CONTENTS

1                      Introduction   1

1.1                   Background to the Study   1

1.2                   Procedures and Guidelines   2

1.3                   Previous Studies Undertaken at the Site   3

1.4                   Scope of this Study   3

1.5                   Report Structure   3

2                      REview of Prelimininary Qualitative Landfill Gas Hazard Assessment  5

3                      LANDFILL GAS Hazard ASSESSMENT METHODOLOGY   8

3.1                   Landfill Gas Hazard Assessment Criteria and Methodology  8

4                      Nature of SENT Landfill  13

4.1                   Landfill History   13

4.2                   Historical Landfill Gas Control  13

4.3                   SENTX   15

4.4                   Landfill Gas Control for the SENTX   15

5                      Potential for the development to intercept landfill gas  17

5.1                   Geology and Hydrogeology   17

5.2                   Utilities   17

6                      Detailed Design of Infrastructure area   19

6.1                   Description of the Infrastructure Area   19

6.2                   Construction Methodology   20

6.3                   Sensitive Target Facilities for Landfill Gas Risk   20

7                      Qualitative assessment of risks due to landfill gas   27

7.1                   Introduction   27

7.2                   Source   27

7.3                   Pathway   28

7.4                   Targets   30

7.5                   Source-Pathway-Target Analysis   42

8                      Recommendations   52

8.1                   General Hazards related to Landfill Gas   52

8.2                   General Recommended Precautionary and Protection Measures – Design Phase   53

8.3                   General Recommended Precautionary and Protection Measures – Construction Phase   53

8.4                   General Recommended Precautionary and Protection Measures – Operational, Restoration and Aftercare Phases  57

9                      Environmental Monitoring and Audit  59

10                    Conclusion   60

 

1                                          Introduction

1.1                                   Background to the Study

The South East New Territories Landfill Extension (SENTX) is located adjacent to the existing SENT Landfill with a portion piggybacking onto the south slope of the SENT Landfill.  The Project location and layout plan shown is shown in Figure 1.1.  SENTX will be a new source of landfill gas generation and there are potential risks associated with any development close to a landfill site relating to the generation and sub-surface migration of landfill gas. 

In 2008, the Environmental Impact Assessment (EIA) Report (AEIAR-117/2008) for the SENTX (hereafter referred to as the approved EIA Report) was approved under the EIA Ordinance (EIAO) and an Environmental Permit (EP) (EP-308/2008) based on the outline design proposed in the 2007 SENTX Feasibility Study was granted on 15 August 2008.  Since then, the Hong Kong SAR Government has decided to reduce the scale of SENTX assessed in the approved EIA Report and SENTX will only receive construction waste.

The Design, Build and Operate (DBO) Contract of SENTX was awarded to Green Valley Landfill Limited (GVL).  In line with the changes proposed by the Environmental Protection Department (EPD), GVL has reviewed the outline design and made a few changes to the scheme to develop a final scheme for SENTX (hereafter referred to “the latest scheme”) in 2016.  The changes, including the landfill profile and lining materials, site layout, design of the process equipment, construction method and sequence and filling operation were made taking into account GVL’s expertise in landfill operation, practical site operation considerations and the availability of process equipment in the market.  An VEP for the latest scheme was applied in December 2016 and a new EP was granted (EP-308/2008/B) on 20 January 2017.

Per requirement of EP Condition 2.5, the Permit Holder shall, at least one month before the commencement of construction of the Project, submit to the Director of Environmental Protection for approval a detailed landfill gas hazard assessment, which shall include a review of the preliminary qualitative risk assessment in the approved EIA report, preparation of a detailed qualitative risk assessment, preparation of detailed design of gas protection measures and the establishment of maintenance and monitoring programmes to ensure the continued performance of the proposed control measures.  The submissions shall be certified by the Environmental Team (ET) Leader and verified by the Independent Environmental Checker (IEC).  

ERM was appointed by GVL to undertake the detailed landfill gas hazard assessment (DLFGHA).  The assessment has included a review the preliminary qualitative risk assessment as presented in the approved EIA Report and taken into account the design changes of the latest scheme, to assess the potential risk due to landfill gas migration based on the latest construction methodology and building design at the infrastructure area of SENTX and to recommend appropriate measures to ensure SENTX can be constructed and operated without undue risk to safety. 

The design of the landfill gas management system and the landfill gas precautionary measures to be adopted on-site have been performed by a landfill gas specialist consultant appointed by GVL, who has comprehensive knowledge on landfill characteristics, potential landfill gas hazards and appropriate precautionary measures to minimise hazards.  Moreover, the landfill gas management system and landfill gas precautionary measures will be checked and certified by the ET Leader who led the environmental team which include an experienced landfill gas hazard specialist.  

For the purpose of this DLFGHA Report, the SENTX schemes assessed in the approved EIA Report and the latest SENTX scheme are referred to as “the EIA Scheme” and “the latest scheme” respectively.  The assessment follows the “source-pathway-target” analysis approach adopted in the approved EIA Report and the EPD’s Guidance Note on Qualitative Landfill Gas Hazard Assessment (Guidance Note) ([1]).

It should also be noted that this Report is related to the potential landfill gas hazards due to the operation of the existing SENT Landfill to the construction and operation of the SENTX and the operation of the SENTX to the infrastructure facilities of the SENTX and establishment of the necessary control measures to minimise the risks identified.

1.2                                   Procedures and Guidelines

Under Annex 7 of the Technical Memorandum on EIA Process (EIAO-TM), an evaluation of the potential risk posed by landfill gas is required for any development which is proposed within 250m of the edge of waste, known as Landfill Consultation Zone.  As the SENTX site falls within the SENT Landfill Consultation Zone (see Figure 1.2), a Qualitative Landfill Gas Hazards Assessment (QLFGHA) is required to assess the potential risk due to landfill gas migration from the SENT Landfill to the construction and operation of the SENTX.  In addition, the operation of the SENTX will generate lower quantities of landfill gas which may pose risk to the operation of the infrastructure facilities of the SENTX.  This assessment considered both landfill gas sources (i.e. existing SENT Landfill and SENTX).

A Practice Note for Professional Person (ProPECC PN 3/96) ([2]) and Guidance Note or the assessment of the hazards which landfill gas may present to developments close to landfills have been issued by the EPD. 

1.3                                   Previous Studies Undertaken at the Site

A number of previous studies have been undertaken at the SENTX.  The documents which have been used as background material for the preparation of this assessment include the following:

·                South East New Territories (SENT) Landfill Extension - Feasibility Study: Environmental Impact Assessment Report, ERM-Hong Kong, Limited 2007;

·                Environmental Review of the Revised Scheme of SENT Landfill Extension: Environmental Review Report, ERM-Hong Kong, Limited 2012;

·                South East New Territories (SENT) Landfill Extension: Environmental Review Report, ERM-Hong Kong, Limited 2016; and

·                Landfill Gas Hazard Assessment: Guidance Note, EPD 1997.

GVL has also provided the landfill gas monitoring data for the SENT Landfill covering the period from January 2017 to December 2017.

