The Extension
will be located adjacent to the SENT Landfill with a portion piggybacking onto the
south slope of the SENT Landfill. Figure
3.1a shows the location of the SENT Landfill and the Extension. The Extension will be a new source of
landfill gas generation.
There are potential risks associated with
developments close to a landfill site due to sub-surface migration of landfill
gas. This Section describes the
methodology and presents the findings of a qualitative landfill gas hazard
assessment of the Extension.
Under Annex
7 of the EIAO-TM, an evaluation
of the potential risk posed by landfill gas is required for any development
which is proposed within 250m of the edge of waste, known as Landfill
Consultation Zone. As the Extension
Site falls within the SENT Landfill Consultation Zone (see Figure 8.2a), a
Qualitative Landfill Gas Hazards Assessment (QLFGHA) is required to assess the
potential risk due to landfill gas migration from the SENT Landfill to the
Extension.
A Practice
Note for Professional Person (ProPECC PN 3/96) ([1])
and Guidance Note([2]) for the assessment of the hazards which
landfill gas may present to developments close to landfills have been issued by
the EPD. These documents provide 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 ProPECC PN 3/96 and Guidance Note
apply to all developments proposed within 250m of the edge of the waste
boundary, known as the Landfill Consultation Zone.
As the Extension is located adjacent to
the existing development in the TKOIE and future industrial development in TKO
Area 137, a QLFGHA will be required for developments within 250m of the
Extension (see Figure 8.2a).
The following tasks have been undertaken
as part of this study:
·
review
of background information (including landfill gas monitoring data) and studies
related to the SENT Landfill and the Extension;
·
identification
of the nature and extent of the sources, including the likely concentrations
and/or amounts of landfill gas emissions which might have the potential for
impacts on the Extension and impacts from the Extension to the potential
receivers;
·
identification
of possible pathways through the ground, underground cavities, utilities or
groundwater, and the nature of these pathways through which the landfill gas
must traverse if they were to reach the Extension and the new consultation
zone;
·
identification
of the potential receivers associated with the Extension which are sensitive to
the impacts of landfill gas emissions;
·
qualitative
assessment on the degree of risk which the landfill gas emissions may impose on
the receivers for each of the source-pathway-receiver combinations; and
·
design of suitable level of precautionary measures and
contingency plan for the Extension and the potential receivers, if needed.
In accordance with the Guidance Note on Landfill Gas Hazard
Assessment, 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) is determined as follows:
Major Recently
filled landfill site at which there is little or no control to prevent
migration of gas or at which the efficacy of the 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 6 months, is less than 5% (v/v).
Generally, three types of pathway are
considered for the transmission of landfill gas. They are:
·
Man-made pathways, eg utility
connections, stormwater channels, etc,
·
Natural pathways such as rock jointing planes, fissures and
other naturally occurring phenomena which may promote or give rise to the
transmission of gas over distances; and
·
A combination of the previous two
categories. An example of the
latter may be, for instance, where a specific geological feature promotes gas
transmission but which stops short of directly linking the landfill and
target. A man made connection,
however may also co-exist near the edge of the geological feature, which in
combination with the former, may act to link the two sites. In this instance, careful assessment of
the likelihood of the mechanism acting to link the two pathways needs to be
undertaken before assigning an appropriate pathway classification.
The broad classification of a Pathway is
as follows:
|
Very short/direct |
Path length of less than 50m for
unsaturated permeable strata and fissured rock or less than 100m for man-made
conduits |
|
Moderately short/direct |
Path length of 50 to 100m for
unsaturated permeable soil or fissured rock or 100 to 250 m for man-made conduits |
|
Long/indirect |
Path length of 100 to 250m for
unsaturated permeable soils and fissured rock |
In classifying the pathway, however,
adjustment to the above general guidelines will often be required to take
account of other factors which will affect the extent of 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);
·
the
nature of the strata over the potential pathway;
·
the
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 · This would include any developments
where there is a possibility of additional structures being erected directly
on the ground on an ad hoc basis
and thereby without due regard to the potential risks. |
|
Medium
Sensitivity |
· Other buildings, structures or service
voids where there is access only by authorised, well trained personnel, such
as the staff of utility companies, who have been briefed on the potential
hazards relating to landfill gas and the specific safety procedures to be
followed. · Deep excavations. |
|
Low
Sensitivity |
· Buildings/structures which are less
prone to gas ingress by virtue of their design (such as those with a raised
floor slab). · Shallow excavations. · Developments which involve essentially
outdoor activities but where evolution of gas could pose potential problems. |
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 the
three indicated categories.
Following the determination of the
categories for the 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 8.4a which is
extracted from the EPD’s Guidance Note on Landfill Gas Hazard
Assessment. The potential
implications associated with the various qualitative risk categories are
summarised in Table 8.4b. It should be noted that the different
levels of risk determine the likely extent of the protection measures required to ensure the safety of a development, but with the possible
exception of the very high risk category, development is not precluded for any
of the assessed levels of risk.
