9 Landfill
Gas Hazard.. 9-1
9.1 Introduction.. 9-1
9.2 Relevant
Legislation, Standards and Guidelines. 9-1
9.3 Potential
Hazard Associated with Landfill Gas and Leachate. 9-1
9.4 Landfill
Gas Assessment Criteria and Methodology.. 9-2
9.5 Description
of the Environment. 9-5
9.6 Qualitative
Landfill Gas Assessment. 9-5
9.7 Recommended
Precautionary and Protection Measures. 9-11
9.8 Evaluation
of Residual Impacts. 9-14
9.9 Environmental
Monitoring and Audit (EM&A) Requirements. 9-14
9.10 Contingency
Plan for Construction and Operational Phases. 9-14
9.11 Conclusion.. 9-15
List of TABLES
Table 9.1.... Classification of Risk
Categories. 9-4
Table 9.2.... General
Categorization of Risk.. 9-5
Table 9.3.... Generic
Protection Measures for Planning Stage Categorization.. 9-5
Table 9.4.... Definition
of Control Terms. 9-5
Table 9.5.... Summary
of LFG monitoring data during 2016 to 2020.. 9-8
Table 9.6.... Summary
of Surface Gas Emissions during 2016 to 2020.. 9-9
Table 9.7.... Summary
of Qualitative Source-Pathway-Target Analysis. 9-10
Table 9.8.... Actions
in the Event of Gas Being Detected in Excavations. 9-13
List of Figures
Figure 9.1 Landfill Gas Consultation
Zone for Shuen Wan Restored Landfill
List of Appendices
Appendix
9.1 Geology
Map for Project Site
Appendix
9.2 Locations
of LFG Monitoring
Appendix
9.3 LFG
Monitoring Data for Shuen Wan Restored Landfill
Appendix
9.4 Facilities Surrounding the Project Site
9.1.1
This Section provides an
evaluation of the potential landfill gas hazards arising from the construction
and operation of the proposed Project. Mitigation measures have been proposed
if considered necessary to minimize the identified landfill gas hazards.
9.2.1
The relevant legislation,
standards and guidelines applicable to the present study for the assessment of
landfill gas (LFG) hazards include:
n Section 1.1 (f)
in Annex 7 and Section 3.3 in Annex 19 of Technical Memorandum on Environmental
Impact Assessment Process (EIAO-TM);
n Landfill Gas
Hazard Assessment Guidance Note (EPD/TR8/97) (Guidance Note)
n Practice Note
for Professional Person (ProPECC) PN3/96 – Landfill Gas Hazard Assessment for
Developments adjacent to Landfills
9.2.2
These
legislation and guidelines recommended that in general, a qualitative
assessment of the risk posed by LFG is required for a development which is
proposed within a 250m “Consultation Zone” around any landfill site, to ensure
appropriate precautionary measures would be designed and implemented to
safeguard the development.
Landfill Gas
9.3.1
Methane, one of the major components
of landfill gas, is flammable and will burn when mixed with air between
approximately 5% by volume and 15% by volume (the Lower Explosive Limit (LEL)
and Upper Explosive Limit (UEL) respectively). If a mixture of methane and air with a
composition between the LEL and UEL is ignited in a confined space, the
resulting combustion may give rise to an explosion. 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 - this can only be confirmed by using
appropriately calibrated methane detectors. Methane is also an asphyxiant.
9.3.2
Carbon Dioxide, the other major
component of landfill gas, is asphyxiating and causes adverse health effects at
relatively low concentrations. The
long-term Occupational Exposure Limit (OEL) is 0.5% by volume. Like methane, in the pure form, it is
odourless and colourless and its presence (or absence) can only be confirmed by
using appropriately calibrated detectors.
9.3.3
Gas Buoyancy. 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 than air or heavier than air. As a result, landfill gas may collect in
the bottoms of trenches or excavations, or may rise up and accumulate beneath
structures and foundations.
Leachate
9.3.4
The main problem associated
with leachate is its potential for corrosion of steel and concrete structures
and pollution of receiving waters. Leachate
also presents a potential health risk to anyone who comes into contact with
it. In particular, it may cause
severe irritation if there is contact with skin or eyes. Many of the compounds likely to be
present in the leachate are toxic, if present at sufficiently high
concentration.
