11
Biogas
Risk Assessment
11.1 Introduction
11.1.1 It is proposed
that mud would be left in place at the west and east corners of the WDII
reclamation within the Causeway Bay Typhoon Shelter. The other areas of the Typhoon Shelter would
be fully dredged. The marine sediment
may contain organic matter. When marine
sediments rich in organic matter are covered over by reclamation fill,
anaerobic degradation of the organic matter in the sediments would generate
biogas (methane) which could pose a potential risk to the overlying future
landuse. The issue of methane risk is
not a requirement in the TM on EIA Process but warrants investigation, as
required in the Study Brief. The
potential gas emission from the proposed reclamation within the Causeway Bay
Typhoon Shelter is assessed in this section.
11.2
Assessment
Methodology and Criteria
Field Sampling and Laboratory Analysis
11.2.1 Vibrocores MV17
and MV18 are located within the western and eastern corners of the Typhoon
Shelter, respectively, in the areas where marine mud is proposed to be left in
place (Figure 6.1). Total Organic Carbon (TOC) and Sediment Oxygen Demand (SOD)
levels in the sediment samples collected from vibrocores MV17 and MV18 are used
for the assessment of methane hazard. The
results are presented in Table 11.1. It
is noted that the extent of dredging may change subject to more detailed
engineering evaluation. Further analysis
of sediment samples at other locations would be required should sediment be
proposed to be left in-situ in areas additional to the western and eastern
corners of the Causeway Bay Typhoon Shelter.
Table
11.1 Sample Locations and Levels of Total
Organic Carbon and Sediment Oxygen Demand
|
Location |
Sample
Depth (m) |
Moisture (%
w/w) |
TOC (%
dry wt) |
SOD* (mg/kg) |
|
|
MV17 |
0 - 0.9 1.0 - 1.9 |
61.2 32.6 |
4.82 1.04 |
13351 2804 |
|
|
Mean |
47 |
2.93 |
8078 |
||
|
MV18 |
0 – 0.9 |
46.6 |
1.57 |
9775 |
|
|
1 – 1.9 |
36.1 |
1.86 |
6901 |
||
|
Mean |
41 |
1.72 |
8338 |
||
Note: * A
testing duration of 5 days and 20 days was used for the determination of
SOD. The test results for the 20-day SOD
test are adopted as a higher value was obtained. Sample weight = 1.5 g.
Background
11.2.2 Typically,
landfill gas hazard assessment has been undertaken using guidance or standards
based on the concentrations of gases (methane and carbon dioxide), rather than
mass flow rates. Such guidance usually
recommends restrictions on development in areas where the gas concentration
exceeds a stated proportion of the lower explosive limit (LEL) of methane,
which is 5% (v/v). Typical margins of
safety are in the range of 1 - 20% of LEL (0.05 - 1% (v/v)).
11.2.3 Most of the
guidance on this subject has been developed for application to sanitary
landfill sites and much less has been written on the subject of standards or
guidance for levels of methane arising from other sources, such as natural peat
formations, marshland, rice paddies, coal measures and other organic deposits
of anthropogenic origin, such as marine sediments. In fact, methane arises naturally in many
areas that have apparently been safely developed or redeveloped without any
regard for gas protection measures.
Development of Guide Levels
11.2.4 There is no
primary legislation in Hong Kong covering hazards to development caused by
landfill gas, or methane gas generated from anthropogenic organic
deposits. The most relevant guidance is
the guideline, “Landfill Gas Hazard
Guidance Note” issued by the Environmental Protection Department
(EPD). The guidance note states that no
works and no entry to the development site should be allowed and the personnel
on-site should be evacuated if the methane concentration of the development
site exceeds 1.0% (v/v).
11.2.5 Perhaps the best
example of methane problems arising from anthropogenic organic deposits is that
of the London Docklands. During
redevelopment of this area, where disused docks containing contaminated silts
and sediments had been backfilled, methane concentrations of 20 - 30% were
commonly found in monitoring boreholes([1]),
but in the majority of cases emission velocities from a 50 mm diameter
borehole were below 0.01 m s-1.
Carpenter therefore recommended that development should not take place
where emission rates exceed 0.05 m s-1 in a 50 mm diameter
borehole. This is the same as a flux of
0.05 m3 m-2 s-1 or 4,320 m3
m-2 per day through a surface with a cross-sectional area equivalent
to the cross-sectional area of the borehole (that is, p r2,
where r is the radius of the borehole).
Reference is also made to Carpenter’s work in the ICRCL guidance on the
development and after-use of landfill sites([2]).
11.2.6 The UK Department
of the Environment Waste Management Paper on Landfill Completion([3])
recommends as a completion criterion that methane emission rates from
monitoring boreholes should fall consistently below 0.015 m3
per hour. The completion criterion must
be met in order for monitoring to be discontinued and for the operator to
surrender the licence that obliges him to maintain aftercare of the site. It is generally taken as an indication that
the site does not pose continuing threat to the environment. For a borehole with a diameter of 100 mm (the
minimum recommended in Waste Management Paper 26A) having a cross
sectional area of 7,854 mm2, this is equivalent to 45.84 m3
m-2 per day. For larger
boreholes of up to 250 mm diameter, the equivalent rate would be
7.35 m3 m–2 per day.
