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CONTENTS
1.3 Previous Studies
Undertaken at the Site
2 REview
of Prelimininary Qualitative Landfill Gas Hazard Assessment
3 LANDFILL
GAS Hazard ASSESSMENT METHODOLOGY
3.1 Landfill Gas Hazard
Assessment Criteria and Methodology
4.2 Historical Landfill
Gas Control
4.4 Landfill Gas Control
for the SENTX
5 Potential
for the development to intercept landfill gas
6 Detailed
Design of Infrastructure area
6.1 Description of the
Infrastructure Area
6.3 Sensitive Target
Facilities for Landfill Gas Risk
7 Qualitative
assessment of risks due to landfill gas
7.5 Source-Pathway-Target
Analysis
8.1 General Hazards
related to Landfill Gas
8.2 General Recommended
Precautionary and Protection Measures – Design Phase
8.3 General Recommended
Precautionary and Protection Measures – Construction Phase
9 Environmental
Monitoring and Audit
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 (
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 |
·
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 3.7d –Workshop 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 3.7e – Female toilet & changing room, Male
toilet & changing room and Accessible toilet
of Workshop/Laboratory 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 3.7f - Electrical room
and Plumbing
room of Workshop/ Laboratory 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 3.7g - Hose reel pump room of
Workshop/Laboratory 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 3.8a - General manager office, Administration general office, File
& general storage and Special storage & office furniture/equipment of
GVL 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 3.8b - Male toilet & changing room,
Female toilet
& changing room and Accessible toilet
of GVL 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 3.8c - Electrical room
and Plumbing room of GVL 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 3.8d - Multi-purpose room & Conference
room, Reception, Conference room 1, Conference room 2 and Pantry of
GVL 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 GVL and EPD staff and invited guests/ visitors |
Medium to High (a) |
Low |
|
Target 3.9a - Special storage & office
furniture/equipment of EPD 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 3.9b - Special store room
of EPD 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 3.9c - Male toilet & changing room, Female
toilet & changing room and Accessible toilet
of EPD 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 EPD staff or invited guests or visitors |
Medium to High (a) |
Low |
|
Target 3.9d - Plumbing room
and Electrical
room of EPD 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 3.9e - General office, Reception, Boot
cleaning area, Pantry and Conference room of
EPD 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 EPD staff and invited guests or visitors |
Medium to High (a) |
Low |
|
Note: (a) During
the preliminary assessment there is not detailed of the accommodation
schedule of the buildings within the infrastructure area and therefore the
target categorisations is conservative assumed to be medium and high. This has been re-assessed based on the
natural of the room or area presented in the detailed design. |
|||
7.5
Source-Pathway-Target
Analysis
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 7.2 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.
The source-pathway-target
analysis shows that landfill gas risk posed by the SENT Landfill and the SENTX
under the latest scheme is very low to medium within the SENTX Site boundary
during both the construction and operation phases. Whereas the risk posed by the SENTX to SENTX
infrastructure area ranges from very low to 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 7.2 Qualitative Assessment of
Landfill Gas Hazard Associated with the SENTX in the Latest Scheme
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
8
Recommendations
The detailed
design of the SENTX 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 SENT Landfill and the SENTX, and various measures have been
incorporated to minimise the potential risks.
This
section of the Report provides advice
and recommendations for further control measures in addition to those measures
being incorporated by GVL in the detailed design of the SENTX and other general
good practices to be implemented during the construction, operation, aftercare
and restoration of the SENTX to ensure that the SENTX development is safe with
respect to landfill gas hazard.
8.1
General Hazards
related to Landfill Gas
All contractors participating
in the works and operational staff during the operation/restoration and
aftercare phases 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 its 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
General Recommended
Precautionary and Protection Measures – Design Phase
8.2.1
For
the Construction and Operation of the SENTX
According
to the source-path-target analysis in Section
7, the risk category at the SENTX during construction and operation (waste
tipping) is medium. This implies
that engineering measures will be required during the detailed design stage to
ensure that the construction and waste tipping at the SENTX is safe. As the SENTX will be designed, built and
operated by an experienced landfill contractor (i.e. GVL who has extensive
experience in management landfill in Hong Kong and overseas), relevant
engineering measures will be identified and implemented in accordance with the SENTX
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. Migration of landfill gas from the SENT
Landfill to the SENTX will be captured by the landfill gas management system at
the SENTX. The implementation of
the recommended landfill gas control measures will be reviewed and checked by
the Independent Consultant jointly employed by the GVL and EPD under the SENTX Contract.