9.4.1
The risk associated with LFG
has been evaluated based on the “Source – Pathway – Target” model in accordance
with the Guidance Note.
Source: the location,
nature and likely quantities/concentrations of LFG which have the potential to
affect the Project site.
Pathway: the ground and
groundwater conditions, through which the LFG must pass if they are to reach
the Project site.
Target: the elements of the
development which are sensitive to the effects of the hazardous materials.
Source
9.4.2
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
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.
Pathway
9.4.3
Generally,
three types of pathways are considered for the transmission of LFG. They are:
n Man-made
pathways e.g. utility connections, stormwater channels, etc.;
n 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
n A combination of
the previous 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, 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.
9.4.4
The broad classification of the
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 50-100m
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
9.4.5
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 gas migration
including the following:
n a broad assessment of the specific permeability of the soil;
n spacing,
tightness and direction of the fissures/ joints;
n topography;
n depth and
thickness of the medium through which the gas may migrate (which may be
affected by groundwater level);
n nature of the
strata over the potential pathway;
n number of
different media involved; and
n depth to
groundwater
table and groundwater flow patterns.
Target
9.4.6
The
different elements of the proposed Project which will be sensitive to the
impacts of landfill gas will be identified. Such potential “target” will
include building basements and ground level rooms, underground carparks,
service ducts and manhole, unventilated excavations and other confined spaces
at or below ground level. 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 authorized, 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 can pose potential problems.
|
9.4.7
The above examples
of different categories within each criterion 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 needs to be taken of any particular
circumstances when assigning a target to one of three indicated categories.
Assessment of Risk
Criteria
9.4.8
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 9.1. The potential
implications associated with the various qualitative risk categories are
summarized in Table 9.2 below.
Table 9.1 Classification
of Risk Categories
|
Source
|
Pathway
|
Target Sensitivity
|
Risk Category
|
|
Major
|
Very short/direct
|
High
|
Very high
|
|
Medium
|
High
|
|
Low
|
Medium
|
|
Moderate 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
|
|
Moderate 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
|
|
Moderate short/direct
|
High
|
Medium
|
|
Medium
|
Low
|
|
Low
|
Very low
|
|
Long/indirect
|
High
|
Medium
|
|
Medium
|
Low
|
|
Low
|
Very low
|
Table 9.2 General
Categorization of Risk
|
Category
|
Level of Risk
|
Implication
|
|
A
|
Very
high
|
The type of development being
proposed is very undesirable and less sensitive form of development should be
considered. At the very least, extensive engineering measures, alarm systems
and emergency action plans are likely to be required.
|
|
B
|
High
|
Significant engineering
measures will be required to protect the planned development.
|
|
C
|
Medium
|
Engineering measures will
be required to protect the proposed development.
|
|
D
|
Low
|
Some precautionary measures
will be required to ensure that the planned development is safe.
|
|
E
|
Very
low
|
The
risk is so low that no precautionary measures are required.
|
9.4.9
Five generic forms of
protection will be used in mitigating the hazards to developments. These
generic forms corresponding to the five risk levels are set out in Table 9.3 and the control terms used are defined in Table 9.4.
Table 9.3 Generic
Protection Measures for Planning Stage Categorization
|
Level of Risk
|
Generic Protection
Measures
|
|
Very
high
|
For the planned development
active control of gas, supported by barriers and detection systems. Another,
less sensitive form of development should also be considered.
|
|
High
|
Active control of gas,
including barriers and detection systems.
|
|
Medium
|
Use of “semi-active” or
enhanced passive controls. Detection systems in some situations.
|
|
Low
|
Passive control of gas
only.
|
|
Very
low
|
No precautionary
measures required.
|
Table 9.4 Definition
of Control Terms
|
Term
|
Definition
|
|
Active
control
|
Control of gas by
mechanical means e.g. ventilation of spaces with air to dilute gas, or
extraction of gas from the development site using fans or blowers.
|
|
“Semi-active
control”
|
Use of wind driven cowls
and other devices which assist in the ventilation of gas but do not rely on
electrically powered fans.
|
|
Passive
control
|
Provision of barriers to
the movement of gas e.g. membranes in floors or walls, or in trenches,
coupled with high permeability vents such as no-fines gravel in trenches or
voids/permeable layers below structures.
|
|
Detection
systems
|
Electronic systems based
upon, for example, catalytic oxidation or infra-red measurement principles,
which can detect low concentrations of gas in the atmosphere and can be
linked to alarms and/or telemetry systems.
|
9.5.1
The proposed Project site, i.e.
