|
Detailed Landfill Gas Hazard
Assessment Report |
Contents
1.3 PREVIOUS STUDIES UNDERTAKEN AT THE SITE
2 REVIEW OF PRELIMINARY QUALITATIVE RISK ASSESSMENT
3 LANDFILL GAS HAZARD ASSESSMENT METHODOLOGY
4.2 HISTORICAL LANDFILL GAS AND LEACHATE CONTROL
4.3 LANDFILL GAS CONTROL FOR THE NENTX
5 POTENTIAL FOR THE DEVELOPMENT TO INTERCEPT LANDFILL GAS
6.2 DESCRIPTION OF THE INFRASRUCTURE AREA
6.4 SENSITIVE TARGET FACILITIES FOR LANDFILL GAS
RISK
7 QUALITATIVE ASSESSMENT OF RISK DUE TO LANDFILL GAS
7.6 WITHIN LANDFILL EXTENSION SITE
7.7 OUTSIDE LANDFILL EXTENSION SITE
7.9 SOURCE-PATHWAY-TARGET ANALYSIS
8.1 GENERAL HAZARDS RELATED TO LANDFILL GAS
8.2 PRECAUTIONARY AND PROTECTION MEASURE - DESIGN
PHASE (FOR THE CONSTRUCTION OF NENTX)
8.4 PRECAUTIONARY AND PROTECTION MEASURE -
CONSTRUCTION PHASE
8.6 PRECAUTIONARY AND PROTECTION MEASURE –
OPERATION, RESTORATION AND AFTERCARE PHASES
9 ENVIRONMENTAL MONITORING AND AUDIT
Figures
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Project
Layout Plan |
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|
250m
Consultation Zone of NENTX Landfill |
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|
Typical Detail of the Composite Liner System |
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|
Location of Landfill
Gas Monitoring Wells of NENT Landfill |
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NENTX Base
Liner System |
|
|
Geological
Map |
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|
Preliminary
Layout Plan of Proposed Utility Services of NENTX |
|
|
Location of
Proposed Infrastructure Building |
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|
Proposed Lower/Ground Floor Plan of Infrastructure Area |
|
|
Temporary
Site Office of the Existing NENT |
|
|
Typical
Details of Gas Protection Measures |
|
|
Location of
Landfill Gas Monitoring Wells of NENTX |
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Appendices
|
Landfill Gas (LFG)
Management System for NENTX |
|
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Typical Details of Low
Gas Permeability Paints |
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|
Typical Details of Gas
Detection System |
1.1.1
The North East New Territories Landfill Extension (the NENTX Project) is
located adjacent to the existing North East New Territories (NENT) Landfill at
Ta Kwu Ling. The extension site is located in a valley covering mainly the
existing NENT Landfill Stockpile and Borrow Area that was formed to the east of
the existing landfill as part of the original site development of the landfill,
project location and layout plan shown in Figure 1.1. NENTX 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.
1.1.2
The NENTX is a designated project. The Environmental Impact Assessment
(EIA) Report (AEIAR-111/2007) and an Environmental Monitoring and Audit Manual
were approved on 20 September 2007. The project is governed by an Environmental
Permit (EP) (EP-292/2007) which was granted on 26 November 2007. A further of
EP (FEP) was applied and the FEP (FEP-01/292/2007) was subsequently granted on April
2022.
1.1.3
As per requirement of EP condition 2.11 and FEP Condition 2.9, 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).
1.1.4
Aurecon Hong Kong Limited (Aurecon) was appointed by Veolia Environmental Services Hong Kong Limited (Veolia) as the ET 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 NENTX and to recommend appropriate measures to ensure NENTX
can be constructed and operated without undue risk to safety.
1.1.5
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 Veolia, 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 leads the environmental team which includes an
experienced landfill gas hazard specialist.
1.1.6
For the purpose of this DLFGHA Report, the NENTX schemes assessed in the
approved EIA Report and the latest NENTX 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).
1.1.7
It should also be noted that this Report is related to the potential
landfill gas hazards due to the operation of the existing NENT Landfill to the
construction and operation of NENTX and the operation of NENTXof NENTX to
the infrastructure facilities of the NENTX and establishment of the necessary
control measures to minimise the risks identified.
1.2.1
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 NENTX site falls within the NENT 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 NENT Landfill to the construction
and operation of the NENTX. This assessment considered both landfill gas
sources (i.e. existing NENT Landfill and NENTX). A Practice Note for
Professional Persons (ProPECC PN 3/96 “Landfill Gas Hazard Assessment for Development
adjacent to Landfills”) (2) and Guidance Note on the assessment of the hazards
which landfill gas may present to developments close to landfills have been
issued by the EPD.
·
Annex 7 and Annex 19 of the
Technical Memorandum on EIAO (TM-EIAO);
·
North East New Territories
(NENT) Landfill Extension – Feasibility Study: Environmental Impact Assessment
Report, Ove Arup & Partners Hong Kong Ltd. 2007
·
Landfill Gas Hazard
Assessment: Guidance Note, EPD 1997
·
ProPECC PN 3/96 “Landfill
Gas Hazard Assessment for Development adjacent to Landfills (2), EPD 1996
1.4.1
The following tasks have been undertaken as part this assessment:
· review of background information (including landfill gas monitoring data) and studies related to the NENT Landfill and the NENTX;
· identification of the nature and extent of the NENT Landfill and NENTX which might have potential impacts on the construction and operation of NENTX;
· 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 NENTX; and
· identification of the potential targets associated with the NENTX which are sensitive to the landfill gas risk.
1.5.1
Following this introductory section, the remainder of this Report is
arranged as follow:
· 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 NENT Landfill and NENTX 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 NENTX 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 the NENTX site;
· Section 8 provides further recommendations for precautionary and protection measures to be adopted during the design, construction and operation of the NENTX 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 NENTX; and
· Section 10 concludes the findings and recommendations of this DLFGHA.