8.2.2
For
the Operation of the Infrastructure Area at the SENTX
The
infrastructure area at the SENTX is considered to have very low to low risk
with the incorporation of the landfill gas control measures in the design of
the SENTX infrastructure area. These
measures include a combination of passive and active systems and are summarised
in Table 7.2.
In
addition to the control measures listed in Table
7.2, landfill gas monitoring wells will be installed between the SENTX
waste boundary and the SENTX infrastructure area to monitor the migration of
landfill gas, if any.
After
the incorporation of the control measures in the detailed design of the SENTX
infrastructure areas and with continuous landfill gas monitoring in all
occupied on-site buildings, the resulting landfill gas hazards of the
identified targets will be low. It
is therefore considered that no further control measures (as those described in
Table 7.2) will be required.
8.3
General Recommended
Precautionary and Protection Measures – Construction Phase
The
construction works to be undertaken at the SENTX Site will involve construction
workers and others with risks resulting from contact with landfill gas. For example, when laying of underground
pipes/utilities in trenches or other situations, personnel may have to enter
confined spaces. Precautionary
measures to be adopted by GVL during construction at the SENTX Site are
outlined in Paragraphs 8.3 to 8.49 of EPD’s Guidance Note. With respect to the nature of the construction works for SENTX, the
following recommendations referenced from the Guidance Note should be
adopted:
Safety Measures
·
During
all works, safety procedures should be implemented to minimise the risks of
fires and explosions and asphyxiation of workers (especially in confined space).
·
Safety
officers, specifically trained with regard to landfill gas related hazards and
the appropriate actions to take in adverse circumstances, should be present on
the site throughout the works, in particular, when works are undertaken below
grade.
·
All
personnel who work on site and all visitors to the site should be made aware of
the possibility of ignition of gas in the vicinity of the works.
·
Those
staff who work in, or have responsibility for “at risk” areas, including bore
piling and excavation works, should receive appropriate training on working in
areas susceptible to landfill gas.
·
Any
offices/quarters set up on site should take precautions against landfill gas
ingress, such as landfill gas monitoring and alarm devices. Other storage premises, e.g. shipping
containers, where this is not possible should be well ventilated prior to
entry.
·
Adequate
precautions to prevent the accumulation of landfill gas under site buildings
and within storage shed should be taken by raising buildings off the ground
where appropriate and “airing” storage containers prior to entry by personnel
and ensuring adequate ventilation at all times.
·
Smoking
and naked flames should be prohibited within confined spaces. “No Smoking” and “No Naked Flame”
notices in Chinese and English should be posted prominently around the
construction site. Safety notices
should be posted warning of the potential hazards.
·
During
the construction works, adequate fire extinguishers and breathing apparatus
sets should be made available on site and appropriate training given in their
use.
·
During
piping assembly or conduiting construction, all valves/seals should be closed
immediately after installation. As
construction progresses, all valves/seals should be closed as installed to
prevent the migration of gases through the pipeline/conduit. All piping/ conduiting should be capped
at the end of each working day.
·
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.
Monitoring
Monitoring
will be undertaken when construction works are carried out in confined space
within the consultation zone. 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.
·
Monitoring
of excavations should be undertaken as follows:
For excavations deeper than 1m,
measurements should be made:
-
at
the ground surface before excavation commences;
-
immediately
before any worker enters the excavation;
-
at
the beginning of each working day for the entire period the excavation remains
open; and
-
periodically
through the working day whilst workers are in the excavation.
For excavations between 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.
·
If
methane (flammable gas) or carbon dioxide concentrations are in excess of the
trigger levels or that of oxygen is below the levels specified in the Emergency Management in the following
section, then evacuation will be initiated.
Actions in the Event of Gas Being Detected
Depending
on the results of the measurements, actions required will vary and should be
set down by the Safety Officer or another appropriately qualified person. As a minimum these should encompass
those actions specified in Table 8.2.
Table 8.2 Actions in the Event of
Gas Being Detected
|
Parameter |
Measurement |
Action |
|
O2 |
< 19% v/v |
Increase underground ventilation to restore O2
to >19% v/v |
|
|
< 18% v/v |
Stop works Evacuate all personnel Increase ventilation further to restore O2
to >19% v/v |
|
CH4 |
> 10% LEL |
Prohibit hot works Increase ventilation to restore CH4
to <10% LEL |
|
|
>20% LEL |
Stop works Evacuate all personnel Increase ventilation further to restore CH4
to <10% LEL |
|
CO2 |
>0.5% v/v |
Increase ventilation to restore CO2
to <0.5% v/v |
|
|
> 1.5% v/v |
Stop works Evacuate all personnel Increase ventilation further to restore CO2
to <0.5% v/v |
Emergency Management
In order to ensure that
evacuation procedures are implemented in the event of the trigger levels
specified in Table 8.2 above being
exceeded, it is recommended that a person, such as the Safety Officer, is
nominated, with deputies, to be responsible for dealing with any emergency
which may occur due to landfill gas.