11.2.7 The above derived
values are not the equivalent of gas fluxes through a freely-venting
surface. For landfills in the UK in
particular, the guidance assumes a capping layer of low permeability. In these cases a borehole installed through the
cap acts to release static gas pressure in the fill. As a result, the flow of
gas from the borehole will represent the flux through a freely-venting surface
of greater cross-sectional area than the borehole itself because gas will be
drawn from the surrounding area to the borehole under the influence of the
pressure gradient. It is difficult to
estimate the radius of influence of such boreholes. In sanitary landfills, radii of influence of
25 m can be achieved under active pumping at pressures of 5 mbar. Assuming a linear relationship and a static
pressure of 0.5 mbar the radius of influence of the borehole would reduce
to 2.5 m. If the emission from the
borehole is assumed to be equivalent to the flux over an area of radius
2.5 m, the resultant flux would range from 18.3 L m-2
per day (based on the recommendation of the Waste Management Paper No. 26A) to
432 L m-2 per day (based on Carpenter’s guidance level).
11.2.8 The UK Department
of the Environment Waste Management Paper No. 26A on Landfill Completion also
recommends a maximum acceptable rate of methane ingress into a building constructed
on a disused landfill site. This
criterion was developed to determine when monitoring of landfill gas emissions
at a restored landfill can be discontinued and when the site can be used for
unrestricted development. It is assumed
that the most sensitive ‘at risk’ room or void has a height of 2.5 m and a very
low rate of ventilation of 1 air change per week. For WDII, it is considered more appropriate
to adopt a height of 1 m to represent the void space (to allow for smaller void
spaces such as utilities or services ducts) and a ventilation rate of 1 air
change per day (this is in line with rates of natural ventilation for closed
rooms). The maximum safe rate of methane
ingress was then defined as that at which it would take 1 day for the methane
concentration to reach 1% (v/v). This is
20% of the lower explosive limit (LEC) for methane and provides a safety factor
of 5. The corresponding daily maximum
“safe” rate of methane gas emission per unit area is calculated to be 10 L m-2
per day([4]).
11.2.9 The EPD’s
guidance note on landfill gas hazard and the UK guidance values (that is, the
UK completion criterion for landfills; the Carpenter’s guidance level and the
“safe” gas emission rate), will be adopted as the assessment criteria (Table
11.2).
Table 11.2 Methane Hazard Assessment Criteria
|
1 (1) |
Notes:
(1)
Guideline value from Landfill Gas Hazard Guidance Note, EPD, HK.
(2)
UK Landfill Completion Criterion from Department of the Environment (1993),
Landfill Completion. Waste Management Paper No. 26A, London: HMSO.
(3) Carpenter’s guidance levels.
(4) Maximum “safe” rate of gas emission
derived for WDII, as based on Department
of the Environment (1993), Landfill Completion.
Waste Management Paper No. 26A.
London: HMSO.
11.3
Calculation
of Potential Gas Emissions
11.3.1 From Table 11.1,
the calculated mean TOC level at vibrocore location MV17 is 2.93% on a dry
weight basis and the calculated mean SOD level is 8,078 mg kg-1. Based on an average moisture content of 47%,
dry matter made up 53% of the sediment on average. At vibrocore location MV18, the calculated
mean TOC level is 1.72% on a dry weight basis and the calculated mean SOD level
is 8,338 mg kg-1.
Based on an average moisture content of 41%, dry matter made up 59% of
the sediment on average.
11.3.2 The potential
methane gas emission was estimated based on the assumption that marine mud
would be left in-situ at the WDII
reclamation within the western and eastern corners of the Causeway Bay Typhoon
Shelter. The quantity of mud was
estimated to be 42,422 m3 and
6,732 m3 at the western and eastern corners, respectively :
Volume of mud left in-situ at western corner = 21,211
m2 x 2 m = 42,422 m3
Volume of mud left in-situ at eastern corner = 3,366
m2 x 2 m = 6,732 m3
11.3.3 The average depth
of sediment at the reclamation area of concern is approximately 4.5 m. The hydraulic and water quality studies undertaken
in 1989([5])
indicated that the siltation rate in the Victoria Harbour was around 50 mm per
year. This suggests that the age of
sediment in the reclamation area at depths of 4.5 m below seabed would be
around 90 years. Other sediment and
ecological studies([6],[7])
have demonstrated that the Redox Potential Discontinuity (RDP) depth (or the oxygenated
surface layer) of marine sediment in the Victoria Harbour is generally limited
to the top 10 cm. Anaerobic degradation
of the organic matter may occur in sediments that are below 10 cm from the
seabed. Based on the range of half-lives
of 0.5 to 5 years, most of the readily degradable organic matter would have
been anaerobically degraded and the current methane generation potential of
this sediment will be insignificant. If
these sediments were not degraded anaerobically for the past 90 years, there is
no reason to believe that they will do so after the reclamation. The methane generation potential of the
sediment to be left in-situ will
therefore be estimated based on the top 2 m depth of sediment.