Table 2.1 Qualitative risk assessment of
LFG hazards associated with NENT Landfill Extension
2.1.2
Precautionary and protection measures during design, construction and
operation/restoration phases of the NENTX have been recommended in the approved
EIA Report.
2.1.3
The design phase of suitable level of significant engineering measures
will be required to protect the planned development and activities within. According
to the LFG Guidance Note, an active gas control system supported by gas
barriers and monitoring systems will be required to protect the planned
development for “High” risk level. Detailed examples of these measures can be
reference to EPD’s Guidance Note.
3.1.1
A relatively simple procedure is used to evaluate the degree of risk which
landfill gas may create for a particular development. The procedure is based on
the Source - Pathway -Target method recommended in the Guidance Note on
Landfill Gas Hazard Assessment, EPD, 1997 as described below:
·
Source - the location, nature and likely quantities/ concentration of landfill
gas which have the potential to affect the landfill extension.
·
Pathway - the ground and groundwater conditions, through which the landfill
gas must pass if they are to reach the development.
·
Target - the elements of the development that are sensitive to the
effects of the landfill gas.
3.1.2
The landfill gas source, pathway and targets are categorised for the
hazard assessment. An assessment of the overall risk is made based on the risk category
as summarised in below, following the combination of existing NENT Landfill and
NENTX fall into during the construction, operation, restoration and aftercare
stages.
3.2.1
The classification of the Source (i.e. the landfill) is determined as
follows:
·
Minor - Landfill sites at
which gas controls have been installed and proven to be effective by
comprehensive monitoring which has demonstrated that there is no migration of gas
beyond the landfill boundary (or any specific control measures) and at which
control of 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 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 or 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.
3.3.1
The type of pathway can be broadly classified based on various geological
features of the landfill extension site such as permeability of soil, spacing,
tightness and direction of fissures/ joints; topography; depth and thickness of
the medium through which the gas may migrate( also affected by groundwater
level); nature of strata over the potential pathway; number of media involved;
and depth to groundwater table and flow patterns, etc. In general, the type of
pathway can be broadly classified 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 |
|
·
Moderate 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 |
3.4.1
Different types of targets may be broadly 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 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 gas could pose potential problems. |
3.4.2
The above examples of the different categories should be used as a
general guide only and particular aspects of a building or development may
render it more or less sensitive than indicated. Account should be taken of any
particular circumstances when assigning a target to one of the three
categories.
3.4.3
A qualitative assessment of the overall risk is made based on the risk
category as summarised in Table 3.1, which is extracted
from Guidance Note. For the purpose of categorising the landfill extension
site, the category is based upon the highest level of risk determined for any
of the potential impacts identified in Table 3.1, in which the
general implications fall into different overall risk categories as show in Table 3.2.
Table 3.1 Classification
of Risk Category
|
Source |
Pathway |
Receiver
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 |
High |
|
|
Medium |
Medium |
||
|
Low |
Very
Low |
||
|
Minor |
Very
Short/Direct |
High |
High |
|
Medium |
Medium |
||
|
Low |
Low |
||
|
Moderately
short/Direct |
High |
High |
|
|
Medium |
Medium |
||
|
Low |
Very
Low |
||
|
Long/Indirect |
High |
High |
|
|
Medium |
Medium |
||
|
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. |
4.1.1
NENT Landfill is located at Ta Kwn Ling, New Territories and the landfill
is one of the three strategic landfills in operation in the HKSAR and was
designed with a capacity of approximately 35Mm3 of waste. The landfill
commissioned in 1995 and receives municipal waste, construction waste and
special waste from the North East New Territories, and Yuen Long and Sha Tin Transfer
Stations solely by road, with an occupying about 108ha total site area and 64ha
landfilling area of land.
4.1.2
The Environmental Protection Department (EPD) of the HKSAR Government
commissioned a study in Year 2000 on the Extension of Existing Landfills and
Identification of Potential New Waste Disposal Sites. Amongst the potential
sites recommended in this territory-wide study is the extension of the existing
NENT Landfill, with a target capacity of about 19 Mm3. The proposed extension, of about 70 ha, is
located immediately east of the existing NENT Landfill. A large proportion of
the Extension area is in fact the Stockpile and Borrow Area of the existing
landfill.
4.2.1
The existing NENT Landfill has been incorporated with an efficient and effective
Landfill Gas (LFG) management system (Appendix A) in which a coordinated approach to LFG monitoring, collection,
extraction, flaring and utilisation in accordance with international best
practices for landfill operations. The
LFG extraction system includes extraction wells and pipework installed within
the landfill cap connecting a pumping station on-site. It also incorporates a
separate collection system connected to perimeter extraction wells isolated
from the main system so that it is operated independently for migration control.
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 NENT Landfill are presented in Figure 4.1.
Under the existing contract, Veolia 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 boundary of NENT Landfill have been
reviewed. Figure 4.2 shows the locations of monitoring wells and the landfill gas monitoring
results of the existing NENT Landfill are summarised in Table 4.1.
Table 4.1 Summary of Landfill Gas Monitoring Results of the Monitoring Wells for Existing
NENT (From January 2017 to December 2021)
Notes:
*For the gas concentration of Methane, <0.1% gas of monitoring result was detected for all monitoring wells which were under the detection limit
therefore 0% of Methane gas concentration has been assumed for the Table 4.1 calculation.
4.2.2
Nil or minimal concentration of methane have been observed for all
monitoring wells along the boundary of NENT Landfill which indicate that there
is no sub-surface off-site migration of methane of the landfill.
4.2.3
The average Carbon Dioxide (CO2) concentrations detected in
all the these monitoring wells ranged from 0.6% to 4.1% (v/v) while the maximum
gas concentrations ranged from 1.8% to 9.0% (v/v). Abnormality of CO2
level was record in some monitoring wells, the cause of abnormalities were the presence
of the carbonate source and natural biological activities in the soil and it
was considered due to natural source and not due to landfill activities and all
the abnormalities has been reviewed by the IC and it was concluded that all of
them were not originated from the existing NENT Landfill.
