Table
of Contents
12 Landfill Gas
Hazard Assessment
12.2 Landfill Gas Risk Assessment Methodology
12.3 Nature of the SENT Landfill and TKO Stage
II/III and I Restored Landfills
12.4 Proposed Desalination Plant and Fresh
Water Mains
12.5 Potential for the Development to Intercept Landfill Gas
12.6 Qualitative Assessment of Risks due to Landfill Gas
Landfill Gas Hazard Assessment Guidance Note (hereinafter referred to as Guidance Note) for the assessment of the
hazards which landfill gas may present to developments close to landfills has
been published by the EPD of the Government of the Hong Kong SAR. This provides an assessment framework to be
followed when evaluating the risks related to developments described under Section 6.5, Chapter 9 of the Hong Kong
Planning Standards and Guidelines.
The Guidance Note applies to
all developments proposed within 250m of the edge of a landfill site, known as
the landfill Consultation Zone. Part of
the desalination plant and the indicative area of natural slope mitigation
works fall within the SENT Landfill Extension Consultation Zone; and part of
the 1,200 mm diameter fresh water mains along Wan Po Road falls within the SENT
Landfill and SENT Landfill Extension Consultation Zones, TKO Stage II/III Restored Landfill and TKO Stage I
Restored Landfill Consultation Zones (see Figure 12.1). Thus a Qualitative Landfill Gas Hazard
Assessment (QLGHA) must be carried out.
A
number of previous studies have been undertaken at, or involving, the SENT
Landfill, SENT Landfill Extension, TKO Stage II/ III Restored Landfill and TKO
Stage I Restored Landfill. The documents
which have been used as background materials for the preparation of this
assessment include the following:
·
Environmental
Review for Extension of Fill Bank Operation at TKO Area 137, ERM-Hong
Kong, 2013;
·
SENT
Landfill Annual Environmental Audit, ERM-Hong Kong, 2013;
·
Environmental
Review of the Revised Scheme of SENT Landfill Extension, ERM-Hong
Kong, 2011;
·
SENT
Landfill Extension EI A, ERM-Hong Kong, 2008;
·
Qualitative
Landfill Gas Hazard Assessment for TVB City,
ERM-Hong
Kong, 2004;
·
Environmental
Review of Urban Landfills and TKO Landfills – Feasibility Study, ERM-Hong Kong,
2003;
·
TKO Area 86,
Wan Po Road Substation: Qualitative
Landfill Gas Hazard Assessment, ERM-Hong Kong, 2001;
·
MTR TKO
Extension Phase II EIA – TKO Stage I Landfill Qualitative Landfill Gas and
Leachate Hazard Assessment, ERM-Hong Kong, 1998;
·
Restoration
of TKO Landfills: Detailed EIA, ERM-Hong
Kong, 1997;
·
MTR TKO
Extension Phase II EIA, ERM-Hong Kong, 1997; and
·
Landfill Gas Hazard Assessment Guidance Note,
EPD, 1997.
The EPD has also provided the landfill gas
and groundwater monitoring data for the SENT Landfill covering the period from January 2013 to June 2014 and
March 2013 to March 2014, respectively; and the TKO Stage II/III Restored
Landfill and TKO Stage I Restored Landfill covering the period from January
2013 to May 2014 and January 2013 to April 2014, respectively.
The
following tasks have been undertaken as part of this section:
·
review of background information and studies related to
the SENT Landfill, SENT Landfill Extension, TKO Stage II/ III Restored Landfill
and TKO Stage I Restored Landfill;
·
identification and investigation of the sources of
landfill gas and leachate which have the potential to affect the development
from both desktop studies and the previous monitoring conducted at the landfill
boundary;
·
identification of viable pathway through the ground,
underground cavities, utilities or groundwater and the conditions of these
pathways through which the landfill gas must pass if they are to reach the
development;
·
review of the Project design and identification of
elements which are sensitive to the effects of landfill gas during construction
and subsequent operation;
·
qualitative assessment of the degree of risk which
landfill gas pose to the development taking account of each
source-pathway-target combination; and
·
recommendation of protection and precautionary
measures (if required) to minimise any risks due to landfill gas and leachate
hazards during the construction and operation of the development.
The remaining section is structured as
follows:
·
Section 12.2 describes
the QLGHA methodology and the framework within which the identified levels of
risk may be compared;
·
Section 12.3
describes
the SENT Landfill, TKO Stage II/III Restored Landfill and TKO Stage I Restored
Landfill, including its history and the engineering measures taken to control
landfill gas and leachate as well as the completed restoration works;
·
Section 12.4
describes
the Project design and reviews the sensitivity of key elements of the
development to the possible presence of landfill gas and leachate;
·
Section 12.5
reviews
the geology and hydrology of the area and evaluates the potential pathways
through which landfill gas and leachate may impact the development;
·
Section 12.6 evaluates
the qualitative risk of landfill gas and leachate to impact the development;
and
·
Section 12.7
provides
recommendations for precautionary and protection measures to be adopted during
the construction and operation of the development based on the findings of the
hazard assessment.
In accordance with the Guidance Note, the risk due to landfill gas may be evaluated based
upon the following three criteria:
·
Source – the rate and concentration of 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 sub-sections below.
The
classification of the Source (ie the landfill) should
be undertaken 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 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. |
|
Medium |
·
Landfill site at which
some form of gas control has been installed (eg
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. |
|
Minor |
·
Landfill sites at which
gas controls have been installed and proven to be effective by comprehensive monitoring
which has demonstrated that there is no migration of gas beyond the landfill
boundary (or any specific control measures) and at which control of gas does
not rely solely on an active gas extraction system or any other single
control measure which is vulnerable to failure; or ·
Old landfill sites where
the maximum concentration of methane within the waste, as measured at several
locations across the landfill and on at least four occasions over a period of
at least 3 months (preferably longer), is less than 5 % by volume (v/v). |
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 50 m for unsaturated permeable
strata and fissured rock or less than 100m for man-made conduits. |
|
Moderately short/ direct |
Path length of 50-100 m for unsaturated permeable soil or fissured
rock or 100-250 m for man-made conduits. |
|
Long/ indirect |
Path length of 100-250 m 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 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 gas may migrate (which may be affected by
groundwater level);
·
nature of
the strata over the potential pathway;
·
number of
different media involved; and
·
depth to
groundwater table and groundwater flow patterns.
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 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. |
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.
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 12.1 which is extracted from the Guidance Note. The potential implications associated with
the various qualitative risk categories are summarised
in Table 12.2 below.
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 risk category,
development is not precluded for any of the assessed levels of risk.
Table 12.1 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 12.2 Summary of General Categorisation 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 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. |
SENT Landfill is located on the western edge
of Clear Water Bay Peninsula in the south-eastern corner of the New Territories
(refers to Figure 12.1). The site covers an area of about 100 ha, half
of which has been reclaimed from Shek Biu Wan (Junk Bay).
To the north and east of the landfill lies Clear Water Bay Country Park;
to the west lies reclaimed as industrial estate next to the Wan Po Road (where
part of the fresh water mains is located); and to the south reclamation (TKO
Area 137 where the desalination plant is located).
The landfill is one of three strategic
landfills in operation in Hong Kong and was designed to receive approximately
40 million tonnes of waste over a period initially
projected as 15 to 17 years. The
landfill commenced operation in 1995 and accepts domestic, commercial and
industrial (C&I), construction and demolition, chemical and clinical
wastes, dewatered sewage sludge and stablised
incineration residues.
