11                Landfill Gas

 

11.1          Landfill Gas Parameters

 

11.1.1      Although a comprehensive landfill gas management system has been constructed under the Restoration Contract for Urban Landfills, Tseung Kwan O Stage I Landfill and Tseung Kwan O Stage II & III Landfill, the efficiency of the system has not been demonstrated at this time.  Landfill gas monitoring should be carried out to identify any migration between the landfills and the proposed development and to ensure the safety of the Contractor’s personnel.

11.1.2      Certain information, such as the proposed routing of utilities in relation to the sensitive elements of the development should be determined at the detailed design stage.  A review of the qualitative source-pathway-target analysis as presented in the EIA report is considered necessary if there are substantial changes of the proposed development.  Monitoring should be carried out and reported in a similar manner to that for the Restoration Contract to provide comparable data.  An example of landfill gas monitoring data form as used in the Restoration Contract is presented in Appendix B.  The presentation format for landfill gas monitoring should be based on this format and agreed with EPD.

11.2          Monitoring Equipment

 

11.2.1      Intrinsically safe, portable gas detectors should be used when working in any confined space, which has the potential for the presence of landfill gas, and the risk of explosion or asphyxiation.  The monitoring equipment should alarm (both audibly and visually) in the event that the concentrations of the following are exceeded:

·       Methane – higher than 10 % LEL;

·       Carbon dioxide – higher than 0.5 %; and

·       Oxygen – lower than 18 % by volume.

11.2.2      Pre-entry and routine monitoring should be conducted at utilities’ manholes and chambers.  The routine monitoring should be conducted on a regular basis and the frequency of the monitoring would be subject to the results of the initial monitoring exercise.  The landfill gas monitoring instrument should:

·       Comply with the EPD’s Landfill Gas Hazard Assessment – Guidance Note as intrinsically safe;

·       Be capable of continuous monitoring of methane, oxygen and carbon dioxide;

·       Be capable of continuous barometric pressure and gas pressure measurement;

·       Normally operate in diffusion mode unless required for spot sampling, when it shall be capable of operating by means of an aspirator or pump;

·       Have low battery, fault and over range indication incorporated;

·       Store monitoring date, and shall be capable of being down-loaded directly to a PC; and

·       Measure in the following ranges:

­          Methane 0-100% LEL & 0-100% v/v;

­          Oxygen 0-25% v/v;

­          Carbon dioxide 0-100% v/v;

­          Barometric pressure mBar (absolute);

­          Gas pressure (relative to atmosphere) Pascals; and

­          Temperature 0-100oC.

 

11.2.3      The equipment used to monitor LFG concentrations in excavations may be the same as that described for routine landfill gas monitoring of manholes in the section above, as long as safety parameters described above can be achieved.

11.3          Monitoring Locations

 

Monitoring of Construction Works

 

11.3.1      During construction activities within 250m Consultation Zone of Sai Tso Wan Landfill, Tseung Kwan O Stage I Landfill and Tseung Kwan O Stage II & III Landfill, it is recommended that pre-entry monitoring should be undertaken, where applicable, in accordance with the requirements of the Factories and Industrial Undertaking (Confined Spaces) Regulation.  For excavations of 1m depth or more, landfill gas should be monitored before entry and periodically during the works.  If drilling is required, the procedures for safety management and working procedures described in the EPD’s Landfill Gas Hazard Assessment – Guidance Note should be adopted.

Operational Landfill Gas Monitoring

 

11.3.2      During operation, when service voids, manholes or inspection chambers within the proposed site are entered for maintenance, monitoring and a checklist system of safety requirements should be performed before entry in accordance with the Code of Practice on Safety and Health at Work in Confined Spaces.

11.3.3      The maintenance companies should undertake a landfill gas surveillance exercise at the utility manholes/inspection chambers.  The surveillance exercise should be undertaken using an intrinsically safe portable instrument, appropriately calibrated and capable of measuring the requirements as mentioned in section 11.2.2.  The monitoring should be undertaken for the duration of the site occupancy, or until such time that EPD agrees that surveillance is no longer required.  Depending on the results of the measurements, actions required will vary and should be set down by appropriately qualified person.  As a minimum these should encompass those actions specified in Table 11.1.

11.4          Limit Levels and Event and Action Plan (EAP) for Landfill Gas

 

11.4.1      The Limit levels and relevant Action Plans for landfill gas detected in utilities and any on-site areas following construction are shown in Table 11.1 below.

