation,
regular monitoring of landfill gas should be done at the monitoring wells
mentioned in Section 8.2.1.5 above as well as underground service
voids and manholes by the STF contractor. Monitoring would be
required to verify the effectiveness and to ensure the continued performance of
the implemented protection measures.