TKO
Stage II/III Restored Landfill is located at approximately 0.5 km to the north
of the SENT Landfill and 1 km to the south of the TKO Stage I Restored Landfill
(refers to Figure 12.1). TKO Stage II/III Restored Landfill received
waste from 1988 to 1994. A total of approximately 12.6 million tonnes of domestic, C&I, construction, chemical,
clinical and special wastes were landfilled.
The landfill was constructed in a reclaimed bay, the boundary being
defined by hills on three sides. The
maximum waste depth is about 90 m and the landfill covers an area of 42
hectares.
TKOL
Stage I Restored Landfill is located at the southeast of the TKO New Town. To the east of the landfill is Wan Po Road
(refers to Figure 12.1). TKO Stage I Restored Landfill received waste
from 1978 to 1990 and from 1992 to 1995.
A total of approximately 15.2 million tonnes
of domestic, C&I, construction, chemical, clinical and special wastes were
landfilled. The landfill was reclaimed
from the sea and functioned as a “disperse-and-attenuate” site, with wastes
being deposited above the marine mud.
The finished landfill is dome-shaped, covering a base area of 68
hectares with a maximum waste depth of about 70m.
The landfill has been designed to incorporate
extensive measures to obtain, collect and treat/ utilise
landfill gas. Such measures include the
state-of-the-art technologies (including a composite liner systems, active
landfill gas extraction and landfill gas treatment and utilisation)
in accordance with international best practices for landfill operations. Presently a skid mounted mobile flare unit
with an electrically driven blower is utilised to
extract and flare gas in a ‘candlestick’ type flare. It is deployed to extract from a number of
temporary gas wells and collectors beneath ‘green liner’ of the landfill. In addition, the landfill contractor is
undertaking routine maintenance and checking of the landfill gas extraction
system to ensure it is operating satisfactorily. As the landfill is lined and landfill gas is
collected and treated, it effectively controls sub-surface off-site migration
of landfill gas. Typical details of the
composite liner system (including an impermeable liner) installed at the SENT
Landfill are presented in Figure 12.2.
A comprehensive environmental monitoring programme has been implemented to monitor landfill gas
generated within the landfill and at the perimeter boreholes around the
landfill (refers to Figure 12.3). Under the
existing contract, the landfill contractor will be required to continue the
control and monitoring of landfill gas following closure of the landfill for a
period of 30 years.
Before restoration,
the TKO Stage I Landfill was not lined and no leachate or gas abstraction
systems were installed during the landfill construction; and the TKO Stage
II/III Landfill was originally not fully lined and no leachate or gas
abstraction system was installed during the landfill operation.
A
comprehensive environmental monitoring programme has
been implemented to monitor the levels of leachate within the landfill,
off-site leachate migration/ groundwater contamination and concentration of
landfill gas at the perimeter boreholes around the landfill. As the landfill is lined and leachate is
extracted for treatment, the leachate head within the landfill is minimised.
During the
development of both TKO Stage II/III Landfill and TKO Stage I Restored
Landfill, no measure was implemented to achieve the collection of leachate from
beneath the waste.
The EIA Report
(AEIAR-117/2008) for the SENT Landfill Extension was approved under the EIA Ordinance (EIAO) in May 2008 and an
Environmental Permit (EP-308/2008) was granted on 15 August 2008. Since then, the administration has decided to
reduce the scale of the extension scheme assessed in the approval EIA Report,
and the SENT Landfill Extension will only receive construction waste. Thus an Environmental Review was undertaken
in 2011 to address the findings of the approved EIA Report as a result of the
changes to the SENT Landfill Extension and a Variation
of an Environmental Permit (EP-308/2008/A)
was granted on 6 January 2012.
The SENT
Landfill Extension with a void space of about 6.5 million m3 will
occupy 13 hectares in part of the TKO Area 137 and be located immediately to
the south of the existing infrastructure area of the SENT Landfill (refers to Figure
12.1).
The date of
commencement is to be confirmed and the construction period of the SENT
Landfill Extension is expected to take about 2 years.
Under the
SENT Landfill Extension contract requirements, the contractor will be required
to control off-site landfill gas migration such that the methane and carbon
dioxide concentrations at the perimeter monitoring wells will not exceed 1% v/v
and 1.5% v/v above the background levels, respectively. A landfill gas management system will be
installed and operated. As presented in
the approval EIA Report (AEIAR-117/2008), the SENT Landfill Extension 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.
It is
envisaged that construction waste (with high portion of inert materials) will
generate significantly lower quantities of landfill gas than municipal solid
waste and sludge. Thus, the quantity of
landfill gas generated in the latest extension scheme is expected to be lower
than that estimated in the approved EIA Report (AEIAR-117/2008). 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.
The TKO
Stage II/III Landfill was subject to restoration works commissioned by the
Government in 1997. A restoration works
contract was awarded to a restoration contractor who designed and implemented a
system of controls to achieve the contractual limits on environmental emissions and is responsible for the
management of installed environmental control facilities and environmental
impacts arising from the landfill for a period of 30 years following completion
of the works.
The
TKO Stage II/III Landfill was originally not fully lined and no leachate
or gas abstraction system was installed during the landfill operation. The major restoration works regarding the
management of landfill gas and leachate under the contract included:
·
placing of a
membrane over the waste;
·
placing of
soil above the membrane and the installation of a comprehensive surface water
drainage system;
·
installation
of a landfill gas collection and abstraction system;
·
utilisation of landfill gas abstracted;
·
treatment of
surplus landfill gas in a landfill gas flare; and,
·
collection
and storage of leachate and landfill gas condensate for treatment.
There are a total of 50 vertical wells were
installed within the landfill to extract landfill gas.
The leachate
collected is pumped to the Leachate Pre-treatment Works (LPTW) at the landfill
for treatment prior to be discharged to sewer leading to the TKO Sewage
Treatment Works (TKOSTW).
Active groundwater
control in the form of groundwater abstraction wells, were installed at TKO
Stage II/III Restored Landfill, upstream of the waste boundary. The wells reduce the ingress of groundwater
to the wastes and hence minimise leachate
generation. Groundwater collected from
groundwater extraction wells at TKOL-II/III is used in the LPTW and the excess
groundwater is discharged to the stormwater
drains.
Sixteen groundwater wells (DG1 to DG7, UG1 to
UG9) were monitored quarterly throughout the aftercare period which is 30 years
from the landfill restoration works completion date (refers to Figure
12.4).
The
landfill restoration works were completed in February 1999. Monitoring and auditing of potential
environmental impacts through landfill gas and leachate measurements around the
perimeter of the site are currently ongoing.
This monitoring will be continued until the end of the landfill
aftercare period.
The restoration works for TKO Stage I Landfill had been
undertaken since 1997 under the same restoration works contract for TKO Stage
II/III Landfill.
The
major restoration works undertaken by the restoration contractor for TKO Stage
I Landfill include those specified for TKO Stage II/III Restored Landfill. There are 49 vertical wells to extract
landfill gas from the deeper parts of the site and 10 horizontal collectors to
extract landfill gas from the shallower areas of waste and from beneath the afteruse platforms.
There
is a landfill gas interception/venting trench, which was installed prior to the
restoration contract, running along the southern perimeter of the
landfill. Landfill gas intercepted by
this trench is vented to atmosphere via passive vents.
A
high-density polyethylene barrier and passive vent were also installed
at the north-west of the site as part
of the development of the former Hong Kong Oxygen site.
The capping and
surface water drainage systems installed at TKO Stage I Landfill have the
effect of limiting the leachate generation rate. To reduce the leachate levels within the
waste, leachate was also envisaged to be abstracted from on-site wells at the
eastern edge of the landfill by pumping and temporarily held in an on-site
storage facility. The leachate would
then be collected by truck and tankered to the LPTW
at the TKO Stage II/III Landfill for treatment.