 

 

 

Table 11.1       Limit Levels and Action Plan for Landfill Gas

 

Parameter

Limit Level

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)

·         Post “No Smoking” signs

·         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%

11.5          Mitigation Measures

Construction Phase

 

11.5.1      The EIA report has recommended specific mitigation measures to be incorporated in the detailed design.  During construction phase, the following precautionary measures should be implemented:

·       The Contractor should take special precautions regarding the possible presence of landfill gas.  The Contractor should ensure that the site workers are aware of potential hazards of working in confined spaces such as manholes and service chambers, and that appropriate monitoring procedures as per the Code of Practice on Safety and Health at Work in Confined Spaces (Labour Department, Hong Kong) are in place to prevent hazards due to asphyxiating atmospheres in confined spaces;

·       All relevant construction workers should undergo training about the risks and indications of landfill gas and should be thoroughly skilled in first aid and emergency and evacuation techniques;

·       A no smoking policy should be strictly adopted.  The possibility of methane rich air being taken into diesel engine plant should not be overlooked, although this is unlikely to occur;

·       Welding, flame cutting or other hot works should only be carried out in trenches or confined area when controlled by a ‘permit to work’ procedure, properly authorised by the Safety Officer.

·       Ground level construction plant should be fitted with vertical exhausts at least 0.6m above ground level and with spark arrestors. 


·       During piping assembly or conduiting construction at underground or at ground level, all valves/seals should be closed immediately after installation.  As construction progresses, all valves/seals should be closed as installed to prevent the migration of gases through the pipeline/conduit.  All piping/conduiting should be capped at the end of each working day.

·       Mobile offices, equipment stores, mess rooms etc. should be located on an area which has been proven to be gas free and on-going monitoring should be carried out to ensure that these areas remain gas free.  Alternatively, such buildings should be raised clear of ground with a minimum clear separation distance (as measured from the highest point on the ground surface to the underside of the lowest floor joist) of 500mm.  However, in this case, it is highly recommended that all the site offices, equipment stores and mess rooms should be located outside the 250m LFG consultation zone.

·       As a minimum, no work should be undertaken in the absence of fire extinguishers;

·       Adequate fire extinguishing equipment, fire-resistant clothing and breathing apparatus sets should be made available on site;

·       The monitoring frequency and areas to be monitored should be specified prior to commencement of ground-works, either by the Safety Officer, or by an appropriately qualified person.  The minimum monitoring frequency would be twice a day (i.e. the morning before site works commence and after the lunch break) during all excavation works below ground level and should always be undertaken by appropriately qualified personnel.  All measurements should be recorded and included in the site diary. 

·       For excavations deeper than 1 m, measurements should be carried out:

­          at the ground surface before excavation commences;

­          immediately before any worker enters the excavation;

­          at the beginning of each half working day (i.e. morning and afternoon) for the entire period the excavation remains open; and

­          periodically through the working day whilst workers are in the excavation.

·       For excavations between 300 mm and 1 m deep, measurements should be carried out:

­          directly after the excavation has been completed; and

­          periodically whilst the excavation remains open.

·       Depending on the results of the measurements, actions required will vary and should be set down by the Safety Officer of the Contractor or other appropriately qualified person.  As a minimum these should encompass those actions specified in Table 11.1.

·       Utility companies should be advised of the proposed development being in the vicinity of the TKO Stage I & II/III Landfill and Sai Tso Wan Landfill for their attention to observe the relevant regulations and requirements in such regards when inspection or works are being undertaken at the nearby area. 


Operational Phase

 

11.5.2      The EIA report has recommended the following specific mitigation measures during the operational phase:

·       As this project only includes road works or drainage works with no involvement of any building structure, the main precautionary measures during the operational phase will be mainly during the inspection or maintenance of the utilities services at the area within the 250m LFG Consultation Zone.

·       An assumed presence of landfill gas should be adopted at all times by maintenance workers.  All maintenance workers inspecting any manhole should be fully trained in the issue of landfill gas hazard.  Any manhole which is large enough to permit to access to personnel should be subject to entry safety procedure.  Such work in confined spaces should be controlled by the Factories and Industrial Undertakings (Confined Spaces) Regulations of the Factories and Industrial Undertakings Ordinance.  Following the Code of Practice on Safety and Health at Work in Confined Spaces (Labour Department, Hong Kong) ensures compliance with the above regulations.  A strictly regulated “work permit procedure” should be implemented and the relevant safety procedures must be rigidly followed.  Adequate communication with maintenance staff should be maintained with respect to landfill gas hazard.

·       The utility companies should undertake a landfill gas surveillance exercise at the utility manholes/inspection chambers.  The surveillance exercise has been described in section 11.3.3.

 

11.5.3      The implementation schedule for the mitigation measures is presented in Appendix C.