6.14 BIOGAS ASSESSMENT

Introduction

6.14.1 This Section provides an assessment of the potential hazards associated with biogas generation from the marine sediment to be left in place under the proposed NLDFS developments, including the artificial lake of Water Recreation Centre in the Theme Park, Penny’s Bay Reclamation Stage II, Yam O Reclamation, Northshore Reclamation, Siu Ho Wan Reclamation, SDU Reclamation, Road P1 Reclamation, Theme Park (Phase III) Extension Reclamation and TCT East Reclamation (hereby refer as undredged areas). Where potential hazards are identified, mitigation measures will be recommended.

Potential Sources of Biogas

6.14.2 In terms of general engineering practice on construction of reclamation, in areas of the reclamation where either settlement effects, construction programme constraints or foundation stability are critical, these areas will be dredged to remove the marine sediment. Since this full-dredge and fill method enable quicker settlement of fills, such arrangement is adopted at Penny’s Bay Reclamation Stage I due to limited construction time for Theme Park Phase I. However, it is also recognised that there is a need to minimise the volume of sediment to be dredged and disposed of in accordance with EPD's policy directive to reduce disposal of dredged materials. In this regard, it was recommended that for the construction of Penny’s Bay Reclamation Stage II, Yam O Reclamation, Northshore Reclamation, Siu Ho Wan Reclamation, SDU Reclamation, Road P1 Reclamation, Theme Park (Phase III) Extension Reclamation and TCT East Reclamation dredging should be assumed only along the seawalls and the remainder of the sites sediments would be left in place. In addition, although artificial lake of the Water Recreation Centre in Theme Park will be built within the boundary of the Penny’s Bay Reclamation Stage I, limited excavation (approximately top 1 m) will be carried out and impermeable membrane will be put on surface of the sediment prior to artificial lake construction. A combination of ground improvement techniques have been assumed, including wick drains and surcharging would be used to accelerate the settlements of the undredged areas.

6.14.3 When marine sediments rich in organic matter are covered with reclamation fill/impermeable membrane and the in situ oxygen is used up, anaerobic degradation of the organic matter in the sediments occurs and generates biogas (comprising mainly methane and carbon dioxide). Biogas may be generated from the in place sediment underneath the undredged areas and hence the potential risk of methane to subsequent developments at the reclamation is of concern. The following sections assessed the potential risks associated with biogas arising from the sediment within these proposed developments.

BIOGAS RISK ASSESSMENT CRITERIA

Introduction

6.14.4 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 Landfill Gas Hazard Assessment Guidance Note ^([18]) issued by the EPD. The guidance note recommends that methane gas should be monitored periodically in all excavations, manholes and chambers and any confined spaces during construction. No works and no entry to the excavation areas or confined spaces should be allowed and the personnel on-site should be evacuated if the methane concentration measured during the monitoring exceeds 20% lower explosive limit (LEL) (or 1.0% gas).

6.14.5 In assessing the risks relating to the generation and emission of biogas, there are two aspects which need to be considered as follows:

· the likely rate of gas emission; and

· the maximum rate of gas emission which is acceptable.

6.14.6 A potential hazards will exist, and therefore mitigation measures will be required, if the likely rate of gas emission exceeds the maximum acceptable rate of gas emission. Thus, assessment of the risks relating to the generation and emission of biogas entails determination of the above two parameters.

Maximum Safe Level of Methane Ingress/Emission

6.14.7 Ideally, in order to determine the potential risks involved, specific details about the particular ‘targets’ which are at risk should be taken into account. This would include the following information about each of the rooms or confined spaces in the development which are at or below ground level, and would include the following:

· size of room;

· rate of ventilation (number of air changes per hour);

· occupancy (nature and frequency of use of the room); and

· presence of any sources of ignition.

6.14.8 With this information it is possible to calculate the maximum safe rate for which gas could enter any of the particular void spaces (rooms). Allowance could then be made for potential variations in the rate of gas ingress or rate of ventilation, by application of a suitable margin of safety, to derive an acceptable average rate of gas ingress.

6.14.9 For the proposed future development at the undredged areas, it is difficult to specify all the potential ‘at risk’ areas (rooms or voids) or to know the specific details (size, rate of ventilation etc) of such rooms or voids, as such information is not yet available at this stage of the Study. However, various assumptions and approximations can be made based on the type of development which is proposed and the likely features of such developments.

6.14.10 A literature search revealed no references relating specifically to the estimation of the potential risks due to biogas from marine sediments. The most comprehensive report on the assessment of the hazards due to methane from the ground is provided in a report by the Construction Industry Research and Information Association (CIRIA) on the potential hazards and risks from all sources of methane from the ground  ^([19]) which includes a review of the approach adopted in several countries. In general, there is little or no guidance as to how to determine whether or not special building measures are required to protect against potential dangers from methane. Where guidance has been identified, this tends to be based on measurements of methane concentrations within the ground.

6.14.11 However, the process of estimating safe rates of biogas emission for reclaimed land or impermeable membrane over sediment left in situ which is to be developed is very similar to estimating safe emission rates of landfill gas when deciding whether or not an old landfill site can be regarded as being sufficiently stabilised so that it no longer poses a danger. The assessment of a safe rate of landfill gas emission is necessary for determining when a site no longer needs to be monitored and when it can be used for unrestricted development. The assumption in this case is that any type of building could be safely constructed on top of the landfill and therefore requires the adoption of a universally applicable safe rate of methane emission.

6.14.12 A criterion for deciding when the rate of landfill gas emissions have dropped sufficiently that they no longer pose a danger is specified in the UK Department of the Environment’s Waste Management Paper No. 26A: Landfill Completion as follows:

"Methane emission rate from any boreholes drilled into the waste to be less than 0.015m³hr^-1".

6.14.13 This figure was derived based on assumptions about the area of influence of a freely venting borehole on a restored landfill (assumed to be 100 m²) and based on assumptions about the maximum acceptable rate of ingress of methane into any building constructed on the landfill. In WMP26A, the following assumptions were made about the most sensitive ‘at risk’ room or void in which gas might accumulate:

· height of void space = 2.5 m (based on a standard room); and

· rate of natural ventilation = 1 air change per week.

6.14.14 In the case of developments at the undredged areas, it is considered that it would be more appropriate to use the following assumptions about the most sensitive void spaces:

· height of void space = 1 m (to allow for smaller void space such as cable pits) ; and

· rate of natural ventilation = 1 air change per day (in line with commonly assumed rates of natural ventilation for closed rooms).

6.14.15 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%. This is 20% of the lower explosive limit (LEL) for methane and therefore provides a safety factor of 5 to allow for variations in, for example, rates of gas emission across the area of the site or over time compared with those measured at particular places and during the evaluation period.