Ten groundwater monitoring wells (DG1 to DG6,
D113, D114, D115 and D120) were monitored quarterly throughout the aftercare
period (refers to Figure 12.5).
The
landfill restoration works were completed in February 1999. Monitoring and auditing of potential
environmental impacts through landfill gas and leachate measurements around the
perimeter of the site are currently ongoing.
This monitoring will be continued until the end of the landfill
aftercare period.
As
discussed in Section 12.3, the desalination plant is located in TKO
Area 137 with
a reserved site area of about 10 hectares. TKO Area 137 is located
to the south of the SENT Landfill and the TKO Industrial Estate. About 9 km of 1,200 mm diameter fresh water
mains will be laid along Wan Po Road, Po Hong Road and Tsui Lam Road. Figure 12.1 indicates that
part of the desalination plant and fresh water mains along the Wan Po Road lie
within the SENT Landfill Consultation
Zone,
SENT Landfill Extension Consultation Zone, TKO Stage I Restored Landfill
Consultation Zone and TKO Stage II/III Restored Landfill Consultation Zone,
respectively.
Figure 12.11 (Drawing No. 178901/B/PDR/40020) shows the preliminary general layout plan and
elevation plan of the desalination plant, respectively. The ground surface level of the desalination
plant is at about +5.0 mPD. The desalination plant includes the following
buildings:
·
Incoming Switchgear Room;
·
132kV Substation;
·
Sludge Filter Press Building;
·
Sludge Thickener (I);
·
Chlorine Store (I) & (II);
·
Underground Clear Water Storage Tank (I);
·
Chlorine Contact Tank (I);
·
Post Treatment (I);
·
Chemical Building;
·
Maintenance Workshop;
·
Reverse Osmosis Building (I) and Secondary Stage Media
Filter (I);
·
Clarifier - Dissolved Air Flotation (DAF) (I);
·
First Stage Media Filters (I);
·
Administrative Building and Laboratory;
·
Intake Pumping Station;
·
Standby Generator and Switchgear Room;
·
Treated Water Pump Station and Switchroom
(III); and
·
Sodium Hypochlorite Storage Tank and Dosing Station.
The buildings located within the SENT Landfill Extension
Consultation Zone are the Incoming Switchgear Room, 132 kV Substation, Treated
Water Pump Station and Switchroom (III), Chlorine
Contact Tank(I) & (II), Treated Water Tank and Standby Generator and
Switchgear Room (see Figure 12.10). A brief description of these buildings is given below. Since parts of the TKO Area 137 are located
within the existing SENT Landfill Consultation Zone and the new Landfill
Consultation Zone of the Extension, the area that is not covered by the
existing consultation zone is defined as Additional Area Covered by
Consultation Zone in the figures.
The Incoming Switchgear Room is a single storey building located at the
north-west corner of the Project Site.
The room will be located on the ground floor with the provision of
mechanical ventilation.
The 132 kV Substation has two storeys housing the 132 kV
transformers. The substation is located
at the north-west corner of the Project Site.
The
substation will be unmanned most of the time and most of the areas are open to
the atmosphere.
The Treated
Water Pump Station and Switchroom (III) is a two-storey building located at the north-east
corner of the Project Site. Mechanical
ventilation will be provided and the buildings will be unmanned most of the
time.
The Chlorine Contact Tank (I) & (II) are two single storey buildings
housing the chlorine contact tanks, ventilation and scrubber systems. Mechanical ventilation will be provided and
access to the building will be by authorisation.
The Treated Water Tank is an open-air structure located between the
Chlorine Contact Tank (I) & (II) and Treated Water Pump Station and Switchroom
(III) along the northern boundary of the Project
Site. The working area within the
building is open to the atmosphere.
The Standby Generator and Switchgear Room is a single storey building located next to the Incoming Switchgear Room and 132 kV Substation. There will be mechanical ventilation.
Table 12.3 provides a summary of the dimensions of the rooms of the
abovementioned buildings and the ventilation design.
Table 12.3 Dimensions of the Rooms/ Buildings within
the SENT Landfill Extension Consultation Zone and Ventilation Information
|
Item |
Buildings |
Floor Area (m2) |
Height (m) |
Room Volume (m3)
(a) |
Type of
Ventilation |
No. of Storey |
Design Air Change
Per Hour (ACH) |
|
1 |
Incoming Switchgear Room |
400 |
6 |
2,400 |
Mechanical
Ventilation |
1 |
5 |
|
2 |
132 kV Substation |
486 |
16 |
7,776 |
Mechanical
Ventilation |
2 |
4-6 |
|
3 |
Treated Water Pump Station and Switchroom
(III) |
793 |
18 |
14,274 |
Mechanical
Ventilation |
2 |
4-6 |
|
4 |
Chlorine Contact Tank(I) |
850 |
3 |
2,550 |
Mechanical
Ventilation |
1 |
4-6 |
|
5 |
Chlorine Contact Tank (II) |
850 |
3 |
2,550 |
Mechanical
Ventilation |
1 |
4-6 |
|
6 |
Treated Water Tank (b) |
500 |
2 |
N/A |
N/A |
1 |
N/A |
|
7 |
Standby Generator and Switchgear Room |
154 |
5 |
770 |
Mechanical
Ventilation |
1 |
4-6 |
|
Note: (a)
Room volume was calculated by multiplying the preliminary area and
height of the structures. (b)
Treated
Water Tank is an open-air structure. |
|||||||
The DN1200
fresh water mains will transfer the fresh water output from the desalination
plant to the TKOPFWSR. The water mains
will be located at about 1.0 m - 1.5 m below ground level whilst part of it
will be laid along the Wan Po Road.
The majority of areas
in the 132 kV Substation and Treated Water Tank will be insensitive
to the potential effects of landfill gas as they will be open to atmosphere;
however, the Incoming Switchgear Room, Treated Water Pump Station and Switchroom
(III), Standby Generator and Switchgear Room and Chlorine Contact Tank (I)
& (II), which are enclosed, will have the potential to accumulate landfill
gas. Nevertheless, these rooms/
buildings will be provided with mechanical ventilation to prevent the
accumulation of landfill gas. The
potential for landfill gas to reach the upper floors will be low.
Any utility pits and
manholes along the underground fresh water mains within the landfill gas
consultation zones are of high potential for ingress of landfill gas. However, the access to these areas will be
rare and by authorised and well trained personnel only.
The
desalination plant is located at the TKO Area 137, which was formed by public
fill generated from construction projects in Hong Kong as it has been operating
as a fill bank since 2002. The future
final level of the desalination plant is at approximately +5.0 mPD. Based on
records of the ground investigation undertaken as part of the SENT Landfill Extension EIA, the level of groundwater table
is approximately at +2.8mPD, leaving an unsaturated layer of 2.2m. It is considered that this permeable layer
between the SENT Landfill, the SENT Landfill Extension and the desalination
plant must be conservatively considered as conducive to landfill gas migration.
Figure 12.1 and Figure 12.6 (Drawing No. 178901/B/EIA/64029(TEMP)), Figure 12.7 (Drawing No. 178901/B/EIA/64028(TEMP)), Figure 12.8 (Drawing No. Drawing No.
178901/B/EIA/64027(TEMP)) and Figure 12.9 (Drawing No. 178901/B/EIA/64026(TEMP)) show the
cross-sections through the SENT
Landfill Extension, SENT Landfill, TKO Stage II/ III Restored Landfill and TKO
Stage I Restored Landfill, respectively, as well as the types and profiles of
soils and groundwater tables. The lowest
level of groundwater is approximately at + 1.5mPD at the Project Site and along
Wan Po Road.
