6.1
Introduction
6.1.1
Since it is a current policy to
leave mud in place whenever feasible, to minimise the amount of dredging, most
of the reclamation for both the Full Reclamation or Miminised Reclamation options
will take place over existing marine sediment. It is proposed that reclamation would
involve the placement of marine sand and/or public dump on top of marine
sediments, with the installation of vertical band drains to accelerate
consolidation. Dredging would only
occur for the seawall foundation.
6.1.2
Some of this marine sediment
contains levels of 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 Full Reclamation and Minimized Reclamation options at Yau Tong Bay is
assessed in the following section.
6.2
Assessment Methodology and Criteria
Field Sampling and
Laboratory Analysis
6.2.1
Sediment samples were collected
at nine vibrocore locations as indicated in Figure 5.1. Four of these locations were along the
proposed seawall and bored pile wall for the Full Reclamation option and five
were within the boundary of the proposed reclamation area (Full or Minimized
Reclamation options). Depending on
the depth of mud at each location, on average five to seven
sediment samples were collected throughout the strata. The collected samples were packed in ice
and transported to MateriaLab for analysis of Total Organic Carbon (TOC)
content and Sediment Oxygen Demand (SOD).
6.2.2
As the marine sediment along
the proposed seawall and bored pile wall will be dredged or excavated, the
potential impact of gas emissions would be mainly from the reclamation where
marine sediment will be left in-situ.
Thus TOC and SOD levels in the sediment samples collected from the five
vibrocores within the boundary of the reclamation area are used for the
assessment of methane hazard for the Full Reclamation and Minimized
Reclamation. Relevant sample
locations and the results are presented in Table 6.1. Sediment samples in the top 5 m of
sediment at each vibrocore location were selected because of their
comparatively higher levels of TOC and SOD. The highest SOD levels were measured
within the top 3m of sediment at vibrocore locations V6 and V8A, and within the
top 2m of sediment at vibrocore locations V7 and V9B.
Table 6.1 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)
|
|
V5
|
0-0.5
0.3-0.5
1.3-1.5
2.3-2.5
3.3-3.5
5.3-5.5
8.2-8.4
|
30.18
39.13
51.38
18.13
33.17
20.12
22.7
|
1.8
2.5
3.4
0.012
3.2
0.43
0.01
|
388
246
549
285
421
48
272
|
|
V6
|
0-0.5
0.4-0.6
1.4-1.6
2.4-2.6
3.45-3.65
5.7-5.9
|
65.41
37.96
42.83
36.4
33.04
20.62
|
3.6
1.9
2.2
1.9
1.4
0.01
|
1361
1509
907
2008
542
50
|
|
V7
|
0-0.5
1.15-1.35
2.15-2.35
3.15-3.35
6.25-6.45
9.25-9.45
11.7-11.9
|
44.89
46.83
29.36
30.79
40.06
16.67
14.36
|
2.6
2.5
0.55
0.45
2.60
0.01
0.01
|
409
3274
158
46
431
16
19
|
|
V8A
|
0-0.5
1.8-2.0
2.7-2.9
3.7-3.9
4.7-4.9
7.7-7.9
|
57.86
39.72
55.62
26.09
26.46
20.37
|
3.4
3.7
3.8
0.58
<0.01
0.06
|
1399
705
3037
142
88
154
|
|
V9B
|
0-0.5
1.7-1.9
2.7-2.9
3.7-3.9
6.8-7.0
7.8-8.0
|
62.95
68.99
43.5
40.99
20.98
17.51
|
3.2
16.0
2.8
0.88
0.78
0.13
|
1514
1735
191
333
279
90
|
|
Mean (all samples):
|
36.10
|
2.08
|
706.44
|
|
Mean (top 5m):
|
41.81
|
2.71
|
951.00
|
Background
6.2.3
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)).
6.2.4
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 which have apparently been safely
developed or redeveloped without any regard for gas protection measures.