1.4                                   Scope of this Study

The following tasks have been undertaken as part of this assessment:

·             review of background information (including landfill gas monitoring data) and studies related to the SENT Landfill and the SENTX;

·             identification of the nature and extent of the SENT Landfill and SENTX which might have potential impacts on the construction and operation of SENTX;

·             identification of possible pathways through the ground, underground cavities, utilities or groundwater, and the nature of these pathways through which the landfill gas must traverse if they were to reach the SENTX;

·             identification of the potential receivers associated with the SENTX which are sensitive to the landfill gas risk;

·             qualitative assessment on the degree of risk which the landfill gas migration may impose on the identified targets for each of the source-pathway-target combinations; and

·             design of suitable level of precautionary measures and contingency plan for the SENTX and the potential targets, if needed.

1.5                                   Report Structure

The remainder of the report is structured as follows:

·             Section 2 summarises the findings and recommendations of the preliminary QLFGHA in the approved EIA Report;

·             Section 3 describes the methodology of the DLFGHA and the framework within which the identified levels of risk may be compared;

·             Section 4 describes the SENT Landfill and SENTX site, including its history and the measures taken to control landfill gas;

·             Section 5 reviews the geology and hydrogeology of the area and evaluates the potential pathways through which landfill gas may impact the targets;

·             Section 6 describes the design of the infrastructure area of SENTX and reviews the sensitivity of key elements of the development to the possible presence of landfill gas;

·             Section 7 evaluates the qualitative risk of landfill gas to impact the SENTX site;

·             Section 8 provides further recommendations for precautionary and protection measures to be adopted during the design, construction and operation of the SENTX based on the findings of the hazard assessment;

·             Section 9 describes the environmental monitoring and audit requirements with respect to landfill gas hazards associated with the construction, operation/restoration and aftercare of the SENTX; and

·             Section 10 concludes the findings and recommendations of this DLFGHA.

2                                          REview of Prelimininary Qualitative Landfill Gas Hazard Assessment

The source-pathway-target analysis in the approved EIA Report shows that landfill gas risk posed by the SENT Landfill and SENTX under the EIA Scheme ([3])  is medium to high within the SENTX Site boundary during both the construction and operation phases.  Whereas the risk posed by SENTX to the adjacent developments ranges from very low to low depending on the nature and location of the adjacent developments. 

The findings of the preliminary QLFGHA in the approved EIA Report are summarised in Table 2.1.

Table 2.1        Qualitative Assessment of Landfill Gas Hazard Associated with SENTX in the EIA Scheme

Source

Pathway

Target

Qualitative Risk

SENT Landfill - potential for gas generation over time, but comprehensive and proven mitigation installed 

(category: medium)

Sub-surface soil, reclamation fill materials of the unsaturated zone between the SENT Landfill and SENTX with potential direct anthropogenic conducts, distance to waste boundary <50m

(category: very short/direct)

 

Target 1 (Construction site of SENTX) –demolition and excavation at the existing infrastructure area and construction of trenches using open cut method

(category: medium sensitivity)

Medium

SENT Landfill - potential for gas generation over time, but comprehensive and proven mitigation installed.

(category: medium)

SENTX - potential for low gas generation over time, comprehensive and proven mitigation to be installed.

(category: medium)

Sub-surface soil, reclamation fill materials of the unsaturated zone between the SENT Landfill and SENTX with potential direct anthropogenic conducts, distance to waste boundary <50m

(category: very short/direct)

Target 2 (Tipping face of SENTX) –  waste tipping in the open air, absence of confined space, access by drivers/operators of waste collection vehicles who may not have knowledge on landfill gas hazards

(category: medium sensitivity)

Medium

SENTX - potential for gas generation over time, comprehensive and proven mitigation to be installed

(category: medium)

Sub-surface soil, reclamation fill materials, potential direct anthropogenic conducts, distance to waste boundary  <50m

(category: very short/direct)

Target 3 (Infrastructure area of SENTX) – Ground level offices and pump rooms of unrestricted staff access, underground confined spaces with restricted access, some with source of ignition

(category: medium to high sensitivity)

 

Medium to High

Sub-surface soil, reclamation fill materials, no direct anthropogenic conducts

-        Distance to waste boundary <50m (category: very short/direct)

-        Distance to waste boundary between 50m to 100m (category: moderate short/direct)

-        Distance to waste boundary >100m (category: long/indirect)

Target 4 (Adjacent existing and new development) –according to ProPECC PN 3/96, project proponents and are required to carry out landfill gas hazard assessment and implement suitable landfill gas protection measures

(category: low sensitivity)

Very Low to Low

Precautionary and protection measures during design, construction and operation/restoration phases of the SENTX have been recommended in the approved EIA Report.

At the design phase, recommended measures to be adopted to protect building against landfill gas ingress include a combination of passive (e.g. gas barrier and gas vents) and active control measures (e.g. creation of positive pressure zone below or within building structure and continuous ventilation of spaces with air by fan) and installation of landfill gas management system to contain, manage and control landfill gas.  Detailed examples of these measures can be reference to EPD’s Guidance Note.

Precautionary measures to be adopted during construction stage including safety measures, routine monitoring, actions in event of gas being detected and emergency management etc. are outlined in Paragraphs 8.3 to 8.49 of EPD’s Guidance Note. 

Taking account of the measures recommended in the preliminary qualitative landfill gas hazard assessment of the approved EIA Report, GVL will incorporate appropriate control measures in the detailed design of the SENTX, which are described in Section 6.  Under this detailed qualitative landfill gas hazard assessment, the updated risk assessment has taken account the accommodation schedule of the SENTX infrastructure area, and the appropriate landfill gas control measures incorporated in the detailed design.

3                                          LANDFILL GAS Hazard ASSESSMENT METHODOLOGY

3.1                                   Landfill Gas Hazard Assessment Criteria and Methodology

3.1.1                           General

In accordance with the Guidance Note on Landfill Gas Hazard Assessment, EPD, 1997, the risk due to landfill gas may be evaluated based upon the following three criteria:

·             Source - the rate and concentration of landfill gas generation by the landfill;

·             Pathway - the nature of and length of potential pathways through which landfill gas can migrate and leachate flow, such as geological strata, utility services; and

·             Target - the level of vulnerability of various elements of the development to landfill gas.

Each of these criteria is further described in the subsections below.

3.1.2                           Source

The classification of the Source (i.e. the landfill) is determined as follows:

Major             Recently filled landfill site at which there is little or no control to prevent migration of gas or at which the efficacy of the landfill gas control measures has not been assessed; or

Any landfill site at which monitoring has demonstrated that there is significant migration of landfill gas beyond the site boundary.

Medium           Landfill site at which some form of landfill 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 landfill gas; or

Landfill site where comprehensive monitoring has demonstrated that there is no migration of landfill gas beyond the landfill boundary but where the control of landfill gas relies solely on an active gas extraction system or any other single control system which is vulnerable to failure.

Minor            Landfill sites at which landfill gas controls have been installed and proven to be effective by comprehensive monitoring which has demonstrated that there is no migration of landfill gas beyond the landfill boundary (or any specific control measures) and at which control of landfill gas does not rely solely on an active gas extraction system or any other single control measure 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 6 months, is less than 5% (v/v).