11.3.4 The capping of
the reclamation would likely create underlying anaerobic conditions that favour
degradation of organic matter by microbial activity in the contaminated
sediment. The end product of this
degradation is biogas, which mainly consists of methane (CH4) and
carbon dioxide (CO2).
11.3.5 The rate of biogas
generation is dependent on the amount of organic matter, degradability of
organic matter, extent of anaerobic conditions, temperature, and transport
medium for bacteria (water). Although
the available information is limited, a theoretical calculation can be made for
an estimate of biogas generation within the reclamation area.
11.3.6 From experience
in several anaerobic degradation projects (with waste as well as sludge), it is
known that the biogas formation can be described as a first order degradation process.
This process is characterised by high gas generation rates at the start,
followed by an exponential decrease over the course of time. Biogas generation can be calculated based on
the available data on organic matter content or sediment oxygen demand (SOD).
11.3.7 Not all organic
carbon present in the sediment would be biodegradable. The SOD represents the biodegradable fraction
of the organic carbon present in the sediment and, thus, is convertible to
methane. Under anaerobic conditions, all
of the oxygen demand of degradable organic material is preserved in the methane
formed. The following equation shows
that 4 g of oxygen demand would have a total yield of 1 g of methane
(the molar mass of methane is half the molar mass of oxygen and two moles of
oxygen are required to oxidise one mole of methane).
CH4 + 2O2 = CO2 + 2H2O
No. of moles 1 2
11.3.8 As an example, a
sediment with a SOD of 200 mg m-3 will ultimately generate
50 mg of methane, equivalent to 0.07 litre of methane per m3 of
sediment at standard temperature and pressure (STP).
11.3.9 It is assumed
that 50% of the gas produced from anaerobic degradation of organic matter of
the sediment is methane (the remainder being carbon dioxide). This is true for substrates such as
carbohydrates that are neither highly oxidised nor highly reduced:
.files/image001.gif){kind=small}
2CH2O CH4 +
CO2
11.3.10 On that basis,
the mass of methane generated from unit mass of TOC is calculated as follows:
.files/image002.gif){kind=small}
2C CH4 +
CO2
2 x
12 = 24 16
that
is, methane potential = 16/24 = 0.67 times TOC
11.3.11 Assuming a SOD in
the material to be contained in the future reclaimed land of
8,078 mg kg-1, the total methane potential would be
2,019 mg kg-1, or assuming a dry matter content of 53%,
3,803 mg kg-1. However,
SOD represents only a fraction of the organic carbon present in the
sediment. Based on a sediment TOC of
2.9% of dry matter and assuming that half is converted to methane (the
remainder being carbon dioxide), methane potential would be about
19,625 mg kg-1 dry matter.
This implies that only 20% of TOC represents readily biodegradable
organic matter. Use of TOC to estimate
methane potential therefore provides an over-estimate of that potential. Furthermore, some organic substrates which are
degradable aerobically (and which therefore contribute to SOD) are not degradable
at all in anaerobic conditions.
Therefore, basing potential methane yield on SOD itself probably
provides an over-estimate of methane potential.
11.3.12 It is difficult
to estimate the half-life of substrates in systems such as contaminated marine
sediment. However, at low substrate
concentrations in engineered systems such as facultative ponds, half-lives of
substrates in the anaerobic could be of the order of half a year. In landfills, the average half-life of
organic substrates could be 5 years. Hence,
for conservatism, the methane potential is calculated based on TOC rather than
SOD because the former represents the extreme worst case, assuming all organic
matter is biodegradable and convertible to methane. In fact, as indicated above, probably only
20% of the organic carbon is readily degradable. Thus, an analysis based on TOC alone will
overestimate the impact by a factor of about five.
11.3.13 Based on the
range of half-lives of 0.5 - 5 years, the peak annual methane potential would
be between 13 and 75% of the total, that is, 2,551 – 14,719 mg kg-1. The peak annual methane potential
corresponding to a half-life of decay of 0.5 years, is actually not significant
in terms of development because after two years over 90% will have degraded and
the flux will have fallen proportionately to a rate less than that of the lower
figure after the same time. Therefore,
the peak annual methane potential based on a half-life of 5 years is adopted
for the calculations of potential methane gas emission for the comparison with
the UK methane hazard assessment criteria.
A half-life of 2 years is also adopted for the calculation of potential
methane gas emission to represent a worst-case scenario (as a half-life of 2
years will result in a higher flux rate at 2 years after reclamation than that
resulting from a half-life of 5 years).
Table 11.3 shows calculations of the daily methane flux from the WDII
reclamation.
11.3.14 The methane
concentrations of the boundary layer at the surface of the WDII reclamation is
also estimated as shown in Table 11.3 for the comparison with the guideline
value (1% v/v) as stipulated in EPD’s Landfill Gas Hazard Guidance Note. The boundary layer is assumed to be 1 m to
represent the worst case scenario.
