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

Oxygen

< 19 %

·          Ventilate to restore oxygen to > 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

 

> 20 % LEL (i.e. > 1 % by volume)

·          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.