3.1.3                           Pathway

Generally, three types of pathway are considered for the transmission of landfill gas.  They are:

·      Man-made pathways e.g. utility connections, stormwater channels, etc.,

·      Natural pathways such as rock jointing planes, fissures and other naturally occurring phenomena which may promote or give rise to the transmission of gas over distances; and

·      A combination of the previous two categories.  An example of the latter may be, for instance, where a specific geological feature promotes gas transmission but which stops short of directly linking the landfill and target.  A man made connection, however may also co-exist near the edge of the geological feature, which in combination with the former, may act to link the two sites.  In this instance, careful assessment of the likelihood of the mechanism acting to link the two pathways needs to be undertaken before assigning an appropriate pathway classification.

The broad classification of a Pathway is as follows:

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 to 100m for unsaturated permeable soil or fissured rock or 100 to 250 m for man-made conduits

Long/indirect

Path length of 100 to 250m for unsaturated permeable soils and fissured rock

In classifying the pathway, however, adjustment to the above general guidelines will often be required to take account of other factors which will affect the extent of landfill gas migration including the following:

·               a broad assessment of the specific permeability of the soil;

·               spacing, tightness and direction of the fissures/joints;

·               topography;

·               depth and thickness of the medium through which the landfill gas may migrate (which may be affected by groundwater level);

·               the nature of the strata over the potential pathway;

·               the number of different media involved; and

·               depth to groundwater table and groundwater flow patterns.

3.1.4                           Target

Different levels of vulnerability or sensitivity of potential targets for landfill gas have been classified as follows:

High Sensitivity

·      Buildings and structures with ground level or below ground rooms/voids or into which services enter directly from the ground and to which members of the general public have unrestricted access or which contain sources of ignition.

·      This would include any developments where there is a possibility of additional structures being erected directly on the ground on an ad hoc basis and thereby without due regard to the potential risks.

Medium Sensitivity

·      Other buildings, structures or service voids where there is access only by authorised, well trained personnel, such as the staff of utility companies, who have been briefed on the potential hazards relating to landfill gas and the specific safety procedures to be followed.

·      Deep excavations.

Low Sensitivity

·      Buildings/structures which are less prone to landfill gas ingress by virtue of their design (such as those with a raised floor slab).

·      Shallow excavations.

·      Developments which involve essentially outdoor activities but where evolution of landfill gas could pose potential problems.

The above examples of different categories within each criteria are to be used as a general guide only and specific aspects of a development may render it more or less sensitive than indicated.  Account has been taken of any particular circumstances when assigning a target to one of the three indicated categories.

3.1.5                           Assessment of Risk Criteria

Following the determination of the categories of source, pathway and target in which the landfill, pathway and development fall, a qualitative assessment of the overall risk may be made by reference to Table 3.1 which is extracted from the Guidance Note.  The potential implications associated with the various qualitative risk categories are summarised in Table 3.2.  It should be noted that the different levels of risk determine the likely extent of the protection measures required to ensure the safety of a development, but with the possible exception of the very high risk category, development is not precluded for any of the assessed levels of risk.

Table 3.1        Classification of Risk Category

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 3.2        Summary of General Categorisations of Risk

Level of Risk

Implication

Very high

At the very least, extensive engineering measures and alarm systems are likely to be required.  An emergency actions plan should also be developed so that appropriate actions may be immediately taken in the event of high landfill gas concentrations being detected within the development.

 

High

Significant engineering measures will be required to protect the planned development.

 

Medium

Engineering measures required to protect the development.

 

Low

Some precautionary measures will be required to ensure that the planned development is safe.

 

Very low

No protection or precautionary measures are required.

4                                          Nature of SENT Landfill

4.1                                      Landfill History

SENT Landfill is located on the western edge of Clear Water Bay Peninsula in the south-eastern corner of the New Territories.  The site covers an area of about 100 ha, half of which has been reclaimed from Shek Biu Wan (Junk Bay).   To the north and east of the site lies Clear Water Bay Country Park; to the west lies land reclaimed as industrial estate (TKO Industrial Estate, TKOIE) and to the south a reclamation (TKO Area 137) intended for industrial uses. 

The landfill is one of the three strategic landfills in operation in the HKSAR and was designed with a capacity of approximately 43 Mm3 of waste.  The landfill commenced operation in 1994 and accepts domestic, commercial & industrial (C&I), construction, and clinical wastes, sewage sludge and stabilised incineration residues.  However, since 6 January 2016, the SENT Landfill is designated to receive only construction waste to address the odour issues associated with the operation of the landfill and its landfill gas generation has diminished significantly.

4.2                                      Historical Landfill Gas and Leachate Control

The landfill has been designed to incorporate extensive measures to contain, collect, and treat/utilise (for landfill gas only) landfill gas and leachate.  Such measures include the state-of-the-art technologies (including a composite liner systems, active landfill gas and leachate extraction, landfill gas and leachate treatment and landfill gas utilisation) in accordance with international best practices for landfill operations.  The landfill gas extraction system contains three blowers, each with spare parts.  While only one blower is in operation at one time, the other blowers serve as emergency backup.  The landfill contractor is undertaking routine maintenance and checking of the landfill gas extraction system to ensure it is operating satisfactorily.  As the site is lined and landfill gas and leachate are collected and treated, it effectively controls sub-surface off-site migration of landfill gas and leachate.  Typical details of the composite liner system (including an impermeable liner) installed at the SENT Landfill are presented in Figure 4.1.

A comprehensive environmental monitoring programme has been implemented to monitor landfill gas generated within the landfill and at the perimeter monitoring wells along the site boundary of the landfill.  Under the existing contract, the landfill contractor will be required to continue the control and monitoring of landfill gas and leachate following closure of the landfill for a period of 30 years.  Recent monitoring results from the monitoring wells located along the southern boundary of SENT Landfill (the boundary next to the SENTX) have been reviewed.  Figure 4.2 shows the locations of these monitoring wells and the landfill gas monitoring results are summarised in Table 4.1 (see Annex A for details).

Table 4.1        Summary of Landfill Gas Monitoring Results of the Perimeter Monitoring Well of the SENT Landfill (From January 2017 to December 2017)

Location

Methane (% gas)

Carbon Dioxide (% gas)

Range

Average

Range

Average

GP-1

0.0 – 0.0

0.0

0.0 – 3.7

1.4

GP-2 (deep)

0.0 – 0.1

0.0

0.0 – 2.3

0.2

GP-2 (shallow)

0.0 – 0.1

0.0

0.0 – 3.6

1.1

GP-3 (deep)

0.0 – 0.0

0.0

0.0 – 3.1

0.5

GP-3 (shallow)

0.0 – 0.0

0.0

0.0 – 1.9

0.4

GP-4 (deep)

0.0 – 0.0

0.0

0.0 – 2.4

0.7

GP-4 (shallow)

0.0 – 0.0

0.0

0.0 – 5.7

1.5

GP-5 (deep)

0.0 – 0.0

0.0

0.0 – 3.2

0.3

GP-5 (shallow)