6.14.16 The maximum safe rate of gas emission was therefore calculated as follows:

1.0 m x 0.01 d^-1 (equivalent to m³ CH₄ m^-2 d^-1)

= 0.01 m³ CH₄ m^-2 d^-1

= 10 litres m^-2 d^-1

6.14.17 There are two points to note about this approach to determining safe rates of gas emission:

(a) The assumption is that the gas entering the room is derived from the same area of site as the area of the room at risk. This is a reasonable assumption for the assumed low rates of ventilation of the rooms as they are unlikely to act as ‘collectors’ for wider areas of ground.

(b) The time scale used for assuming accumulation of the gas, one day, is in line with CIRIA’s assumption that an ‘unventilated’ room will have one air change per day. Typical ‘natural’ ventilation rates may vary from 0.5 to 3 air changes per hour.

6.14.18 The suggested maximum rate of methane emission per unit area provides a reasonable general guide for determining whether the rates of methane emission pose an unacceptable risk to unrestricted development on a potentially gassing site. However, this specified emission rate should be used only as a guide and the likely risk for a particular development should be reassessed in the light of a review of the proposed development plans for the site.

6.14.19 The London Scientific Services has recommended that development should not take place on sites where significant gas concentrations (>1% gas v/v) have been recorded with methane emission rates in excess of 0.05 m s^-1 from a 50 mm diameter monitoring borehole. This is equivalent to a methane flux of approximately 84.7 litres m^-2 d^-1 ([20]) assuming that the borehole vents gas from an area of ground of 100 m². It should be noted that the limit recommended by the London Scientific Services should be considered as the ‘absolute’ cut-off level for development to be built in areas where methane gas may be generated.

Estimation of Likely Rates of Gas Emission

6.14.20 As discussed above, the derivation of acceptable rates of gas emissions for determining when a landfill site may be regarded as being safe, as specified in WMP26A, were based on measurement of gas emissions from boreholes drilled into the waste. This method is appropriate and practical for a completed landfill because the heterogeneous and unpredictable nature of landfilled waste does not lend itself to accurate modelling whereas by taking measurements from several boreholes across the surface of a site over a period of time allows the spatial and temporal variations of gas emissions to be taken into account.

6.14.21 This is particularly important because, at the very low rates of gas emission being considered, it is not so much the rate of gas generation which is important but the rate of gas emission from the ground. As the gas (landfill gas or ‘biogas’) is generated, a ‘reservoir’ of gas will accumulate within the pore spaces of the waste (or reclamation fill/sediment left in situ under impermeable membrane) and, as the pressure builds up within the reservoir, gas will vent to atmosphere (or migrate laterally). The rate at which this occurs will depend in part on the difference in pressure between the gas in the reservoir and the atmosphere. Thus, a major factor influencing the rate of gas emission is likely to be change in atmospheric pressure: as atmospheric pressure falls there will be an increase in emission rate and when atmospheric pressure rises emission rates will decrease. The exact way in which emission rates respond to changes in atmospheric pressure is quite complex and depends on a number of factors including the nature of the gas reservoir and the nature of the pathways along which gas is vented to atmosphere. Empirical evidence from the monitoring of landfill gas indicates that there can be variable lag times between changes in atmospheric pressure and impacts on rates of gas emission. As a general rule, however, the highest rates of gas emission are to be expected during or immediately following a rapid and/or prolonged fall in atmospheric pressure which occurs following a period of steady atmospheric pressure. For any particular site it is very difficult to predict to the exact relationship between changing atmospheric pressure and gas emission rates.

6.14.22 In the absence of monitoring data, such as in the case of the proposed undredged areas, reliance has to be placed on estimates of gas generation. It must therefore be appreciated that the predicted rates of gas generation represent the average rates of gas emission and that instantaneous rates of gas emission may vary considerably from the average. The need to theoretically predict gas generation rates rather than being able to measure them also presents another level of inaccuracy although it would be expected that the organic material producing the biogas in the case of marine sediments beneath a reclamation (at Penny’s Bay Reclamation Stage II, Yam O Reclamation, Northshore Reclamation, Siu Ho Wan Reclamation, SDU Reclamation, Road P1 Reclamation, Theme Park (Phase III) Extension Reclamation and TCT East Reclamation) or sediment left in situ under impermeable membrane (at the artificial lake of the Water Recreation Centre of Theme Park) will be more homogeneous than the waste materials which give rise to landfill gas and any spatial variations will therefore be less marked.

6.14.23 Various methods are used for the theoretical estimation (prediction) of the rate of biogas (or landfill) gas generation. A number of biological methane potential (BMP) tests have been developed ([21]) ([22])^([23]) and these are generally regarded as providing the best estimate of potential yield of gas. They involve incubation of samples of the subject material and collection/measurement of the evolved gas. They do have a number of serious disadvantages in terms of their widespread use as practical tests, however, for the following reasons:

· they are relatively specialised and complex tests which can only be performed by specialised laboratories;

· they require specially prepared innoculum and nutrient solutions;

· they take between 3 and 10 months to carry out; and

· there is no recognised standard methodology.

6.14.24 As a substitute for BMP tests, a number of other biological, chemical and physical tests have been employed to estimate potential biogas generation. The most commonly used analyses are as follows:

· calorific value;

· volatile solids content;

· total organic carbon (TOC);

· chemical oxygen demand (COD); and

· biochemical oxygen demand (BOD).

6.14.25 Of these, it is considered that those tests which provide a measure of the biologically degradable organic material present in the sample, as opposed to that material which may be chemically degraded, are likely to be more representative. Thus, TOC and sediment oxygen demand (SOD), a specific type of BOD test were used to estimate the methane generating potential of the sediment at the undredged areas.

Sediment Sampling and Testing Programme

6.14.26 The reclamation area, depth of sediment and volume of sediment left in situ for the undredged areas are shown in Table 6.14a. For the biogas assessment, sediment samples were taken from seventeen (17) vibrocore stations (see Figures 6.5c and 6.14a) within the proposed reclamation. At each of the vibrocore stations, subsamples were taken at seabed level and thereafter at the following depths: 0.9 m, 1.9 m, 2.9 m and 5.9 m below seabed or until the bottom of the marine sediment. The sediment subsamples were analysed for TOC and SOD.

Table 6.14a - Sediment Sampling and Testing Programme for the Undredged Areas

Location		Undredged Area (ha)	Depth of Sediment (m)	Volume of sediment left In Situ (m^3)
Artificial lake of Water Recreation Centre in Theme Park		12	6 – 14 ^(a)	1.7 M ^(a)
Penny’s Bay Reclamation Stage II		80	2 – 26 ^(a)	10 M ^(b)
Yam O Reclamation		10	10 – 15 ^(c)	1.3 M ^(b)
Northshore Reclamation		65	10 – 15 ^(c)	8.1 M ^(b)
Siu Ho Wan Reclamation		30	5 – 10 ^(c)	2.3 M ^(b)
SDU Reclamation		1	5 – 10 ^(c)	0.075 M ^(b)
Road P1 Reclamation		8	5 – 10 ^(c)	0.6 M ^(b)
Theme Park (Phase III) Extension Reclamation		80	2 – 20 ^(a)	8.8 M ^(b)
TCT East Reclamation		74	2 – 18 ^(a)	7.4 M ^(b)
Note:	(a) Source: Scott Wilson communication 97240/2/11/53672 dated 2 February 2000. (b) Estimation is made according to current engineering information. (c) Source: Scott Wilson communication 97240/2/14/53493 dated 25 January 2000.