Development of
Guide Levels
6.2.5
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).
6.2.6
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, but in the majority of cases emission velocities from a 50 mm
dia. borehole were below 0.01 ms-1. Carpenter therefore recommended that
development should not take place where emission rates exceed 0.05 ms-1
in a 50 mm diameter borehole.
This is the same as a flux of 0.05 m3m-2s-1
or 4320 m3m-2d-1 through a surface with a
cross-sectional area equivalent to the cross-sectional area of the borehole
(i.e. 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.
6.2.7
The UK Department of the
Environment Waste Management Paper on Landfill Completion recommends as a completion criterion that methane emission rates
from monitoring boreholes should fall consistently below 0.015 m3h-1. The completion criterion is that which
must be met in order for monitoring to be discontinued and for the operator to
surrender the licence which 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 m3m-2d-1. For
larger boreholes of
up to 250 mm diameter, the equivalent
rate would be 7.35 m3 m–2d-1.
6.2.8
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 25m 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 2.5m.
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-2d-1 (based on the recommendation of
the Waste Management Paper No. 26A) to 432 l m-2d -1
(based on Carpenter’s guidance level).
6.2.9
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 2.5m and a very low rate of ventilation of 1 air change per week. For Yau Tong Bay Development, 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.
6.2.10
The EPD’s guidance note on
landfill gas hazard and the UK
guidance values (i.e. 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 6.2).
Table 6.2 Methane Hazard Assessment Criteria
Notes:
1.
Guideline value from Landfill Gas Hazard Guidance Note, EPD, HK.
2.
UK Landfill Completion Criterion from Department of 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 YTB from Department
of the Environment (1993) Landfill Completion. Waste Management Paper No. 26A. London: HMSO.
6.3
Calculation of Potential Gas Emissions
6.3.1
From Table
6.1, the calculated mean TOC level is 2.71% on a dry weight basis and the
calculated mean SOD level is 951 mg kg-1. Based on an average moisture content of
41.8%, dry matter made up 58.2% of
the sediment on average.
6.3.2
The potential methane gas
emission was estimated based on the assumption that all marine mud would be
left in-situ within the reclamation
area for both the Full Reclamation and Minimized Reclamation, and that only the
dredged mud from the seawall foundation would be disposed off-site. The quantity of mud was estimated to be
900,000 m3 for the
Full Reclamation:
Volume of mud left in-situ = Total area of
reclamation (excluding the volume
of
mud dredged from seawall foundation)
x
depth of sediment
= 180,000 m2
x 5 m
= 900,000 m3
6.3.3
The quantity of mud was
estimated to be 600,000 m3 for the Minimized Reclamation
option.
6.3.4
The capping of the reclamation
would likely create underlying anaerobic conditions which 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).
6.3.5
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.
6.3.6
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 characterized 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).
6.3.7
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 oxidize one mole of methane).
CH4 + 2O2 = CO2 + 2H2O
No. of moles 1 2
6.3.8
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).
6.3.9
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 oxidized nor highly reduced:
2CH2O ---> CH4 + CO2
6.3.10
On that basis, the mass of
methane generated from unit mass of TOC is calculated as follows:
2C ---> CH4 + CO2
2 x 12 = 24 16
i.e. methane
potential = 16/24 = 0.67 times TOC
6.3.11
Assuming a SOD in the material
to be contained in the future reclaimed land of 951 mg kg-1, the
total methane potential would be 237.8 mg kg-1, or
assuming a dry matter content of 58.2%, 408.6 mg kg-1. However, SOD represents only a fraction
of the organic carbon present in the sediment. Based on a sediment TOC of 2.71% of dry
matter and assuming that half is converted to methane (the remainder being
carbon dioxide), methane potential would be about 18,172.4 mg kg-1
dry matter. This implies that only
2.25% 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.