Table
11.3 Calculation of Methane Flux
from the WDII Reclamation
|
|
Marine
Sediment – Western Corner |
Marine
Sediment – Eastern Corner |
Methane
Hazard Assessment Criteria |
||
|
Half-life
cycle of 5years |
Half-life
cycle of 2years |
Half-life
cycle of 5years |
Half-life
cycle of 2years |
||
|
Volume (m3) |
42,422 |
42,422 |
6,732 |
6,732 |
- |
|
Density (kg m-3) |
1,370 |
1,370 |
1,370 |
1,370 |
- |
|
Dry matter (% w/w) |
53 |
53 |
58.6 |
58.6 |
- |
|
Dry matter (kg m-3) |
726.10 |
726.10 |
802.82 |
802.82 |
- |
|
TOC (%) |
2.93 |
2.93 |
1.72 |
1.72 |
- |
|
TOC (kg m-3) |
21.27 |
21.27 |
13.81 |
13.81 |
- |
|
CH4 potential (kg m-3) |
14.25 |
14.25 |
9.25 |
9.25 |
- |
|
Peak annual CH4 potential (kg) |
78,609 |
175,359 |
8,097 |
18,062 |
- |
|
Total area (m2) |
21,211 |
21,211 |
3,366 |
3,366 |
- |
|
Total potential CH4 flux (kg m-2 per year) |
3.71 |
8.27 |
2.41 |
5.37 |
- |
|
Total potential CH4 flux (g m-2 per year) |
3,706.06 |
8,267.36 |
2,405.44 |
5,365.98 |
- |
|
Total potential CH4 flux (mol m-2 per year) |
231.63 |
516.71 |
150.34 |
335.37 |
- |
|
Total potential CH4 flux (L m-2 per year) |
5,188.48 |
11,574.30 |
3,367.62 |
7,512.38 |
- |
|
Total potential CH4 flux (L m-2 per day)
(assuming 20% of TOC biodegradable) |
2.84 |
6.34 |
1.85 |
4.12 |
18(a) – 432(b) 10(d) |
|
Total potential CH4 flux (L m-2 per day)
(assuming 100% of TOC biodegradable) |
14.22 |
31.71 |
9.23 |
20.58 |
|
|
Potential CH4 concentration (% v/v) at the surface boundary
layer (assuming 20% of TOC biodegradable) |
0.28 |
0.63 |
0.19 |
0.41 |
1(c) |
|
Potential CH4 concentration (% v/v) at the surface boundary
layer (assuming 100% of TOC biodegradable) |
1.42 |
3.17 |
0.92 |
2.06 |
|
Notes
(a) UK Landfill Completion Criterion from Department of the Environment (1993),
Landfill Completion. Waste Management
Paper No. 26A. London: HMSO.
(b) Carpenter’s guidance level from Carpenter, R. J. (1988), Building
development on disused landfill sites - overcoming the landfill gas
problem. In: Proc. 5th International
Solid Wastes Conference, Copenhagen, Denmark, Vol., 153-160. London: Academic
Press.
(c) Guideline value from Landfill Gas Hazard Guidance Note, EPD, HK.
(d) Maximum “safe” rate of gas emission derived
for WDII, as based on Department of the
Environment (1993), Landfill Completion.
Waste Management Paper No. 26A.
London: HMSO.
11.3.15
The above analysis is based on a number of broad
assumptions that might affect the precision of the estimates. Furthermore, it takes no account of
biological methane oxidation that will probably occur in the upper layers of
the sediment. In the case of a uniform
emission through a permeable, aerobic reclamation layer, methane (or part of
it) can be oxidised microbiologically.
In the literature, oxidation efficiencies can be found of 2% up to 100%([8]). For landfills, covered by a very permeable
top layer, oxidation efficiencies were found in the range of 0 - 50%([9]). High efficiencies will only occur when the
fill material is well aerated (for example, by diffusion of air) and the gas is
able to emit uniformly over the surface area.
Low efficiencies, however, will occur when the fill material is poorly
permeable for gases and when the gas generation rate is rather high, so that
concentrated emissions can take place via fissures, or other preferential
pathways (for example, gravel layers).
11.4
Evaluation
of Significance of Potential Gas Emissions
Significance of potential methane emissions with
reference to the UK Guidance Values
11.4.1 As shown in Table
11.3, the predicted potential methane flux from the marine sediment left in-situ at the western corner of the
WDII reclamation within the typhoon shelter is higher than that predicted from
the eastern corner of the typhoon shelter reclamation. The assessment of biogas risk associated with
the WDII reclamation will therefore be based on the methane flux calculation
for the western corner of the typhoon shelter to represent a worst-case
scenario.
