10. Landfill Gas Hazard

10.1. Introduction

This chapter presents the assessment of the potential impacts of landfill gas (LFG) hazards due to the proposed WENT Landfill Extension. The LFG assessment has been conducted based on the Landfill Gas Hazard Assessment (LFGHA) Guidance Note.

10.2. Environmental Legislation, Policies, Plans, Standards and Criteria

10.2.1. Relevant environmental legislation, policies, plans, standards and guidelines regarding the assessment of LFG hazards include:

· Section 3.4.9, and Appendix H of EIA Study Brief (ESB-243/2012)

· Section 1.1(f) in Annex 7 of the Technical Memorandum on Environmental Impact Assessment Ordinance (EIAO (TM))

· Section 3.3 in Annex 19 of the EIAO (TM)

· Landfill Gas Hazard Assessment for Development Adjacent to Landfills (ProPECC PN 3/96)

· Landfill Gas Hazard Assessment Guidance Note (1997) (EPD/TR8/97)

10.2.2. The above listed guidance notes in general recommends the assessment of LFG hazards for developments within the 250m Consultation Zone around a landfill. As the proposed future WENT Landfill Extension will include the Subject Site within the 250m consultation zone, the landfill gas hazard assessment is necessary for the Project.

10.3. Description of the Environment

10.3.1. The Subject Site is located at the existing ash lagoons in Tsang Tsui, Tuen Mun, overlooking the Deep Bay in the north-western New Territories. The Subject Site within area of approximately 30,000m²(3 hectares) located at the west portion of the Middle Ash Lagoon, was leased to China Light & Power Company Ltd. (CLP) for the storage of pulverized fuel ash (PFA). A location plan of this potential site is shown in Figure 1.1.

10.3.2. Other than the Subject Site, the remaining Middle Ash Lagoon area will be taken over by EPD as an extension of WENT Landfill (WLES). According to EPD’s current preliminary planning, an Integrated Waste Management Facilities (IWMF) is proposed to be constructed directly to the east of the Subject Site. Further to the east, a Sludge Treatment Facilities (STF) development is currently under construction.

10.3.3. The baseline environmental condition at Subject Site is addressed in terms of distance from the existing WENT landfill. The Subject Site is located more than 1km away from the existing WENT landfill boundary. However, with the proposed WENT Landfill Extension in the future, the Subject Site would be located adjacent to the boundary, as Figure 10.1 shows.

10.4. Description of Assessment Methodology

10.4.1. The methodology and requirements of this assessment follows the procedure set out in Landfill Gas Hazard Assessment (LFGHA) Guidance Note. The potential risks were evaluated based on the combination of Source, Pathway and Target, which is described in Sections 10.4.2 to 10.4.5 below.

Source

10.4.2. The Source is classified as listed below:

Minor – Landfill sites at which gas controls have been installed and proven to be effective by comprehensive monitoring has demonstrated that there is no migration of gas beyond the landfill boundary (or any specific control measures). Also, the control of gas does not rely solely on an active gas extraction system or any other single control measures, which is vulnerable to failure. Or, old landfill sites where the maximum concentration of methane within the waste, as measured at several locations across the landfill and on at least four occasions over a period of at least 3 months (preferably longer), is less than 5% by volume (v/v).

Medium – Landfill site at which some form of gas control has been installed (e.g. lined site or one where vents or barriers have been retrospectively installed) but where there are only limited monitoring data to demonstrate its efficacy to prevent migration of gas. Or, landfill site where comprehensive monitoring has demonstrated that there is no migration of gas beyond the landfill boundary but where the control of gas relies solely on an active gas extraction system or any other single control system, which is vulnerable to failure.

Major – Recently filled landfill site at which there is little to no control to prevent migration of gas or at which the efficacy of the gas control measures has not been assessed. Or, any landfill site at which monitoring has demonstrated that there is significant migration of gas beyond the site boundary.

Pathway

10.4.3. Pathway is the path (such as geological features on ground and underground) of which hazardous emissions could migrate as listed below:

Very Short / Direct – Path length of less than 50m for unsaturated permeable strata and fissured rock or less than 100m for man-made conduits.