Tai Po Sewage Treatment Works (TPSTW), is located in Tai Po Industrial Estate
(TPIE) and approximately 13.8 ha in area. It is adjacent to the western
boundary of Shuen Wan Restored Landfill and fall within 250m consultation zone
of the landfill as shown in Figure 9.1.
Assessment Methodology
9.6.1
The potential risk of LFG from
the Shuen Wan Restored Landfill to the proposed Project has been assessed in
accordance with the Guidance Note and Appendix G of the EIA Study Brief
(ESB-321/2019). The assessment shall include the following:
(i)
Review of background
information and studies related to Shuen Wan Restored Landfill.
(ii)
Identification of the nature and
extent of the sources, including the likely concentrations/amounts of hazardous
emissions which might have the potential for causing impacts on the Project.
(iii) Identification of possible pathways through the ground, underground
cavities, utilities or groundwater and the nature of these pathways through
which hazardous emissions must traverse if they were to reach the facilities
within the Project site.
(iv) Identification of the potential targets associated with the Project
which are sensitive to the impacts of the hazardous emissions.
(v)
Qualitative assessment on the
degrees of risk which the hazardous emissions may pose to the target for each
of the source-pathway-target combinations.
(vi) Design of suitable level of precautionary measures and types of
protection measures and contingency plan for the construction and operation of
the developments proposed under the Project.
(vii)Identification
of monitoring requirements for assessing the adequacy and performance of the
implemented protection measures.
Desktop Study
9.6.2
The following information and
documents have been reviewed for the preparation of this assessment:
n Hong Kong Geological
Survey Map (HGM20 series scale 1:20,000) (Edition II – 2008);
n Approved EIA
report of “Shuen Wan Golf Course” (AEIAR-221/2019);
n Approved EIA
report of “Tai Po Sewage Treatment Works Stage V” (AEIAR-081/2004);
n Shuen Wan
Landfill
Restoration – Annual Environmental Audit Report (2016-2020)
History of Shuen Wan
Restored Landfill
9.6.3
The Shuen Wan Restored Landfill
was opened for the waste reception in 1973 and closed in 1995. The waste
received included municipal, construction as well as industrial and commercial.
The landfill site is located to the east of the of Tai Po and is approximately
55 hectares (ha) in area. The restoration works for Shuen Wan Restored Landfill
commenced in December 1996 and was completed in December 1997. It has been
subsequently used as a temporary golf driving range since April 1999. According
to the EIA report of “Shuen Wan Golf Course”
(AEIAR-221/2019), an 18-hole golf course, serving both members and the
public will be constructed on the restored landfill in 2021 and operated in end
2023 tentatively.
9.6.4
Since the restoration works,
monthly environmental monitoring (e.g. LFG, groundwater, leachate, surface
water, etc.) and annually environmental audit have been conducted.
Restoration Facilities
for Shuen Wan Restored Landfill
9.6.5
The restoration facilities for
Shuen Wan Restored Landfill mainly consist of landfill capping system, LFG
management system and leachate management system as described below:
(I)
LFG Management System
9.6.6
The LFG management system
consists of active gas extraction system, LFG utilisation system, flaring
system for LFG, passive vent trenches/pipes and gas monitoring probes located
both on site and off site.
9.6.7
Active gas
extraction from the waste is used to relieve the pressure build-up in the waste
and to prevent LFG from migrating off site laterally or vertically. The
extracted LFG is delivered to the Hong Kong and China Gas Company through the
LFG utilisation plant for town gas production. However, when LFG utilisation
plant is under maintenance, flaring system will be switched from standby mode
to active mode for LFG treatment. Meanwhile, passive vent trenches and pipes
use existing variations in landfill pressures and gas concentrations to vent
LFG as a "fail-safe" gas migration barrier in case the gas extraction
system fails. In addition, methane and carbon dioxide are monitored by the Gas
Monitoring Probes (GMPs) on a monthly basis in order to ensure the
effectiveness of LFG management system.