Table 8.4a 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 8.4b Summary
of General Categorisations of Risk
|
Level of Risk |
Implication |
|
Very high |
At the very least, extensive engineering measures and
alarm systems are likely to be required.
An emergency actions plan should also be developed so that appropriate
actions may be immediately taken in the event of high 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
The
landfill is one of the three strategic landfills in operation in the SAR and
was designed to receive approximately 43 Mm3 of waste over a period
initially projected as 15 to 17 years. The landfill commenced
operation in 1994 and accepts domestic, commercial & industrial (C&I),
construction, and clinical wastes, sewage sludge and stablised
incineration residues. However, the
landfill is filling faster than originally projected and the latest forecast shows
that the landfill will be full by around 2012.
In the landfill gas generation forecast undertaken as
part of this Study, it has been estimated that the landfill will typically
generate between 100 and 140 Mm3 of gas per year throughout its
operational life. Gas generation
was predicted to peak at about 150 Mm3 per year in 2014,
approximately 2 years after the landfill is closed.
The landfill has been designed to
incorporate extensive measures to contain, 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. The landfill gas extraction system
contains two skids, each with spare parts.
While only one skid is in operation at one time, the other skid serves
as an emergency backup. The
landfill contractor is undertaking routine maintenance and checking of the
landfill gas extraction system to ensure it is operating satisfactorily. As the site is lined and landfill gas 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
8.5a.
A comprehensive environmental monitoring
programme has been implemented to monitor landfill gas generated within the
landfill and at the perimeter boreholes around the landfill. 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. Recent monitoring results from the
boreholes located along the southern boundary of SENT Landfill have been
reviewed. Figure 8.5b shows
the locations of these boreholes and the monitoring results are summarised in Table 8.5a (see Annex C for details). The monitoring results indicate that
there is no sub-surface off-site migration of methane at the southern part of
the landfill. Elevated carbon
dioxide concentrations (over 15% v/v) were recorded in GP-4 (deep), GP-9,
GP-10, GP-11, P-1, P-2, P-3, P-4 and P-5.
Table 8.5a Landfill
Gas Monitoring Results at SENT Landfill (Jan 2006 – Jul 2007)
|
Location |
Methane (% gas) |
Carbon Dioxide (% gas) |
||
|
|
Range |
Average |
Range |
Average |
|
GP-1 |
0.0 – 0.0 |
0.0 |
0.0 – 3.1 |
0.9 |
|
GP-2 (deep) |
0.0 – 0.0 |
0.0 |
0.0 – 2.4 |
1.0 |
|
GP-2 (shallow) |
0.0 – 0.1 |
0.0 |
0.0 – 3.5 |
2.2 |
|
GP-3 (deep) |
0.0 – 0.0 |
0.0 |
0.0 – 0.3 |
0.0 |
|
GP-3 (shallow) |
0.0 – 0.0 |
0.0 |
0.0 – 3.3 |
0.2 |
|
GP-4 (deep) |
0.0 – 0.0 |
0.0 |
0.0 –
12.6 |
1.8 |
|
GP-4 (shallow) |
0.0 – 0.1 |
0.0 |
0.0 – 5.2 |
1.6 |
|
GP-5 (deep) |
0.0 – 0.0 |
0.0 |
0.0 – 0.1 |
0.0 |
|
GP-5 (shallow) |
0.0 – 0.1 |
0.0 |
0.0 – 1.9 |
0.5 |
|
GP-6 |
0.0 – 0.0 |
0.0 |
0.0 – 2.6 |
1.1 |
|
GP-7 |
0.0 – 0.1 |
0.0 |
0.0 – 0.9 |
0.2 |
|
GP-8 |
0.0 – 0.0 |
0.0 |
0.0 – 5.2 |
1.2 |
|
GP-9 |
0.0 – 0.0 |
0.0 |
0.0 –
12.3 |
4.5 |
|
GP-10 |
0.0 – 0.1 |
0.0 |
0.0 –
21.1 |
7.1 |
|
GP-11 |
0.0 – 0.1 |
0.0 |
0.0 –
16.0 |
7.7 |
|
GP-15 |
0.0 – 0.0 |
0.0 |
0.0 – 0.2 |
0.0 |
|
P-1 |
0.0 – 0.1 |
0.0 |
0.0 –
15.0 |
8.0 |
|
P-2 |
0.0 – 0.1 |
0.0 |
0.0 –
10.2 |
5.2 |
|
P-3 |
0.0 – 0.0 |
0.0 |
0.0 –
16.8 |
6.8 |
|
P-4 |
0.0 – 0.1 |
0.0 |
6.2 –
19.2 |
15.0 |
|
P-5 |
0.0 – 0.0 |
0.0 |
0.0 –
13.8 |
4.3 |
|
P-6 |
0.0 – 0.1 |
0.0 |
0.0 – 1.7 |
0.5 |
|
P-7 |
0.0 – 0.1 |
0.0 |
0.0 – 0.0 |
0.0 |
|
P-8 |
0.0 – 0.0 |
0.0 |
0.0 – 0.0 |
0.0 |
|
P-9 |
0.0 – 0.0 |
0.0 |
0.0 – 0.2 |
0.0 |
As SENT
Landfill is a large operating landfill, 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 reputable 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. The regular
landfill gas monitoring results at the perimeter boreholes undertaken by the
landfill contractor indicate that an insignificant amount of methane was
recorded in the perimeter boreholes along the southern boundary. However, according to the Guidance Note, a carbon dioxide
concentration greater than 5% v/v above background levels in any monitoring
well indicates significant migration.