0.0 – 0.0

0.0

0.0 – 3.3

1.2

GP-6

0.0 – 0.1

0.0

0.0 – 5.3

3.3

GP-7

0.0 – 0.0

0.0

0.0 – 3.5

0.7

GP-8

0.0 – 0.1

0.0

0.0 – 12.7

3.8

GP-9

0.0 – 0.0

0.0

0.0 – 0.2

0.1

GP-10

0.0 – 0.0

0.0

0.0 – 9.2

3.2

GP-11

0.0 – 0.1

0.0

0.0 – 4.4

1.0

GP-15

0.0 – 0.0

0.0

0.0 – 0.0

0.0

P-1

0.0 – 4.6

0.1

0.0 – 16.2

5.8

P-2

0.0 – 0.1

0.0

0.1 – 8.8

2.1

P-3

0.0 – 0.0

0.0

0.0 – 7.0

3.0

P-4

0.0 – 0.0

0.0

0.0 – 9.0

4.0

P-5

0.0 – 0.0

0.0

0.1 – 11.9

3.6

P-6

0.0 – 0.0

0.0

0.0 – 2.3

0.8

P-7

0.0 – 0.0

0.0

0.0 – 0.1

0.0

P-8

0.0 – 0.0

0.0

0.0 – 0.2

0.0

P-9

0.0 – 0.0

0.0

0.0 – 0.3

0.0

Nil or minimal concentration of methane have been observed for all perimeter monitoring wells along the southern boundary of SENT Landfill which indicate that there is no sub-surface off-site migration of methane at the southern part of the landfill.

Positive carbon dioxide readings are not in themselves indicative of a landfill gas presence, however, it is acknowledged that under some circumstances, the methane component of landfill gas may be oxidised leaving reduced concentrations of oxygen and relatively high concentrations of carbon dioxide.  Under these circumstances, elevated readings of carbon dioxide and reduced oxygen concentrations could be assumed to be associated with a potential landfill gas presence.  Carbon dioxide concentrations in monitoring wells are generally expected to be of the order of 1% v/v, however, under some circumstances, according to geological and hydrogeological conditions, concentrations of 3% v/v or more may reasonably be expected.  Oxygen concentrations in monitoring wells are typically 20% v/v.

Low concentrations of carbon dioxide have been observed in these monitoring wells.  The average carbon dioxide concentrations detected in all the these perimeter monitoring wells ranged from 0.0% to 5.8% (v/v) while the maximum gas concentrations ranged from 0.1% to 16.2% (v/v).  Elevated carbon dioxide concentration (over 15% v/v) was recorded in P-1.In summary, minimal concentrations of methane and low concentrations of carbon dioxide were detected in the perimeter monitoring wells along the southern boundary of SENT Landfill.  This suggests that off-site landfill gas migration in this area is under effective control.  In addition, as the landfill is lined and leachate is extracted for treatment, the leachate head within the landfill is controlled at low level overall.  The groundwater monitoring results do not indicate that the groundwater is contaminated with leachate.  As discussed above, the risk of landfill gas generation from groundwater will be very low. 

4.3                                      SENTX

SENTX will occupy 13 ha in TKO Area 137 located immediately south of the existing infrastructure area of existing SENT Landfill.  SENTX will piggyback onto the southern slope of the existing SENT Landfill.

SENTX is designed to receive approximately 6.5 Mm3 of construction waste over a period of approximately 6 years.  It is anticipated that construction and operation of SENTX will commence in early 2019 and early 2021, respectively.

Although SENTX will only accept construction waste which contains much less organic materials which can be decomposed compared to municipal solid waste, low levels of landfill gas will still be generated from decomposition of the organic materials in construction waste and therefore could pose a low landfill gas risk to the operation of the SENTX infrastructure facilities.

4.4                                      Landfill Gas Control for the SENTX

Similar to the SENT Landfill, SENTX will be designed and constructed to incorporate extensive measures to contain, collect, and treat landfill gas and leachate.  These measures include a composite liner systems, active landfill gas and leachate extraction and landfill gas treatment and utilisation system ([4]) in accordance with international best practices for landfill operation.  These measures can effectively control sub-surface off-site migration of landfill gas and leachate, as demonstrated by the monitoring data at the existing SENT Landfill.  The design of the composite liner system for the SENTX will be similar to those currently being used in the SENT Landfill.  Details of the composite liner system designed for the SENTX is shown in Figure 4.3.

A comprehensive environmental monitoring programme will be implemented during the construction, operation, restoration and aftercare of the SENTX to monitor landfill gas generated within the SENTX and at the perimeter monitoring wells along the site boundary of SENTX and off-site leachate migration/ groundwater contamination.  With reference to the performance standard stipulated in the SENTX contract, GVL is required to control the migration of landfill gas such that the concentration of methane and carbon dioxide at the perimeter monitoring wells shall not exceed 1% v/v and 1.5% v/v above the background concentration (measured before the operation of the SENTX), respectively.   GVL will be required to continue the control and monitoring of landfill gas and leachate following closure of the landfill for a period of 30 years.

In conclusion, the potential for landfill gas generation from SENTX is low and the most significant source of landfill gas is associated with the decomposition of municipal solid waste (MSW) landfilled at SENT Landfill, which is located over 500 m away (from waste boundary) from the SENTX infrastructure area.

5                                          Potential for the development to intercept landfill gas

5.1                                      Geology and Hydrogeology

SENTX is located partly on reclaimed land in TKO Area 137 and partly on the southern slope of the existing SENT Landfill. 

It is understood that TKO Area 137 was formed by public fill (inert construction waste, e.g. soils, rock, broken concretes, bentonite, asphaltic concrete, etc.) generated from construction projects in Hong Kong as it has been operating as a fill bank since 2002.  The future final level of TKO Area 137 is at approximately +5.5mPD.  Based on records of the ground investigation, the level of groundwater table is approximately at +2.8mPD, leaving an unsaturated layer of 2.7m.   Although there is a very small potential for landfill gas dissolved in groundwater to be released at remote locations, it is considered that this permeable layer between the SENT Landfill, SENTX and the adjacent existing and future development should be conservatively considered as conducive to landfill gas migration, noting that the level of groundwater may vary with time .  Sections through the SENT Landfill and SENTX are presented in Figures 5.1 and 5.2. 

5.2                                      Utilities

It is known that underground utilities (landfill gas collection pipes, electricity, telecommunications and Towngas) exist in the existing infrastructure area of SENT Landfill.  However, these underground utilities do not connect to TKO Area 137.  It is also understood that currently there are no man-made underground utilities in TKO Area 137, except a surface water box culvert to connect the existing SENT Landfill and the berthing area in TKO Area 137.  Layout plan for the utility services in the infrastructure area of the existing SENT Landfill is presented in Figure 5.3.

At present, services (electricity, telecommunications and Towngas) exist running parallel to Wan Po Road near TKOIE and there are no man-made underground service channels, tunnels or culverts run contiguously to SENTX. 

In future, the utilities to be provided from existing SENT Landfill site to the SENTX site include electricity, telecommunication cables, water mains, sewers, storm water drains, Towngas, Fire Services, landfill gas and leachate pipes along the existing SENT landfill access road.  A preliminary layout plan for the proposed utility services to SENTX is presented in Figure 5.4. 

Electricity and telecommunication cables will be laid underground in ducts which could provide a direct man-made pathway from SENT Landfill to SENTX. 