In an emergency situation,
the nominated person or his deputies, shall have the necessary authority and
shall ensure that the confined space is evacuated and the necessary works
implemented for reducing the concentrations of gas. The following organisations shall also
be contacted as appropriate:
·
Hong
Kong Police Force;
·
Fire
Services Department; and
·
Environmental
Protection Department.
8.4
General Recommended
Precautionary and Protection Measures – Operational, Restoration and Aftercare
Phases
The SENTX Contractor
GVL
will be responsible to train and to ensure that their staff take appropriate
precautions at all times when entering enclosed spaces or plant rooms. GVL 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. GVL will be responsible to prepare and
implement emergency plan in case off-site landfill gas migration is detected
and the trigger levels specified in Table
8.3 are being exceeded.
Table 8.3 Actions in the Event of Gas
Being Detected
|
Parameter |
Measurement |
Action |
|
O2 |
< 19% v/v |
Increase ventilation of the concerned room(s)
to restore O2 to >19% v/v |
|
|
< 18% v/v |
Stop works Evacuate all personnel Increase ventilation further to restore O2
to >19% v/v |
|
CH4 |
> 10% LEL |
Prohibit hot works Increase ventilation of the concerned room(s)
to restore CH4 to <10% LEL |
|
|
>20% LEL |
Stop works Evacuate all personnel Increase ventilation of the concerned room(s)
further to restore CH4 to <10% LEL Investigate the potential landfill gas
ingress points and sources, and undertake remedial actions (if necessary) to
stop the gas ingress |
|
CO2 |
>0.5% v/v |
Increase ventilation of the concerned room(s)
to restore CO2 to <0.5% v/v |
|
|
> 1.5% v/v |
Stop works Evacuate all personnel Increase ventilation further to restore CO2
to <0.5% v/v |
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, particularly in areas where a gas membrane has been installed or
involving excavation or penetration beneath the floor slab. Any penetrations of the membrane must be
repaired as soon as possible after detection or works completion using similar
products.
9
Environmental
Monitoring and Audit
GVL should
undertake regular monitoring of landfill gas within the SENTX and along the SENTX
waste boundary as required by the SENTX Contract Specification and the approved
EM&A Manual.
Since
SENTX will receive construction waste only, it is recommended that monitoring
of landfill gas at the perimeter landfill gas monitoring wells will be required
at monthly intervals during the operation, restoration and aftercare of the
SENTX. Bulk gas analysis for at
least 2 of the perimeter wells should be undertaken at quarterly intervals
throughout the operation, restoration and aftercare of the SENTX. In addition, monitoring of service voids,
utilities and manholes along the SENTX site boundary and within the SENTX site should
be undertaken at monthly intervals throughout the operation, restoration and
aftercare of the SENTX. GVL should also
undertake surface emission monitoring of methane gas (or flammable gas) in
areas between the SENTX site boundary and the waste boundary and location of
vegetation stress at quarterly intervals throughout the operation, restoration
and aftercare phases. Actions described
in Table 8.3 will be taken if an
abnormal level of landfill gas is detected.
10
Conclusion
The
source-pathway-target analysis in the approved
EIA Report shows that landfill gas risk posed by the SENT Landfill and the SENTX
is medium to high during both construction and operation phases within the
SENTX site under the EIA Scheme.
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. Hence, SENTX has a low potential for
landfill gas generation and SENT Landfill is considered to be the most
significant potential source. The
source classification of SENTX has changed from medium to minor.
Since
the source-pathway-target analysis shows that the risk levels associated with
the latest scheme (SENTX will accept construction waste only, with the
incorporation of the landfill gas control measures adopted in this report) is very
low to medium within the SENTX Site boundary during both the construction and
operation phases and the risk posed by the SENTX to SENTX infrastructure area
ranges from very low to low, no further control measures will be required in
addition to those described in Table 7.2. With the proposed landfill gas control measures
in place, the potential risk of landfill gas migration to the respective
targets will be minimal.