(II)
Leachate Management System
9.6.8
The leachate management system
consists of the active leachate extraction system, leachate pumping chambers and leachate monitoring wells.
9.6.9
The active leachate extraction
system is used to pump the leachate from the platform and slopes to the
leachate pumping chambers. Three leachate pumping chambers are located on the
perimeter of the site. They collect the leachate intercepted by both peripheral
drains beneath part of the capping layers at 30mPD, 40mPD and 38mPD platforms
and side slope, followed by a series of horizontal sub-drains. The leachate
collected in the chambers is pumped to the Leachate Pre-treatment Works and
subsequently conveyed to the Tai Po Sewage Treatment Works for treatment.
(III)
Landfill Capping Layer
9.6.10
The components of the landfill
capping layer include the following (from top to bottom).
n An
approximately 800mm – 850mm thick soil layer of general cover, hydroseeded to
act as the growing medium;
n A geocomposite
drainage layer consists of filtration geotextile, high density polyethylene
(HDPE) geonet, very flexible polyethylene (VFPE) geomembrane and cushion
geotextile; and
n A 500mm think
final intermediate soil cover.
Geological Assessment
9.6.11
According to the geological map
of Hong Kong Geological Survey, the Project site (i.e. TPSTW) and major portion
of the Shuen Wan Restored Landfiill was reclaimed land (refer to Appendix 9.1). Site investigation records from Ginfo
website of Geotechnical Engineering Office (GEO) have been studied. The upper
most is a fill layer and underneath the fill is marine deposits or alluvium in
the Project site. Rocks of various degrees of decomposition present below the
marine deposit / alluvium layer or immediately underneath the fill layer. With
reference to the approved EIA report of “Shuen Wan Golf Course”
(AEIAR-221/2019), the geological formation beneath Shuen Wan Restored Landfill
mainly comprises reclamation fill, underlain by Quaternary superficial deposits
of fine and medium marine sands, debris flow deposits, alluvium and fill. No
major fault lines or fissures have been identified across the Project Site as
well as the Shuen Wan Restored Landfill for LFG migration.
Source
Landfill Gas
9.6.12
Comprehensive LFG monitoring
has been conducted in monthly basis since the restoration works in December
1996. LFG monitoring in Annual Environmental Audit Reports for the Shuen Wan
Restored Landfill in the past five years (2016 to 2020) has been reviewed for
this assessment.
9.6.13
22 multiple level gas
monitoring probes (GMPs) located at the boundary of the landfill site and 46
off-site single level gas monitoring probes (P-series) have been monitored
under LFG management system. The LFG monitoring probe
locations are indicated in Appendix 9.2. Levels of methane (CH4)
and carbon dioxide (CO2) recorded at the monitoring probes have been
summarized in Table 9.5 and the monitoring data are tubulated in Appendix 9.3.
Table 9.5 Summary
of LFG monitoring data during 2016 to 2020
|
Monitoring Year
|
CH4 (% v/v)
|
CO2 (% v/v)
|
|
Range
|
Average
|
Range
|
Average
|
|
Multiple Level Gas Monitoring Probes (GMPs)
|
|
2016
|
0.0 – 0.0
|
0.0
|
0.0
– 3.1
|
0.23
|
|
2017
|
0.0 – 0.0
|
0.0
|
0.0
– 3.7
|
0.13
|
|
2018
|
0.0 – 0.0
|
0.0
|
0.0
– 2.3
|
0.21
|
|
2019
|
0.0 – 0.0
|
0.0
|
0.0
– 2.6
|
0.14
|
|
2020
|
0.0 – 0.0
|
0.0
|
0.0
–1.7
|
0.17
|
|
Single Level Gas Monitoring Probes (P-series)
|
|
2016
|
0.0 – 0.0
|
0.0
|
0.0
– 6.9
|
0.22
|
|
2017
|
0.0 – 0.0
|
0.0
|
0.0
– 6.3
|
0.27
|
|
2018
|
0.0 – 0.0
|
0.0
|
0.0
– 10.4
|
0.27
|
|
2019
|
0.0 – 0.0
|
0.0
|
0.0
– 2.9
|
0.14
|
|
2020
|
0.0 – 0.0
|
0.0
|
0.0
–5.1
|
0.22
|
|
Standard Compliance Level
|
1.0%
v/v (i.e. 1.0% v/v above natural background)
|
8.6% v/v (i.e. 1.5% v/v above
natural background)
|
|
Level Indicating Significant Migration
|
5.0% v/v (i.e. 5.0% v/v above natural background)
|
12.1% v/v (5.0% v/v above natural background)
|
Note: According
to Annual Environmental Audit Report for Shuen Wan Restored Landfill, the
natural background levels for methane and carbon dioxide are 0.0% v/v and 7.1%
v/v respectively.