6.14.27 To determine the total quantity of oxygen that will be required to biologically stabilise the organic matters present in the sediment samples and hence to determine the total amount of biodegradable organic carbon in the sediment. SOD values were used to calculate the total biodegradable organic carbons in the sediment.

Sediment Testing Results

6.14.28 The analytical results of SOD and TOC analyses ^([24]) for the sediment samples are presented in Table 6.14b.

6.14.29 Artificial Lake of Water Recreation Centre in Theme Park: The depth averaged TOC concentrations at seabed, 1 m, 2 m, 3 m and 6 m below seabed vary from 0.6% to 1.2% (w/w) dry matter (with an average level of 0.83%). Depth averaged moisture content range from 38.7% to 45.9% (with an average level of 42.6%). Depth averaged SOD_24hours levels of the samples vary from 466 to 1,072 mg O₂ kg^-1 (w/w) dry matter (with an average level of 740 mg O₂ kg^-1). Depth averaged SOD_5days levels of the samples vary from 1,941 to 3,228 mg O₂ kg^-1 (w/w) dry matter (with an average level of 2,396 mg O₂ kg^-1).

6.14.30 Penny’s Bay Reclamation Stage II: The depth averaged TOC concentrations at seabed, 1 m, 2 m, 3 m and 6 m below seabed vary from 0.7% to 1.0% (w/w) dry matter, with overall average of 0.84%. Depth moisture content of the sediment samples range from 41.0% to 49.6% (with an average level of 45.30%). Depth averaged SOD_24hours levels of the samples vary from 713 to 913 mg O₂ kg^-1 (w/w) dry matter (with an average level of 733 mg O₂ kg^-1). Depth averaged SOD_5days levels of the samples vary from 2,316 to 2,676 mg O₂ kg^-1 (w/w) dry matter (with an average level of 2,493 mg O₂ kg^-1).

6.14.31 Yam O Reclamation: Depth averaged moisture content at seabed, 1 m, 2 m, 3 m and 6 m below seabed range from 35.6% to 48.4% (with an average level of 43.19%). The depth averaged TOC concentrations vary from 0.7% to 1.0% (w/w) dry matter, with overall average of 0.83%. Depth averaged SOD_24hours levels of the samples vary from 546 to 2,097 mg O₂ kg^-1 (w/w) dry matter (with an average level of 1,329 mg O₂ kg^-1). Depth averaged SOD_5days levels of the samples vary from 180 to 2,724 mg O₂ kg^-1 (w/w) dry matter (with an average level of 1.442 mg O₂ kg^-1).

6.14.32 Northshore Reclamation: Depth averaged moisture content at seabed, 1 m, 2 m, 3 m and 6 m below seabed has an average level of 27.70%. The depth averaged TOC concentrations consists of an overall average of <0.2% (w/w) dry matter. Depth averaged SOD_24hours and SOD_5days are 200 mg O₂ kg^-1 and 1,173 mg O₂ kg^-1 (w/w) dry matter, respectively.

6.14.33 Siu Ho Wan Reclamation: Depth averaged moisture content depth averaged results range from 46.1% to 51.9% (with an average level of 48.43%). The depth averaged TOC concentrations vary from 0.8% to 0.9% (w/w) dry matter, with overall average of 0.83%. Depth averaged SOD_24hours levels of the samples vary from 1,155 to 3,618 mg O₂ kg^-1 (w/w) dry matter (with an average level of 2,189 mg O₂ kg^-1). Depth averaged SOD_5days levels of the samples vary from 2,960 to 4,530 mg O₂ kg^-1 (w/w) dry matter (with an average level of 3,758 mg O₂ kg^-1).

6.14.34 SDU and Road P1 Reclamations: Depth averaged moisture content depth averaged at seabed, 1 m, 2 m, 3 m and 6 m below seabed range has an average level of 45.17%. The depth averaged TOC concentrations consists of overall average of 0.47% (w/w) dry matter. Depth averaged SOD_24hours and SOD_5days are 938 mg O₂ kg^-1 and 2,727 mg O₂ kg^-1 (w/w) dry matter, respectively.

6.14.35 Theme Park (Phase III) Extension Reclamation: No sediment sampling is carried out in this reclamation, however, the sediment quality is likely to be similar to Penny’s Bay Reclamation Stage II and TCT East Reclamation.

6.14.36 TCT East Reclamation: The depth averaged TOC concentrations at seabed, 1 m, 2 m, 3 m and 6 m below seabed vary from 0.4% to 1.0% (w/w) dry matter, with overall average of 0.70%. Depth averaged moisture content range from 26.3% to 47.5% (with an average level of 45.30%). Depth averaged SOD_24hours levels of the samples vary from 297 to 819 mg O₂ kg^-1 (w/w) dry matter (with an average level of 521 mg O₂ kg^-1). Depth averaged SOD_5days levels of the samples vary from 1,057 to 2,710 mg O₂ kg^-1 (w/w) dry matter (with an average level of 1,725 mg O₂ kg^-1).

6.14.37 The SOD results indicate that the longer the sediment is incubated the higher the value of the SOD. This suggested that readily biodegradable organic matters will be degraded within the 24 hours but it will take a longer time to assimilate the less readily biodegradable organic matters. From SOD_5days curves in the sediment quality report, it is possible that some biodegradable organic matters have not yet been assimilated and the SOD value could be higher if it allows to incubate for a longer period. For the purpose of determining the total biodegradable portion of the TOC, the SOD_5days value will therefore provide a better estimate. Based on the SOD_5days results, the proportion of the biodegradable organic matters in TOC is about 14% ^([25]). It is comparable with the figure (15%) reported in the South East Kowloon Development EIA Study.