The EPD has
provided the monitoring data for:
·
the SENT Landfill monitoring wells covering the period
from January 2013 to June 2014 and from March 2013 to March 2014 for landfill
gas and groundwater, respectively;
·
the TKO Stage II/III
Restored Landfill monitoring wells
covering the period from January 2013 to May 2014 and from January 2013
to April 2014 for landfill gas and groundwater, respectively; and
·
the TKO
Stage I Restored Landfill monitoring
wells covering the period from January 2013 to May 2014 and from January
2013 to April 2014 for landfill gas and groundwater, respectively.
A number of
landfill gas monitoring wells were installed and used to monitor off-site
landfill gas migration for the SENT Landfill, TKO Stage II/III Restored Landfill and TKO Stage I
Restored Landfill. Recent landfill gas
monitoring results of these wells are summarised in Table 12.4, Table 12.5 and Table 12.6.
Details of the monitoring results are presented in Annex 12A.
Table 12.4 Summary of Landfill Gas Monitoring Results
of the SENT Landfill (From January 2013 to June 2014)
|
Location |
Methane (% v/v) |
Carbon Dioxide (% v/v) |
Oxygen (% v/v) |
|||
|
Range |
Average |
Range |
Average |
Range |
Average |
|
|
Monitoring Wells
along the Southern Boundary of the SENT Landfill |
||||||
|
P1 |
0.0 - 0.0 |
0.0 |
0.0 – 13.9 |
5.7 |
0.2 – 20.7 |
11.6 |
|
P2 |
0.0 - 0.0 |
0.0 |
0.0 - 9.9 |
6.0 |
1.1 - 20.7 |
10.5 |
|
P3 |
0.0 - 0.0 |
0.0 |
0.0 – 11.5 |
5.6 |
0.1 – 20.7 |
11.0 |
|
P4 |
0.0 - 0.0 |
0.0 |
0.1 – 20.4 |
12.3 |
0.0 – 20.5 |
3.8 |
|
P5 |
0.0 - 0.0 |
0.0 |
0.0 – 15.1 |
3.1 |
4.1 – 20.7 |
17.1 |
|
P6 |
0.0 - 0.0 |
0.0 |
0.2 – 1.5 |
0.8 |
19.3 – 20.6 |
19.9 |
|
GP9 |
0.0 - 0.0 |
0.0 |
0.3 – 9.0 |
4.5 |
1.5 – 20.4 |
14.0 |
|
GP10 |
0.0 - 0.0 |
0.0 |
3.9 – 11.7 |
8.7 |
0.1 – 19.0 |
11.8 |
|
GP11 |
0.0 - 0.0 |
0.0 |
0.1 – 16.8 |
6.3 |
4.9 - 20.7 |
15.0 |
|
GP12 |
Filled with Water |
|||||
|
Monitoring Wells
along Wan Po Road |
||||||
|
P7 |
0.0 - 0.0 |
0.0 |
0.0 - 0.0 |
0.0 |
20.7 |
20.7 |
|
P8 |
0.0 - 0.0 |
0.0 |
0.0 - 0.1 |
0.0 |
20.5 – 20.7 |
20.7 |
|
P9 |
0.0 - 0.0 |
0.0 |
0.0 - 0.1 |
0.0 |
20.6 – 20.7 |
20.7 |
|
P10 |
0.0 - 0.0 |
0.0 |
0.0 - 0.3 |
0.0 |
20.3 – 20.7 |
20.6 |
|
P11 |
0.0 - 0.0 |
0.0 |
0.0 - 0.7 |
0.1 |
20.2 – 20.7 |
20.6 |
|
P12 |
0.0 - 0.0 |
0.0 |
0.0 – 2.7 |
1.2 |
14.7 – 20.7 |
18.1 |
|
P13 |
0.0 - 0.0 |
0.0 |
0.0 - 0.6 |
0.2 |
19.8 – 20.7 |
20.5 |
|
P14 |
0.0 - 0.0 |
0.0 |
0.4 – 6.2 |
3.6 |
15.0 – 20.4 |
17.5 |
|
P15 |
0.0 - 0.0 |
0.0 |
0.0 – 0.9 |
0.2 |
19.8 – 20.7 |
20.5 |
|
P16 |
0.0 - 0.0 |
0.0 |
0.0 – 0.6 |
0.1 |
20.0 – 20.7 |
20.6 |
|
P17 |
0.0 - 0.0 |
0.0 |
0.0 – 0.7 |
0.2 |
19.5 – 20.7 |
20.5 |
|
P18 |
0.0 - 0.0 |
0.0 |
0.0 – 0.8 |
0.2 |
20.1 – 20.7 |
20.6 |
|
P19 |
0.0 - 0.0 |
0.0 |
0.0 – 7.9 |
0.9 |
13.1 - 20.8 |
19.8 |
|
P20 |
0.0 - 0.0 |
0.0 |
0.0 – 21.9 |
9.7 |
1.1 – 20.8 |
11.7 |
|
P21 |
0.0 – 0.0 |
0.0 |
0.0 – 16.8 |
5.2 |
2.9 – 20.8 |
15.4 |
|
P22 |
0.0 - 0.0 |
0.0 |
0.0 - 10.8 |
0.8 |
5.9 – 20.7 |
19.6 |
|
P23 |
0.0 - 0.4 |
0.0 |
2.9 – 22.4 |
11.4 |
0.4 – 18.0 |
9.7 |
|
P24 |
0.0 - 0.0 |
0.0 |
0.0 – 13.3 |
4.7 |
7.1 – 20.8 |
16.1 |
|
GP15 |
0.0 - 0.0 |
0.0 |
0.0 – 0.2 |
0.0 |
20.5 – 20.7 |
20.7 |
|
GP16 |
0.0 - 0.0 |
0.0 |
0.0 – 1.1 |
0.4 |
18.8 – 20.7 |
20.2 |
|
GP17 |
0.0 - 0.0 |
0.0 |
0.0 – 3.5 |
0.6 |
4.9 – 20.7 |
17.7 |
|
GP18 |
0.0 - 0.0 |
0.0 |
0.0 – 11.4 |
6.0 |
1.1 – 20.7 |
14.9 |
|
GP19 |
0.0 - 0.0 |
0.0 |
0.0 - 0.5 |
0.1 |
20.2 – 20.7 |
20.6 |
|
GP20 |
0.0 - 0.0 |
0.0 |
0.1 – 16.3 |
6.6 |
4.4 – 20.6 |
16.0 |
|
GP21 |
0.0 - 0.0 |
0.0 |
0.0 - 18.0 |
1.6 |
3.5 – 20.7 |
19.1 |
|
GP22 |
0.0 - 0.0 |
0.0 |
0.0 – 8.3 |
0.7 |
14.0 – 20.8 |
20.1 |
Table 12.5 Summary of Landfill Gas Monitoring Results
of the TKO Stage II/III Restored Landfill (From January 2013 to May 2014)
|
Location |
Methane (% v/v) |
Carbon Dioxide (% v/v) |
Oxygen (% v/v) |
|||
|
Range |
Average |
Range |
Average |
Range |
Average |
|
|
DG1 |
0.0 - 0.0 |
0.0 |
0.0 – 3.7 |
0.6 |
16.1 – 20.5 |
19.6 |
|
DG2 |
0.0 - 0.0 |
0.0 |
0.0 – 4.8 |
1.6 |
16.5 – 20.7 |
18.8 |
|
DG3 |
0.0 - 0.0 |
0.0 |
0.2 – 6.5 |
1.7 |
9.4 – 20.4 |
18.0 |
|
DG4 |
0.0 - 0.0 |
0.0 |
0.1 – 7.3 |
3.1 |
12.1 – 20.1 |
17.2 |
|
DG5 |
0.0 - 0.0 |
0.0 |
0.0 – 0.3 |
0.1 |
19.9 – 20.7 |
20.3 |
|
DG6 |
0.0 - 0.0 |
0.0 |
0.2 – 1.1 |
0.7 |
18.2 – 20.4 |
19.5 |
|
DG7 |
0.0 - 0.0 |
0.0 |
0.0 – 2.7 |
0.8 |
15.7 – 20.6 |
19.2 |
|
Note: DG6 is
the monitoring well nearest to Wan Po Road.