9.6.14
The results reveal that no
methane gas was detected during January 2016 to December 2020 for all gas
monitoring probes. The carbon dioxide levels at the monitoring probes ranged
from 0.0% to 10.4% during 2016 to 2020. Most of them were well below the
standard compliance level of 8.6% v/v (i.e. 1.5% v/v above natural background of
7.1% v/v) stipulated in the Guidance Note. One exceedance of the standard
compliance of carbon dioxide at single level gas monitoring probes P63bS
(10.4%) was observed during June 2018.
9.6.15
Surface gas emission survey
were also performed by EPD and the locations of the surface gas emission survey
are indicated in Appendix 9.2. The survey results during 2016 to 2020 are summarized in Table 9.6. It is observed that no exceedance of action
(1000ppm methane) and target limit (1.0% or 10,000ppm methane) for surface gas
emissions was recorded during January 2016 to December 2020.
Table 9.6 Summary
of Surface Gas Emissions during 2016 to 2020
|
Monitoring Year
|
CH4 (ppm)
|
|
2016
|
6.4 – 9.8
|
|
2017
|
7.1 – 9.4
|
|
2018
|
7.0 – 8.6
|
|
2019
|
7.0 – 8.5
|
|
2020
|
7.0 – 9.3
|
|
Action Limit
|
1,000
ppm methane
|
|
Target Limit
|
1.0% v/v or 10,000 ppm methane
|
9.6.16
With reference to Section 3.10
of the Guidance Note, concentration of greater than 1% v/v methane or 1.5% v/v
carbon dioxide (above background levels in each case) indicate less than
adequate control of the gas at source.
From the LFG monitoring results, one exceedance of 1.5% v/v above
natural background (i.e. 8.6% v/v) at P63bS (10.4%) was observed during June
2018. Considering the single
exceedance episode at single monitoring location, the high level of carbon
dioxide measured were unlikely caused by LFG migration from the Shuen Wan
Restored Landfill. However, as a conservative assessment, the source level due
to Shuen Wan Restored Landfill is classified as Medium.
Leachate
9.6.17
Capping layer and leachate
extraction system has been installed at Shuen Wan Restored Landfill since the
restoration in 1997 for the control of flow direction and leachate level. The leachate collected in the chambers
is pumped to the Leachate Pre-Treatment Works and eventually discharged to the
Tai Po Sewage Treatment Works for treatment. Hence, the effect of leachate on
the construction and operation of the proposed Project would not be expected.
Pathway
Natural Pathways
9.6.18
For the natural pathways, the
presence of natural cavities is directly related to the prevailing local
geology. Any presence of fault lines may act as a preferential pathway for LFG
migration. The geological formation
beneath the Project Site mainly comprises of natural earth overlying marine
deposits or alluvium. There are no major fault lines persisting between Shuen
Wan Restored Landfill and the Project Site directly. The Project works will be
undertaken at the western side of the TPSTW and no underground works would be
carried out outside the proposed upgrading/expansion boundary. The shortest
path length between the western waste boundary of the Shuen Wan Restored
Landfill and the upgrading/expansion boundary would be approximately 11m (refer
to Figure 9.1). As such, the natural pathways are
categorized as Very Short/Direct.