For fresh water (including Fire Services pipes) and salt water pipes (force mains), it is anticipated that the pipes will be filled with water at all the time and there is no risk of landfill gas migration through these pipes. 

A proposed effluent pipe to DSD sewerage connection at SENT Landfill will also be a force main and it is anticipated that the pipe will be filled with effluent at all the time and there is no risk of landfill gas migration through this pipe. 

Stormwater drains will be connected to gullies and open to atmosphere and the risk of landfill gas migration through this route to the buildings of the SENTX infrastructure area will be low. 

Groundwater manhole MH-01 will be relocated from the existing location to the proposed new location close to Wan Po Road (see Figure 5.5) with a gravity connection drain which is not directly connected to the infrastructure area of the SENTX or the temporary construction site office.    

Landfill gas collection pipes will not act as a potential man-made migration pathway for sub-surface landfill gas migration.

6                                          Detailed Design of Infrastructure area

Based on the findings and recommendations of the preliminary QLFGHA of the approved EIA Report and the 2011 ERR, GVL has incorporated landfill gas control measures in the detailed design of the SENTX infrastructure facilities.  This assessment has taken account of these control measures in the evaluation of the landfill gas hazard with respect to the nature and characteristics of the targets.  The adequacy of these control measures will be evaluated and if necessary further control measures will be recommended in this Report.

6.1                                      Description of the Infrastructure Area

The new infrastructure area will be located immediately south of the SENTX waste boundary (see Figure 1.1) and the proposed layout plan of the infrastructure area is presented in Figure 6.1.  It should be noted that the new infrastructure area is located over 250m from the SENT Landfill site boundary.  Therefore, the key risk source for the new infrastructure area is the SENTX.

The infrastructure area includes the following buildings/ structures:

·             Fire Service Building;

·             Equipment Storage Area;

·             Vehicle Washing Area;

·             Maintenance Area;

·             Maintenance Building;

·             EPD Building;

·             GVL Building;

·             Workshop/Laboratory Building;

·             Gas Plant Building;

·             Bioplant Building;

·             Dangerous Goods Storage Area;

·             Security Office; and

·             Weighbridge Office.

As the Vehicle Washing Area, Equipment Storage Area, Maintenance Area and Dangerous Goods Storage Area are operated in open space or non-enclosed building, there will be no or very low potential for landfill gas accumulation at these facilities.  Therefore, they are not further assessed in this report.

The approximate distance from the SENTX waste boundary to the each of the target facilities at the new infrastructure area are presented in Table 6.1.  All facilities are operated/managed by GVL.

Table 6.1      Key Target Facilities in the New Infrastructure Area

Key Facilities

Approximate Distance from the SENTX (Waste Boundary)

Distance from SENT (Waste Boundary)

1.        Weighbridge Office

9 m

500 m

2.        Security Office

20 m

3.        Bioplant Building

37 m

>500 m

4.        Maintenance Building

41 m

5.        Fire Service Building

46 m

6.        Gas Plant Building

71 m

7.        Workshop/Laboratory Building

81 m

8.        GVL Building

86 m

9.        EPD Building

119 m

6.2                                      Construction Methodology

Blasting and slope formation works at the hillside for the infrastructure area is not required under the latest scheme.  The site formation works for the landfill lining system will require some open excavation works due to slightly lower formation level for the basal liner and the handling of the stockpile material above +6mPD tentatively left within the SENTX site boundary at the time of site handover ([5]).

6.3                                      Sensitive Target Facilities for Landfill Gas Risk

6.3.1                              EPD Building

The EPD building is a two-storey building covering a total area of approximately 350 m2.  Ground level rooms include general office, reception, boot cleaning area, pantry, store rooms, conference room, plumbing room, electrical room and toilets & changing rooms (see Figure 6.2).  These areas will be provided with mechanical ventilation or air conditioning with natural ventilation.  The designed air change for each room are presented in Table 6.2.

Table 6.2        Designed Air Change of the Ground Floor Rooms of the EPD Building

No.

Room

No. of Air Change Per Hour (a)

1

General office

5

2

Reception

5

3

Boot cleaning area

5

4

Pantry

7

5

Special storage & office furniture/equipment

5

6

Conference room

6

7

Special store room

5

8

Plumbing room

5

9

Electrical room

5

10

Male toilet & changing room

10

11

Female toilet & changing room

10

12

Accessible toilet

10

Note:

(a)   Refer to fresh air change rate.  A higher air change rate is maintained with air re-circulation.

 

6.3.2                              GVL Building

The GVL building consists of two floors and covers a total area of approximately 580 m2.  Ground level rooms include offices, multi-purpose room, conference rooms, reception, electrical room, pantry, store rooms, plumbing room and toilets & changing rooms (see Figure 6.3).  These areas will be provided with mechanical ventilation or air conditioning with natural ventilation.  The designed air change of the ground floor rooms of the GVL Building are presented in Table 6.3.

Table 6.3        Designed Air Change of the Ground Floor Rooms of the GVL Building

No.

Room

No. of Air Change Per Hour (a)

1

General manager office

5

2

Multi-purpose room & Conference room

6

3

Administration general office

5

4

Reception

5

5

Electrical room

5

6

Conference room 1

6

7

Conference room 2

6

8

Pantry

7

9

File & general storage

5

10

Special storage & office furniture/equipment

5

11

Plumbing room

5

12

Male toilet & changing room

10

13

Female toilet & changing room

10

14

Accessible toilet

10

Note:

(a)   Refer to fresh air change rate.  A higher air change rate is maintained with air re-circulation.

6.3.3                              Workshop/ Laboratory Building

The Workshop/laboratory building is a two-storey building covering a total area of approximately 260 m2.  Ground level rooms include reception, store rooms, electrical room, plumbing room, workshop, toilets & changing rooms and hose reel pump room (see Figure 6.4[CL1] ).  These areas will be provided with mechanical ventilation or air conditioning with natural ventilation.  The designed air change for each ground floor room are presented in Table 6.4.

Table 6.4        Designed Air Change of the Ground Floor Rooms of the Workshop/ Laboratory Building

No.

Room

No. of Air Change Per Hour (a)

1

Reception

6

2

Dangerous goods storage room

6

3

Dry gas bottle room

6

4

Storage

7

5

Electrical room

5

6

Plumbing room

5

7

Workshop

Natural ventilation

8

Male toilet & changing room

10

9

Female toilet & changing room

10

10

Accessible toilet

10

11

Hose reel pump room

5

Note:

(a)   Refer to fresh air change rate.  A higher air change rate is maintained with air re-circulation.

 

6.3.4                              Maintenance Building

The Maintenance building is a two-storey building covering a total area of approximately 980 m2.  The maintenance area is designed with two-storey headroom, where other rooms are single storey in height.  Ground level rooms include maintenance area, managers and supervisor’s room, reception, pantry and lunch room, plumbing & electrical room, toilets & showers, first aid room, parts/tools storage and hose reel pump room (see Figure 6.5).  These areas will be provided with mechanical ventilation or air conditioning with natural ventilation.  The designed air change for each room are presented in Table 6.5.

Table 6.5        Designed Air Change of the Ground Floor Rooms of the Maintenance Building

No.