4.2.4
The Site is in a valley to the southeast of the existing NENT Landfill.
The Valley is encircled by three ridgelines and exits to the southwest through
a small gorge.
4.2.5
The valley has an area of about 70 ha and overlaps with the stockpile and
borrow area of the existing NENT Landfill. NENTX is designed with an estimated
void space of landfill capacity of not less than 19Mm3 to receive
Municipal Solid Waste (MSW) over a period of approximately 10 years. It is
anticipated that construction and operation of NENTX will commence in 2022 and
2026, respectively.
4.3.1
The future landfill extension itself is a source to generate significant
amount of LFG during the operation and aftercare phases. With the LFG control
measures in the future NENTX, it is anticipated that the source of LFG will be
properly controlled within the site similar to the operation in existing NENT
Landfill.
4.3.2
NENTX will be designed and constructed to incorporate extensive measures
to contain, collect, and treat landfill gas and leachate. These measures include
a composite liner system, active gas extraction systems and gas control systems
in accordance with international best practices for landfill operation. These
measures can effectively control sub-surface off-site migration of landfill gas
and leachate. The base liner systems are designed as secure contaminant systems
consisting of multi-layer impermeable liners, to contain leachate and landfill
gas, LFG generated during waste deposition. Details of the base liner system
designed for the NENTX is shown in Figure 4.3.
4.3.3
A comprehensive environmental monitoring programme will be implemented
during the construction, operation, restoration and aftercare of the NENTX to
monitor landfill gas generated within the NENTX and at the gas monitoring wells
along the site boundary of NENTX and off-site leachate migration/ groundwater
contamination. With reference to the performance standard stipulated in the NENTX
contract, Veolia 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 NENTX), respectively. Veolia 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.
4.3.4
In conclusion, the future landfill extension itself is a source to
generate amount of LFG during the Operation and Aftercares phases and another
source of LFG is associated with the decomposition of MSW landfilled at NENT
Landfill, which is located within (from waste boundary) from some NENTX
infrastructure area.
5.1.1
The NENTX site has a variable thickness of superficial deposits across
the majority of its area, comprising fill, colluvium and soil derived from in-situ
weathering. The solid geology of the site comprises a mixture of volcanic tuff
deposits, sightly metamorphosed volcanic tuff deposits and a small area of
meta-sedimentary rock. The rockhead levels within the site are largely
reflected by the topography, with low-lying valleys having been formed due to
the presence of geological faults.
Four major
faults are presented within the site (see Figure
5.1):
Fault 1
is a located along the northern boundary of the study area and striking
approximately west-northwest.
Fault 2
strikes north-northeast through the study area, most likely extending through
the existing waste reception area to the south of the site and then following
the topographic valley northeast from this. Previous works has recorded this
fault as dipping at 80O to the southeast, with the material on the south
eastern side having been downthrown.
Fault 3
follows the approximate alignment of the existing haul road through the centre
of the site area, trending west-northwest to east-southeast, and extends beyond
the site boundaries.
Fault 4
is mostly like a large splay fault associated with Fault 2. The main trend of
the fault is north-northeast along the topographic valley to the east of Fault
2, with a small potion trending east-northeast to the south of the haul road
before joining with Fault 2 in the low-lying area within the southern part of
the site.
5.2.1
It is known that underground utilities (landfill gas collection pipes,
electricity, telecommunications etc.) exist
in the existing infrastructure area of NENT Landfill in Portion D. Also, the new
infrastructure area will integrate within the existing NENT supporting
facilitates located in Portion D of the site in the future. The utilities to be provided from existing NENTX
include electricity, telecommunication cables, water mains, sewer, storm water
drains, landfill gas and leachate pipes etc. along the NENTX. Electricity and
telecommunication cables will be laid underground in ducts which could provide
a direct man-made pathway from NENT Landfill to NENTX. A preliminary layout plan for the proposed utility
services to NENTX is presented in Figure 5.2.
6.1.1
Based on the findings and recommendations of the preliminary QLFGHA of
the approved EIA Report, Veolia has incorporated landfill gas control measures
in the detailed design of the NENTX 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.
6.2.1
In accordance with the EIA and Specification requirements, the new
infrastructure area (see Figure
6.1 - Location of Proposed Infrastructure Buildings and Figure
6.2a to 6.2e - the proposed ground
floor plan of the infrastructure area) will integrate within the existing NENT
supporting facilities located in Portion D of the site. According to the consultation
zone of Guidance Notes (GN), it should be noted that the new infrastructure
area is located within 250m from the existing NENT and NENTX waste boundary.
Therefore, the key risk source for the new infrastructure area is the NENT and
NENTX. The infrastructure area includes the
following building/ structures:
·
Integrated Office Building
·
Operations and Maintenance Building and Maintenance Workshop Building
·
Fire Services Building
·
Process Building
6.2.2
As the Portion D will also house other supporting arrangements such as
car parking, vehicle maintenance yards and landfill vehicle wash bay are
operated in open space or non-enclosed building, there will be no or very low
potential for landfill gas cumulation at these facilities. Therefore, they are
not further assessed in this report.
6.2.3
The approximate distance from the NENTX waste boundary defined in the GN to
the each of the target facilities at the new infrastructure area are presented
in Table 6.1. All facilities are operated/managed by Veolia.