6.3.12
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 2.25% of the organic carbon is readily degradable. Thus, an analysis based on TOC alone
will overestimate the impact by a factor of about forty.
6.3.13
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, i.e. 2,362 – 13,629 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 6.3 shows calculations of the peak annual methane potential and
the daily potential methane flux from the Yau Tong Bay Reclamation for the Full
Reclamation option as this represents the worst case scenario with a greater
volume of marine mud left in-situ. Although it should be noted that the
potential methane flux would be the same for the Full Reclamation or Minimum
Reclamation Options as the flux represents an area emission rate (i.e. rate at
which gas is emitted per unit area of the reclamation).
6.3.14
The methane concentrations of
the boundary layer at the surface of the Yau Tong Bay Reclamation is also
estimated as shown in Table 6.3 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 a conservative scenario.
Table 6.3 Calculation
of Methane Flux from the Yau Tong Bay Reclamation
|
|
Marine Sediment
Full Reclamation Option
|
Methane Hazard Assessment Criteria
|
|
Half-life cycle of 5
years
|
Half-life cycle of 2
years
|
|
Volume
(m3)
|
900,000
(600,000)e
|
900,000
(600,000)e
|
|
|
Density
(kg m-3)
|
1,750
|
1,750
|
|
|
Dry
matter (% w/w)
|
58.19
|
58.19
|
|
|
Dry
matter (kg m-3)
|
1018.33
|
1018.33
|
|
|
TOC (%)
|
2.71
|
2.71
|
|
|
TOC (kg
m-3)
|
27.60
|
27.60
|
|
|
CH4
potential (kg m-3)
|
18.49
|
18.49
|
|
|
Peak
annual CH4 potential (kg)
|
2,163,298
(1,442,199) e
|
4,825,819
(3,217,213) e
|
|
|
Total
area (m2)
|
180,000
(120,000)
e
|
180,000
(120,000)
e
|
|
|
Total potential CH4 flux
(kg m-2 yr-1)
|
12.02
|
26.81
|
|
|
Total potential CH4 flux
(g m-2 yr-1)
|
12018.32
|
26810.10
|
|
|
Total potential CH4 flux
(mol m-2 yr-1)
|
751.15
|
1675.63
|
|
|
Total potential CH4 flux
(l m-2 yr-1)
|
16825.65
|
37534.15
|
|
|
Total potential CH4 flux
(l m-2 dy-1) (assuming 2.25% of TOC biodegradable)
|
1.04
|
2.31
|
18a – 432b
10d
|
|
Total potential CH4 flux
(l m-2 dy-1) (assuming 15% of TOC biodegradable)
|
6.91
|
15.42
|
|
Total potential CH4 flux
(l m-2 dy-1) (assuming 50% of TOC biodegradable)
|
23.05
|
51.42
|
|
Total potential CH4 flux
(l m-2 dy-1) (assuming 100% of TOC biodegradable)
|
46.10
|
102.83
|
|
Potential CH4
concentration (% v/v) at the surface boundary layer (assuming 2.25% of TOC
biodegradable)
|
0.10
|
0.23
|
1c
|
|
Potential CH4
concentration (% v/v) at the surface boundary layer (assuming 100% of TOC
biodegradable)
|
4.61
|
10.28
|
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., pp 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 YTB, as based onDepartment of the Environment (1993) Landfill Completion. Waste Management Paper No. 26A. London: HMSO.
e Values
in brackets are for the Minimum Reclamation Option. The total potential methane flux is the
same for the Full Reclamation or Minimum Reclamation Options as the flux
represents an area emission rate (i.e. rate at which gas is emitted per unit
area of the reclamation).
6.3.15
The above analysis is based on
a number of broad assumptions which 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%. For landfills, covered
by a very permeable top layer, oxidation efficiencies were found in the range
of 0-50% . High efficiencies will
only occur when the fill material is well aerated (e.g. 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 (e.g. gravel layers).