Moderately short / Direct – Path length of 50-100m for unsaturated permeable soil or fissured rock or 100-250m for man-made conduits.

Long / Indirect – Path length of 100-250m for unsaturated permeable soils and fissured rock.

Target

10.4.4. Target is any development classified as listed below:

High Sensitivity – Buildings and structures with ground level or below ground rooms/voids where services enter directly from the ground and to which members of the general public have unrestricted access which contain sources of ignition. This includes any developments where there is a possibility of additional structures being erected directly on the ground on ad hoc basis and thereby without due regard to the potential risks.

Medium Sensitivity – Other buildings, structures or services voids where there is access only by authorized, well trained personnel, such as the staff of utility companies, who have been briefed on the potential hazards relating to landfill gas, the specific safety procedures to be followed; and deep excavations.

Low Sensitivity – Buildings/structures which are less prone to gas ingress by virtue of their design such as those with a raised floor slab; shallow excavations; and developments which involves essentially outdoor activities but where evolution of gas could pose potential problems.

Qualitative Risk Assessment

10.4.5. Once the source, pathway and target are identified and categorized as listed in the above sections, an assessment can be made with reference to the classification of risk category as shown in Table 10.1. With the use of this table, the level of risk for the development can be determined. Table 10.2 subsequently describes the implications associated with the risk category.

Table 10.1 Qualitative Risk Assessment Matrix

Source	Pathway	Target Sensitivity	Risk Category
Major	Very short/direct	High	Very High
		Medium	High
		Low	Medium
	Moderately short/direct	High	High
		Medium	Medium
		Low	Low
	Long/indirect	High	High
		Medium	Medium
		Low	Low
Medium	Very short/direct	High	High
		Medium	Medium
		Low	Low
	Moderately short/direct	High	High
		Medium	Medium
		Low	Low
	Long/indirect	High	Medium
		Medium	Low
		Low	Very Low
Minor	Very short/direct	High	High
		Medium	Medium
		Low	Low
	Moderately short/direct	High	Medium
		Medium	Low
		Low	Very Low
	Long/indirect	High	Medium
		Medium	Low
		Low	Very Low

Table 10.2 Summary of General Categorization of Risk

Category	Level of Risk	Implication
A	Very high (Undesirable)	The type of development being proposed is very undesirable and a less sensitive form of development should be considered. At the very least, extensive engineering measures, alarm systems and emergency action plans are likely to be required.
B	High	Significant engineering measures will be required to protect the planned development.
C	Medium	Engineering measures will be required to protect the proposed development.
D	Low	Some precautionary measures will be required to ensure that the planned development is safe.
E	Very Low (Insignificant)	The risk is so low that no precautionary measures are required.

10.4.6. Five generic forms of protection will be used in mitigating the hazards to development. In general, higher risk categories will warrant the use of more active or more sophisticated combinations of control measures. The generic forms corresponding to the five risk levels are set out in Table 10.3, and the definition of control terms are described in Table 10.4.

Table 10.3 Generic Protection Measures for Planning Stage Categorization

Category	Generic Protection Measures
A	For the planned development active control of gas, supported by barriers and detection systems. Another, less sensitive form of development should also be considered.
B	Active control of gas, including barriers and detection systems⁽ⁱ⁾.
C	Use of “semi active” or enhanced passive controls. Detection systems in some situations.
D	Passive control of gas only.
E	No precautionary measures required.

Note:

(i) The gas protection measures required to allow the safe development of a Category a risk development will need to be more extensive than those for a Category B risk development.

Table 10.4 Definition of Control Terms

Term	Definition
Active control	Control of gas by mechanical means i.e. ventilation of spaces with air to dilute gas, or extraction of gas from the development site using fans or blowers.
'Semi active' control	Use of wind driven cowls and other devices which assist in the ventilation of gas but do not rely on electrically powered fans.
Passive control	Provision of barriers to the movement of gas i.e. membranes in floors or walls, or in trenches, coupled with high permeability vents such as no-fines gravel in trenches or voids/permeable layers below structures.
Detection systems	Electronic systems based upon, for example, catalytic oxidation or infrared measurement principles, which can detect low concentrations of gas in the atmosphere and can be linked to alarms and/or telemetry systems.