Table 6.14b - Sediment Analysis Results

Vibrocore Station	Depth Below Seabed (m)	Moisture Content (% w/w) ^(a)	TOC (% dw) ^(b)	SOD24hours (mg O2 kg^-1 dw)	SOD5days (mg O2 kg^-1 dw)
Artificial Lake of Water Recreation Centre of Theme Park
NNS5	0.4-0.9	36.9	1.3	1,547	3,940
	0.9-1.5	26.6	0.3	281	1,335
	2.9-3.5	34.7	0.5	567	1,796
	5.3-5.9	42.9	0.9	601	2,832
NNS6	0.0-0.6	51.7	1.1	596	2,635
	0.9-1.5	50.8	0.9	650	2,547
	2.9-3.5	45.6	0.9	722	2,364
	5.3-5.9	48.9	0.7	958	2,166
Depth Average	0.0-0.9	44.3	1.2	1,072	3,288
	0.9-2.9	38.7	0.6	466	1,941
	2.9-3.9	40.2	0.7	645	2,080
	5.9-6.2	45.9	0.8	780	2,274
Overall Average		42.26	0.83	740	2,396
Penny’s Bay Reclamation Stage II
NNS1	0.5-0.9	27.5	0.7	283	753
	0.9-1.5	37.7	0.8	685	1,734
	2.9-3.5	44.5	0.8	865	2,355
	5.3-5.9	33.0	0.5	616	1,533
NNS2	0.0-0.6	57.7	1.0	840	3,364
	0.9-1.5	47.9	0.7	1,042	2,482
	2.9-3.5	50.6	1.0	1,200	2,767
	5.3-5.9	48.2	0.7	893	2,664
NNS3	0.25-0.09	61.2	1.0	1,127	2,938
	0.9-1.5	54.4	1.0	965	2,105
	2.9-3.5	49.1	0.9	1,465	2,633
	5.9-5.9	50.7	0.9	1,264	3,150
NNS4	0.0-0.6	58.5	0.9	1,092	2,923
	0.9-1.5	40.9	0.7	535	2,944
	2.9-3.1	54.3	1.1	1,035	2,947
	5.3-5.9	49.5	0.8	792	2,600
Depth Average	0.0-0.9	41.0	0.9	659	2,495
	0.9-2.9	45.2	0.8	645	2,316
	2.9-3.9	49.6	1.0	913	2,676
	5.9-6.2	45.4	0.7	713	2,487
Overall Average		45.3	0.84	733	2,493
Yam O Reclamation
SS6	0.3-0.9	44.5	1.2	2,932	865
	0.9-1.5	39.7	0.7	1,7758	376
	2.9-3.5	43.1	0.9	2,097	533
	5.2-5.8	48.4	1	1,459	180
SS17	0.2-0.8	43.7	0.4	1,030	3,860
	0.9-1.5	40.8	0.8	823	3,017
	1.9-2.5	47.8	0.8	808	3,055
SS19	0.2-0.8	50.1	0.7	761	3,447
	0.9-1.5	26.4	0.6	316	1,294
	1.9-2.5	37.6	<0.2	286	1,365
Depth Average	0.0-0.9	46.1	0.8	1,574	2,724
	0.9-1.9	35.6	0.7	966	1,526
	1.9-2.9	42.7	0.8	547	2,210
	2.9-3.9	43.1	0.9	2,097	533
	5.9-6.2	48.4	1.0	1,459	180
Overall Average		43.19	0.83	1,329	1,442
Northshore Reclamation
SS20	0.1-0.7	32.2	<0.2	373	1.721
	0.9-1.5	23.2	<0.2	26	625
Overall Average		27.70	<0.2	200	1,173
Siu Ho Wan Reclamation
SS1	0.4-0.9	38	0.5	945	2,085
	0.9-1.5	47	0.8		2,138
	2.9-3.5	46.2	0.8	1,035	2,801
SS16	0.0-0.6	56.7	1.1	2,640	6,975
	0.9-1.5	56.8	1	3,618	5,433
	2.9-3.5	45.9	0.8	1,275	3,118
Depth Average	0.0-0.9	47.4	0.8	1,793	4,530
	0.9-1.9	51.9	0.9	3,618	3,786
	2.9-3.9	46.1	0.8	1,155	2,960
Overall Average		48.43	0.83	2,189	3,758
SDU and Road P1 Reclamations
SS5	0.0-0.6	44.1	0.4	810	2,412
	0.9-1.5	48.5	<0.2	918	2,986
	2.9-3.5	45.9	0.8	1,085	2,783
Overall Average		45.17	0.47	938	2,727
TCT East Reclamation
SS12	0.03-0.63	16.5	<0.2	75	749
SS13	0.0-0.6	60.8	2	936	3,742
	0.9-1.5	33.6	0.5	391	1,435
	2.9-3.5	27.6	0.5	354	1,271
	5.3-5.9	39.8	0.8	743	2,037
SS14	0.3-0.9	52.2	0.9	1,109	2,845
	0.9-1.5	55.5	1.2	898	3,072
	2.9-3.5	24.9	<0.2	240	843
	5.0-5.6	21.5	0.4	76	645
SS15	0.07-0.67	60.6	0.9	1,157	3,502
	0.9-1.4	27.4	0.8	299	872
Depth Average	0.0-0.9	47.5	1.0	819	2,710
	0.9-1.9	38.8	0.8	560	1,793
	2.9-3.9	26.3	0.4	297	1,057
	5.9-6.2	30.7	0.6	410	1,341
Overall Average		35.81	0.70	521	1,725
Notes: (a) w/w = by weight (b) dw = dry weight

Estimation of Potential Gas Emissions

6.14.38 The generation rate of biogas will depend on a number of parameters including concentrations and biodegradability of the organic matters in the sediment, age of the reclamation, redox potential, temperature, moisture content, presence of toxic matters which may inhibit biological activities. These parameters may vary at different depths and locations within the reclamation area and their interactions are complex and difficult to predict. For the purpose of this preliminary assessment, the potential methane risk will be assessed based on the theoretical biogas production rate.

6.14.39 The formation of biogas under anaerobic conditions can be described as a first order degradation process. This process is characterised by high gas generation rates at the early stage of the process, followed by an exponential decrease over the course of time. Although it is difficult to predict the extent of anaerobic conditions, the generation of biogas can be estimated theoretically based on the available data on TOC and the SOD.

6.14.40 Upon completion of reclamation, sediment would be left in situ and estimated volume is shown in Table 6.14a. The depths of the sediments for the undredged areas range from 2 to 26 m (see Table 6.14a). Assuming a conservative siltation rate of 200 mm yr^-1 in the Northshore Lantau and Penny’s Bay areas ^([26]) the ages of sediment in the undredged areas from 2.0 to 26 m below seabed would vary from 10 to 130 years old.

6.14.41 Other sediment and ecological studies ^{([27]) ([28])} have demonstrated that the Redox Potential Discontinuity (RDP) depths (or the oxygenated surface layer) of marine sediment in Hong Kong is generally limited to the top 10 to 30 cm. Anaerobic degradation of the organic matter may occur in sediments which are blow the oxygenated layer. Based on the range of half lives of 0.5 to 5 years, most of the readily degradable organic matters should have been anaerobically degraded and the current methane generation potential of these sediments will be insignificant. If these sediments were not degraded anaerobically for the last 10 to 130 years, there is no reasons to believe that they will after the reclamation. The methane generation potential of the sediment to be left in place will therefore be estimated based on the top 2 m of the sediment.