Table
6.1 Key Target Facilities in the New
Infrastructure Area
|
Key Facilities |
Approximately Distance from the NENTX |
Distance from NENT |
|
Integrated Office Building |
215m |
135m |
|
Fire Services Building |
205m |
165m |
|
Operations and Maintenance Building and Maintenance Workshop
Building |
105m |
100m |
|
Process Building |
195m |
30m |
Remarks: Under Annex 7 of the Technical Memorandum on EIA Process
(EIAO-TM) and Landfill Gas Hazard Assessment: Guidance Note, EPD 1997, 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 NENTX proposed infrastructure area(Integrated Office
Building, Fire Services Building, Operations and Maintenance Building and
Maintenance Workshop Building and Process Building) are falls within the NENT and NENTX Landfill
Consultation Zone, a Qualitative Landfill Gas Hazards Assessment (QLFGHA) is
required to assess the potential risk due to landfill gas migration from the NENT
Landfill/ NENTX(Assessment details please refer to the below Section).
6.3.1
Simple excavation and slope formations works will be carried out during
the construction stage. The site
formation works for the landfill lining system will require some open
excavations works due to slightly lower formation level for the basal liner. The temporary works will involve the formation
of temporary ditches along the sides of excavations and associated drainage
works, and material storage areas.
Integrated Office Building
6.4.1
The Integrated Office Building is designed as 3-storey building, ground
floor covering a total area of approximately 500 m2. Ground level rooms
include Storage, Laboratory, Female/ Male Toilets, Shower & Lockers, Accessible
Toilet, Pantry, M& E Room etc. (see Figure 6.2a). 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 Integrated Building
|
Room |
No. of Air Change Per Hour (*) |
|
|
1. |
Lobby & Waiting Area (Reception) |
5 |
|
2. |
Storage |
6 |
|
3. |
Laboratory |
10 |
|
4. |
Storage Room (1) |
6 |
|
5. |
Male Toilet, Shower & Lockers |
15 |
|
6. |
Female Toilet, Shower & Lockers |
15 |
|
7. |
Accessible Toilet |
15 |
|
8. |
Pantry |
10 |
|
9. |
Staff Rest Room |
5 |
|
10. |
Employer/ ER (Open Plan Office) |
6 |
|
11. |
Conference/ Meeting Room |
6 |
|
12. |
Conference/ Meeting Room |
6 |
|
13. |
Conference/ Meeting Room |
6 |
|
14. |
Interview Room |
5 |
|
15. |
M&E Room |
10 |
|
16. |
Special Storage Room cum Monitoring
Room |
6 |
|
Note: *Refer to fresh
air change rate. A higher air change rate is maintained with air re-circulation.
|
||
Fire & Water Services Building
6.4.2
The Fire Services Building is 2-story high structure, ground floor covering
a total area of approximately 250 m2. Ground level rooms include Fire Services Control
Room, Fire Services
Pump and Tank Room, Flushing Water and Tank Room and Sprinkler and Street Hydrant
Pump and Tank Room. (seeSee
Figure 6.2b). These areas will
be provided with mechanical ventilation or air conditioning with natural
ventilation. The designed air change for each room is presented in Table 6.3.
Table 6.3 Designed Air Change
of the Ground Floor Rooms of Fire & Water Services Building
|
No. |
Room |
No. of Air Change Per Hour (*) |
|
1. |
FS Control Room |
5 |
|
2. |
FS Pump and Tank Room |
5 |
|
3. |
Flushing Water and Tank Room |
5 |
|
4 |
Sprinkler and Street Hydrant Pump and
Tank Room |
5 |
|
Notes: *Refer to fresh air change rate. A higher air
change rate is maintained with air re-circulation. |
||
Operations and Maintenance Building and Maintenance Workshop Building
6.4.3
The Operations and Maintenance Building is functional, rectangular
3-storey building, ground floor covering a total area of approximately 200 m2.
Ground level rooms include Parts/ Tools Storage, Dry Gas Bottle Room, Plumbing
Room, ELE Room and Pumping Room (see Figure 6.2c). These areas will be provided with mechanical ventilation. The designed
air change for each room is presented in Table 6.4.
Table 6.4 Designed Air Change
of the Ground Floor Rooms of Operations and Maintenance Building
|
No. |
Room |
No. of Air Change Per Hour (*) |
|
1. |
Workshop Area |
6 |
|
2. |
Parts/ Tools Storage |
6 |
|
3. |
Dry Gas Bottle Room |
10 |
|
4. |
Plumbing Room |
10 |
|
5. |
ELE Room |
10 |
|
Notes: *Refer to fresh air change rate. A higher air
change rate is maintained with air re-circulation. |
||
Process Building
6.4.4
The Process Building will be constructed with front and rear façades on
two different site formation platform levels. The building’s Lower Ground floor
will be constructed abutting a retaining wall at its rear and partly adjacent
to sloping ground on one side. However, all of the Process Building’s rooms
will be located above the respective ground level at the front and rear façades
of the building, including the rooms of the Lower Ground floor. The building
will house the electrical switch room and workshop in its lower ground floor
level; blower room, toilets, changing room etc. on the ground floor. (seeSee
Figure 6.2d&e). These areas will
be provided with mechanical ventilation or air conditioning with natural
ventilation. The designed air change for each room is presented in Table 6.5 & Table 6.6.
Table 6.5 Designed Air Change
of the Lower Ground Floor Rooms of Process Building
|
No. |
Room |
No. of Air Change Per Hour (*) |
|
1. |
HV Switch Room |
10 |
|
2. |
CLP Room |
10 |
|
3. |
LV Room LFG |
10 |
|
4. |
Common Process Workshop |
10 |
|
Notes: *Refer to fresh air change rate. A higher air
change rate is maintained with air re-circulation. |
||
Table 6.6 Designed Air Change
of the Ground Floor Rooms of Process Building
|
No. |
Room |
No. of Air Change Per Hour (*) |
|
1. |
Office |
5 |
|
2. |
Storage (1) |
6 |
|
3. |
Storage (2) |
6 |
|
5. |
Common Pantry |
15 |
|
6. |
Hose Reel Pump Room |
10 |
|
7. |
Blower Room for SBRS |
10 |
|
8. |
SNG Plant Control Room |
10 |
|
9. |
SNG MCC Room |
10 |
|
10. |
Males
Showers, Toilets & Lockers |
15 |
|
11. |
Disable Toilet |
15 |
|
Notes: *Refer to fresh air change rate. A higher air
change rate is maintained with air re-circulation. |
||
7.2.1
The existing NENT Landfill and the NENTX will both be the source of
potential risk of landfill gas migration. NENT and NENTX have the same
potential for landfill gas generation. The
source of LFG at the existing NENT landfill and extension NENT Landfill are
categorised as Medium. The 250m
consultation zones for the NENT Landfill and the NENTX are shown in Figure 1.2.