Hence the potential of off-site migration of landfill gas cannot be
eliminated.
Given the
size of the SENT Landfill, the multiple landfill gas controls and the recent
landfill gas monitoring data, it would be reasonable to classify it as a
“medium” source.
The Extension will occupy 15 ha in TKO
Area 137 located immediately south of the existing infrastructure area of SENT
Landfill. The Extension will
piggyback onto the southern slope of the existing SENT Landfill and will occupy
approximately 5 ha of the natural slope at the edge of the Clear Water Bay
Country Park (CWBCP). The two
landfills will be separated by the cap of the SENT Landfill and the liner of
the Extension.
The
Extension is designed to receive approximately 17 Mm3 of waste over
a period of approximately 6 years. It
is anticipated that the Extension will commence operation in 2013 and will
accept similar waste types to those currently disposed of at SENT Landfill.
It has been estimated that the landfill
will typically generate between 14 and 52 Mm3 of gas per year
throughout its operational life.
Gas generation is predicted to peak at 73 Mm3 per year in
2020, around two years after the Extension is closed.
Similar to the SENT Landfill, the
Extension will be designed to incorporate extensive measures to contain,
collect, and treat/utilise landfill gas.
These measures include a composite liner systems, active landfill gas
extraction and landfill gas treatment and utilisation system in accordance with
international best practices for landfill operation. These measures can effectively control
sub-surface off-site migration of landfill gas, as demonstrated by the
monitoring data at the SENT Landfill.
The design of the composite liner system for the Extension will be
similar to those currently being used in the SENT Landfill. Details of the composite liner system
designed for the Extension is shown in Figure 3.3c.
A comprehensive environmental monitoring
programme will be implemented during the construction, operation, restoration
and aftercare of the Extension to monitor landfill gas generated within the
Extension and at the perimeter boreholes around the Extension. With reference to the performance
standard stipulated in the SENT Landfill contract, the landfill contractor is
required to control the migration of landfill gas such that the concentration
of methane and carbon dioxide at the perimeter boreholes shall not exceed 1%
v/v and 1.5% v/v, respectively. The
Extension contract will adopt the same standard. Since the Extension Site is formed by
reclamation of public fill, carbon dioxide may occur naturally in the fill
material. It is thus more
reasonable to adopt 1.5% v/v above the background concentration as the
standard. The Extension contractor
will be required to continue the control and monitoring of landfill gas
following closure of the landfill for a period of 30 years.
Although the Extension is not as large as the
SENT Landfill, the facility should still be acknowledged as a potential source
of landfill gas. Similar to the
SENT Landfill, the Extension will be designed and constructed to incorporate
international best practices to contain, manage and control waste, leachate, and landfill gas. It will be operated by an
experienced waste management contractor.
Under the 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
Extension as a “medium” source.
The potential pathways
through which landfill gas may enter the Extension Site and the developments adjacent
to the Extension are threefold; namely:
·
through
transmission along natural pathways such as fissures or joints in rock;
·
man-made
pathways such as through permeable backfill in utilities trenches; or
·
a combination of both.
The
likely potential for each mode of transmission are clearly dependent on the
geological and hydrogeological conditions, which are
discussed below.
The Extension is located partly on reclaimed land in TKO
Area 137, partly on the southern slope of the SENT Landfill and partly on the
natural slope of CWBCP.
It is understood that TKO Area 137 was formed by
public fill generated from construction projects in
It is known that underground utilities (leachate and landfill gas collection pipes, electricity,
telecommunications and Towngas) exist in the existing
infrastructure area of SENT Landfill.
However, these underground utilities do not connect to TKO Area 137 or
the TKOIE. It is also understood that
there are no man-made underground utilities in TKO Area 137, except a box
culvert connect the existing SENT Landfill and the berthing area in TKO Area
137. A layout plan for the utility
services in the infrastructure area is presented in Figure 8.5e.
It is known that services (electricity,
telecommunications and Towngas) exist
running parallel to
In future, leachate
and landfill gas pipes at the existing SENT Landfill will be connected to the
infrastructure area of the Extension.
At present,
the potential pathways for migration of landfill gas from the SENT Landfill to
the Extension Site in TKO Area 137 are considered to comprise only natural
features and reclamation fill. No
direct anthropogenic migration pathways (man-made underground utilities) have
been identified as connecting the SENT Landfill to the Extension site in TKO
Area 137. As the design of the
Extension will piggyback onto the southern slope of the SENT Landfill, gaps in
the SENT Landfill capping system, if any, and the underground utilities at the
infrastructure area may form migration pathways for landfill gas to migrate to
the portion of Extension directly over the infrastructure area and the southern
slope of the SENT Landfill. Taking
into account the distance between the SENT Landfill and the Extension and the
presence of possible migration pathways between the two landfills, the pathway
for landfill gas migration from the SENT Landfill to the Extension should be
classified as “very short/direct”.