11.4.2 Taking the UK
landfill completion criterion (that is, 18 L m-2 per day)
as the standard, the predicted methane emission from the WDII reclamation based
on a half-life of 5 years (2.84 L m-2 per day), assuming
20% of TOC biodegradable, is only 15.8% of this guide value. This provides a safety factor of
approximately 6. Based on a half-life of
2 years, the predicted methane emission (6.34 L m-2 per
day) is 35% of the UK landfill completion criterion, providing a safety factor
of approximately 3. The predicted
methane emission based on the assumption of 100% TOC biodegradable, though
which is highly unlikely, is also considered as a conservative estimate. The methane emission (14.22 L m-2
per day) for a half-life of 5 years is found to be approximately 79% of the UK
landfill completion criterion. Under the
worst case scenario of a half-life of 2 years, the predicted methane emission
(31.71 L m-2 per day) is found to be approximately 1.8 times greater
than the UK landfill completion criterion.
11.4.3 The calculations
show that the predicted methane emission based on a half-life of 5 years,
assuming 100% of TOC biodegradable, is about 3.3% of the Carpenter’s guidance
level (that is, 432 L m-2 per day), providing a safety factor of
approximately 30. The predicted methane
emission based on a half-life of 2 years, assuming 100% of TOC biodegradable,
is about 7.3% of the Carpenter’s guidance level, providing a safety factor of
approximately 14. Under this worst case
scenario, the predicted methane emission is well below the upper UK guide
value, which is the level at which development would be restricted according to
Carpenter’s guidelines.
11.4.4 Taking the “safe”
rate of gas emissions derived for WDII from Waste Management Paper No. 26A on
Landfill Completion as the standard (that is, 10 L m-2 per
day), the predicted methane emission from the WDII reclamation, assuming 20% of
TOC biodegradable, is about 3.5 times less than this guide value. The predicted methane emission based on the
worst case assumption of 100% TOC biodegradable is also considered as a
conservative estimate. The calculations show
that the predicted methane emission (14.22 L m-2 per day), assuming
100% of TOC biodegradable, is approximately 1.4 times greater than the “safe”
emission rate (that is, 10 L m-2
per day). Taking the worst case scenario
of a half-life of 2 years, the calculations show that the predicted methane
emissions are approximately 1.6 times less and 3.2 times greater than the
“safe” emission rate, assuming 20% and 100% of TOC biodegradable,
respectively.
11.4.5 The derived
maximum “safe” rate of gas emissions is based on a number of assumptions
regarding the size and rate of ventilation of the ‘at risk’ room, and the
permeability of the ground surface at the site.
It is therefore necessary to consider the most sensitive ‘at risk’
features of the proposed development at the WDII reclamation in order to
determine the likelihood of methane emissions posing a significant risk and the
need for mitigation measures.
11.4.6 The proposed land
use at the western corner of the reclamation within the Causeway Bay Typhoon
Shelter is either commercial use (office and retail) or a hotel
development. The estimated site area of
the proposed development is 6,964 m2. Any biogas emissions from the sediment
beneath the reclamation will tend to pose limited risk to the above ground
rooms in this proposed high rise development.
Rooms located at the ground level of the building, however, such as
transformer and refuse collection rooms for example, may be susceptible to
ingress of any biogas generated from the reclamation. These rooms are usually provided with a
relatively high rate of mechanical ventilation.
Other at risk voids may include utility (services) voids and lift
pits. The most sensitive ‘at risk’ room
is considered to be the proposed underground car park which would be
susceptible to ingress and accumulation of any biogas emissions from the
reclamation. It will therefore be
necessary to ensure adequate ventilation of the underground car park to prevent
the accumulation of any methane gas emissions to dangerous concentrations. This precautionary measure and other recommended
gas protection measures for both the ground level and underground rooms at the
proposed development are discussed in Section 11.5. An entertainment complex is under
consideration for the eastern corner of the WDII reclamation within the typhoon
shelter, with uses such as an indoor interactive theme park, game stalls,
restaurants, shops and car parking. The
estimated site area of the proposed complex is 7,736 m2. The most sensitive ‘at risk’ rooms are
considered to be the proposed basement food court and underground car park
which would be susceptible to ingress and accumulation of any biogas emissions
from the reclamation. From Table 11.3,
it can be seen that the predicted methane emission based on the worst case
assumption of 100% TOC biodegradable and a half-life of 2 years is approximately
2 times greater than the “safe” emission rate
(that is, 10 L m-2 per day).
It will therefore be necessary to ensure adequate ventilation of the
basement floors to prevent the accumulation of any methane gas emissions to
dangerous concentrations. Precautionary
measures are discussed in Section 11.5.
11.4.7 The development
will include some cover in the form of non-permeable concrete or asphalt
layers. These layers decrease the
possibilities for bacteriological oxidation of methane in the top layer. This might result in more concentrated
emissions or accumulation of methane in cavities. Therefore, if the precautionary principle is
to be applied, it is recommended to undertake methane gas monitoring in the
immediate post-reclamation period to measure methane concentrations in the
fill.