7.4.1
The future landfill extension itself is a source to generate significant
amount of LFG during the operation and aftercare phases. LFG hazards may be hazard
to front-line workers within the site especially where the LFG is extracted, transported,
and processed. With the LFG control measures and Waste to Energy Facility in
the future NENTX, it is anticipated that the source of LFG will be properly
controlled within the site similar to the operation in existing NENT landfill.
7.4.2
The NENTX will be designed as a containment landfill with LFG collection
and management systems to eliminate any off-site migration of LFG. By virtue of
the effective control and utilisation of LFG being implemented in the existing
NENT landfill based on the past monitoring data, it is likely that the NENTX
will be designed to adopt similar LFG control measures so as to ensure future
landfill extension compliance of environmental and safety requirements. The source of LFG at the future landfill extension
was categorised as Medium.
7.5.1
The potential pathways through which landfill gas may enter the NENTX
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 conditions, which are discussed below.
NATURAL PATHWAYS
7.6.1
The major concern is the presence of Faults 1 and 3 as shown in Figure 5.1 across the existing
NENT Landfill towards the NENTX, which as natural pathways for preferential LFG
migration. These pathways of fissured rock are less than 50m to the landfill
extension site and are categorised as Very Short/Direct and the superficial
deposits located below the formation level to the south of the landfill extension
site may act as natural pathways for LFG migration towards the Waste Reception
Area of the existing NENT Landfill. These pathways of unsaturated permeable
strata are less than 50m to the landfill extension site and are categorised as
Very Short/Direct as presented in the approved EIA.
7.6.2
There are man-made pathways in the vicinity of the site consisting of
services routes leading to the existing landfill. These pathways to sensitive
receivers are classified as Very Short/Direct towards the landfill extension
site in the approved EIA Report, as landfill workers could be the targets
during landfill operation, piping/ conduit construction, and/ or drilling/
boring operation.
Also, the potential pathways or
sub-surface migration of landfill gas from the NENTX waste boundary to the
future infrastructure area are considered to comprise both reclamation fill and
the future utilities connecting the infrastructure area. It is known that the
new infrastructure area will integrate within the existing NENT supporting
facilities located in Portion D of the site in the future and the underground
utilities (landfill gas collection pipes, electricity, telecommunications
etc.) to be provide to the NENTX. Among
all utilities, electricity and telecommunications cables will provide a direct
man-made pathway for the transmission of landfill gas from NENT to NENTX.
Therefore, landfill gas control measures such as seal cable duct with bentonite
will be implemented to minimise the potential risk.
7.6.3
Considering the distance and the presence of possible migration pathways
between the NENTX waste boundary and NENTX infrastructure area. And based on
the detailed design of the new infrastructure area, the pathway for landfill
gas migration from the NENTX 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 NENTX waste
boundary: <50m as Very Short/Indirect, 50-100m as Moderately Short/Indirect,
100-250m as Long/Indirect. (seeSee Table 7.1)
NATURAL PATHWAYS
7.7.1
The presence of faults lines in NENT Landfill Extension site are
identified as the potential natural pathways for potential LFG migration to the
vicinities of sensitive receivers as shown in Figure 5.1. Fault 4 is the
nearest fault line to Tong To Shan Tsuen at a distance of ~280m which is
classified as “Long/Indirect” pathway extending from the northern bound of the
future landfill extension site. Mitigation measures will include installation
of proper liner to act as barriers and sealing of faults line ends to prevent
off-site LFG migration.
7.7.2
Although there are man-made pathways in the vicinity of the site
consisting of services routes leading to the existing landfill, they are far from
sensitive receivers and are classified as Long/Indirect toward the adjacent
occupied development in the approved EIA
Report.
Table 7.1 Classification of Landfill Gas Migration Pathway
|
Targets |
Pathway Description |
Classification |
|
1. Integrated Office Building |
Path length of 100 to 250m for unsaturated
permeable soil |
Long/Indirect |
|
2. Fire & Water Services
building |
Path length of 100 to 250m for unsaturated
permeable soil |
Long/Indirect |
|
3.Operations and Maintenance Building and Maintenance Workshop
Building |
Path length of 100 to 250m for unsaturated
permeable soil |
Long/Indirect |
|
4.Process Building |
Path length of 100 to 250m for unsaturated
permeable soil |
Long/Indirect |
WITHIN LANDFILL EXTENSION SITE
7.8.1
Potential receivers sensitive to LFG hazards associated with the NENT
Landfill Extension include the workers and staff of NENT Landfill Extension
Site. The targets identified in the latest scheme are presented below.
Target 1 - Construction Phase of the NENTX
7.8.2
As shown in Figure 1.2, some of NENTX infrastructure
site falls within the 250m landfill consultation zone of the NENT Landfill. Excavation
for construction of new landfill bowl and deep unventilated excavation works are
expected. The excavation area and the minimal 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 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 occupied on-site buildings). This target was thus classified as “High Sensitivity” in the approved
EIA Report which is also applicable for the latest scheme.
Target 2 - Operation Phase of the
NENTX (Infrastructure Area)
7.8.3
All of proposed new infrastructure area of NENTX will be within the 250m
Landfill Consultation Zone of the NENTX and existing NENT. The assessment of
potential targets for landfill gas sensitivity in the infrastructure area have
been selected from the below ground and ground floor rooms of the buildings and
structures. Services ducts or other confined spaces at basement or ground floor
level and waste reception area of the existing NENT were classified as “high sensitivity” in the approved EIA Report.