The
potential pathways for sub-surface migration of landfill gas from the Extension
to the future infrastructure area are considered to comprise both reclamation
fill and the future utilities connecting the infrastructure area. At present, no direct anthropogenic
migration pathways (man-made underground utilities) have been identified as
connecting the Extension to the adjacent existing and future development in TKO
Area 137. Taking into account the distance
and the presence of possible migration pathways between the Extension and the
future infrastructure area, the pathway for landfill gas migration from the
Extension to the future infrastructure area should be classified as “very
short/direct”.
The potential pathways for sub-surface
migration of landfill gas from the Extension to the adjacent existing and
future developments in TKO Area 137 are considered to comprise only natural
features and reclamation fill. At
present, no direct anthropogenic migration pathways (man-made underground
utilities) have been identified as connecting the Extension to the adjacent
existing and future development in TKO Area 137. Taking into account the distance between
the Extension and the adjacent existing and future development, and the
presence of possible migration pathways, the pathway for landfill gas migration
from the Extension to the adjacent existing and future developments in Area 137
should be classified as according to the distance between the development and
the Extension: <50m as very short/direct, 50-100m as moderately
short/indirect, 100-250m as long/indirect.
Landfill gas related impacts may occur in
areas at or below ground, at the Extension and the adjacent existing and future
development in TKO Area 137.
As shown in Figure 8.2a, the
majority of the Extension Site falls within the 250m Landfill Consultation Zone
of the SENT Landfill. Demolition
and minor excavation at the existing infrastructure area are expected. The excavation area and the areas of
confined space and trenches, if any, are at a higher risk of exposure to
landfill gas. However, in general,
any excavation work or work involving the construction of trenches will use the
open cut method, although there may be deep excavations. Any migration of landfill gas will
easily be dispersed and diluted upon contact with the atmosphere. Tunnel construction will involve working
in confined spaces by trained workers.
This target is thus classified as “medium sensitivity”.
As discussed earlier, the SENT Landfill
and the Extension are separated by the capping system of the SENT Landfill and
the liner system of the Extension.
It is not expected that there will be landfill gas migration from the
SENT Landfill to the tipping face.
In addition, all landfill equipment is designed to work under conditions
where flammable gas may present.
The majority the waste tipping face will
be carried out within the 250m Landfill Consultation Zone of the SENT
Landfill. Waste tipping will be in
the open air which will not involve working at confined spaces and by
definition, the Extension is also a source of landfill gas, thus any migration
of landfill gas to the Extension will either be dispersed and diluted upon
contact with the atmosphere or be captured in the landfill gas collection
system of the Extension. However,
it is also noted that drivers/operators of waste collection vehicles will have
access to the waste tipping face for disposal of waste and they may not have
knowledge on landfill gas hazards.
In view of the above, this target is thus classified as “medium
sensitivity”.
The proposed infrastructure area of the
Extension will be outside the 250m Landfill Consultation Zone of the SENT
Landfill but within the 250m Landfill Consultation Zone of the Extension. Underground confined spaces (such as
manhole and utility pits) and ground level offices and pump rooms with
underground utility connections are places where landfill gas can potentially
accumulate and may have sources of ignition (e.g. electrical or other
equipment). Restricted access to
the manhole, utility pits and pump rooms by authorised and well trained
personnel is expected. These
rooms/voids are thus considered to have “medium” sensitivity. Ground level offices and some
ground level rooms are generally of unrestricted staff access or public access
accompanied by site staff. These
rooms/voids are thus considered to have “high” sensitivity. Nevertheless, the landfill will be
operated by an experienced landfill contractor who is fully aware of the
potential landfill gas hazards and with their staff well trained on the
potential hazards relating to landfill gas and the specific safety
procedures. All ground floor rooms
of the buildings on-site will be equipped with a gas
detection and alarm system. This
target is classified as “medium to high sensitivity”.
Parts of the TKOIE and TKO Area 137 are
within the existing Landfill Consultation Zone of the SENT Landfill and the new
Landfill Consultation Zone of the Extension. The additional Landfill Consultation
Zone, not covered by the existing one, is shown in Figure 8.2a. The area within this additional
consultation zone is located at TKO Area 137, which is planned for industrial
uses.
The potential hazards posed by landfill
gas migration and the need for protection measures for developments close to
landfill sites are outlined in Chapter 9 of the HKPSG. It is a requirement that project
proponents of developments adjacent to landfills undertake a landfill gas
hazard assessment and submit the findings to EPD for vetting. As recommended in ProPECC
PN 3/96, the project proponents and professionals (Authorised Persons)
responsible for the developments adjacent to landfills should:
(i)
carry
out a landfill gas hazard assessment to evaluate the degree of risk associated
with the proposed development;
(ii)
design
suitable precautionary/protection measures to render the proposed development
as safe as reasonably practicable;
(iii)
ensure
that the precautionary/protection measures will be implemented and constructed
in accordance with the design; and
(iv)
establish a maintenance and monitoring programme for ensuring
the continued performance of the implemented protection measures.