Significance of potential methane emissions with
reference to EPD’s Landfill Gas Hazard Guidance Note
11.4.8 The predicted
methane concentration at the surface boundary layer within the western corner
of the WDII reclamation has been compared with the assessment criteria to
represent a worst-case scenario.
11.4.9 The methane
concentration at the surface boundary layer from the WDII reclamation is
estimated to be 0.28% (v/v) (assuming 20% of TOC biodegradable and a half-life
of 5 years) which is 3.5 times less than the guide value of 1% (v/v), as
stipulated in EPD’s Landfill Gas Hazard Guidance Note. Based on the worst case scenario of a
half-life of 2 years, the estimated methane concentration of 0.63% (v/v) is
below the EPD’s guide value.
11.4.10 On considering
the highly unlikely event of assuming 100% TOC biodegradable, the methane
concentration at the surface boundary layer from the WDII reclamation is
estimated to be 1.42% and 3.17% based on a half-life of 5 and 2 years,
respectively, which is in exceedance of the EPD’s guide value. It is therefore recommended that the above
precautionary approach be adopted.
Mitigation requirements are discussed in Section 11.5 below.
11.5
Mitigation
Measures and Further Work
11.5.1 The methane
calculations provided above are based on numerous theoretical assumptions and
there is virtually no precedent established on practical grounds against which
they can be tested. It is therefore
recommended to establish gas monitoring boreholes immediately after reclamation
and prior to development to determine actual rates of methane gas emissions
generated from the marine sediment underlying the reclamation. The predicted methane emissions based on the
conservative assumption of 100% biodegradable TOC are well below the upper UK
guidance value (which is the level at which development would be restricted
according to Carpenter’s guidelines).
The recommended monitoring requirements are detailed below.
11.5.2 The potential
biogas risk has been assessed based on the predicted peak methane generation
potential and total daily methane flux (based on the TOC results and assuming
all organic matter is
biodegradable). Based on this
conservative approach, the predicted daily methane flux is higher than the UK
“safe” rate of methane gas emission (as derived from Waste Management Paper No.
26A for methane ingress into an ‘at risk’ room within a building constructed on
a restored landfill site). As discussed
in Section 11.2.8, the UK maximum “safe” rate of landfill gas emissions is
based on a number of assumptions regarding the size and rate of ventilation of
the ‘at risk’ room or void space. This
criterion was developed to determine when monitoring of landfill gas emissions
at a restored landfill can be discontinued and when the site can be used for
unrestricted development.
11.5.3 As sensitive ‘at
risk’ rooms may be present at the proposed developments at the western and
eastern corners of the typhoon shelter, both at ground level and below ground,
it is recommended that a precautionary principle should be applied. Given that mitigation measures to prevent the
ingress and / or accumulation of any methane gas emissions generated from the
reclamation may be very costly, it is recommended that monitoring of gas
emission rates should be undertaken at the area of the proposed developments
within the western and eastern corners at the WDII reclamation. The results of the gas monitoring should be
reviewed to determine the extent of mitigation measure requirements to be
incorporated in the detailed design of the proposed development.
Gas
Monitoring
11.5.4 Monitoring should
be undertaken via purposely installed monitoring wells within boreholes drilled
into the fill material. The boreholes
should be drilled down to the level of the groundwater (mean sea water level)
and standard landfill gas-type monitoring wells installed. During the drilling of boreholes, the safety
and working procedures described in the EPD Landfill Gas Hazard Assessment
Guidance Note (1997) should be followed.
11.5.5 Concentrations of
methane gas should be measured using intrinsically safe, portable gas
monitoring instruments. Fluxes should
also be measured if the emission velocities are not too low. It is recommended that monitoring should be
undertaken monthly for a period of at least one year prior to the commencement
of construction works on the reclamation.
Details of the recommendations for methane gas monitoring are given in
the EM&A Manual.
Precautionary Gas
Protection Measures
General Guidelines
11.5.6 At this stage it
is difficult to formulate specific guidelines on what measures would be
required for the measured rates of gas emission as this would depend on the
detailed design of the individual buildings to be constructed. The following criteria may be used as general
guidelines. The maximum “safe” rate of
methane gas emission of 10 L m-2 per day derived for WDII from the
Waste Management Paper No. 26A on Landfill Completion is proposed to be adopted
as the trigger value.
Scenario 1
11.5.7 If rates of
methane emission are consistently much less than the trigger value (10 L m-2
per day), including monitoring occasions when atmospheric pressure is falling
rapidly, then it is considered that the buildings will not require gas
protection measures.
11.5.8 The trigger value
is an area emission rate (that is, rate at which gas is emitted per unit area
of the reclamation). In order to convert
this into an emission rate from a borehole, it is necessary to make an
assumption about the "area of influence" of a freely venting borehole
which depends on a number of factors. A
key factor is the ease by which gas can escape from the surface of the
site. For a site with cover in the form
of low permeability paving or concrete, it would be expected that a borehole
would have a much greater area of influence than if the site had soft
landscaping.