OUTSIDE LANDFILL EXTENSION SITE
7.8.4
According to the Wo Keng Shan Outline Zoning Plan (OZP) S/NE-WKS/10 extracted
from the “Statutory Planning Portal” of Planning Department (PlanD) website at https://www2.ozp.tpb.gov.hk/gos/default.aspx?planno=S%2fNE-WKS%2f10&lang=0#, the planned landuse
to the south of the landfill extension site consist mainly of “Green Belt” with
minor area for “Agriculture” and “Village” Type Development with the landuse governed
by the Town Planning Ordinance.
7.8.5
The northern part of the consultation zone of NENTX falls within the Tong
To Shan Archaeological Site which also imposes restrictions on any proposed
development/ re-development.
7.8.6
There are 2 nearest LFG receivers outside the NENTX including LFG1 (Wo
Keng Shan Tsuen) and LFG2 (Tong To Shan Tsuen. And LFG1 lies within the
original 250m consultation zone (at ~100m from the landfill site boundary). It
is therefore categorised under “High Sensitivity”) mentioned in approved EIA
Report.
7.8.7
LFG2 is a village house marginally outside the proposed new demarcation
of 250 consultation zone corresponding to the latest footprint of NENTX site area
(at ~270m from the NENTX site boundary). Although it is currently abandoned and
unoccupied, it will be prudent to pay due attention to the implementation of
all necessary protective measures if LFG2 were occupied in future.
7.8.8
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 Appendix B for the proposed products to be used); and
(b)
Ventilation
- ground floor rooms will be provided
with mechanical or natural ventilation to prevent potential accumulation of
landfill gas; and
(c)
Gas alarm
- all occupied on-site buildings will
be provided with gas alarm (see Appendix C for the common type
product to be used);
- check the fixed gas detector by the
calibration gas at least once per quarter in order to
verify the accuracy and alarm function
of the fixed gas detector;
- clear calibration / checking record
will be maintained; and
- 2 action trigger levels will be set
up (If the first action trigger level to be set at 10% LEL is
triggered, the Registered Safety
Officer (RSO) will be informed. If the second action
trigger level to be set at 20% LEL is
trigger, all personnel within the permanent building will
be evacuated.
7.8.9
Specification of gas protection measures are shown in Figure 7.2. Taking into
account the combination of landfill gas control measures that has been
incorporated in the detailed design of the NENTX infrastructure area, Veolia’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 NENTX infrastructure area is
conservatively classified as low.
Table 7.2 Sensitivity of all Targets in the Infrastructure Area
|
Description/ Proposed
Mitigation Measures |
Sensitivity |
||
|
Preliminary analysis |
With incorporation of
Control Measures in the Detailed Design |
||
|
Target 2.1a - Storage
Room, Employer/ ER Office, Conference/ Meeting Room and Special Storage Room
cum Monitoring Room of Integrated Office Building |
Mitigation Measures: |
High |
Low |
|
Target 2.1b - Lobby &
Waiting Area, Staff Rest Room and Interview Room of Integrated Office
Building |
Mitigation Measures: |
High |
Low |
|
Target 2.1c - Female/
Male Changing Room & Toilets, Accessible Toilet of Integrated Office
Building |
Mitigation Measures: |
High |
Low |
|
Target 2.1d - Laboratory and
Pantry of Integrated Office Building |
Mitigation Measures: |
High |
Low |
|
Target 2.1e - M&E
Room of Integrated Office Building |
Mitigation Measures: |
High |
Low |
|
Target 2.2a - Fire
Services Control Room, Fire Services Pump & Tank Room and Sprinkler
and Street hydrant Pump & Tank Room of Fire & Water
Services Building |
Mitigation Measures: |
High |
Low |
|
Target 2.2b - Flushing
Water & Tank Room and Sprinkler & Street Hydrant Pump & Tank Room
of Fire & Water Services Building |
Mitigation Measures: |
High |
Low |
|
Target
2.3a - Workshop Area, Parts/ Tools Storage of Operation &
Maintenance Building |
Mitigation Measures: |
High |
Low |
|
Target
2.3b - Dangerous Goods Storeroom, Dry Gas Bottle Room, Plumbing Room
and ELE Room of Operation &
Maintenance Building |
Mitigation Measures: |
High |
Low |
|
Target
2.4a - HV Switch Room of Process Building |
Mitigation Measures: |
High |
Low |
|
Target
2.4b – CLP Room of Process Building |
Mitigation Measures: |
High |
Low |
|
Target
2.4c - LV Room LFG of Process Building |
Mitigation Measures: |
High |
Low |
|
Target
2.4d - Common Process Workshop of Process Building |
Mitigation Measures: |
High |
Low |
|
Target 2.4e
- (Infrastructure Area of the NENTX - Office
of Process Building) (category: Low
sensitivity) |
Mitigation Measures: |
High |
Low |
|
Target
2.4f - Storage Room of Process Building |
Mitigation Measures: |
High |
Low |
|
Target 2.4g - Common
Pantry, Males Showers,
Toilets & Locker and Disable Toilet of Process Building |
Mitigation Measures: |
High |
Low |
|
Target 2.4h - Hose Reel Pump Room, Blower Room, SNG Plant
Control Room, SNG MCC Room of Process Building |
Mitigation Measures: |
High |
Low |
7.9.1
On the basis of the source, pathways and targets identified above, a
source-pathway-target analysis for the latest scheme has been undertaken and is
presented in Table 7.3 according to the assessment framework described in EPD’s Guidance Notes.
Different combination of source, pathway, and target result in a range of
overall potential hazards.
7.9.2
The source-pathway-target analysis shows that landfill gas risk posed by
the NENT Landfill and the NENTX under the latest scheme is low to high within
the NENTX waste boundary during both the construction and operation phases.