Design information for the uses located
within the 250m Landfill Consultation Zone of the Extension is not available at
this stage. The Extension will
impose constraints on the landuse and suitable
landfill gas protection measures will/may be required depending on the landuse.
With consideration of these above
requirements, this target is classified as “low sensitivity”.
On the basis of the source, pathways and
targets identified above, a source-pathway-target analysis has been undertaken
and is presented in Table 8.5b
according to EPD’s assessment framework. Different combination of source, pathway
and target result in a range of overall potential hazards.
Table 8.5b Qualitative
Assessment of Landfill Gas Hazard Associated with the Extension
|
Source |
Pathway |
Target |
Qualitative
Risk |
|
SENT landfill - potential for gas generation
over time, but comprehensive and proven mitigation installed (category: medium) |
Surface soil, reclamation fill materials,
part of the work site piggyback onto the waste slope of SENT Landfill and the
existing infrastructure area with potential direct anthropogenic conducts,
distance to waste boundary
<50m (category: very short/direct) |
Target 1 (Construction site of the
Extension) – Open cut construction method, may have deep excavation, working
in confined space by trained workers (category: medium sensitivity target) |
Medium |
|
SENT landfill - potential for gas
generation over time, but comprehensive and proven mitigation installed The Extension - potential for gas generation over time, comprehensive
and proven mitigation to be installed
(category: medium) |
Surface soil, reclamation fill materials,
part of the tipping face piggyback onto the waste slope of SENT Landfill and
the existing infrastructure area with potential direct anthropogenic
conducts, distance to waste boundary <50m (category: very short/direct) |
Target 2 (Tipping face of the Extension)
– waste tipping in the open air,
absence of confined space, access by drivers/operators of waste collection
vehicles who may not have knowledge on landfill gas hazards (category: medium sensitivity) |
Medium |
|
The Extension - potential for gas generation over time,
comprehensive and proven mitigation to be installed (category:
medium) |
Surface soil, reclamation fill materials, potential direct
anthropogenic conducts, distance to waste boundary <50m (category: very
short/direct) |
Target 3 (Infrastructure area of the Extension) – Ground
level offices and pump rooms of unrestricted staff access, underground
confined spaces with restricted access, some with source of ignition (category:
medium to high sensitivity) |
Medium to High |
|
Surface soil, reclamation fill materials, no direct
anthropogenic conducts -
Distance to
waste boundary <50m (category: very
short/direct) -
Distance to
waste boundary between 50m to 100m (category:
moderate short/direct) -
Distance to
waste boundary >100m (category:
long/indirect) |
Target 4 (Adjacent existing and new development)
–according to ProPECC PN 3/96, project
proponents and are required to carry out landfill gas hazard assessment and
implement suitable landfill gas protection measures (category: low
sensitivity) |
Very Low to Low |
The source-pathway-target analysis shows that
landfill gas risk posed by the SENT Landfill and the Extension is medium to
high within the Extension Site boundary during both the construction and
operation phases. Whereas the risk
posed by the Extension to the adjacent developments ranges from very low to low
depending on the nature and location of the adjacent developments.
This section of the report provides
general advice and recommendations for the avoidance of environmental impacts
related to landfill gas during the construction and operation of the
Extension. It is recommended that
the project proponent of any development within the 250m Landfill Consultation
Zone of the Extension should strictly follow the requirements for landfill gas
hazard assessment stipulated in the ProPECC 3/96 and the
Guidance Note and undertake a
detailed QLFGHA to assess the hazard potential and to identify the
precautionary measures.
Implementation of the landfill gas control
measures, and restoration works undertaken will also significantly reduce the
potential for off-site migration of landfill gas to the adjacent developments.
All contractors participating in the works
and operational staff 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
gives some warning of its presence. However, the absence of odour should not
be taken to mean that there is no methane.
Methane levels can only be reliably confirmed by using appropriately
calibrated portable methane detectors.
·
Methane is a flammable gas and will burn when mixed with air
between approximately 5 and 15% (v/v).
If a mixture of methane and air with a composition between these two values
is ignited in a confined space, the resulting combustion may give rise to an
explosion. Methane is also an asphyxiant.
·
Carbon dioxide, the other major component of landfill gas is an
asphyxiating gas and causes adverse health effects at relatively low
concentrations. The long-term
Occupational Exposure Limit (OEL) is 0.5% (v/v). Like methane, it is odourless and
colourless and its presence (or absence) can only be confirmed by using
appropriately calibrated portable detectors.