Room

No. of Air Change Per Hour (a)

1

Maintenance area

- (b)

2

Maintenance manager’s room

5

3

Operations supervisor’s room

5

4

Operations manager’s room

5

5

Reception

5

6

Pantry and lunch room

7

7

Plumbing & electrical room

5

8

Female toilet & shower

10

9

Male toilet & shower

10

10

First aid room

8

11

Parts/tools storage

5

12

Fire hose reel pump room

4

Notes:

(a)   Refer to fresh air change rate.  A higher air change rate is maintained with air re-circulation.

(b)   Not a confined room

 

6.3.5                              Bioplant Building

The Bioplant building consists of two floors and covers a total area of approximately 340 m2.  Ground level rooms include blower room, chemical storage room, workshop and FM200 cylinder room (see Figure 6.6).  These rooms will be provided with mechanical ventilation or air conditioning with natural ventilation.  The designed air change of the ground floor rooms of the GVL Building are presented in Table 6.6.

Table 6.6        Designed Air Change of the Ground Floor Rooms of the Bioplant Building

No.

Room

No. of Air Change Per Hour (a)

1

Blower room

5

2

Chemical storage room

6

3

Workshop

10

4

FM200 cylinder room

6

Note:

(a)   Refer to fresh air change rate.  A higher air change rate is maintained with air re-circulation.

 

6.3.6                              Fire Service Building

The Fire service building covers a total area of approximately 340 m2.  Ground level rooms include fire service pump room, fire service control room and store room (see Figure 6.7).  These areas will be provided with mechanical ventilation or air conditioning with natural ventilation.  The designed air change of the ground floor rooms of the Fire Service Building are presented in Table 6.7.

Table 6.7        Designed Air Change of the Ground Floor Rooms of the Fire Service Building

No.

Room

No. of Air Change Per Hour (a)

1

Fire Service pump room

5

2

Fire Service control room

5

3

Store room

5

Note:

(a)   Refer to fresh air change rate.  A higher air change rate is maintained with air re-circulation.

 

6.3.7                              Gas Plant Building

The Gas plant building is a single-storey building covering a total area of approximately 390 m2.  Ground level rooms include switch and transformer rooms, control room, toilet and store room (see Figure 6.8).  These rooms will be provided with mechanical ventilation or air conditioning with natural ventilation.  The designed air change for each room are presented in Table 6.8.

Table 6.8        Designed Air Change of the Ground Floor Rooms of the Gas Plant Building

No.

Room

No. of Air Change Per Hour (a)

1

LV main switch room

10 – 30

2

CLP transformer room

10 – 30

3

HV switch and transformer room

10 – 30

4

Rec transformer room

10 – 30

5

Control room

6

6

Toilet

10

7

Store room

7

Note:

(a)   Refer to fresh air change rate.  A higher air change rate is maintained with air re-circulation.

 

6.3.8                              Weighbridge Office

The weighbridge office is the closest facility to the SENTX site.  It covers a total area of approximately 45 m2 and consists of the control room and toilet (see Figure 6.9).  These rooms will be provided with mechanical ventilation or air conditioning with natural ventilation.  The designed air change for each room are presented in Table 6.9.

Table 6.9        Designed Air Change of the Ground Floor Rooms of the Weighbridge Office

No.

Room

No. of Air Change Per Hour (a)

1

Control room

6

2

Toilet

10

Notes:

(a)   Refer to fresh air change rate.  A higher air change rate is maintained with air re-circulation.

6.3.9                              Security Office

The security office consists of the control room and toilet (see Figure 6.10) covering a total area of approximately 16 m2.  These rooms will be provided with mechanical ventilation or air conditioning with natural ventilation.  The designed air change for each room are presented in Table 6.10.

Table 6.10      Designed Air Change of the Ground Floor Rooms of the Security Office

No.

Room

No. of Air Change Per Hour (a)

1

Control room

6

2

Toilet

10

Notes:

(b)   Refer to fresh air change rate.  A higher air change rate is maintained with air re-circulation.

7                                          Qualitative assessment of risks due to landfill gas

7.1                                   Introduction

This section reviews the information presented in the preceding sections and evaluates the data presented with reference to the assessment definitions given in the Guidance Note on Landfill Gas Hazard Assessment.  The qualitative assessment of the potential hazard from landfill gas to the proposed targets is then concluded.

7.2                                   Source

The existing SENT Landfill and the SENTX will both be the source of potential risk of landfill gas migration.  However, SENTX has a low potential for landfill gas generation and SENT Landfill is considered to be the most significant potential source.  The 250m consultation zones for the SENT Landfill and the SENTX are shown in Figure 1.2.

7.2.1                           SENT Landfill

As the SENT Landfill is a large operating landfill, the landfill is a significant potential source of landfill gas.  The SENT Landfill was designed and constructed to incorporate international best practices to contain, manage and control waste and landfill gas.  It is operated by an experienced international waste management contractor.

The potential off-site migration of landfill gas is assessed taking into account the comprehensive and highly effective collection and management system installed and operated.   The regular landfill gas monitoring results at the perimeter monitoring wells undertaken by the landfill contractor indicate that an insignificant amount of methane was recorded in the perimeter monitoring wells along the southern boundary.  However, according to the Guidance Note, a carbon dioxide concentration greater than 5% v/v above background levels in any monitoring well indicates significant migration.  Hence, the potential of off-site migration of landfill gas cannot be eliminated.

Given the size of the SENT Landfill, the multiple landfill gas controls implemented and the recent landfill gas monitoring data, the SENT Landfill was classified as a “medium” source of potential landfill gas risk as presented in the approved EIA Report which is also applicable to the latest scheme.

7.2.2                           SENTX

The original EIA Scheme for SENTX with a void space of about 17 Mm3 would have occupied 15 ha in TKO Area 137, located immediately to the south of the existing infrastructure area of the SENT Landfill.  Under the SENTX contract requirements, the contractor will be required to control off-site landfill gas migration such that the methane and carbon dioxide concentration at the perimeter wells will not exceed 1% v/v and 1.5% v/v above the background levels, respectively.  A gas collection and management system will be installed and operated.  As presented in the approved EIA Report, SENTX (which will accept MSW, sludge and construction waste) was classified as a “medium” source taking into account the multiple landfill gas control measures to be installed, the recent landfill gas monitoring data of the SENT Landfill, and stringent contract requirements for controlling off-site landfill gas migration.

The latest scheme of SENTX with a void space of about 6.5 Mm3 will occupy 13 ha in TKO Area 137 and be located immediately to the south of the existing infrastructure area of the SENT Landfill.  SENTX in the latest scheme is a lot smaller than the EIA Scheme and will only receive construction waste.  It is envisaged that construction waste (with high portion of inert materials) will generate minimal quantities of landfill gas compared to MSW and sludge.  Hence, the quantity of landfill gas generated in the latest scheme is expected to be much lower.  Nevertheless, landfill gas control measures will still be implemented to ensure that any landfill gas generated can be collected and treated in a controlled and safe manner.  Taking into account the nature of waste to be received in the latest scheme, the landfill gas control measures to be installed, the recent landfill gas monitoring data of the SENT Landfill and the stringent contract requirements for controlling off-site landfill gas migration, the SENTX in the latest scheme is conservatively classified as a “minor” source.