Whereas the risk posed by the NENTX to NENTX infrastructure area is low with
respect to the nature of the targets and the gas control measures incorporated
in the detailed design of the buildings of the new infrastructure area.
Table 7.3 Qualitative Assessment of Landfill Gas Hazard Associated with the NENTX in the Latest Scheme
8.1.1
The detailed design of the NENTX infrastructure area has taken into
account the findings and recommendations of the preliminary qualitative
landfill gas hazard assessment and the potential risk associated with
sub-surface migration of landfill gas from the NENTX and various measures have
been incorporated to minimise the potential risks.
8.1.2
This section of the Report provides advice and recommendations for
further control measures in addition to those measures being incorporated by Veolia
in the detailed design of the NENTX and other general good practices to be
implemented during the construction, operation, aftercare and restoration of
the NENTX to ensure that the NENTX development is safe with respect to landfill
gas hazard.
8.1.3
According to the LFG Guidance Note, engineering measures will be required
to protect the planned development with risk category at “High” Level (Table 3.2). Recommendations for protection and precautionary measures for
implementation in NENT Landfill Extension during the various Project phases are
discussed as follows.
8.1.4
All contractors participating in the works and operational staff should
be made aware of the potential of methane and carbon dioxide present in the
soil and all works should be undertaken on the basis of an "assumed
presence of landfill gas". In addition, the following properties of
landfill gas should be noted.
·
Methane is odourless and colourless, although in landfill gas it is
typically associated with numerous highly odoriferous compounds which gives
some warning of it presence. However,
the absence of odour should not be taken to mean that there is no methane.
Methane levels can only be reliably confirmed by using appropriately calibrated
portable methane detectors.
·
Methane is a flammable gas and will burn when mixed with air between
approximately 5 and 15% (v/v). If a mixture of methane and air with a
composition between these two values is ignited in a confined space, the
resulting combustion may give rise to an explosion. Methane is also an
asphyxiant.
·
Carbon dioxide, the other major component of landfill gas is an
asphyxiating gas and causes adverse health effects at relatively low concentrations.
The long-term Occupational Exposure Limit (OEL) is 0.5% (v/v). Like methane, it
is odourless and colourless and its presence (or absence) can only be confirmed
by using appropriately calibrated portable detectors.
·
Gas density. Methane is lighter than air whereas carbon dioxide is
heavier than air. Typical mixtures of landfill gas are likely to have a density
close to or equal to that of air. However, site conditions may result in a
ratio of methane to carbon dioxide which may make the gas mixture lighter or
Heavier than air. As a result, landfill gas may accumulate in either the base
or top of any voids or confined spaces.
8.2.1
Based on the source-path-target analysis in Section 7, the risk category
at the NENT during construction is high. This implies that significant
engineering measures will be required during the detailed design stage to
ensure that the construction at the NENTX is safe. As the NENTX will be designed,
built, and operated by an experienced landfill contractor (Veolia), relevant
engineering measures will be identified and implemented in accordance with the NENTX
Contract Specification requirements. These measures will include the placement
of liner and installation of landfill gas management system to contain, manage
and control landfill gas. The implementation of the recommended landfill gas
control measures will be reviewed and checked by the Independent Consultant
jointly employed by the Veolia and EPD under the NENTX Contract.
8.3.1
The infrastructure area at the NENTX is considered to have low risk with
the incorporation of the landfill gas control measures in the design of the
NENTX infrastructure area. (seeSee Table 7.2)
8.3.2
Proposed landfill gas monitoring wells (see Figure 8.1) will be installed between the NENTX waste boundary and the NENTX
infrastructure area to monitor the migration of landfill gas, if any.
8.3.3
For the control measures in the detailed design of the NENTX infrastructure
area and with continuous landfill gas monitoring, the level of resulting landfill
gas hazards of the identified target will be low. Therefore, no further control
measures will be required.
8.4.1
The construction works to be undertaken at the NENTX Site will involve
construction workers and others with risks resulting from contact with landfill
gas. According to the LFG Guidance Note, engineering will be required to
protect the planned development with risk category at” High” level (Table 3.2). Recommendations for protection and precautionary measures for
implementation in NENTX are discussed as follows.
·
Special precautions should be taken in all respects of works against
the possible presence of LFG due to proximity of the landfill extension site to
the existing NENT Landfill. Potential hazards of exposure to LFG, e.g.
Ignition, explosion, asphyxiation, toxicity, etc. should be fully aware and alerted.
·
Prominent LFG safety warning signs should be erected on-site to alert
all personnel and visitors of the hazards during excavation works. “No smoking
or burning should be permitted on-site in the working area”, and prominent “No
smoking” and “No Naked Flames” signs should be erected on-site where
appropriate. No worker should be allowed to work alone at any time in excavated
trenches or confined area on-site.
·
Those staff who work in, or have responsibility for 'at risk' areas,
including all excavation workers, supervisors and engineers working within the
Consultation Zone, should receive appropriate training on working in areas
susceptible to landfill gas, fire and explosion hazards.
·
Adequate firefighting equipment should be provided on-site.
Construction equipment should be equipped with a vertical exhaust at least 0.6m
above ground installed with spark arrestors. Electrical motors and extension
cords should be explosion-proof and intrinsically safe when being used on-site.
·
Welding, flame-cutting or other hot works may only be carried out in
confined spaces when controlled by a “permit to work” procedure, properly authorised
by the Safety Officer. The permit to work procedure should set down clearly the
requirements for continuous monitoring of methane, carbon dioxide and oxygen
throughout the period during which the hot works are in progress. The procedure
should also require the presence of an appropriately qualified person who shall
be responsible for reviewing the gas measurements as they are made, and who
shall have executive responsibility for suspending the work in the event of
unacceptable or hazardous conditions. Only those workers who are appropriately
trained and fully aware of the potentially hazardous conditions which may arise
should be permitted to carry out hot works in confined areas.