·
Gas density. Methane
is lighter than air whereas carbon dioxide is heavier than air. Typical mixtures of landfill gas are
likely to have a density close to or equal to that of air. However, site conditions may result in a
ratio of methane to carbon dioxide which may make the gas mixture lighter or
heavier than air. As a result,
landfill gas may accumulate in either the base or top of any voids or confined
spaces.
The construction works to be undertaken at
the Extension Site and the adjacent developments present construction workers
and others with risks resulting from contact with landfill gas. For example, when laying
of underground pipes/utilities in trenches or other situations, personnel may
have to enter confined spaces.
Precautionary measures to be adopted by the contractors at the Extension
Site and the adjacent development site within the landfill consultation zone
are outlined in Paragraphs 8.3 to 8.49 of EPD’s Guidance Note. The following guidance has been
extracted from and appended to this and to ensure a robust and comprehensive
set of measures to protect workers are provided.
·
During
all works, safety procedures will be implemented to minimise the risks of fires
and explosions and asphyxiation of workers (especially in confined space).
·
Safety
officers, specifically trained with regard to landfill gas related hazards and
the appropriate actions to take in adverse circumstances,
will be present on all worksites throughout the works.
·
All
personnel who work on site and all visitors to the site will be made aware of
the possibility of ignition of gas in the vicinity of the works.
·
Those
staff who work in, or have responsibility for 'at risk' areas, including bore
piling and excavation works, will receive appropriate training on working in
areas susceptible to landfill gas.
·
Any
offices/quarters set up on site will take precautions against landfill gas
ingress, such as being raised off the ground. Other storage premises, e.g. shipping
containers, where this is not possible will be well ventilated prior to entry.
·
Adequate
precautions to prevent the accumulation of landfill gas under site buildings and
within storage shed will be taken by raising buildings off the ground where
appropriate and 'airing' storage containers prior to entry by personnel and
ensuring adequate ventilation at all times.
·
Smoking
and naked flames will be prohibited within confined spaces. 'No Smoking' and 'No Naked Flame'
notices in Chinese and English will be posted prominently around the
construction site. Safety notices
should be posted warning of the potential hazards.
·
Welding,
flame-cutting or other hot works may only be carried out in confined spaces
when controlled by a 'permit to work' procedure, properly authorised by the
Safety Officer. The permit to work
procedure will set down clearly the requirements for continuous monitoring of
methane, carbon dioxide and oxygen throughout the period during which the hot
works are in progress. The
procedure will also require the presence of an appropriately qualified person
who shall be responsible for reviewing the gas measurements as they are made,
and who shall have executive responsibility for suspending the work in the
event of unacceptable or hazardous conditions. Only those workers who are appropriately
trained and fully aware of the potentially hazardous conditions which may arise
will be permitted to carry out hot works in confined areas.
·
During
the construction works, adequate fire extinguishers and breathing apparatus
sets will be made available on site and appropriate training given in their
use.
Monitoring will be undertaken when construction
works are carried out in confined space within the consultation zone. The monitoring requirements and
procedures specified in Paragraphs 8.23 to 8.28 of EPD’s
Guidance Note are highlighted
below:
·
The monitoring
equipment used will be capable of measuring methane, carbon dioxide and oxygen
concentrations. The equipment will
be intrinsically safe and calibrated according to the manufacturers
instructions.
·
When
portable monitoring equipment is to be used, the frequency and areas to be
monitored will be set down prior to commencement of the works either by the
Safety Officer or by an appropriately qualified person.
·
All
measurements will be made with the monitoring tube located not more than 10 mm from
the surface.
·
A
standard form, detailing the location, time of monitoring and equipment used
together with the gas concentrations measured, will be used when undertaking
manual monitoring to ensure that all relevant data are recorded.
·
Monitoring
of excavations will be undertaken as follows:
For excavations deeper than 1m, measurements will be
made:
(i)
At
the ground surface before excavation commences;
(ii)
Immediately
before any worker enters the excavation;
(iii)
At the
beginning of each working day for the entire period the excavation remains
open; and
(iv) Periodically through the working day
whilst workers are in the excavation.
For
excavations between 300mm and 1m deep, measurements will be made:
(i)
Directly
after the excavation has been completed; and
(ii)
Periodically
whilst the excavation remains open.
For excavations less than 300mm deep, monitoring may be omitted, at the
discretion of the Safety Officer or other appropriately qualified person.
·
If methane
(flammable gas) or carbon dioxide concentrations are in excess of the trigger
levels or that of oxygen is below the level specified in the Emergency Management in the following
section, then evacuation will be initiated.
Depending on the results of the
measurements, actions required will vary and will be set down by the Safety
Officer or another appropriately qualified person. As a minimum these should encompass
those actions specified in Table 8.6a.