Estimation of Methane Generation Potential Based on TOC

6.14.42 It is assumed that 50% of the gas produced from anaerobic degradation of organic matter of the sediment is methane and the remainder is carbon dioxide. The degradation process can be represented by the following equation:

2 C₆H₁₂O₆ -------------> 6 CH₄ + 6 CO₂

6.14.43 On this basis, the mass of methane generated from unit mass of TOC can be calculated as follows:

12 C --------------> 6 CH₄ + 6 CO₂

12 x 12 = 144 6 x 16 = 96

6.14.44 The theoretical mass of methane generated will equal to 0.67 times of the mass of TOC in the sediment.

6.14.45 Based on the range of half-lives of 0.5 to 5 years ⁽²⁸⁾, the estimated total theoretical methane generation potential and daily methane flux from the undredged areas are presented in Tables 6.14c and 6.14d, respectively.

Table 6.14c - Calculation of Total Methane Generation Potential from the Undredged Areas Based on TOC

Parameter
Artificial Lake of Water Recreation Centre of Theme Park
Volume of Sediment to be left in situ	240,000 m^3
Assumed Density of sediment	1500 kg m^-3
Average dry matter of the undredged area	57.74% (w/w)
Mass of dry matter	866.06 kg m^-3
Average TOC level of the undredged area	0.83% (w/w) dry matter
Mass of TOC	7.15 kg m^-3
CH4 generation potential ^(a)	4.79 kg m^-3
Total CH4 generation potential	1.15 x 10^6 kg or 1.61 x 10^6 m^3
Penny’s Bay Reclamation Stage II
Volume of Sediment to be left in situ	1,600,000 m^3
Assumed Density of sediment	1500 kg m^-3
Average dry matter of the undredged area	54.71% (w/w)
Mass of dry matter	820.58 kg m^-3
Average TOC level of the undredged area	0.84% (w/w) dry matter
Mass of TOC	6.92 kg m^-3
CH4 generation potential ^(a)	4.64 kg m^-3
Total CH4 generation potential	7.42 x 10^6 kg or 1.04 x 10^7 m^3
Yam O Reclamation
Volume of Sediment to be left in situ	200,000 m^3
Assumed Density of sediment	1500 kg m^-3
Average dry matter of the undredged area	56.81% (w/w)
Mass of dry matter	852.20 kg m^-3
Average TOC level of the undredged area	0.83% (w/w) dry matter
Mass of TOC	7.10 kg m^-3
CH4 generation potential ^(a)	4.76 kg m^-3
Total CH4 generation potential	9.52 x 10^5 kg or 1.33 x 10^6 m^3
Northshore Reclamation
Volume of Sediment to be left in situ	1,300,000 m^3
Assumed Density of sediment	1500 kg m^-3
Average dry matter of the undredged area	72.30% (w/w)
Mass of dry matter	1084.50 kg m^-3
Average TOC level of the undredged area	<0.2% (w/w) dry matter
Mass of TOC	2.17 kg m^-3
CH4 generation potential ^(a)	1.45 kg m^-3
Total CH4 generation potential	1.89 x 10^6 kg or 2.64 x 10^6 m^3
Siu Ho Wan Reclamation
Volume of Sediment to be left in situ	600,000 m^3
Assumed Density of sediment	1500 kg m^-3
Average dry matter of the undredged area	51.57% (w/w)
Mass of dry matter	773.50 kg m^-3
Average TOC level of the undredged area	0.83% (w/w) dry matter
Mass of TOC	6.45 kg m^-3
CH4 generation potential ^(a)	4.32 kg m^-3
Total CH4 generation potential	2.59 x 10^6 kg or 3.63 x 10^6 m^3
SDU and Road P1 Reclamations
Volume of Sediment to be left in situ	180,000 m^3
Assumed Density of sediment	1500 kg m^-3
Average dry matter of the undredged area	54.83% (w/w)
Mass of dry matter	822.50 kg m^-3
Average TOC level of the undredged area	0.47% (w/w) dry matter
Mass of TOC	3.84 kg m^-3
CH4 generation potential ^(a)	2.57 kg m^-3
Total CH4 generation potential	4.63 x 10^5 kg or 6.48 x 10^5 m^3
Theme Park (Phase III) Extension Reclamation
Volume of Sediment to be left in situ	1,600,000 m^3
Assumed Density of sediment	1500 kg m^-3
Average dry matter of the undredged area ^(b)	59.45% (w/w)
Mass of dry matter	891.68 kg m^-3
Average TOC level of the undredged area ^(b)	0.77% (w/w) dry matter
Mass of TOC	6.86 kg m^-3
CH4 generation potential ^(a)	4.60 kg m^-3
Total CH4 generation potential	7.36 x 10^6 kg or 1.03 x 10^7 m^3
TCT East Reclamation
Volume of Sediment to be left in situ	1,480,000 m^3
Assumed Density of sediment	1500 kg m^-3
Average dry matter of the undredged area	64.19% (w/w)
Mass of dry matter	962.78 kg m^-3
Average TOC level of the undredged area	0.70% (w/w) dry matter
Mass of TOC	6.67 kg m^-3
CH4 generation potential ^(a)	4.49 kg m^-3
Total CH4 generation potential	6.64 x 10^6 kg or 9.03 x 10^6 m^3
Notes: (a) The theoretical mass of methane generated equals to 0.67 times of the mass of TOC in the sediment. (b) Average between Penny’s Bay Reclamation Stage II and TCT East Reclamation is used.

Table 6.14d-1 Page 72

Table 6.14d-2 Page 73

Table 6.14d-3 Page 74

Table 6.14d-4 Page 75

Table 6.14d-5 Page 76

Table 6.14d-6 Page 77

Table 6.14d-7 Page 78

Table 6.14d-8 Page 79

6.14.46 The higher flux rate, corresponding to a half life of decay of 0.5 years, is not significant in terms of future development because after two years over 95% of the biodegradable organic matter would have been decomposed and the potential methane flux would have fallen proportionally to a rate less than that of the lower figure (corresponding to a half life of decay of 5 years) after the same time. Therefore, the half life of decay of 5 years represents the worse case after two years.

6.14.47 The above calculation is based on a number of broad assumptions which might affect the precision of the estimates. It assumes that all the organic matter is readily degradable (worse case scenario) and it takes no account of biological oxidation of methane that would probably occur in the upper layers (aerobic zone) of the fill. Oxidation efficiencies of 2% to 100% ⁽²⁹⁾ were reported in the Tsuen Wan Bay Further Reclamation Study. High oxidation efficiencies will occur when the fill material is well aerated and the gas is able to emit uniformly over the surface area. While low oxidation efficiencies will occur when the fill material is poorly permeable for gases and when the gas generation rate is rather high or when surface is low permeability e.g. hard surface such as roads, buildings. However, it is difficult to predict the likely oxidation efficiency for the reclamation area.

Estimation of Methane Generation Potential Based on SOD

6.14.48 The amount of methane gas produced per kg of SOD can be estimated as follows:

6.14.49 Assume that the starting compound is glucose (C₆H₁₂O₆), the conversation of glucose to carbon dioxide and methane under anaerobic conditions can be represented by the following balanced equation:

C₆H₁₂O₆ ------------> 3 CO₂ + 3 CH₄

180 g 132 g 48 g

6.14.50 It should be noted that although the glucose has been converted to carbon dioxide and methane, the methane generation potential may be estimated by the oxygen requirement for complete conversion of the glucose to carbon dioxide and water under aerobic conditions.