Referring to Figure 12.4,
monitoring wells UG1, UG2 and UG3 are close to Wan Po Road. However, no monitoring data of these wells
is available on request. |
||||||
Table 12.6 Summary of Landfill Gas Monitoring Results
of the TKO Stage I Restored Landfill (From January 2013 to May 2014)
|
Location |
Methane (% v/v) |
Carbon Dioxide (% v/v) |
Oxygen (% v/v) |
|||
|
Range |
Average |
Range |
Average |
Range |
Average |
|
|
D113 |
0.0 - 0.0 |
0.0 |
0.2 – 6.4 |
2.1 |
6 - 21 |
17.8 |
|
D114 |
0.0 - 0.0 |
0.0 |
0.0 – 0.7 |
0.2 |
19.5 – 21.1 |
20.1 |
|
D115 |
0.0 - 0.0 |
0.0 |
0.1 – 6.1 |
1.2 |
13.2 – 21.0 |
19.0 |
|
D118 |
0.0 - 0.0 |
0.0 |
0.0 – 0.7 |
0.3 |
19.0 – 20.7 |
20.1 |
|
D120A |
0.0 - 0.0 |
0.0 |
0.0 – 0.6 |
0.1 |
18.1 – 20.8 |
19.9 |
|
D157 |
0.0 - 0.0 |
0.0 |
0.0 - 0.0 |
0.0 |
19.8 – 20.8 |
20.3 |
|
D126 |
0.0 - 0.0 |
0.0 |
0.0 – 1.4 |
0.4 |
18.5 – 20.7 |
19.8 |
|
D130R |
0.0 - 0.0 |
0.0 |
0.0 – 0.9 |
0.3 |
18.9 – 20.6 |
20.0 |
According to
the above monitoring data, zero concentration of methane has been observed in
all the monitoring wells of the SENT Landfill along its southern boundary, and the
TKO Stage II/III Restored Landfill and TKO Stage I Restored Landfill along Wan
Po Road near the fresh water mains. Low
concentrations of methane have been observed in monitoring well P23 in the SENT
Landfill along Wan Po Road where maximum concentration is less than 1%
(v/v). This suggests that the off-site
landfill gas migration in these areas is under effective control.
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 concentration of oxygen and
relatively high concentrations of carbon dioxide and reduced oxygen
concentrations are 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% or more may reasonably be expected. Oxygen concentrations in monitoring wells are
typically 20% (v/v).
The average carbon dioxide concentration
detected in the selected monitoring wells of the SENT Landfill, TKO Stage
II/III Restored Landfill and TKO Stage I Restored Landfill ranged from 0.0% to
11.4% (v/v), 0.1% to 3.1% (v/v) and 0.0% to 2.1 % (v/v), respectively. Low concentrations (below 5% (v/v) on
average) of carbon dioxide concentrations have been observed in the monitoring
wells for the TKO Stage II/III Restored Landfill and TKO Stage I Restored
Landfill. The monitoring wells for the
SENT Landfill where significant carbon dioxide concentrations have been
detected are P1 to P4 and GP9 to GP11 along the southern boundary of the landfill,
as well as P20 and P23 along Wan Po Road.
The maximum gas concentrations ranged from 6.2% to 22.4% (v/v)
indicating an elevated concentration compared to the normal background level.
Low levels of oxygen concentrations were
detected in monitoring wells P1 to P4 and GP9 to GP11 along the SENT Landfill
southern boundary, which further support the occurrence of landfill gas
migration.
In summary, zero concentration of methane and
low concentrations of carbon dioxide was detected in the monitoring wells for
the TKO Stage II/III Restored Landfill and TKO Stage I Restored Landfill along
Wan Po Road near the fresh water mains.
This suggests that off-site landfill gas migration in these areas is
under effective control. For the
monitoring wells along the southern boundary of the SENT Landfill, although
methane was not measured, elevated carbon dioxide and reduced oxygen levels
have been detected. In addition, low
concentrations of methane have been measured in the SENT Landfill monitoring
well P23 along Wan Po Road. These
observations suggest that there is still a potential for off-site migration of
landfill gas in these areas near the water mains. However, it is likely the migration, if any,
is limited and the area of influence is close to the landfill boundary. In general, any excavation work or work
involving the construction of trenches will use the open cut method. Any migration of landfill gas will easily
be dispersed and diluted upon contact with the atmosphere. Thus migration of landfill gas presents less
of risk to the excavation area of the water mains located at about 1.0 m - 1.5
m below ground level.
The results
of recent groundwater monitoring in the SENT Landfill, TKO Stage II/III
Restored Landfill and TKO Stage I Restored Landfill are presented in Annex 12B. Levels of pH are likely to be of interest to
designers to assess the likely durability requirements for below grade
structures, whilst ammoniacal nitrogen, total organic
carbon (TOC) and chemical oxygen demand (COD) parameters are of interest when
assessing pollution potential due to organic material within the groundwater.
The pH
levels measured at the monitoring wells for the SENT Landfill, TKO Stage II/III
Restored Landfill and TKO Stage I Restored Landfill ranged from 2.84 to 8.35,
6.62 to 7.21 and 5.46 to 7.36, respectively.
The monitoring wells where low pH levels have been continuously detected
are MW1-2, MW1-3 and MW1-5 in the SENT Landfill and D113 in the TKO Stage I
Restored Landfill. A low pH may result
from carbon dioxide dissolving into groundwater, although also an ambient
feature of the groundwater itself.
Considering ammoniacal nitrogen, TOC and COD, the concentrations
measured across the monitoring wells for the SENT Landfill, TKO Stage II/III
Restored Landfill and TKO Stage I Restored Landfill were generally low. The risk of generation of landfill gas from
groundwater is therefore considered to be low.
The ground
water level recorded in the groundwater monitoring wells for the SENT Landfill along
Wan Po Road ranged from 1.89 m to 11.55 m below ground (refers to Annex 12B). Although there is a very small potential for
landfill gas dissolved in groundwater to be released at remote locations; the
groundwater table is generally regarded as an effective barrier to landfill gas
movement, noting that the level of groundwater may vary with time.
Information
about groundwater level at the monitoring wells for the TKO Stage II/III Restored Landfill and TKO Stage
I Restored Landfill as well as the Project Site is unknown. However, with reference to SENT Landfill Extension EIA, the level of groundwater table of TKO Area 137
is approximately at +2.8 mPD. The formation level of the desalination plant
is around +5.0 mPD and hence leaving an unsaturated
layer of about 2.2 m. It is considered
that this permeable layer between the SENT Landfill, the SENT Landfill
Extension and the desalination plant will be conducive to landfill gas
migration.
With the
consultations with utility service providers (including Drainage Services
Department, CLP, Towngas, Towngas
Telecommunications, Hong Kong Broadband Network (HKBN), Hutchison
Communications (HGC) and WSD),
it is known that utilities run parallel to Wan Po Road (which is located within
the Consultation Zones of the SENT Landfill, SENT Landfill Extension, TKO Stage
II/III Restored Landfill and TKO Stage I Restored Landfill) (Figures
12.12 – 12.14). The detailed
records of underground utilities in the vicinity of the Project Site and within
the Consultation Zones are presented in Annex
12C, and the locations of the underground utilities along Wan Po Road are
highlighted in yellow in the related drawings.
