6.1 Although a comprehensive landfill gas management system has been constructed under the Restoration Contract for Shuen Wan Landfill, the efficiency of the system has not been demonstrated at this time. Landfill gas monitoring shall be carried out to identify any migration between the landfill and the site and to ensure the safety of the Contractor’s personnel.
6.2 Certain information, such as the proposed routing of utilities in relation to the sensitive elements of the development will be determined at the detailed design stage. It is recommended in the EIA that a review shall be carried out to identify whether the detailed design results in any fundamental changes to the landfill gas hazard assessment presented in the EIA and whether additional monitoring is required.
6.3 Monitoring shall 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 C. The presentation format for landfill gas monitoring shall be based on this format and agreed with EPD.
6.4 Intrinsically safe, portable gas detectors shall 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 shall 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.
6.5 Pre-entry and routine monitoring shall be conducted at utilities’ manholes and chambers. The routine monitoring shall be conducted on a regular basis and the frequency of the monitoring will be subject to the results of the initial monitoring exercise. The landfill gas monitoring instrument shall:
· comply with the EPD’s Landfill Gas Hazard Assessment – Guidance Noteas 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.
6.6 The equipment used to monitor LFG concentrations in excavations may be the same as that described for routine landfill gas monitoring of manholes described in Section 6.4 above, as long as safety parameters described above can be achieved.
6.7 During construction activities within 250m Consultation Zone of Shuen Wan Landfill, excavations of 1m depth or more, landfill gas shall 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 shall be adopted.
6.8 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.
6.9 The Limit levels and relevant Action Plans for landfill gas detected in utilities and any on-site areas following construction are shown in Table 6.1 below.
Table 6.1 Limit Levels and Action Plan for Landfill Gas
|
Parameter |
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% |
6.10 The EIA report has recommended specific mitigation measures to be incorporated in the detailed design. All personnel who work on the Site and all visitors to the Site shall be made aware of the possibility of ignition of gas in the vicinity of excavations. Safety notices (in Chinese and English) shall be posted at prominent position around the Site warning of the potential hazards.
6.11 Service runs within the consultation zone (i.e. 250 m around the boundary of Shuen Wan Landfill) shall be designated as “special routes” and utilities companies shall be informed of this and implement precautionary measures. These shall include ensuring that staff aware of the potential hazards of working in confined spaces such as manholes and service chambers, and that appropriate monitoring procedures are in place to prevent hazards due to asphyxiating atmospheres in confined spaces. Detailed guidance on entry into confined spaces is given in Code of Practice on Safety and Health at Work in Confined Spaces (Labour Department, Hong Kong).
6.12 Notwithstanding, the requirement for the review of the detailed design, mitigation measures to be incorporated in the detailed design include:
· No worker shall work alone at any time in or immediately adjacent to any excavation. At least one other worker shall be available to assist with rescue if needed;
· Construction equipment shall be equipped with a vertical exhaust at least 0.6 m above grade and/or with spark arrestors;
· Electrical motors and electrical extension cords, if utilized in the excavation area, shall be explosive-proof or intrinsically safe;
· Welding, flame-cutting or other hot works shall only be carried out in trenches or confined spaces when controlled by a ‘permit to work’ procedure. The Permit to Work shall be authorized by the Safety Officer and shall include information of monitoring for LFG during the hot works. Under the procedure, a qualified person shall attend outside the trenches or confined space during the hot works to review gas monitoring results and suspend the works in the event of unacceptable or hazardous conditions. Only those workers who are appropriately trained and fully aware of the potentially hazardous conditions shall be permitted to carry out hot works in confined areas;
· Forced ventilation shall be provided for workers, if in a trench deeper than 1 m;
· Permit no smoking or burning on site and eliminate all other sources of ignition in the working area. ‘No smoking’ and ‘No naked flames’ notices shall be posted prominently on the construction site and, if necessary, special areas designated for smoking;
· During piping assembly or conduiting construction, all valves/seals shall be closed immediately after installation. As construction progresses, all valves/seals shall be closed as installed to prevent the migration of gases through the pipeline/conduit. For the large diameter pipes (say >600mm diameter), pipe end shall be capped on one side during construction stage in order to prevent migration of gases through pipes to other areas. Force ventilation shall be provided before operation of the pipeline. Forced ventilation and gas monitoring shall be performed before staff entering and working in such large diameter pipe;
· The monitoring frequency and areas to be monitored shall be set down prior to commencement of ground-works either by the Safety Officer or an approved and appropriately qualified person;
· Routine monitoring shall be carried out in all excavations;
· All measurements in excavations shall be made with the monitoring tube located not more than 10 mm from the exposed ground surface. Monitoring shall be performed properly to make sure that the area is free of landfill gas before any man enters the area;
· For excavations deeper than 1 m, measurements shall 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;
· For excavations between 300 mm and 1 m deep, measurements shall 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 shall vary and shall be set down by the Safety Officer or other approved and appropriately qualified person;
· The landfill gas precautionary measures involved with excavation and piping works shall be included in the Safety Plan in the Construction Phase of the Project. The suitability of the above precautionary measures shall depend on the real situation but however, the proposed precautionary measures shall comply in accordance the Landfill Gas Hazard Assessment Guidance Note (EPD/TR8/97);
· The cracks on the ground level at the working area shall be monitored for methane, carbon dioxide and oxygen using appropriately calibrated portable gas detection equipment periodically during ground works construction.
6.13 Where there are any temporary site offices, or any other buildings which have enclosed spaces with the capacity to accumulate landfill gas, then they shall either:
· be located on an area which has been proven to be free of landfill gas (by survey with portable gas detectors) and monitored manually by the Safety Officer or an approved and appropriately qualified person to ensure that hazardous concentration of landfill gas does not occur; or
· be raised clear of the ground. If buildings are raised clear of the ground, a minimum, clear separation distance (as measured from the highest point on the ground surface to the underside of lowest floor joist) shall be 500mm.
6.14 Such offices or buildings shall be provided with some kinds of control of gas by mechanical means e.g. forced ventilation using fans or blowers.
6.15 The EIA report has recommended the following specific mitigation measures during the operational phase:
· When operators enter voids, manholes or inspection chambers within the proposed site for maintenance, monitoring and a checklist system of safety requirements shall be performed before entry in accordance with Code of Practice on Safety and Health at Work in Confined Spaces (Labour Department, Hong Kong).
· For newly built permanent building structures (e.g. plant rooms at the basement of buildings into which operators frequently enter) within the 250m Consultation Zone forced ventilation and gas detection system shall be installed in such rooms or buildings.
· Forced ventilation shall be used if methane of more than 0.5 % (by volume) in the internal atmosphere (e.g. in service voids, manholes, inspection chambers or rooms as mentioned above) is detected.
· No person shall enter or remain in any confined spaces or trenches where the carbon dioxide concentration exceeds 1.5 % (by volume).
· Oxygen concentration shall be monitored and no person shall enter or remain in any confined spaces or trenches where the oxygen content of air has fallen below 18 % by volume.
· All the access to these confined spaces will be restricted only to authorized personnel who shall be aware of the LFG hazard. No general public have permitted or allowed to access these confined spaces, manholes or inspection chambers.
6.16 The implementation schedule for the mitigation measures is presented in Appendix B.