Table 8.6a Actions
in the Event of Gas Being Detected
|
Parameter |
Measurement |
Action |
|
O2
|
<
19% v/v |
Increase
underground ventilation to restore O2 to >19% v/v |
|
|
<
18% v/v |
Stop
works Evacuate
all personnel Increase
ventilation further to restore O2 to >19% v/v |
|
CH4
|
>
10% LEL |
Prohibit
hot works Increase
ventilation to restore CH4 to <10% LEL |
|
|
>20%
LEL |
Stop
works Evacuate
all personnel Increase
ventilation further to restore CH4 to <10% LEL |
|
CO2
|
>0.5%
v/v |
Increase
ventilation to restore CO2
to <0.5% v/v |
|
|
>
1.5% v/v |
Stop
works Evacuate
all personnel Increase
ventilation further
to restore CO2 to <0.5% |
In order to ensure that evacuation procedures
are implemented in the event of the trigger levels specified in Table 8.6a above being exceeded, it is
recommended that a person, such as the Safety Officer, is nominated, with
deputies, to be responsible for dealing with any emergency which may occur due
to landfill gas.
In an emergency situation the nominated
person, or his deputies, shall have the necessary authority and shall ensure
that the confined space is evacuated and the necessary works implemented for
reducing the concentrations of gas.
The following organisations shall also be contacted as appropriate:
·
·
Fire
Services Department; and
·
Environmental
Protection Department.
The design of the landfill gas management
system and the landfill gas precautionary measures to be adopted on-site will
be performed by a landfill gas specialist consultant appointed by the Extension
contractor, who has comprehensive knowledge on landfill characteristics,
potential landfill gas hazards and appropriate precautionary measures to
minimise hazards. Moreover, the
landfill gas management system and landfill gas precautionary measures will be
checked and certified by a qualified independent consultant The
potential implications associated with the various qualitative risk categories
are summarised in Table 8.4b. During the detailed design stage, a
review of this preliminary qualitative risk assessment will be carried out, a
detailed qualitative landfill gas risk assessment will be prepared and the
report together with the detailed design of gas protection measures will be
submitted to EPD for vetting.
According to the source-path-target
analysis in Section 8.5, the risk
category at the Extension during construction and operation (waste tipping) is
medium. This implies that
engineering measures will be required during the design stage to ensure that
the construction and waste tipping at the Extension is safe. As the Extension will be designed, built
and operated by an experienced landfill contractor, it is anticipated that
relevant engineering measures will be identified and implemented in accordance
with the Extension Contract Specification requirements. These measures will include the
placement of liner and installation of landfill gas management system to
contain, manage and control landfill gas.
Migration of landfill gas from the SENT Landfill to the Extension, if
any, will be captured by the landfill gas management system at the Extension.
The infrastructure area at the Extension
is considered to have medium to high risk.
According to Table 8.4b,
engineering measures to significant engineering measures will be required to
protect the staff working in the infrastructure area. These measures include a
combination of passive and active systems.
Examples of these measures as recommended in EPD’s
Guidance Notes are listed below for
reference.
·
Passive control measures:
·
Gas-resistant
polymeric membranes which can be incorporated into floor or wall construction
as continuous sealed layer (see Figure 8.6a). Membranes should be able to demonstrate
low gas permeability and resistance to possible chemical attack, and may
incorporate aluminium wafers to improve performance.
·
Other
building materials such as dense well-compacted concrete or steel shuttering
which provide a measure of resistance to gas permeation.
·
Creation
of a clear void under the structure which is ventilated by natural air
movements such that any emissions of gas from the ground are mixed and diluted
by air (see Figure 8.6b).
·
Synthetic
composite geotextiles which provide a free-venting
cellular structure and provide preferential pathways for release of gas
·
Active control measures:
·
A void
under the structure, as discussed for passive control, but which is
continuously ventilated by a fan, such that any emissions of gas from the
ground are mixed and diluted in the air flow before discharge to atmosphere
(see Figure 8.6b). The rate of ventilation is usually
expressed in terms of the number of air changes (volume of the void) per hour
and is designed to ensure that, based on the estimated rate at which gas will
enter the void, the landfill gas will be diluted to
safe concentrations. Discharge to
atmosphere usually takes place above the eaves level of the building or, in the
case of high rise structures, at a minimum height of 10 m above ground and away
from air intakes to the building.
·
Construction
of a granular layer incorporating perforated collector pipes which is
continually ventilated by a fan, such that any emissions of gas from the ground
are drawn towards the end of the pipes and diluted in the air flow before
discharge to atmosphere above the eaves level of the building, or in the case
of high rise structures, at a minimum height of 10 m above ground and away from
air intakes to the building.
·
Creation
of a positive pressure zone below the building structure by injection of air from
a blower into the granular layer.
·
Creation
of a positive air pressure zones within building structures to counteract
possible leakage of gas into the building from the ground.
Active control should always be used in
conjunction with passive barriers such as membranes in floors, in order that
there is no leakage of air/gas flow through a floor or wall into a
structure. Gas detection systems
should also be used to monitor gas in extracted air flow, and to monitor
internal spaces inside buildings.
Active systems are normally required for high risk sites where landfill
gas has been measured in the ground at or close to the development site, and
where buildings are close to the source of landfill gas.
Landfill gas may also enter the
building/void via service entries.