Source: Landfill Gas Hazard Assessment Guidance Note, EPD

10.5. Identification, Prediction and Evaluation of Environmental Impacts

10.5.1. The landfill gas risk assessment is conducted based on Section 10.4 above, pertinent to the Project.

Source

10.5.2. WENT Landfill Extensions will be the source of landfill gas hazard to the Subject Site. According to the WENT Landfill Extensions EIA Report (EIA-171/2009), it is likely that WENT Landfill Extension will be designed to adopt similar or more advanced landfill gas control measures as in the existing WENT Landfill. Some of the measures include:

· Multi-layer composite liner systems covering the entire surface area of the landfill site with LFG collection and management systems to eliminate any off-site migration of LFG;

· Active gas extraction systems ;

· Gas control systems installed and comprehensive monitoring conducted to ensure no migration of gas beyond landfill boundary; and

· Any other specific control measures will be applied if necessary.

10.5.3. As such, the source is classified as MEDIUM.

Pathway

10.5.4. The decommissioning and construction works for the Project is expected to begin by the September 2015, and finish by the March 2016. The distance between the Subject Site and its access road to the existing WENT Landfill is more than 1km. According to the WENT Landfill Extensions EIA Report (EIA-171/2009), the first phase of WENT Landfill extension is expected to begin construction in 2016, and come into operation in 2018. While the schedule and program suggest that WENT landfill extension does not come into operation until after the decommissioning and construction works of the Project, a worst-case scenario is taken for the LFG assessment—the Subject Site located immediately to the boundary of the WENT Landfill Extension, which would be the actual scenario when all phases of the extension are expected to be in operation by 2024. As such, the pathway is classified as VERY SHORT/DIRECT. Figure 10.1 shows the WENT Landfill Extension and the 250m consultation zone.

Target

10.5.5. During decommissioning and construction works period, mostly construction workers which have adequate training on the potential hazards relating to landfill gas will be on site, working in an outdoor environment. Drainage works at the site perimeter and road works are expected to take place on ground level and open space area. Excavation works shall be carried out during the construction of retaining structure to the existing slope along the water channel. On the other hand, minimal excavation works will be involved during the construction of perimeter cut-off drains and other drainage and road works, workers are not expected to be working within the excavated area. With reference to Table 10.1, the target sensitivity is identified as MEDIUM.

10.5.6. In addition, a site office is to be built for construction staff and project related personnel only. The site office provides an indoor environment which in turn raises potential hazards of landfill gas if it migrates into confined spaces. The target sensitivity is identified as MEDIUM, since the site office will be restricted and accessed only by authorized, well trained personal, who have been briefed on the potential hazards relating to landfill gas and all relevant safety precautions and procedures to be followed.

Qualitative Risk Assessment

10.5.7. Table 10.5 provides the summary of the qualitative risk assessment of WENT Landfill Extension. The LFG risks arising from decommissioning and construction of the Project is determined to be MEDIUM for construction workers working outdoors, and MEDIUM for workers within the site office.

Table 10.5 Qualitative Risk Assessment Matrix

Source

Pathway

Target

Risk

WENT Landfill Extension

MEDIUM

VERY SHORT/DIRECT

Outdoor construction workers:
MEDIUM

MEDIUM

Site office personnel:

MEDIUM

10.6. Mitigation of Adverse Environmental Impacts

10.6.1. With a medium level of risk associated with landfill gas, engineering and precautionary measures are necessary to protect the safety of the workers on site. It should be noted that the source of landfill gas will be subjected to the development of the WENT Landfill Extension. However, as there are uncertainties on the implementation programme and phasing of the WENT Landfill Extension, the following mitigation measures recommended are only applicable when they WENT Landfill Extension is in operation.

10.6.2. Landfill gas monitoring should be undertaken during ground works construction and in all excavations. Manual monitoring shall be conducted using a range of portable calibrated instruments. Measurements should be made by the Safety Officer or by an appropriately qualified person, directly after the excavation has been completed and periodically whilst the excavation remains open. The Safety Officer should be notified of any excavation works and subsequently to conduct more rigorous monitoring before, during, and after excavated works to ensure gas levels are within action level. Action must be taken in accordance with Table 10.6 when detected gas levels reach the respective level.