Man-made Pathways
(Utilities)
9.6.19
According to the findings of
the utility survey, potential man-made pathway such as electricity cables, gas
pipelines, watermain and sewage pipe are within the boundary of the TPSTW.
However, there are no existing man-made services and utilities connecting
directly between the Project Site and the Shuen Wan Restored Landfill. As such, the man-made pathways are
categorized as Long/Indirect. Relevant
correspondences and layout plans regarding the above man-made services and
utilities are provided in Appendix 9.4.
Target
9.6.20
The Project mainly comprises
the following works:
n Construction and
operation of new treatment facilities, modification / demolition of existing
treatment facilities of TPSTW;
n Providing
effluent reuse facilities; and
n Providing
co-digestion facilities for imported sewage sludge and organic waste /
pre-treated food waste
9.6.21
In accordance with the Guidance
Note, the following potential targets have been identified:
Construction of
Upgrading TPSTW
9.6.22
During the construction phase,
it is expected that deep excavations will be involved for the construction of
some buildings/structures, and works will be undertaken below ground level. No
source of ignition will occur nor be allowed in the excavation areas. According
to the Guidance Note, deep excavations is considered as Medium Sensitivity.
Operation of Upgrading TPSTW
9.6.23
The following proposed
buildings / facilities would be located within the consultation zone of the
Shuen Wan Restored Landfill:
n Proposed
Buildings (e.g. Administration / Maintenance Building and Electrical Room)
n Proposed Biogas
Recovery and Storage Facilities
n Proposed Sewage
Treatment Facilities
n Proposed
Co-digestion and Sludge Related Facilities
n Proposed
Reclaimed Water Plant
9.6.24
Access to
these buildings/facilities is restricted to the authorized contractors,
DSD staffs or other authorized persons who will be briefed on the potential LFG
hazards and the specific safety procedures to be followed. This target is considered as Medium Sensitivity.
Assessment of Hazard
9.6.25
With reference to Table 9.1, source-pathway-target analyses have been
undertaken and the results are presented in Table 9.7.
Table 9.7 Summary
of Qualitative Source-Pathway-Target Analysis
|
Source
|
Pathway
|
Target
|
Risk
|
|
Shuen Wan Restored Landfill
(Medium)
|
Natural pathways
(Very Short/Direct)
|
Construction of upgrading
TPSTW
(Medium sensitivity)
|
Medium
|
|
Operation of upgrading
TPSTW
(Medium sensitivity)
|
Medium
|
|
Man-made pathways
(Long/Indirect)
|
Construction of upgrading
TPSTW
(Medium sensitivity)
|
Low
|
|
Operation of upgrading
TPSTW
(Medium sensitivity)
|
Low
|
9.6.26
The qualitative landfill gas
hazard to the proposed Project would be Low to Medium for the
construction and operation of the upgrading TPSTW.
9.7.1
The overall potential hazard
for the proposed Project associated with the Shuen Wan Restored Landfill would
be Low to Medium for both construction and operation of TPSTW.
Referring to Table 9.2, semi-active or enhanced passive controls as
well as gas detection system are recommended.
Construction Phase
Safety Measures
9.7.2
The following safety measures shall
be implemented during the construction phase:
n 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 excavations. Safety notices should be posted
warning of the potential hazards.
n A Safety Officer, trained in
the use of gas detection equipment and landfill gas-related hazards, should be
present on site during the groundworks trenching and construction stages.
n All staff working in the
Consultation Zone should receive appropriate training on working in areas
susceptible to landfill gas, fire and explosion hazards.
n An excavation procedure or code
of practice to minimize landfill gas related risk should be devised and carried
out.
n No worker
should be allowed to work alone at any time in or near to any excavation areas
within the Consultation Zone. At least one other worker should be available to
assist with a rescue if needed.
n Smoking, naked flames and all other sources of ignition should be
prohibited within 15m of any excavation or ground-level confined space. 'No
smoking' and 'No naked flame' notices should be posted prominently on the
construction site, especially in excavation or trenches.
n Welding, flame-cutting or other hot works should be confined to
open areas at least 15m from any
trench or excavation.