6.4
Evaluation of Significance of Potential Gas Emissions
Significance of
potential methane emissions with reference to the UK Guidance Values
6.4.1
Taking the UK landfill
completion criterion (i.e. 18 l m-2d-1) as the
standard, the predicted methane emission from the Full or Minimized Reclamation
options based on a half-life of 5 years (1.04 l m-2d-1),
assuming 2.25% of TOC biodegradable, is only 5.7% of this guide value. This is therefore insignificant, since
provides a safety factor of approximately 17. Based on a half-life of 2 years, the predicted
methane emission (2.31 l m-2 per day) is 12.8% of the UK
landfill completion criterion, providing a safety factor of approximately
8. 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 (46.10 l m-2 d-1) for a half-life of 5 years is found to be approximately 2.5 times
greater than the UK landfill completion criterion.
Under the worst case scenario of a half-life of 2 years, the predicted
methane emission (102.83 l m-2 per day) is found to be approximately
5.7 times greater than the UK
landfill completion criterion.
6.4.2
The calculations show that the
predicted methane emission based on a half-life of 5 years, assuming 100% of
TOC biodegradable, is about 10.7% of the Carpenter’s guidance level (i.e. 432 l
m-2 d-1), providing a safety factor of approximately
9. The predicted methane emission
based on a half-life of 2 years, assuming 100% of TOC biodegradable, is about
23.8% of the Carpenter’s guidance level, providing a safety factor of
approximately 4. It is noted that
even under the extreme worst case scenario with the predicted methane emission
in exceedance of the UK landfill completion criterion, the 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.
6.4.3
Taking the “safe” rate of gas
emissions derived for YTB from Waste Management Paper No. 26A on Landfill
Completion (i.e. 10 l m-2d-1) as the standard,
the predicted methane emission from the Full or Minimized Reclamation options,
assuming 2.25% of TOC biodegradable, is about 9 times less than this guide
value. The predicted methane
emission based on the worst case assumption of 100% TOC biodegradable, which is
considered a highly unlikely event, is also considered as a conservative
estimate. The calculations show
that the predicted methane emission (46.10 l m-2 d-1), assuming 100% of TOC biodegradable, is approximately 4.6 times
greater than the “safe” emission rate
(i.e. 10 l m-2 d-1). Taking the worst case scenario of a
half-life of 2 years, the calculations show that the predicted methane
emissions are approximately 4 times less and 10 times greater than the “safe”
emission rate, assuming 2.25% and 100% of TOC biodegradable, respectively.
6.4.4
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 Yau Tong Bay in order to determine the likelihood of methane emissions posing a
significant risk and the need for mitigation measures.
6.4.5
The proposed reclamation will
be developed for residential/commercial uses and open space. The proposed high rise residential
buildings with a podium design tend to pose limited risk as these do not have
any below ground rooms and car parking is on lower storeys of the building. Similarly, the commercial towers and
shopping arcade comprising above ground structures pose limited risk. Rooms located at the ground level of the
commercial and residential buildings, such as utility (services) and refuse
collection rooms, may be susceptible to ingress of any biogas generated from
the reclamation if mitigation measures are not included in the design of the
building. Rooms on the ground floor
of schools may also be ‘at risk.’ The most sensitive ‘at risk’ rooms are
considered to be the underground car parks 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 parks 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
structures at the development are discussed in Section 6.5.
6.4.6
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 incorporate gas protection
measures and 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
6.4.7
The methane concentration at
the surface boundary layer from the Full or Minimized Reclamation options is
estimated to be 0.10% (v/v) (assuming 2.25% of TOC biodegradable and a
half-life of 5 years) which is 10 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 0.23% (v/v) is 4
times less than the EPD’s guide value.
6.4.8
On considering the highly
unlikely event of assuming 100% TOC biodegradable, the methane concentration at
the surface boundary layer from the Full or Minimized Reclamation options is
estimated to be 4.61% and 10.28% 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 6.5 below.