6.14.51 The amount of methane formed per kg of SOD can be represented in the following balanced equation for the oxidation of glucose to carbon dioxide and water.

C₆H₁₂O₆ + 6 O₂ --------------> 6 CO₂ + 6 H₂O

180 g 192 g

6.14.52 The SOD of glucose is (192/180) kg, and 1 kg of glucose yields (48/180) kg of methane, so that the ratio of the amount of methane produced per kg of SOD is:

(kg CH₄/kg SOD) = {(48/180)/(192/180)} = 0.25

6.14.53 Therefore, for each kg of SOD, 0.25 kg of methane will be formed.

6.14.54 The volume equivalent of the 0.25 kg methane produced from the equivalent of 1 kg of SOD is:

Vol_CH4 = (0.25 kg) * (10³ g kg^-1) * (1 mol/16 g) * (22.4 l mol^-1) * (10^-3 m³ l^-1)

= 0.35 m³ CH₄ (at standard conditions of temperature and pressure)

6.14.55 Therefore, 0.35 m³ of methane will be generated per kg of SOD converted.

6.14.56 SOD will also include the nitrogenous oxygen demand (ie the oxidation of nitrogen and ammonia), but in the estimation of the methane generation potential it is assumed that the oxygen demand is entirely carbonaceous demand. This represents a conservative approach and the actual methane generation potential is expected to be lower.

6.14.57 The average of SOD_5days levels of the sediment samples as presented in Table 6.14a. The theoretical methane generation potential (in m³ CH₄ kg^-1 dry weight of sediment) will be:

= average SOD_5days (in mg SOD kg^-1 dry weight of sediment) x 10^-6 kg mg^-1 x 0.35 m³CH₄ kg SOD^-1

Table 6.14e - Calculation of Total Methane Generation Potential of the Undredged Areas Based on SOD

Parameter
Artificial Lake of Water Recreation Centre of Theme Park
Volume of Sediment to be left in situ	240,000 m^3
Assumed Density of sediment	1500 kg m^-3
Average dry matter of the undredged area	57.74% (w/w)
Mass of dry matter	866.06 kg m^-3
Total dry matter of sediment	1.47 x 10^9 kg
SOD5days in mg O2 kg^-1 dry weight of sediment	2396 mg O2 kg^-1
Total SOD in sediment	4.98 x 10^5 kg
Total CH4 generation potential	1.74 x 10^5 m^3CH4
Penny’s Bay Reclamation Stage II
Volume of Sediment to be left in situ	1,600,000 m^3
Assumed Density of sediment	1500 kg m^-3
Average dry matter of the undredged area	54.71% (w/w)
Mass of dry matter	820.58 kg m^-3
Total dry matter of sediment	8.21 x 10^9 kg
SOD5days in mg O2 kg^-1 dry weight of sediment	2493 mg O2 kg^-1
Total SOD in sediment	3.27 x 10^6 kg
Total CH4 generation potential	1.15 x 10^6 m^3CH4
Yam O Reclamation
Volume of Sediment to be left in situ	200,000 m^3
Assumed Density of sediment	1500 kg m^-3
Average dry matter of the undredged area	56.81% (w/w)
Mass of dry matter	852.20 kg m^-3
Total dry matter of sediment	1.11 x 10^9 kg
SOD5days in mg O2 kg^-1 dry weight of sediment	1442 mg O2 kg^-1
Total SOD in sediment	2.46 x 10^5 kg
Total CH4 generation potential	8.60 x 10^4 m^3CH4
Northshore Reclamation
Volume of Sediment to be left in situ	1,300,000 m^3
Assumed Density of sediment	1500 kg m^-3
Average dry matter of the undredged area	72.30% (w/w)
Mass of dry matter	1084.50 kg m^-3
Total dry matter of sediment	8.78 x 10^9 kg
SOD5days in mg O2 kg^-1 dry weight of sediment	1173 mg O2 kg^-1
Total SOD in sediment	1.65 x 10^6 kg
Total CH4 generation potential	5.79 x 10^5 m^3CH4
Siu Ho Wan Reclamation
Volume of Sediment to be left in situ	600,000 m^3
Assumed Density of sediment	1500 kg m^-3
Average dry matter of the undredged area	51.57% (w/w)
Mass of dry matter	773.50 kg m^-3
Total dry matter of sediment	1.78 x 10^9 kg
SOD5days in mg O2 kg^-1 dry weight of sediment	3758 mg O2 kg^-1
Total SOD in sediment	1.74 x 10^6 kg
Total CH4 generation potential	6.10 x 10^5 m^3CH4
SDU and Road P1 Reclamations
Volume of Sediment to be left in situ	180,000 m^3
Assumed Density of sediment	1500 kg m^-3
Average dry matter of the undredged area	54.83% (w/w)
Mass of dry matter	822.50 kg m^-3
Total dry matter of sediment	5.55 x 10^8 kg
SOD5days in mg O2 kg^-1 dry weight of sediment	2727 mg O2 kg^-1
Total SOD in sediment	4.04 x 10^5 kg
Total CH4 generation potential	1.41 x 10^5 m^3CH4
Theme Park (Phase III) Extension Reclamation
Volume of Sediment to be left in situ	1,600,000 m^3
Assumed Density of sediment	1500 kg m^-3
Average dry matter of the undredged area ^(b)	59.45% (w/w)
Mass of dry matter	891.68 kg m^-3
Total dry matter of sediment	7.85 x 10^9 kg
SOD5days in mg O2 kg^-1 dry weight of sediment ^(b)	2109 mg O2 kg^-1
Total SOD in sediment	3.01 x 10^6 kg
Total CH4 generation potential	1.05 x 10^6 m^3CH4
TCT East Reclamation
Volume of Sediment to be left in situ	1,480,000 m^3
Assumed Density of sediment	1500 kg m^-3
Average dry matter of the undredged area	64.19% (w/w)
Mass of dry matter	962.78 kg m^-3
Total dry matter of sediment	7.12 x 10^9 kg
SOD5days in mg O2 kg^-1 dry weight of sediment	1725 mg O2 kg^-1
Total SOD in sediment	2.46 x 10^6 kg
Total CH4 generation potential	8.60 x 10^5 m^3CH4
Notes: (a) 1 kg SOD = 0.35 m^3 CH4 (b) Average between Penny’s Bay Reclamation Stage II and TCT East Reclamation is used.

6.14.58 The total methane generation potential calculated based on SOD results range from 6% to 22%(with an average value of 14%) of that calculated from the TOC results as shown in Table 6.14f. The rate of methane generation predicted from the SOD results would be expected to be much lower than that calculated from the TOC data because TOC is a measure of the total organic carbon whereas some of it may not be biodegradable.