11.5.9 To be
conservative, it is proposed to adopt an area of influence of 20 m2 (radius
of 2.5m)([10]),
which would give:
·
Trigger value of 10 L m-2
per day x 20 m2 = 200 L per day emitted from the borehole
11.5.10 The criterion for
“safe” flow rate from a free venting borehole becomes:
·
Flow rate of methane (in
terms of litre per day) < 200 L per day or
·
(Gas flow rate in terms of
litre per day) x (concentration of methane in gas (in % gas)) < 200 L per
day
Scenario
2
11.5.11 If the rate of
methane emission frequently exceeds the trigger value or shows a rising trend
such that future emission rates are likely to exceed the trigger value, then
any buildings to be constructed on that part of the site will require some form
of gas protection measures, that is,
·
(Gas flow rate in terms of
litre per day) x (concentration of methane in gas (in % gas)) > 200 L per day.
11.5.12 The type of gas
protection measures would be dependent on the design and use of the particular
building. A possible measure is the
incorporation of a low gas permeability membrane in the floor slab of the
building. Further investigation may be
required to determine the area of land which is affected by gas emissions. The analysis and assessment of the results
and design of any gas protection measures should be undertaken by suitably
qualified and experienced professionals who are familiar with the properties of
biogas and building protection design measures.
Scenario 3
11.5.13 If there are
occasional exceedances of the trigger value for methane emission rate from a
borehole or if there is a significant fluctuation of the monitoring results
with some readings coming close to the trigger value, then any trends in the
results will need to be assessed to determine their significance and the need
for any building protection measures. It
may be necessary to undertake further monitoring by extending the monitoring
period, for example, if a spuriously high reading is noted towards the end of
the monitoring period or if it seems likely that future emission rates may
exceed the trigger value. The analysis
and assessment of the monitoring results and design of any gas protection
measures should be undertaken by suitably qualified and experienced
professionals who are familiar with the properties of biogas and building
protection design measures.
Scenario 4
11.5.14 If the rate of
methane emission from any borehole frequently exceeds the upper UK guidance
value of 432 L m-2 per day (that is, Carpenter’s guidance level at
which it is recommended that development should not take place), or shows a
rising trend such that future emission rates are likely to exceed this value,
then no buildings should be constructed on that part of the site. That is when:
·
Upper UK guidance value of
432 L m-2 per day x 20 m2
= 8,640 L per day emitted from the borehole; or
·
(Gas flow rate in terms of
litre per day) x (concentration of methane in gas (in % gas)) > 8,640 L per
day.
11.5.15 Depending on the
monitoring results, it may be necessary to incorporate a number of gas
protection measures into the design of the proposed development. Specific details cannot be provided until the
results of the monitoring are available, and the proposed landuse and building
design are known and confirmed. A
combination of different measures may be used for protecting both the ground
level and underground structures at the development against possible risks due
to biogas emissions. Discussions would
need to be held with the developer and architects to determine the protection
measures which are the most appropriate and feasible. Typical gas protection measures that may be
adopted are described below.
Measures to Prevent Ingress of Gas into ‘At Risk’
Rooms
11.5.16 To prevent the
ingress of methane gas into a building, a low gas permeability membrane may be
incorporated in the design of the floor and any below ground walls of
identified ‘at risk’ rooms (for examples, rooms housing electrical equipment,
pumps or switchgear). In addition,
measures should be taken to avoid or seal any openings in the floor (for
example, at services entry points). Such
techniques are commonly used where there is a risk of landfill gas entering a
building and have been employed on a number of developments in Hong Kong.
11.5.17 There are various
proprietary products available in the market and the specific details of their
application will depend on the detailed design of the ‘at risk’ rooms. Possible measures include gas-resistant
polymeric membranes that can be incorporated into the floor or wall
construction as a continuous sealed layer.
Membranes should be able to demonstrate low gas permeability and
resistance to possible chemical attack.
Other building materials such as dense well-compacted concrete or steel
shuttering also enhance resistance to gas permeation. In all cases, extreme care is needed during
the installation of the membrane and subsequent construction works to avoid
damage to the membrane.
Ventilation
within ‘At Risk’ Rooms
11.5.18 As an additional
measure for the protection of specific ‘at risk’ rooms, mechanical ventilation
may be provided to ensure that if any gas enters the room it is dispersed and
cannot accumulate to potentially dangerous concentrations. For particularly sensitive rooms, such as
below ground confined spaces which contain sources of ignition, forced
ventilation may be used in addition to the use of a low gas permeability
membrane.
11.5.19 The basement car
park proposed at the development would be susceptible to ingress and
accumulation of any biogas emissions from the reclamation. The basement car park ventilation system will
be designed to ensure that the car park air quality guidelines given in ProPECC
PN 2/96 Control of Air Pollution in Car
Parks are achieved. The minimum ventilation rate for a basement
car park is 5 to 6 air changes per hour in order to comply with the EPD requirement
on carbon monoxide concentrations within car parks.