7.3                                   Pathway

The potential pathways through which landfill gas may enter the SENTX Site are threefold, namely:

·           through transmission along natural pathways such as fissures or joints in rock;

·           man-made pathways such as through permeable backfill in utilities trenches; or

·           a combination of both. 

The likely potential for each mode of transmission are clearly dependent on the geological and hydrogeological conditions, which are discussed below.

7.3.1                           Classification of Pathways – from SENT Landfill to SENTX

At present, the potential pathways for migration of landfill gas from the SENT Landfill to the SENTX Site in TKO Area 137 are considered to comprise mainly natural features and reclamation fill.  The formation drain below the base liner system of SENT Landfill provides a direct anthropogenic landfill gas migration pathway connecting the SENT Landfill to the SENTX Site.  However, this drain is to be extended as part of the SENTX construction with a gravity connection drain which is not directly connected to the infrastructure area of SENTX or the temporary construction site office (see Figure 5.5).  The design of the SENTX will piggyback onto the southern slope of the SENT Landfill where part of the SENT Landfill capping system will be removed and will form a migration pathway for landfill gas to migrate to the portion of SENTX directly over the southern slope of the SENT Landfill.  As presented in the approved EIA Report, the pathway for landfill gas migration from the SENT Landfill to the SENTX was classified as “very short/direct” taking into account the distance between the SENT Landfill and the SENTX and the presence of possible migration pathways between the two landfills.

Similar to the EIA Scheme, the latest scheme also share a similar footprint at the piggyback section of the SENT Landfill and TKO Area 137.  Whilst it is known that underground utilities to be provided from existing SENT Landfill site to the SENTX site will include electricity, telecommunication cables, water mains, sewers, storm water drains, Towngas, Fire Services, landfill gas and leachate pipes.  Among all utilities, electricity and telecommunication cables will provide a direct man-made pathway for the transmission of landfill gas from SENT Landfill to SENTX.  Nevertheless, landfill gas control measures e.g. seal cable duct with bentonite will be implemented to minimise the potential risks.  Based on the latest scheme, the pathway is classified as “very short/direct” for SENTX construction site and tipping face but “long/indirect” for SENTX infrastructure area.

7.3.2                           Classification of Pathways – from the SENTX Waste Boundary to the SENTX Infrastructure Area

The potential pathways for sub-surface migration of landfill gas from the SENTX waste boundary to the future infrastructure area are considered to comprise both reclamation fill and the future utilities connecting the infrastructure area.  At present, no direct anthropogenic migration pathways (man-made underground utilities) have been identified as connecting the SENTX to the adjacent TKO Area 137.  Taking into account the distance and the presence of possible migration pathways between the SENTX waste boundary and the SENTX infrastructure area, the pathway for landfill gas migration from the SENTX waste boundary to the future infrastructure area was classified as “very short/direct” in the approved EIA Report.

Based on the detailed design of the new infrastructure area, the pathway for landfill gas migration from the SENTX waste boundary to individual target at new infrastructure area should be classified as according to the presence of possible migration pathways and distance between the target and the SENTX waste boundary: <50m as very short/direct, 50-100m as moderately short/indirect, 100-250m as long/indirect (see Table 7.1).

Table 7.1        Classification of Landfill Gas Migration Pathway

Targets

Pathway Description

Classification

1.        Weighbridge Office

Path length of less than 50m for unsaturated permeable strata or less than 100m for man-made conduits

Very short/direct

2.        Security Office

3.        Bioplant Building

4.        Maintenance Building

5.        Fire Service Building

6.        Gas Plant Building

Path length of 50 to 100m for unsaturated permeable soil or 100 to 250 m for man-made conduits

Moderately short/direct

7.        Workshop/Laboratory Building

8.        GVL Building

9.        EPD Building

Path length of 100 to 250m for unsaturated permeable soils

Long/indirect

7.4                                   Targets

Landfill gas related impacts may occur in areas at or below ground, at the SENTX.  The targets identified in the latest scheme are presented below.

7.4.1                           Target 1 – Construction Site of the SENTX

As shown in Figure 1.2, the majority of the SENTX Site falls within the 250m Landfill Consultation Zone of the SENT Landfill.  Demolition and minor excavation at the existing infrastructure area are expected.  The excavation area and the areas of confined space and trenches, if any, are at a higher risk of exposure to landfill gas.  However, in general, any excavation work or work involving the construction of trenches will use the open cut method, although there may be deep excavations.  Landfill gas, if any, migrated to the site can easily be dispersed and diluted in the atmosphere.  Construction works involving working in confined spaces will be undertaken by trained workers. 

The main temporary site office (see Figure 7.1) for construction phase will be provided with multiple landfill gas control measures (including provision of mechanical or natural ventilation and continuous gas monitoring system with gas alarm for all ground floor rooms).  While for other temporary site offices located within the SENTX site boundary but outside the 250m Landfill Consultation Zone of the SENT Landfill (see Figure 7.2), safety measures given in Section 8.3 would not be required as the proposed offices will be constructed and subsequently removed prior to the operation of SENTX, which is considered that the SENTX Landfill Consultation Zone would not be in force.

This target was thus classified as “medium sensitivity” in the approved EIA Report which is also applicable for the latest scheme.

7.4.2                           Target 2 – Operation of the SENTX (Tipping Face)

The majority of the waste tipping face will be carried out within the 250m Landfill Consultation Zone of the SENT Landfill.  Waste tipping will be in the open air which will not involve working at confined spaces and by definition, the SENTX is also a source of landfill gas, thus any migration of landfill gas to the SENTX will either be dispersed and diluted upon contact with the atmosphere or be captured in the landfill gas collection system of the SENTX.  In addition, all landfill equipment is designed to work under conditions where flammable gas may present.  However, it is also noted that drivers/operators of waste collection vehicles will have access to the waste tipping face for disposal of waste and they may not have knowledge on landfill gas hazards.  In view of the above, this target was thus classified as “medium sensitivity” in the approved EIA Report which is also applicable for the latest scheme.

7.4.3                           Target 3 - Operation of the SENTX (Infrastructure Area)

The proposed new infrastructure area of the SENTX will be outside the 250m Landfill Consultation Zone of the SENT Landfill but within the 250m Landfill Consultation Zone of the SENTX.  Landfill gas related impacts are most likely to occur in areas at, or below grade, at the SENTX site.  The assessment of potential targets for landfill gas sensitivity in the infrastructure area have thus been selected from the below ground and ground floor rooms of the buildings and structures.  Ground level offices and pump rooms with underground utility connections and restricted access by authorised and well trained personnel, and ground level offices and rooms with unrestricted staff access were classified as “medium” and “high” sensitivity in the approved EIA Report respectively.