Table 6.14f-1 Page 84

Table 6.14f-2 Page 85

Table 6.14f-3 Page 86

Table 6.14f-4 Page 87

Table 6.14f-5 Page 88

Table 6.14f-6 Page 89

Table 6.14f-7 Page 90

Table 6.14f-8 Page 91

Assessment of Methane Hazard of Developments on the undredged areas

Results of Sediment Testing

6.14.59 As discussed in Section 6.14.6, the estimated rate of methane generation (based on TOC results) for the undredged areas after 2 years following reclamation are summarised in Table 6.14g.

Table 6.14g - Estimated Theoretical Gas Emission Rates of the Undredged Areas Based on TOC

Reclamation	Methane Generation Rate (l m^-2 yr^-1)	Gas Emission Rate (l m^-2 d^-1)
Artificial Lake of Water Recreation Centre in Theme Park	1,315	3.60
Penny’s bay Reclamation Stage II	1,274	3.49
Yam O Reclamation	1,307	3.58
Northshore Reclamation	399	1.09
Siu Ho Wan Reclamation	1,186	3.25
SDU and Road P1 Reclamations	706	1.94
Theme Park (Phase III) Extension ^(a)	1,263	3.46
TCT East Reclamation	1,233	3.38
Note: (a) Average between Penny’s Bay Reclamation Stage II and TCT East Reclamation is used.

6.14.60 If it is based on the SOD results, the estimated rate of methane for the undredged areas after 2 years following reclamation are summarised in Table 6.14h.

Table 6.14h - Estimated Theoretical Gas Emission Rates of the Undredged Areas Based on SOD

Reclamation	Methane Generation Rate (l m^-2 yr^-1)	Gas Emission Rate (l m^-2 d^-1)
Artificial Lake of Water Recreation Centre in Theme Park	142	0.39
Penny’s bay Reclamation Stage II	140	0.38
Yam O Reclamation	84	0.23
Northshore Reclamation	87	0.24
Siu Ho Wan Reclamation	200	0.55
SDU and Road P1 Reclamations	154	0.42
Theme Park (Phase III) Extension ^(a)	129	0.35
TCT East Reclamation	114	0.31
Note: (a) Average between Penny’s Bay Reclamation Stage II and TCT East Reclamation is used.

6.14.61 The potential methane fluxes estimated based on TOC results for the undredged areas (top 2 m) range from 1.09 to 3.60 l m^-2 d^-1 which are within the WMP26A risk assessment criterion of 3.6 l m^-2 d^-1 and well within the suggested risk assessment criterion of 10 l m^-2 d^-1.

6.14.62 The potential methane fluxes estimated using SOD results, which measures the biodegradable portion of the organic matters in the sediment provide a more realistic assessment comparing with those estimated using TOC results. As shown in Table 6.14h, the potential methane fluxes for the undredged areas (top 2 m) estimated using SOD results are well within the WMP26A risk assessment criterion of 3.6 l m^-2 d^-1, the suggested risk assessment criterion of 10 l m^-2 d^-1, and the ‘absolute’ cut-off level of 84.7 l m^-2 d^-1 for the development to be built in areas where methane gas may be generated suggested by the London Scientific Services.

6.14.63 As discussed in Section 6.14.3, the safe emission rate, developed from WMP26A, is based on a number of assumptions about the size and rate of ventilation of ‘at risk’ rooms or voids as well as the general nature of the surface of the ground from which the gas is being generated. It is therefore necessary to consider the particular type of development being planned for the undredged areas and what ‘at risk’ features are likely to be present in order to assess the likelihood of methane emissions presenting a significant risk.

Features of the Undredged Areas

6.14.64 Although there is no detail design on the proposed development, the proposed land used is indicated in Section 2.

6.14.65 Biogas emitted from the organic sediment beneath the reclamation/impermeable membrane will tend to pose limited risk to many of the multi-storey tower block type of tourism related development, residential developments or school because, although no detail design is available typically, these do not have any below ground rooms and for residential use, often have car parking on lower levels with the residential apartments above a podium level. However, each block may have a few ‘at risk’ rooms, the vulnerability of which will depend on the exact details of construction, size ventilation and use. Transformer rooms and refuse collection rooms, for example, are typically located at ground level but are usually provided with a relatively high rate of mechanical ventilation which will serve to disperse and dilute any gas which happens to enter such rooms. Other at risk rooms and voids will include lift pits and utility (services) voids within and around individual tower blocks.

Conclusions

6.14.66 Given that, at this stage, it is not possible to measure the rates of gas emission from the organic sediment within the area of the proposed artificial lake of Water Recreation Centre in the Theme Park, Penny’s Bay Reclamation Stage II, Yam O Reclamation, Northshore Reclamation, Siu Ho Wan Reclamation, SDU Reclamation, Road P1 Reclamation, Theme Park (Phase III) Extension Reclamation and TCT East Reclamation (hereafter refer as undredged areas), an estimate of the future rate of gas generation has been made from the results of analysis of the sediment for TOC and SOD.

6.14.67 Several assumptions and estimations have to be made when making theoretical predictions about possible future rates of methane generation at the undredged areas. The estimated potential rates of methane gas generation based on both TOC and SOD are within the maximum safe rate (3.6 l m^-2 d^-1) of methane emission from landfill sites which is specified by the UK Department of Environment in WMP26A as indicating that such sites can be regarded as no longer posing a threat due to gas and are safe to be developed. They are well within the suggested maximum rate of methane emission per unit area of 10 l m² d^-1, and the limit of 84.7 l m² d^-1 recommended by the London Scientific Services. The former criterion provides a reasonable general guide for determining whether the rates of methane emission pose an unacceptable risk to unrestricted development on a potentially gassing site. The latter criterion represents the absolute ‘cut-off’ level of methane flux which developments should be allowed to build on the potentially gassing site.

6.14.68 Overall, based on the results of the sediment analysis and comparison with published guidance on safe levels of gas emissions, it is considered the predicted rates of gas generation from the undredged areas are within the range which is considered as ‘likely be safe’ and will not constraint the developments on top of the reclamation areas.

6.14.69 Given the inherent uncertainties involved in estimating future rates of gas emissions from theoretical calculations of rates of gas generation and given that mitigation measures for avoiding the potential risks may be very expensive, it would be of benefit to select one of the undredged areas to undertake monitoring of gas emission rates following the reclamation works to confirm the findings of this assessment. It is recommended that the monitoring boreholes should be installed in areas where the predicted methane flux is high (relatively) and at areas to be reclaimed first so that the monitoring results will be available as soon as possible. Should the monitoring results will represent the worst case scenario and show that biogas may be a problem, further monitoring boreholes could then be installed at other reclamation areas. Based on these criteria (see Tables 6.5a, 6.14g and 6.14h), it is recommended that monitoring boreholes should be installed at the Water Recreation Centre of the Theme Park.