There is no underground service channel, tunnel or culvert connecting
the SENT Landfill, SENT Landfill Extension, TKO Stage II/III Restored Landfill
and TKO Stage I Restored Landfill to the areas adjacent to the fresh water
mains along the Wan Po Road. In
addition, there is currently no man-made underground utility in TKO Area 137,
except a box culvert connecting the existing SENT Landfill and the berthing
area in TKO Area 137. However, it is
envisaged that the utilities along Wan Po Road will be extended to TKO Area 137
(including the Project Site) for the developments in the future.
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. The qualitative assessment of the hazard from
landfill gas to the desalination plant and water mains are then concluded.
SENT
Landfill is a large operating landfill, thus the facility must be acknowledged
as a significant potential source of landfill gas. SENT Landfill was designed and constructed to
incorporate international best practices to contain, manage and control waste,
leachate and landfill gas. It is
operated by an experienced international waste management contractor.
The
potential off-site migration of landfill gas should be assessed taking into
account the comprehensive and highly effective collection and management system
installed and operated. As discussed in Section 12.5.2 above, the recent landfill gas monitoring
results indicate that an insignificant amount of methane was recorded at the
monitoring wells along Wan Po Road.
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. Thus the potential of off-site migration of
landfill gas cannot be eliminated.
With
reference to the SENT Landfill Extension EIA and the recent landfill gas monitoring
data at the SENT Landfill, it is classified as a “medium” source.
With reference to the Environmental
Review of the Revised Scheme of SENT Landfill Extension, the SENT Landfill Extension will occupy 13 ha in the TKO Area 137
located immediately south of the existing infrastructure area of the SENT
Landfill. The two landfills will be
separated by the cap of the SENT Landfill and the liner of the SENT Landfill
Extension. The SENT Landfill Extension
is designed to receive approximately 6.5 Mm3 of void space over a
period of approximately 6 years.
Although the SENT Landfill Extension is not as large as
the SENT Landfill, it should still be acknowledged as a potential source
of landfill gas. Similar to the SENT
Landfill, the SENT Landfill Extension will be designed and constructed to
incorporate international best practices to contain, manage and control waste,
leachate and landfill gas. An
experienced waste management contractor will be appointed for the operation of the
SENT Landfill Extension.
Under the SENT Landfill Extension contract requirement, the Extension
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 background level, respectively. The potential off-site migration of landfill
gas should be assessed taking into account the comprehensive and highly
effective collection and management system to be installed and operated.
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, it would be reasonably conservative to classify the SENT Landfill
Extension as a “medium” source.
With
reference to the Environmental Review of Urban Landfills and Tseung Kwan O Landfills – Feasibility Study, the Restoration Contractor of the landfill has designed and
implemented a gas management system for the TKO Stage II/III Restored
Landfill. The system included placement
of a membrane over the waste to contain the landfill gas, installation of a gas
collection and abstraction system, utilisation of the landfill gas as a fuel to
generate electricity and for leachate treatment, and combustion of the surplus
gas in enclosed flares. There are a
total of 50 vertical wells to extract landfill gas.
In consideration of the recent landfill gas monitoring data as discussed in Section 12.5.2 (concentration of methane is continuously at
0.0% (v/v)) and the landfill gas management system installed
at the landfill, the TKO Stage II/III Landfill is therefore classified as a
“medium” source.
The
TKO Stage I Restored Landfill is a landfill site of over 60 ha in area and
estimated to contain between 10.6 and 16.2 million m3 of waste which
was deposited in two phases, the last of which was completed in 1995.
The
landfill has been restored since1999.
The landfill gas control measures included the provision of a passive
gas venting trench along the southern boundary of the landfill and an active
gas extraction system for control of gas.
The recent landfill gas monitoring data as discussed in Section 12.5.2 above demonstrates the no methane was
detected in the perimeter monitoring wells at the site boundary.
Taking the landfill gas control system installed at the landfill and the
recent landfill gas monitoring data (concentration
of methane is continuously at 0.0% (v/v)) into account, the TKO Stage I Restored Landfill has been considered to
constitute a “medium” source in this assessment.
The Incoming
Switchgear Room, 132 kV Substation and Chlorine
Store (I) and (II) of the proposed desalination plant are about 250 m from the southern
boundary of the SENT Landfill Extension.
The potential pathways for sub-surface
migration of landfill gas from the SENT Landfill Extension to these buildings
within the Project Site are considered to comprise only natural features and
reclamation fill. Taking account of
the distance between the SENT Landfill Extension and these buildings, and the
presence of possible anthropogenic migration pathways (such as an extension of
the existing utilities along Wan Po Road to the Project Site), it is considered
that the hazard category of migration pathway to be moderately
short/ direct.
The distances between the fresh water mains and the western boundary of
the SENT Landfill (including the extension), TKO Stage II/III Restored Landfill
and TKO Stage I Landfill are about 50 m, 200 m and 20 m, respectively. The ground is reclaimed fill and presumed to
be moderately high gas permeable. Table 12.7 below classifies
the landfill gas pathways for various parts of the fresh water mains.
Table 12.7 Classification of Landfill Gas Migration
Pathway
|
Pathway |
Pathway Description |
Classification |
|
1. Fresh Water Mains within the SENT Landfill Extension Consultation Zone |
Path length of less than 50 m for
unsaturated permeable strata and fissured rock or less than 100m for man-made
conduits. |
Very short/ direct |
|
2. Fresh Water Mains within the SENT Landfill Consultation Zone |
Path length of less than 50 m for
unsaturated permeable strata and fissured rock or less than 100m for man-made
conduits. |
Very short/ direct |
|
3.
Fresh Water Mains within
the TKO Stage II/III Restored Landfill Consultation Zone |
Path length of 100-250 m for unsaturated
permeable soils and fissured rock. |
Long/ indirect |
|
4. Fresh Water Mains within the TKO Stage I Restored Landfill
Consultation Zone |
Path length of less than 50 m for
unsaturated permeable strata and fissured rock or less than 100m for man-made
conduits. |
Very short/ direct |
In accordance with the Guidance
Note, the desalination plant and fresh water mains can be categorised into
the following targets:
Target 1 – Construction of the Fresh Water Mains
The fresh water mains fall within the SENT Landfill, SENT Landfill
Extension, TKO Stage II/III Restored Landfill and TKO Stage I Restored Landfill
Consultation Zones. The work will involve
the excavation to around 2.5 m below ground to construct a trench for the
installation of the fresh water mains.
In general, open cut method will be used and hence any
migration of landfill gas will easily be dispersed and diluted upon contact with
the atmosphere. In the case that open
cut method is not practical, trenchless method such as pipe jacking by
micro-tunnelling will be adopted. Figure
12.1 shows the proposed locations of pipe jacking. Three of the proposed locations will be located
within the TKO
Stage II/III Restored Landfill and SENT
Landfill Consultation Zones, respectively.
This target is catergoised
as “medium sensitivity” using either open cut or trenchless method.
Target 2 – Construction of the Desalination Plant
The Incoming Switchgear Room, 132 kV Substation, Treated Water Pump Station and Switchroom (III), Chlorine Contact Tank(I) & (II),
Treated Water Tank and Standby Generator and Switchgear Room of the
desalination plant are located within the SENT Landfill Extension Landfill
Consultation Zone. The
construction works will include open excavation to around 1 m below ground to
construct these buildings. Works (including
welding and concreting) will thus be undertaken below ground level. Since the area of excavation is large, and it
is open above, the below ground area can be adequately ventilated. Landfill gas, if any, migrated to the Project
Site can easily be diluted in the atmosphere.