Based on the guidance given in EPD’s Guidance Note, the sensitivity of all targets in the infrastructure area is summarised in Table 7.2.  Multiple options for landfill gas control measures will be utilised in the detailed design, for example:

(a)  Gas barrier

-              impermeable gas membrane to be installed below the base slab of the building; or

-              the internal floor slab of the ground floor rooms will be painted with low gas permeability paints (see Annex B for the proposed products to be used) ([6]); or

-              building are raised above ground with sufficient ventilation to prevent accumulation of landfill gas; and

(b)  Ventilation

-              ground floor rooms will be provided with mechanical or natural ventilation ([7]) to prevent potential accumulation of landfill gas; and

(c)   Gas alarm

-              all occupied on-site buildings will be provided with gas alarm.

Typical details of gas protection measures are shown in Figure 7.3.  Taking into account the combination of landfill gas control measures that has been incorporated in the detailed design of the SENTX infrastructure area, GVL’s expertise in landfill operation who is fully aware of the potential landfill gas hazards and with their staff well trained on the potential hazards relating to landfill gas and the specific safety procedures, the targets in the SENTX infrastructure area are conservatively classified as “very low to low” sensitivity. 

Table 7.2        Sensitivity of all Targets in the Infrastructure Area

Targets

Description/ Proposed Mitigation Measures

Sensitivity

Preliminary analysis

With incorporation of Control Measures in the Detailed Design

Target 3.1a – Control room of Weighbridge Office

·    Above ground room

·    With air conditioning and natural ventilation

·    With gas-proofing coating or liner protection or raised floor

·    With gas alarm

·    Restricted access by authorised personnel

Medium to High (a)

Low

Target 3.1b – Toilet of Weighbridge Office

·    Above ground room

·    With mechanical ventilation (10 air changes per hour)

·    With gas-proofing coating or liner protection or raised floor

·    With gas alarm

·    Restricted access to staff only

Medium to High (a)

Low

Target 3.2a – Control room of Security Office

·    Above ground room

·    With air conditioning and natural ventilation

·    With gas-proofing coating or liner protection or raised floor

·    With gas alarm

·    Restricted access by authorised personnel

Medium to High (a)

Low

Target 3.2b – Toilet of Security Office

·    Above ground room

·    With mechanical ventilation (10 air changes per hour)

·    With gas-proofing coating or liner protection or raised floor

·    With gas alarm

·    Restricted access to staff only

Medium to High (a)

Low

Target 3.3a – Blower room, Chemical storage room, Workshop and FM200 cylinder room of Bioplant Building

·    Above ground room

·    With natural ventilation

·    With gas-proofing coating or liner protection or raised floor

·    With gas alarm

·    Restricted access by competent person

Medium to High (a)

Low

Target 3.4a - Maintenance area of Maintenance Building

·    Above ground room

·    Open area

·    With natural ventilation

·    Restricted access by competent person

Medium to High (a)

Low

Target 3.4b - Maintenance manager’s room, Operations supervisor’s room and Operations manager’s room of Maintenance Building

·    Above ground room

·    With air conditioning and natural ventilation

·    With gas-proofing coating or liner protection or raised floor

·    With gas alarm

·    Restricted access by authorised personnel

Medium to High (a)

Low

Target 3.4c - Female toilet & shower and Male toilet & shower of Maintenance Building

·    Above ground room

·    With mechanical ventilation (10 air changes per hour)

·    With gas-proofing coating or liner protection or raised floor

·    With gas alarm

·    Restricted access to staff only

Medium to High (a)

Low

Target 3.4d - Parts/tools storage of Maintenance Building

·    Above ground room

·    With natural ventilation

·    With gas-proofing coating or liner protection or raised floor

·    With gas alarm

·    Restricted access by authorised personnel      

Medium to High (a)

Low

Target 3.4e - Plumbing & electrical room of Maintenance Building

·    Above ground room

·    With natural ventilation

·    With underground utilities connection

·    With ignition source

·    With gas-proofing coating or liner protection or raised floor

·    With gas alarm

·    Restricted access by competent person

Medium to High (a)

Low

Target 3.4f – Reception and Pantry and lunch room of Maintenance Building

·    Above ground room

·    With air conditioning and natural ventilation

·    With gas-proofing coating or liner protection or raised floor

·    With gas alarm

·    Restricted access to staff only

Medium to High (a)

Low

Target 3.4g - First aid room of Maintenance Building

·    Above ground room

·    With natural ventilation

·    With gas-proofing coating or liner protection or raised floor

·    With gas alarm

·    Restricted to access staff only

Medium to High (a)

Low

Target 3.4h - Fire hose reel pump room of Maintenance Building

·    Above ground room

·    With natural ventilation

·    With underground utilities connection

·    With ignition source

·    With gas-proofing coating or liner protection or raised floor

·    With gas alarm

·    Restricted access to staff only

Medium to High (a)

Low

Target 3.5a – Fire Service pump room of Fire Service Building

·    Above ground room

·    With mechanical ventilation (5 air changes per hour)

·    With underground utilities connection

·    With ignition source

·    With gas-proofing coating or liner protection or raised floor

·    With gas alarm

·    Restricted access by competent person

Medium to High (a)

Low

Target 3.5b – Fire Service control room and store room of Fire Service Building

·    Above ground room

·    With ventilation system as per Fire Services Department requirement

·    With gas-proofing coating or liner protection or raised floor

·    With gas alarm

·    Restricted access by authorised personnel

Medium to High (a)

Low

Target 3.6a – Toilet of Gas Plant Building

·    Above ground room

·    With mechanical ventilation (10 air changes per hour)

·    With gas-proofing coating or liner protection or raised floor

·    With gas alarm

·    Restricted access to staff only

Medium to High (a)

Low

Target 3.6b – Store room and Control room of Gas Plant Building

·    Above ground room

·    With air conditioning and natural ventilation

·    With gas-proofing coating or liner protection or raised floor

·    With gas alarm

·    Restricted access by authorised personnel

Medium to High (a)

Low

Target 3.6c – LV main switch room of Gas Plant Building

·    Above ground room

·    With natural ventilation

·    With underground utilities connection

·    With ignition source

·    With gas-proofing coating or liner protection or raised floor

·    With gas alarm

·    Restricted access by competent person

Medium to High (a)

Low

Target 3.6d – CLP transformer room of Gas Plant Building

·    Above ground room

·    With ventilation system as per CLP requirement

·    With underground utilities connection

·    With ignition source

·    With gas-proofing coating or liner protection or raised floor

·    With gas alarm

·    Restricted access by competent person

Medium to High (a)

Low

Target 3.6e – HV switch and transformer room and Rec transformer room of Gas Plant Building

·    Above ground room

·    With air conditioning and natural ventilation

·    With underground utilities connection

·    With ignition source

·    With gas-proofing coating or liner protection or raised floor

·    With gas alarm

·    Restricted access by competent person

Medium to High (a)

Low

Target 3.7a - Reception of Workshop/ Laboratory Building

·    Above ground room

·    With air conditioning and natural ventilation

·    With gas-proofing coating or liner protection or raised floor

·    With gas alarm

·    Restricted access to staff only

Medium to High (a)

Low

Target 3.7b - Dangerous goods storage room and Dry gas bottle room of Workshop/Laboratory Building

·    Above ground room

·    With natural ventilation

·    With gas-proofing coating or liner protection or raised floor

·    With gas alarm

·    Restricted access by competent person

Medium to High (a)

Low

Target 3.7c – Storage of Workshop/ Laboratory Building