6.5
Mitigation Measures And Further Work
6.5.1
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 guide
value (which is the level at which development would be restricted according to
Carpenter’s guidelines). The recommended monitoring requirements are detailed
below and apply to both the Full and Minimized Reclamation options.
6.5.2
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 paragraph 6.4.4, 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.
6.5.3
As sensitive ‘at risk’ rooms
have been identified at the proposed development, both at ground level and
below ground, it is recommended that a precautionary principle be applied. Gas protection measures are therefore
recommended to be incorporated in the building design. 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 proposed that the
results of the recommended gas monitoring to be undertaken at the YTB
reclamation be reviewed against a trigger value and thereby determine the
extent and type of mitigation measure requirements to be incorporated in the
detailed design of the proposed development.
Gas Monitoring
6.5.4
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. It is recommended that
the monitoring wells be installed in an approximately even distribution across
the reclamation area. Proposed
monitoring locations are indicated in the EM&A Manual.
6.5.5
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 be
undertaken monthly for a period of at least one year prior to the commencement
of construction works on the reclamation.
The results of the gas monitoring should be reviewed to determine
whether the length of the monitoring period should be extended beyond one
year. Details of the
recommendations for methane gas monitoring are given in the EM&A Manual.
Precautionary Gas
Protection Measures
General Guidelines
6.5.6
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 from
the Waste Management Paper No. 26A on Landfill Completion is proposed to be
adopted as the trigger value.
Scenario 1
6.5.7
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.
6.5.8
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.
6.5.9
To be conservative, it is
proposed to adopt an area of influence of 20 m2 (radius of 2.5m),
which would give:
·
Trigger value of 10 L m-2
per day x 20 m2 = 200 L per day emitted from the borehole
6.5.10
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
6.5.11
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.
6.5.12
The type of gas protection
measures would be dependent on the design and use of the particular
building. Possible measures are the
incorporation of a low gas permeability membrane in the floor slab of the
building or mechanical ventilation of ‘at risk’ rooms. 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
6.5.13
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
6.5.14
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.
6.5.15
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 building detailed
designs 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. At this stage, discussions with
the architect have identified feasible gas protection measures that may be
adopted to prevent the ingress and/or accumulation of any methane gas emissions
generated from the reclamation.
These gas protection measures are described below and would apply to
both the Full and Minimized Reclamation options.
Protection of Above Ground Structures
6.5.16
Passive sub-floor ventilation may
be incorporated in the building design for those buildings with no underground
basement or rooms. The general principle of passive sub-floor venting is shown
in Figure 6.1. Passive control measures for buildings
to prevent gas build-up involve the creation of a clear void beneath the
structure allowing natural air movements such that any emissions of gas from
the ground are mixed and diluted by air.
Measures to Prevent Ingress of Gas into ‘At
Risk’ Rooms
6.5.17
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 (e.g. rooms housing electrical equipment, pumps or
switchgear). In addition, measures
should be taken to avoid or seal any openings in the floor (e.g. 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.
6.5.18
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 which 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. Typical design details for gas
impermeable membrane protection are shown on Figure 6.2.
Ventilation within ‘At Risk’ Rooms
6.5.19
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.
6.5.1
Three basement carparks are
proposed at the Yau Tong Bay development which would be susceptible to ingress and accumulation
of any biogas emissions from the reclamation. The storey height of each basement
carpark is 4 m, with 2.4 m clear headroom (minimum) at driveway and
carpark. With reference to the
Waste Management Paper No. 26A (para. 6.2.9),
the maximum safe rate of methane ingress is defined as that at which it would
take 1 week for the methane concentration to reach 1% (v/v). The corresponding daily maximum “safe”
rate of methane gas emission per unit area for the basement carpark is
calculated to be 3.43 1 m-2d-1 (assuming a height of 2.4
m and no ventilation). This
emission rate is expressed as a daily rate and is based on an assumed
accumulation of gas at this daily emission rate over a period of one week.