This target is thus classified as “low sensitivity”.
Target 3 – Operation of the Desalination Plant
The Incoming Switchgear Room, 132 kV Substation, Treated Water Pump Station and Switchroom (III), Chlorine Contact Tank(I) & (II),
Treated Water Tank and Standby Generator and Switchgear Room of the
desalination plant will be located within the 250 m Consultation Zone of the
SENT Landfill Extension. All these
buildings are on the ground level and access to these buildings is restricted
to the authourised contractors, CLP or WSD staffs who
will be briefed on the potential hazards related to landfill gas and the
specific safety procedures to be followed.
Thus this target is considered as “medium sensitivity”.
Target 4 – Operation of the Fresh Water Mains
The fresh water mains will be located at around 1 m to 1.5 m below
ground. Underground confined spaces
(such as utility pits and manholes) along the fresh water mains are places
where landfill gas can potentially accumulate.
Restricted access to the manhole and utility pits by authorised and well
trained personnel is expected. This
target is therefore considered as “medium sensitivity”.
A qualitative assessment of the hazard to the abovementioned targets
based on the catergorisation of the Source, Pathway
and Target terms and the EPD assessment framework is presented in Table 12.8.
Table 12.8 Qualitative
Assessment of Landfill Gas Hazard
|
Source |
Pathway |
Target |
Assessment of Hazard |
|
SENT Landfill. Potential for gas generation over time, but
comprehensive and proven mitigation installed. Monitoring data has shown
effective control on off-site landfill gas migration. (medium source) |
Surface soil, reclamation
fill materials and no direct man-made conduit. Part of the fresh water mains located at 50
m from the landfill. (very
short/ direct) |
Target 1 - Construction of the fresh water mains. (medium
sensitivity) |
Medium |
|
Ditto. |
Target 4 – Operation of the fresh water
mains. (medium
sensitivity) |
Medium |
|
|
SENT Landfill
Extension. Potential for gas
generation over time. Comprehensive
and proven mitigation to be installed. (medium source) |
Surface soil, reclamation fill
materials, and potential direct man-made conduits. The Incoming Switchgear Room, 132 kV
Substation, Treated Water Pump Station
and Switchroom (III), Chlorine Contact Tank(I)
& (II), Treated Water Tank and Standby Generator and Switchgear Room of
the proposed desalination plant located at about 250 m from the southern
boundary of the SENT Landfill Extension. (moderately short/ direct) |
Target 2 - Construction of the desalination plant. (low
sensitivity) |
Low |
|
Ditto. |
Target 3 – Operation of the desalination
plant. (medium
sensitivity) |
Medium |
|
|
Surface soil, reclamation
fill materials, and no direct man-made conduit. Part of the fresh water mains located
within the SENT Landfill 250 m Consultation Zone. (Very short/ direct) |
Target 4 – Operation of the fresh water
mains. (medium
sensitivity) |
Medium |
|
|
TKO Stage II/III Restored
Landfill. Actively producing landfill
gas with gas extraction system as gas control measures. Monitoring data has shown effective
control of landfill gas is being achieved. (medium source) |
Permeable reclamation fill
materials. Part of the freshwater
mains located at about 200 m from the landfill. (Long/ indirect) |
Target 1 - Construction of the fresh water mains. (medium sensitivity) |
Low |
|
Ditto. |
Target 4 – Operation of the fresh water
mains. (medium
sensitivity) |
Low |
|
|
TKO Stage I Restored
Landfill. Actively producing landfill
gas, retrospective gas control measures installed (including active gas extraction
system and a passive gas venting trench along the southern boundary. Monitoring data has shown
effective control of landfill gas is being achieved. (medium source) |
Permeable reclamation fill
materials. Part of the fresh water
mains located at about 20 m from the landfill. (very short/ direct) |
Target 1 - Construction of the fresh water mains. (medium sensitivity) |
Medium |
|
Ditto. |
Target 4 – Operation of the fresh water
mains. (medium
sensitivity) |
Medium |
The qualitative hazard to the proposed desalination plant and fresh
water mains from landfill gas has assessed as being from low to medium for
various targets.
The
qualitative hazard assessment undertaken in Section
12.6 has concluded that the potential for landfill
gas to affect the desalination plant and fresh water mains during the
operational phase is low to medium.
During the construction period, similar hazards may also arise related
to the flammability or the potential asphyxiating properties of landfill gas
and/ or the potentially toxic nature of leachate. Given the results of the qualitative risk
assessment, it should be assumed that the risks due to the landfill gas or
leachate during the construction are low to medium.
This section
provides advice and recommendations for the avoidance of environmental impacts
related to landfill gas and leachate during the construction and operational
phases.
In general,
the restoration and aftercare of the TKO Stage II/III Restored Landfill and TKO
Stage I Restored Landfill, and the control of landfill gas and leachate of the
SENT Landfill and SENT Landfill Extension should not be relied upon to ensure
the safety of adjoining developments; however, it must also be acknowledged
that the restoration works and landfill gas control measures undertaken will
have the effect of lowering the potential for an incident to occur off-site. Allowance for this has been made in the
qualitative assessment undertaken.
All
contractors, CLP or WSD staffs participating in the construction and operation
of the desalination plant and fresh water mains should be aware that 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 give 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 detector;
·
Methane is
flammable gas and will burn when mixed with air between 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 an asphyxiant;
·
Carbon
dioxide,
the other major component of landfill gas, is an asphyxiating gas and causes
adverse health impacts 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.
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.
Notwithstanding
the development of other safety and protection measures and procedures which
will be required through the construction contracts or according to the site
conditions, the following recommendations should also be adopted, including:
·
All staff working in the Consultation Zones should
receive appropriate training on working in areas susceptible to landfill gas,
fire and explosion hazards;
·
No worker should be allowed to work alone at any time in
or near to any excavation areas within the Consultation Zones. At least one other worker should be available
to assist with a rescue if needed. An
excavation procedure to minimize landfill gas related risk should be devised
and carried out;
·
Ground level construction plant used in Consultation
Zones (such as micro-tunnelling machine) should be
fitted with vertical exhausts at least 0.6m above ground level and with spark
arrestors. Any electrical equipment
(such as motors and extension cords) should be intrinsically safe;
·
During piping assembly within Consultation Zones, all
valves should be closed immediately after installation. As construction progress, all valves should
be closed as installed to prevent the migration of gases through the
pipeline. All piping should be capped at
the end of each working day;
·
During construction, adequate fire extinguishing
equipment, fire resistant clothing and breathing apparatus sets should be made
available on site;
·
During all works, safety procedures should be implemented
to minimise the risks of fires and explosions,
asphyxiation of workers and toxicity effects resulting from contact with
contaminated soil and groundwater;
·
During trenching and excavation as well as creation of
confined spaces near to or below ground level, precautions should be clearly
laid down and adhered to rigidly. Gas
detection equipment and appropriate breathing apparatus should be available and
used when entering confined spaces or trenches deeper than 1 metre;
·
The Contractor should make the workers aware of potential
hazards of working in confined spaces (any chamber, manhole or culvert which is
large enough to permit access by personnel).