11.5.20 It is recommended
that several ventilation systems should be installed and evenly distributed
within the basement car park. Therefore,
even during equipment failure, it is unlikely that the entire exhaust system
would break down. To cater for the
situation of power failure, it is recommended that a back-up power supply shall
be provided for the ventilation system so that certain designated exhaust
systems would still operate. Under
normal conditions, the power failure should be rectified within several hours.
Protection
of Utilities or Below Ground Services
11.5.21 Below ground
ducts or trenches for the installation of utilities or services (for examples,
telecommunications, gas, water, electricity supply or drainage connections)
would be particularly prone to the ingress and accumulation of any biogas
emissions. It is therefore important to
prevent such ducts and trenches acting as routes by which gas may enter buildings
by avoiding, as far as possible, the penetration of floor slabs by such
services. In addition, any unavoidable
penetrations should be carefully sealed using puddle flanges, low permeability
sealant and / or membrane.
Precautions during
Construction Works
11.5.22 Special care must
be taken during the first two years of construction activities on the
reclamation. Sub-surface excavations
into the mud layers might encounter gas occasionally, but not at levels likely
to be dangerous provided that the gas vents freely to atmosphere. Emission rates are unlikely to be sufficient
to sustain a flame. These gas bubbles
will only occur for short periods, and therefore, as a precaution, smoking and
naked flames in the vicinity of drilling activities and excavations of 1 m
depth or more should be prohibited.
11.5.23 Precautions may
be required to ensure that there is no risk due to the accumulation of gas
within any temporary structures, such as site offices, during construction
works on the reclamation area. It may be
necessary, for example, to raise such structures slightly off the ground so
that any gas emitted from the ground beneath the structure may disperse to
atmosphere rather than entering the structure.
A minimum clear separation distance of 500 mm, as measured from the
highest point on the ground surface to the underside of the lowest floor joist,
is recommended in the Landfill Gas Hazard
Assessment Guidance Note, EPD (1997).
Precautions Prior to Entry
of Below Ground Services
11.5.24 Following
construction, accumulation of gas within any below ground services can pose a
risk to the staff of the utility companies.
As a good working practice, prior to entry into any confined space
within the reclamation site (such as manholes, underground culverts and utility
casings), the gas atmosphere within the confined space should be monitored for
oxygen, methane and carbon dioxide.
Personnel should be made aware of the potential dangers and advised to
take appropriate precautions.
11.5.25 The working
practices should follow the Landfill Gas
Hazard Assessment Guidance Note, EPD (1997) guidelines as follows:
·
Any chamber, manhole or
culvert that is large enough to permit access to personnel should be subject to
entry safety procedures. 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.
·
The entry or access point
should be clearly marked with a warning notice (in English and Chinese) which
states that there is the possibility of flammable and asphyxiating gases
accumulated within.
·
The warning notice should
also give the telephone number of an appropriate competent person who can
advise on the safety precautions to be followed before entry and during
occupation of the manhole.
·
Personnel should be made
aware of the dangers of entering confined spaces potentially containing
hazardous gases and, where appropriate, should be trained in the use of gas
detection equipment.
·
Prior to entry, the
atmosphere within the chamber should be checked for oxygen, methane and carbon
dioxide concentrations. The chamber may
then only be entered if oxygen is greater than 18% by volume, methane is less
than 10% of the Lower Explosive Limit (LEL), which is equivalent to 0.5% by
volume (approximately), and carbon dioxide is less than 0.5% by volume.
·
If either carbon dioxide or
methane is higher, or oxygen lower than the values given above, then entry to
the chamber should be prohibited and expert advice sought.
·
Even if conditions are safe
for entry, no worker should be permitted to enter the chamber without having
another worker present at the surface.
The worker who enters the chamber should wear an appropriate safety / recovery
harness and, preferably, should carry a portable methane, carbon dioxide and
oxygen meter.
11.5.26 In general, when
work is being undertaken in confined spaces sufficient approved resuscitation
equipment, breathing apparatus and safety torches should be available. Persons involved in or supervising such work
should be trained and practised in the use of such equipment. A permit-to-work system for entry into
confined spaces should be developed by an appropriately qualified person and consistently
employed.
11.6
Conclusion
11.6.1 Organically
enriched material is planned to be left in-situ
at the WDII reclamation within the western and eastern corners of the Causeway
Bay Typhoon Shelter. As methane gas
could be generated under anaerobic conditions, there is a potential for this
gas to be released either during construction or after development of the
reclaimed area.
11.6.2 The calculation
of the total potential methane flux was overly conservative in nature in order
to build-in a large margin of safety.
Conservative assumptions included the following:
·
All the TOC is readily
biodegradable.
·
All the organic matter is
degraded to methane and no re-oxidation in surface layers occurs. (In fact, oxidation may occur in the upper
layers of fill. Methane passing up
through such layers may be partially or even totally destroyed by oxidation).
·
Higher methane fluxes can be
ignored as they are based on a half-life of only 0.5 year, which would result
in 90% of the methane being lost to atmosphere prior to the proposed
development on WDII reclamation.
·
A boundary layer of 1 m at
the surface of the reclamation is assumed as a worst case scenario.