·
For piping assembly or conduit construction, all valves and seals should
be closed immediately after installation to avoid accumulation and migration of
LFG. If installation of large diameter pipes (Diameter>600m) is required,
the pipe ends should be sealed on one side during installation. Forced
ventilation is required prior to operation of the installed pipeline. Force
ventilation should also be required for works inside trenches deeper than 1m.
·
During construction, adequate fire extinguishing equipment,
fire-resistant clothing and breathing apparatus (BA) sets should be made
available on site.
a)
At larger developments, fire drills should be organised at not less
than six monthly intervals.
b)
The developer should formulate a health and safety policy, standards
and instructions for site personnel to follow.
8.5.1
The frequency and location of LFG monitoring within the excavation area
should be determined prior to commencement of works. The monitoring
requirements and procedures specified in Paragraphs 8.23 to 8.28 of EPD’s
Guidance Note are below. The monitoring frequency and areas to be monitored
should be set down prior to commencement of groundworks either by the Safety
Officer or by an appropriately qualified person. Routine monitoring should be
carried out in all excavations, manholes and chambers and any other confined
spaces that may have been created by, for example, the temporary storage of
building materials on the site surface. All measurements in excavations should
be made with the monitoring tube located not more than 10mm from the exposed
ground surface. Monitoring of excavations should be undertaken as follows:
For excavation works deeper than 1m, measurements should be made:
·
at ground surface prior to excavation;
·
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 excavation between 300mm and 1m deep, measurements should be made:
·
directly after the excavation has been completed; and
·
periodically whilst the excavation remains open.
For excavations less than 300mm deep, monitoring may be omitted, at the
discretion of the Safety Officer or other appropriately qualified person.
Actions in the Event of Gas Being Detected
1.1.1
Depending on the results of the measurements, actions required will vary
and should
1.1.1
be
set down by the Safety Officer or other appropriately qualified person. As a
minimum
8.5.2
these should
encompass those actions specified in Table 8.1.
Table
8.1 Action plan for LFG monitoring during
construction phase
|
Parameter |
Monitoring Result |
Action |
|
O2 |
<19% |
Ventilate trench/ void to restore O2 level to>19% |
|
<18% |
Stop works, evacuate all personnel/ prohibit entry, and increase
ventilation to restore O2 level to >19% |
|
|
CH4 |
>10% LEL |
Post “No smoking” signs, prohibit hot works, and ventilate to
attenuate CH4 level to <10% LEL |
|
>20%LEL |
Stop works, evacuate personnel/ prohibit entry, and ventilate to
attenuate CH4 level to <10%LEL |
|
|
CO2 |
>0.5% |
Ventilate to attenuate CO2 level to<0.5% |
|
>1.5% |
Stop works, evacuate personnel/ prohibit entry increase ventilation
to restore CO2 to <0.5% |
8.5.3
The monitoring requirements and procedures specified in Paragraphs 8.23
to 8.28 of EPD’s Guidance Note are highlighted below:
·
Periodically during ground-works construction, the works area should
be monitored for methane, carbon dioxide and oxygen using appropriately
calibrated portable gas detection equipment. The equipment should be
intrinsically safe and calibrated according to the manufacturer’s instructions.
·
The monitoring frequency and areas to be monitored should be set down
prior to commencement of works either by the Safety Officer or by an
appropriate qualified person.
·
Routine monitoring should be carried out in all excavations, manholes
and chambers and any other confined spaces that may have been created by, for
example, the temporary storage of building materials on the site surface.
·
All measurements in excavations should be made with the monitoring
tube located not more than 10mm from the exposed ground surface.
·
A standard form, detailing the location, time of monitoring and
equipment used together with the gas concentrations measured, should be used
when undertaking manual monitoring to ensure that all relevant data are
recorded.
8.6.1
Veolia will be responsible to train and to ensure that their staff take
appropriate precautions at all times when entering enclosed spaces or plant
rooms. Veolia will also undertake regular monitoring of landfill gas at the
perimeter monitoring wells to detect if there are any signs of off-site
landfill gas migration. Veolia will be responsible to prepare and implement an emergency
plan in case off-site landfill gas migration is detected.
8.6.2
A continuous permanent gas monitoring system with alarms will be
installed and operated in all occupied on-site buildings. Any proposed
modifications or additions to the building structure in the infrastructure area
should be subject to a further assessment of landfill gas hazard.
9.1.1
According to the approved EM&A Manual, Veolia should undertake
regular monitoring of landfill gas within the NENTX and along the NENTX waste
boundary, it is recommended that designated monitoring locations at the gas monitoring
boreholes, supplemented by monthly site surveys of the surrounding environment
including natural cracks and fissures, services drains, area with sign of
vegetation death, and any below ground enclosed spaces during the operation,
restoration and aftercare of the NENTX.
9.1.2
If the monitoring results indicate evidence of gas migration, the
monitoring frequency should be increased accordingly, with the implementation
of appropriate measures under the Event and Action Plan.
10.1.1
The source-pathway-target analysis in the approved EIA Report shows that
landfill gas risk posed by the NENT Landfill and the NENTX is high during both
construction and operation phases within the NENTX site under the EIA Scheme.
10.1.2
Based on the Table 7.1, the pathway classification between the NENTX waste boundary and NENTX
infrastructure area has changed from Very Short/Direct for path length less
that 50m in approved EIA Report to Long/Indirect for path length of 100-250m with
latest design and the source-pathway-target analysis shows that the risk levels
associated with the latest scheme is low to high within the NENTX Site boundary
during the construction and operation phases. With the proposed landfill gas
control measures and precautionary measures including engineering design (e.g.,
gas-proofing coating apply on top of all ground floor slabs, gas alarm, with
air conditioning/ mechanical ventilation/ natural ventilation etc.) in place, the potential risk of landfill gas
migration to the respective targets will be minimal.