Such work in confined spaces is controlled by the Factories and Industrial Undertakings (Confined Spaces) Regulations of
the Factories and Industrial Undertakings Ordinance. Following the Safety Guide to Working in Confined Spaces ensures compliance with
the above regulations;
·
Safety officers, specifically trained with regard to
landfill gas and leachate 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, the possible presence of
contaminated water and the need to avoid physical contact with it;
·
Monitoring for landfill gas should be undertaken in all
excavations, manholes, chambers (particularly during pipe jacking) and any
confined spaces through the use of an intrinsically safe portable instrument,
appropriately calibrated and capable of measuring the following concentrations:
|
-
methane |
0-100%
(Lower Explosive Limit (LEL)) and
0-100% (v/v) |
|
-
carbon dioxide |
0-100% (v/v); and |
|
-
oxygen |
0-21% (v/v). |
All measurements should be made prior to the entry with the
use of monitoring tube located not more than 10 mm from the exposed ground
surface. Entry should only be allowed if
oxygen is greater than 18% by volume, methane is less than 10% of the LEL,
which is equivalent to 0.5% by volume (approximately), and carbon dioxide is
less than 0.5% by volume;
·
Monitoring frequency and areas to be monitored should be
specified prior to commencement of groundwork, either by the Safety Officer, or
by an appropriately qualified person.
All measurements should be recorded and documented;
·
For excavations deeper than 1m, measurements should be
carried out:
-
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 carried out:
-
directly after the excavation has been completed; and
-
periodically whilst the excavation remains open.
·
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;
·
Welding, flame-cutting or other hot works should be
confined to open areas at least 15m from any trench or excavation;
·
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 authorised by the Safety
Officer. The ‘permit to work’ procedure
will set out 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 should be responsible for reviewing the gas
measurements as they are made, and who should 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;
·
Enhanced personal hygiene practices including washing
thoroughly after working and eating only in “clean” areas should be adopted
where contact may have been made with any groundwater which is thought to be
contaminated with leachate.
In
the case that drilling operations are required to be carried out within any of
the landfill consultation zones, the
following additional recommendations should also be adopted, including:
·
Proceed drilling with adequate care and precautions
against the potential hazards which may be encountered;
·
Prior to the commencement of the site works, the drilling
contractor should devise a 'method-of-working' statement covering all normal
and emergency procedures (including but not limited to number of operatives,
experience and special skills of operatives, normal method of operations,
emergency procedures, supervisors responsibilities, storage and use of safety
equipment, safety procedures and signs, barriers and guarding). The site supervisor and all operatives must be
familiar with this statement;
·
The drilling rig should be set-up up-wind
of the borehole location. 'No smoking' signs should be placed prominently
adjacent to the drilling area and the working area should be cordoned off;
·
One person should be present at all times
during drilling operations with the sole responsibility of assuring the
observance of all safety procedures. This person should be trained in the use
of all recommended safety equipment.
The construction
of the desalination plant and fresh water mains will be undertaken in
accordance with the construction contract which specifies the relevant
statutory and other recommended environmental, health and safety guidelines to
be followed. In addition, and
specifically, the tenderers will be made aware of hazards associated with
landfill gas and other issues relevant to construction in the vicinity of a
landfill through the Materials and Workmanship Specification and further, will
be provided with copies of this report in order to inform them of the likely
significances of these issues and any additional recommended safety and
precautionary measures that may need to be implemented during the construction
phase.
Prior to the
commencement of works, the employer under the construction contract should
ensure through review and verification of the Contractor’s Safety Plan that
adequate consideration has been given by the contractor to the occurrence and
management of emergency situations and that emergency plans have been developed
to cover contingencies relating to landfill gas and leachate.
The
construction contract also calls for contractor to submit method statement or
statements of working procedures to the engineer prior to the commencement of
specific activities on site. The
method-of working statement should cover the following aspects, inter
alia:
·
number of operatives;
·
experience and special skills of operatives;
·
normal method of operations;
·
proposed monitoring;
·
emergency procedures, including firefighting;
·
supervisors responsibilities;
·
storage and usage of safety equipment;
·
safety procedures; and
·
signs, barriers and guarding for safety control.
It is
recommended that the employer under the construction contract provides a review
of all method statements, where there is a potential for landfill gas and
leachate impacts to occur, to ensure that adequate measures are incorporated in
the procedures to contour any possible adverse effects. The contractor should not be allowed to
commence work prior to the review by these persons and where recommendations
are made, until appropriate adjustments to the work procedure as required by
these persons have been incorporated and agreed.
Monitoring
for methane, carbon dioxide and oxygen should be undertaken during the
construction phase where works involve deep excavation (eg
mainlaying, piling and foundation works etc.) or
entry into confined spaces. The
frequency and areas to be monitored should be documented in the Contractor’s
Safety Plan prior to commencement of the works.
All
monitoring shall be carried out by trained technicians and equipment calibrated
according to the manufacturers’ instructions.
A standard record form, detailing the location, time of monitoring and
equipment used, together with the gas concentrations measured, should be used
to ensure all relevant data are recorded for future reference.
Depending on
the results of gas monitoring and the environment under which is conducted (ie confined space or atmospheric works), appropriate
actions will vary and should be stated in the Contractor’s Safety Plan.
In
emergencies, the following bodies should be contacted:
·
Fire Services Department;
·
Green Valley Landfill Limited (the contractor of the SENT
Landfill);
·
SITA Waste Services Limited (the contractor of the
Restoration of TKO Stage I Restored Landfill and Stage II/III Restored
Landfill); and
·
EPD.
The qualitative assessment has determined that there is a low to medium
potential hazard of landfill gas affecting the identified targets based on the
data and assumptions contained in this section.
However, the following potential measures are recommended for active
consideration by the Project team in the ongoing design, construction and
operational processes in order to minimise the
potential hazards.
Notwithstanding these recommendations, it should be acknowledged that
other methods conceived by the design team may ultimately have a similar effect
to those prescribed.
Where below ground service entries are necessary to the Incoming
Switchgear Room, 132 kV Substation, Treated Water Pump Station and Switchroom (III), Chlorine Contact Tank(I) & (II),
Treated Water Tank and Standby Generator and Switchgear Room, the entry point
should be sealed to prevent gas entry.
In addition, any below grade cable trenches entering the Incoming Switchgear
Room and 132 kV Substation can become the pathway for landfill gas and hence
grilled metal covers should be used.
Landfill gas mainly consists of methane and
carbon dioxide. Explosion will be
resulted when methane mix with air within the 5%, LEL and 15%, Upper Explosive
Level (UEL) in confined spaces and given a source of ignition. It is recommended regular landfill gas
monitoring should be carried out at the Incoming Switchgear Room, 132 kV Substation, Treated Water Pump Station
and Switchroom (III), Chlorine Contact Tank(I) &
(II), Treated Water Tank and Standby Generator and Switchgear Room. The monitoring frequency will be monthly
for the first year of operation. If the
monitoring results show no sign of landfill gas migration, reduce the
monitoring frequency to once every six months.
The manholes
and utility pits within the Project Site and along the fresh water mains. Each manhole/ utility pit should be monitored
with two measurements (at mid depth and base).
Each measurement should be monitored for a minimum of 10 minutes. A steady reading and peak reading should be
recorded at each manhole/ utility pit and for each measurement. The need for venting the manhole/ utility pit
and further monitoring will be reviewed after the initial monitoring.
All construction, operation and maintenance personnel working on-site as
well as visitors should be made aware of the hazards of landfill gas and its
possible presence on-site. This should
be achieved through a combination of posting warning signs in prominent places
and also by access to detailed information on landfill gas hazards and the
designs and procedural means by which these hazards are being minimised on-site.
The contractor under the construction contract and the
WSD should also make arrangement with the landfill and landfill restoration
contractors so that they are advised of all situations which may potentially
threaten the safety of the visitors and workers on-site resulting from any
accidents or failures at the landfill sites.
To ensure that all the aforesaid protection and
precautionary measures and issues pertaining to landfill gas are properly and
consistently addressed, it is recommended that a comprehensive landfill gas
hazard management system be developed.
The system shall describe responsibilities for implementing the landfill
gas precautionary measures and contingency plan in case of landfill gas being
detected.