n Welding, flame-cutting or other hot works may only be carried out
in trenches or confined spaces when controlled by a 'permit to work' procedure,
properly authorized by the Safety Officer.
n The permit to
work procedure should set down clearly the requirements for continuous
monitoring for 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, in attendance
outside the 'confined area', 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.
n Ground level
construction plant used within in Consultation Zone should be fitted with vertical
exhausts at least 0.6m above ground level and with spark arrestors.
n Any electrical
equipment, such as motors and extension cords, should be intrinsically safe.
n During piping
assembly or conduiting construction within Consultation Zone, 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.
n Mobile offices,
equipment stores, mess rooms etc. should be located on an area which has been
proven to be gas free (by survey with portable gas detectors) and ongoing
monitoring / measurement should be carried out, preferably at least at the
beginning of every working day, to ensure that these areas remain gas free.
Alternatively, such buildings should be raised clear of the ground. If
buildings are raised clear of the
ground, a minimum, clear separation distance (as measured from the highest
point on the ground surface to the underside of the lowest floor joist) should
be 500mm.
n Adequate fire
extinguishing equipment, fire-resistant clothing and breathing apparatus sets
should be made available on site.
Fire drills should be organized at
not less than six monthly intervals.
LFG Monitoring
9.7.3
LFG Monitoring shall be
undertaken during construction phase as described below:
n Periodically
during ground-works construction, the works area should be monitored for
methane, carbon dioxide and oxygen using appropriately calibrated portable gas
detection equipment.
n The monitoring
frequency and areas to be monitored should be set down prior to commencement of
ground-works either by the Safety Officer or by an appropriately qualified person.
n 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.
n All
measurements in excavations should be made with the monitoring tube located not
more than 10mm from the exposed ground surface.
n 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.
n 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.
n For excavations
less than 300mm deep, monitoring may be omitted, at the discretion of the
Safety Officer or other appropriately qualified person.
n Depending on
the results of the measurements, actions required will vary and should be set
down by the Safety Officer or other appropriately qualified person. As a
minimum these shall encompass those actions specified in Table 9.8.
Table 9.8 Actions
in the Event of Gas Being Detected in Excavations
|
Parameter
|
Measurement
|
Action
|
|
O2
|
< 19%
|
·
ventilate trench/void to restore O2 to
>19%
|
|
< 18%
|
·
stop works
·
evacuate personnel/prohibit entry
·
increase ventilation to restore O2 to >19%
|
|
CH4
|
> 10% LEL
|
·
prohibit hot works
·
ventilate to restore CH4 to <10% LEL
|
|
>20% LEL
|
·
stop works
·
evacuate personnel/prohibit entry
·
increase ventilation to restore CH4 to
<10% LEL
|
|
CO2
|
> 0.5%
|
·
ventilate to restore CO2 to <0.5%
|
|
> 1.5%
|
·
stop works
·
evacuate personnel/prohibit entry
·
increase ventilation to restore CO2 to
<0.5%
|
n The hazards from landfill gas during
the construction phase shall be minimized by precautionary measures recommended
in the Guidance Note.
n In any emergency situation, the
Safety Officer or other appropriately qualified person, 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 organizations should also
be contacted as appropriate:
·
Hong Kong Police Force (HKPF);
·
Fire Services Department (FSD); and
·
Landfill Operator
Operational Phase
Building Protection
Design
9.7.4
Where below ground service
entries are necessary to the buildings/facilities, the entry point should be
sealed to prevent gas entry.
9.7.5
Where practicable, natural
ventilation through windows and openings, coupled with wind driven cowls and
other devices as required, should be provided at or below the ground floor of
new permanent building structures of the Project.
9.7.6
Where natural ventilation is
not feasible, the floors and walls at the ground level and the below ground
rooms / voids of any proposed permanent structures should consist of gas
resistant material with low gas permeability. Gas detection systems with audio
alarm and forced ventilation should also be provided in such area of the
Project. In addition, a clear void
or a gas vent (e.g. in the form of no-fines gravel in trenches) should be
created under these structures to vent and dilute any gas emitted from the
ground.
9.7.7
The aforementioned gas
detection systems should be calibrated and maintained at regular basis in
according to the recommendation of manufacturer’s instruction. The
operators of the Project should also make sure that the gas detection systems
are in functions during the operational phase of the Project.