6.5.2
The predicted peak daily
methane flux (based on the TOC results, a half-life of 2 years and assuming all
organic matter is biodegradable) is 102.83 1 m-2d-1. Based on this conservative approach, the
predicted daily methane flux is 30 times greater than the calculated maximum
“safe” rate of methane gas emission for the basement carpark. To achieve this recommended daily “safe”
rate, a minimum of 30 air changes per week would be required. The basement carpark ventilation systems
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 normal ventilation rate for the
basement carpark is 5 to 6 air changes per hour in order to comply with the EPD
requirement on carbon monoxide concentrations within carparks. Therefore, the
ventilation rate proposed at the basement carpark (around 120 air changes per
day) to satisfy the air quality guidelines is well above that required to
achieve the daily “safe” rate of methane gas emission per unit area.
6.5.3
Several ventilation systems
would be installed and evenly distributed within the basement carpark. 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 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
6.5.4
Below ground ducts or trenches
for the installation of utilities or services (e.g. 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
6.5.5
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 1m depth or more should be
prohibited.
6.5.6
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 500mm, 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
6.5.7
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.
6.5.8
The working practices should
follow the Landfill Gas Hazard Assessment
Guidance Note, EPD (1997) guidelines as follows:
·
Any chamber, manhole or culvert
which 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 are 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.
6.5.9
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.
6.6
Conclusions
6.6.1
Organically enriched material
is planned to be left in-situ beneath
the YTB Full Reclamation or Minimized Reclamation Options. 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.
6.6.2
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 YTB development.
·
A boundary layer of 1 m at the
surface of the reclamation is assumed as a worst case scenario.
6.6.3
Assuming 100% of TOC is
biodegradable, a highly unlikely event, the predicted methane emission from the
YTB Reclamation (Full Reclamation or Minimized Reclamation options) for a
half-life of 5 years was found to be approximately 2.5 times greater than the UK
landfill completion criterion.
Based on a half-life of 2 years, the predicted methane emission was
found to be approximately 5.7 times greater than the UK
landfill completion criterion.
Under this extreme worst case scenario, the predicted methane emissions
based on half-lives of 5 and 2 years are well below the upper UK guide value,
which is the level at which development would be restricted according to
Carpenter’s guidelines. This
suggests that the methane gas generation potential is not expected to pose a
development constraint to the YTB Full Reclamation or Minimized Reclamation
options. These UK guidelines are considered to be consistent with current Hong Kong guidance.
6.6.4
In view of the exceedance of
the UK landfill completion criterion and the identification of ‘at risk’ rooms
at the development, it is recommended to undertake gas monitoring in the
immediate post-reclamation period and prior to the commencement of construction
works on the reclamation to measure methane concentrations in the fill and to
determine actual rates of methane gas emissions. The review of the monitoring results
would determine the extent and type of gas protection measures to be incorporated
in the building design to prevent the ingress and/or accumulation of any
methane gas emissions to potentially dangerous concentrations. Guidelines on criteria for evaluation of
the gas monitoring results and gas protection measure requirements have been
identified for both the ground level and underground structures at the
development.
6.6.5
Precautionary measures to be
taken prior to entry into any below ground services or confined space within
the reclamation site are also recommended.
As a further precaution, naked flames should not be permitted during
construction involving drilling or excavation.
6.6.6
The proposed monitoring
guidelines and other precautionary mitigation measures should be examined
further at the detailed design stage with regard to the specific design details
of individual buildings. With the
incorporation of the recommended gas protection measures in the design of the
buildings, as necessary depending on the results of the monitoring, together
with the implementation of the other recommended precautionary measures, the
risk to people and property due to biogas emissions from the YTB Full
Reclamation or Minimized Reclamation options is considered to be low.