Measures to prevent gas migration through service entries are listed
below:
·
Gas Barriers
·
Barriers
used to prevent movement of landfill gas through service entries may be made of
clay (or clay-rich soils), bentonite or polymeric membranes
(such as HDPE). The design detail
at the point where the service penetrates the membrane is important and, in the
case of HDPE membranes, pre-formed shrouds are available. The design detail at the point where the
service penetrates the membrane is important and use should be made of
pre-formed shrounds (orr cloaks), skirts and fillets. A schematic for an HDPE flexible
membrane cut-off is shown in Figure 8.6c.
·
It
may be appropriate to consider routing all services through a sealed culvert or
duct which is either completely lined in naturally gas-resistant material (e.g.
clay) or which is lined with an HDPE membrane.
·
In
the case of water pipes and sewers which are not always fully filled, water traps,
such as U-bends, should be provided to effectively seal off the conduit and
prevent gas-phase transport.
·
In
order to prevent the ingress of landfill gas into a building via the interface
between the service pipe and the backfilled soil, it is important that the
annulus around any service entry points is effectively blocked by means of
sealant, collars or puddle flanges as appropriate (see Figure 8.6d)
·
Gas Vents
·
Vent pipes
or griddled manhole covers may be used to avoid build-up of landfill gas in
underground utilities manholes.
Venting stacks may be built into inspection chambers or connected to
collection pipes within high permeability drainage layers adjacent to landfill
gas barriers. A typical vented
manhole arrangement is shown in Figure 8.6e.
·
A
further type of venting arrangement, which may be appropriate to multiple
service entries, comprises a vented gas interceptor cavity through which
service pipes pass, as shown in Figure 8.6f. The aim of this protection measure is to
locate the barrier component within the building sub-structure in a sealed
entry box which is fitted with a vent stack.
In addition to the above precautionary
measures, landfill gas monitoring boreholes will be installed at the edge of
the waste slope between the waste and the new infrastructure area to monitor
the migration of landfill gas, if any.
For future developments in TKOIE and TKO
Area 137 which fall into the Landfill Consultation Zone of the Extension as
shown in Figure 8.2a, the project
proponents should strictly follow the recommendations in the HKPSG and the ProPECC PN 3/96 to carry out landfill gas
hazard assessment for the developments and design/implement suitable
precautionary and protection measures to render the development as safe as
practicable. These adjacent
developments are considered to have very low to low risk.
The Extension Contractor will have a
responsibility to train and to ensure that their staff take
appropriate precautions at all times when entering enclosed spaces or plant
rooms. The Extension Contractor
will also undertake regular monitoring of landfill gas at the perimeter
boreholes to detect if there are any signs of off-site landfill gas
migration. The Extension Contractor
will be responsible to prepare and implement emergency plan in case off-site
landfill gas migration is detected.
A permanent gas monitoring system with
alarms will be installed and operated in all occupied on-site buildings.
All utility companies should be made aware
of the location and features of the development site within the Extension
Landfill Consultation Zone by the future developers of the site during the
respective detailed design stage as part of the QLFGHA. The utilities companies should have a
responsibility to train and ensure their staff to take appropriate precautions
at all times when entering enclosed spaces or plant rooms.
The Extension Contractor will be required
to undertake regular monitoring of landfill gas within the Extension and the
Extension boundary as required by the Extension Contract Specification.
Similar to the Contract Specification of
the existing SENT Landfill, monitoring of landfill gas at the perimeter
landfill gas monitoring wells will be required at weekly (where there is
development within 250m of the Extension Site Boundary) or monthly intervals
(for the other monitoring wells) during the operation and restoration of the
Extension. During the aftercare
phase, monitoring at all perimeter wells will be at monthly intervals. Bulk gas analysis for at least 2 of the
perimeter wells will be required at quarterly intervals throughout the
operation, restoration and aftercare of the Extension. In addition, monitoring of service voids
along the Site boundary and within the Extension Site will be required at
monthly intervals throughout the operation, restoration and aftercare of the
Extension. The Extension Contractor
will also be required to undertake surface emission monitoring of methane gas
(or flammable gas) in areas between the waste boundary and the Extension Site
boundary at quarterly intervals throughout the operation, restoration and
aftercare phases. Actions will be
taken if an abnormal level of landfill gas is detected.
This section has provided a qualitative
assessment on potential hazards associated with landfill gas migration from the
SENT Landfill to the Extension and from the Extension to the adjacent existing
and future developments. Both
landfills are considered as a “medium” source of gas migration due to the comprehensive
and proven landfill gas control measures installed or to be installed. The source-pathway-target analysis shows
that landfill gas risk posed by the SENT Landfill and the Extension is medium
to high during both construction and operation phases within the Extension
Site. Whereas the risk posed by the
Extension to the adjacent developments ranges from very low to low depending on
the nature and location of the these developments.
In general, underground rooms or void
spaces should be avoided as far as practicable at the infrastructure area of
the Extension site. Other
precautionary and protection measures during construction, design and
operation/restoration phases of the Extension have been recommended. It is expected that with the proposed precautionary
measures in place, the potential risk of landfill gas migration to the
respective targets will be minimal.