10.6.3. For site office personnel, which have been identified to having a medium-level risk to which landfill gas may pose to the confined environment, the site offices in which they work in must provide proper ventilation system to keep the air within the confined area well ventilated. Site Offices shall be designated to be raised clear off of the ground with a minimum clear separation distance of 500mm as an alternative precautionary measure. Safety notices should be posted with warnings of the potential landfill gas hazards within site offices. In addition, gas detection equipment should be installed and appropriate breathing apparatus should be made available. On-going gas monitoring should be carried out with the gas detection equipment to ensure that the office areas remain gas free. Action must be taken in accordance with Table 10.6 when detected gas levels reach the respective level:

Table 10.6 Actions in the Event of Landfill Gas Detected in Confined Areas

Parameter	Measurement	Action
Oxygen	<19%	Ventilate to restore oxygen to > 19 %
Oxygen	<18%	1. Stop works 2. Evacuate personnel/prohibit entry 3. Increase ventilation to restore oxygen to > 19 %
Methane	> 10 % LEL (i.e. > 0.5 % by volume)	l Prohibit hot works l Ventilate to restore methane to < 10% LEL
Methane	> 20 % LEL (i.e. > 1 % by volume)	1. Stop works 2. Evacuate personnel/prohibit entry 3. Increase ventilation to restore methane to < 10 % LEL
Carbon Dioxide	>0.5%	Ventilate to restore carbon dioxide to <0.5%
Carbon Dioxide	>1.5%	1. Stop works 2. Evacuate personnel/prohibit entry 3. Increase ventilation to restore carbon dioxide to <0.5%

10.6.4. With reference to the LFGHA Guidance Note (EPD/TR8/97), in all construction works adjacent to landfill sites, safety procedures should be implemented to minimize the risks of:

· Fires and explosions;

· Asphyxiation of workers;

· Confined Spaces; and

· Toxicity effects.

10.6.5. Precautions should be clearly laid down and rigidly adhered to with respect to:

· Trenching and excavation; and

· Creation of confined spaces at, near to or below ground level.

10.6.6. The precautionary measures during the construction stage are detailed in the LFGHA Guidance Note Chapter 8 as shown in Appendix 10.1. These recommended measures, along with engineering matters in providing ventilation to site office, gas detection devices and breathing apparatus, will form part of the specifications of the construction contract to ensure that the construction Contractor is well aware of the hazard arising from construction works during tender stage. The Contractor will be required to submit their landfill gas monitoring plan and precautionary measures to the Architectural or Engineer Representative for their approval before the commencement of work.

10.7. Definition and Evaluation of Residual Environmental Impacts

Residual environmental impact is the net impact remaining with the mitigation measures in place. With mitigation measures in place, the residual impact arising from LFG is expected to be minimal to negligible.

10.8. Identification and Evaluation of Operational Phase Environmental Impact

As defined in Section 2.5.2, operational phase shall be defined as the period when the decommissioning works as detailed in Section 2.6 are completed. The operational phase of the Project shall be an idle buildable land to cater any future developments by the government. The operational phase of the Project will be under an open area; thereby adverse environmental impact is not expected. In light of the anticipation for the operation of WENT Landfill Extension, any such future developments would render the respective project proponent necessary to undertake another set of LFG assessment based on the respective design provisions in order to assess landfill gas hazards.

10.9. Environmental Monitoring & Audit

Further details of the specific EM&A requirements are detailed in Section 12of this report and in the EM&A Manual, together with event action plans and procedures for complaints.

10.10. Conclusion

Landfill Gas Hazard Assessment Study is conducted in accordance with the Landfill Gas Hazard Assessment Guidance Note. Through the use of Source, Pathway and Target analysis, the qualitative risk assessment to evaluate the potential risk associated with the WENT Landfill Extension has been performed. According to the classification of risk category, the Project is classified as MEDIUM for construction workers working outdoors and site office personnel. Provided that mitigation measures are implemented properly, no adverse impact is anticipated.