9.7.8
Forced ventilation should be used if methane
of more than 0.5 % (by volume) in the internal atmosphere (e.g. in voids or
rooms as mentioned above) is detected.
9.7.9
No person should enter or
remain in any confined spaces (e.g. in voids or rooms as mentioned above) where
the carbon dioxide concentration exceeds 1.5 % (by volume).
9.7.10
Oxygen concentration should be
monitored and no person shall enter or remain in any confined spaces (e.g. in
voids or rooms as mentioned above) where the oxygen content of air has fallen
below 18 % by volume.
9.7.11
All the access to these
confined spaces (e.g. in rooms or voids as mentioned above) should be
restricted only to authorised personnel who should be aware of the LFG
hazard. No member of general public should be permitted or allowed to
access these confined spaces, manholes or inspection chambers.
Guidance for Entry into
Manholes and Chambers
9.7.12
When service voids, manholes or
inspection chambers within the proposed site are entered for maintenance, monitoring
and a checklist system of safety requirements should be performed before entry
in accordance with Code of Practice on Safety and Health at Work in Confined
Spaces published by Labour Department.
9.7.13
All the access to the confined
spaces would be restricted only to authorized personnel who should be aware of
the LFG hazard. No member of
general public should be permitted or allowed to access these confined spaces,
manholes or inspection chambers.
9.8.1
Provided that all the recommended mitigation measures and monitoring
programme are properly implemented, no unacceptable residual landfill gas
hazard is expected from the Project.
9.9.1
During the whole construction
phase and the first 2 years of operational phase, the ongoing monitoring and
implementation performance on the recommended precautionary and protection
measures would be provided and reviewed in the monthly EM&A report. The need of further reporting the
ongoing monitoring and implementation performance on the recommended
precautionary and protection measures could be reviewed at the end of the
second year of operation.
9.10.1
Detection of LFG at the Project
site could be the consequence of accidents or emergency situations such as
failures of the LFG management system of Shuen Wan Restored Landfill or
failures of the recommended precautionary and protection measures of the
Project. All construction, operation and maintenance personnel working on-site
should be made aware of the hazards of LFG and the proposed precautionary and
protection measures.
9.10.2
To provide a mechanism to
minimise the impact of such emergency situations and the subsequent management
and rectification of the situations, a contingency plan should be formulated
prior to the construction and commissioning of the Project. The construction
contractor under the construction contract and the plant operators of the
Project should carry out necessary actions according to the procedures of the
contingency plan. An outline of the contingency plan is given below:
·
Descriptions of the proposed LFG hazard
precautionary and protection measures of the Project.
·
Guidelines for regular inspection of the
proposed LFG hazard precautionary and protection measures and routine integrity
checking and maintenance of the proposed building protection measures / devices
/ facilities.
·
Emergency responses such as evacuation
procedures and rectification procedures to reduce LFG concentration (e.g. by
increasing ventilation rate) and, if required, restore normal operation of the
LFG hazard precautionary and protection measures / facilities or liaison with
the restored landfill operator.
·
Roles and responsibilities for implementing
the proposed LFG precautionary and protection measures and contingency plan.
·
The contacts of Government Departments (HKPF
and FSD) to be notified and stakeholders (i.e. suitable personnel of
contractor/DSD for implementing the contingency plan and landfill operator) in
case of emergency situations.
9.11.1
The results of this qualitative
risk assessment for LFG hazards posed by the Shuen Wan Restored Landfill to the
Project site would be Low to Medium during both construction and
operational phases. With proper implementation of the recommended precautionary
and protection measures, the safety of all personnel presence at the Project
site would be safeguarded during the construction and operational phases. Thus, there would be no adverse impact
anticipated on the feasibility of the Project.
9.11.2
During the whole construction phase
and the first 2 years of operational phase, the ongoing monitoring and
implementation performance on the recommended precautionary and protection
measures would be provided and reviewed in the monthly EM&A report. The need of further reporting the
ongoing monitoring and implementation performance on the recommended
precautionary and protection measures could be reviewed at the end of the
second year of operation.