6.14.70 As it is not practicable to install monitoring wells over the impermeable membrane of the artificial lake due to potential leakage of water into monitoring wells, it is recommended that monitoring wells should install in the edge of the artificial lake of Water Recreation Centre where sediment will be left in situ (see Figure 6.14b).

Recommendations

Monitoring

6.14.71 It may take some time for fully anaerobic conditions to be established within the organic sediment and for a reservoir of gas to accumulate within the reclamation fill so monitoring should be undertaken for as long as possible. Ideally, monitoring should be undertaken for a period of at least one year.

6.14.72 If there is only limited time between completion of the reclamation and commencement of construction at Water Recreation Centre, monitoring should commence immediately upon completion of the reclamation so that any trends may be observed and results extrapolated to the period of construction and occupation of the development.

6.14.73 Monitoring should be undertaken via purposely installed monitoring wells installed 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 should be installed. These should be fitted with a removable cap and gas monitoring valve so that gas concentrations may be measured as well as flow rates from the open well.

6.14.74 It is recommended that two monitoring wells should be installed across the area of the Water Recreation Centre (Figure 6.14b). These should be located such as to give an approximately even distribution across the whole area. If they are located within areas designated for open space, it may be possible to retain them for future long-term monitoring. The wells should be monitored as follows:

· concentrations of the following gases should be measured using portable monitoring equipment with gas chromatographic (GC) analysis being undertaken on selected samples to confirm the results:

· methane

· carbon dioxide

· oxygen

· gas flow rates from the open wells - very sensitive techniques (such as micro-anemometer) will need to be used to measure the anticipated very low flow rates. In addition, and if practical, emissions from the surface of the reclaimed land could also be monitored using flux boxes.

6.14.75 It will be important to monitor gas flows from the wells under different meteorological conditions and to include some occasions when atmospheric pressure is falling quite quickly (e.g. immediately proceeding a typhoon).

6.14.76 This analysis and assessment of the results should be undertaken by suitably qualified ad experience professionals who are familiar with the Properties of biogas.

Protection Measures

General Guidelines

6.14.77 At this stage it is difficult to provide precise guidelines on what measures would be required for the specific rates of gas emission which may measured because this would depend on the exact pattern of the results and the design/ purpose of the specific buildings to be erected. However, the following criteria provide the may be used as general guidelines:

Scenario 1

6.14.78 If rates of methane emission are consistently much less than the trigger value (10 l m^-2

d^-1), including monitoring occasions when atmospheric pressure is falling quite quickly, and they do not show any rising trend over time, then the buildings will not require gas protection measures.

6.14.79 The trigger value of 10 litres (methane) m^-2 d^-1 is an "area" emission rate (ie 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 - mainly how easy it is for gas to escape from the surface of the site. Thus, for a site covered in a hard surface (eg paved) it would be expected that any borehole would have a much greater area of influence than if the site had soft landscaping.

6.14.80 Different people have assumed different areas of influence - in WMP 26A, it assumed 100 m² whereas in the South East Kowloon Development Study, the consultant assumed approximately 20 m² (radius of 2.5m). To be conservative, it is proposed to adopt an area of influence of 20m², which would give:

· Trigger value of 10 l m^-2 d^-1 x 20 m² = 200 l d^-1 emitted from the borehole

6.14.81 In practice, such low rates of gas emission will be measured using 'flux box' methods as the velocity of the gas (m s^-1) will be too low to measure, unless very sensitive flow meter is used. Thus, expressing the flow rate as a volume per day or volume per hour will be acceptable. Hence, the criterion for safe flow rate from the free venting boreholes becomes:

· Flow rate of methane (in term of l d^-1) < 200 l d^-1

or

· (Gas flow rate in term of l d^-1) x (concentration of methane in gas (in % gas)) < 200 l d^-1

Scenario 2

6.14.82 If the rates of methane emission from any borehole frequently exceed the trigger value or show 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 when:

· (Gas flow rate in term of l d^-1) x (concentration of methane in gas (in % gas)) > 200 l d^-1

6.14.83 The exact details of the gas protection measures would need to be designed to take into account the design and use of the particular buildings involved but would, most probably, include the installation of a low gas permeability membrane in the floor slab of the building. The exact area of the reclamation over which buildings would need to have gas protection measures would depend on the pattern of the results from the different monitoring boreholes and 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 the way in which buildings may be protected against the impacts of gases derived from the ground.

Scenario 3

6.14.84 If there are occasional exceedances of the methane emission rate trigger value or if there is significant fluctuation of the results obtained with some readings coming close to the trigger value, then the exact pattern and any trends in the results will need to be assessed to determine their significance and whether any building protection measures are required. It might be necessary to undertake additional monitoring by extending the monitoring period, for example, if an apparently spurious high reading is noted towards the end of the monitoring period or if it seems likely that future rates of emission may exceed the trigger value.

6.14.85 Whatever the results obtained from the proposed monitoring of gas emission rates, 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 the way in which buildings may be protected against the impacts of gases derived from the ground.

Scenario 4

6.14.86 If the rates of methane emission from any borehole frequently exceed the Limit value (84.7 l m^-2 d^-1), or show a rising trend such that future emission rates are likely to exceed the limit value, then no buildings should be constructed on that part of the site. That is when:

· Limit value of 84.7 l m^-2 d^-1 x 20 m² = 1,694 l d^-1 emitted from the borehole

or

· (Gas flow rate in term of l d^-1) x (concentration of methane in gas (in % gas)) > 1,694 l d^-1

Typical /Generic Protection Measures

6.14.87 Depending on the results of the monitoring recommended above, it may be necessary (although at this stage it is considered that it is unlikely to be required) to incorporate a number of general protection measures into the design of the redevelopment and to include specific measures in the design of individual buildings. Specific details cannot be provided until the results of this monitoring and exact details of individual building designs are available. A combination of different measures may be used for protecting the development against possible risks due to biogas and discussions would need to be held with the developers and architects to determine which are the most appropriate.

6.14.88 Typical, generic, protection measures which may be employed (depending on the results of the monitoring and exact building designs) include the following:

6.14.89 Ventilation of Gas from the Reclaimed Land: Vertical boreholes/wells could be installed across the area of the reclamation to allow gas to vent to atmosphere thereby preventing the build-up of gas pressures within the fill material. Care would be needed in the design of the venting stacks to ensure that any gas was safely vented without causing nuisance or danger to users of the land. This technique is probably appropriate only if the monitoring indicates a relatively high rate of gas emission.

6.14.90 More specific areas of the development could be targeted such as the footprints of specific buildings or other sensitive areas. In this case the ventilation measures would be designed specifically to prevent the build up of gas pressure beneath a building, for example, and could comprise a network of horizontal gas collection pipes installed underneath the building connected to a number of risers to allow any accumulated gas to be dispersed to atmosphere at a suitable level above ground.

6.14.91 Barriers to Prevent Ingress of Gas into Buildings: There are a number of ways of ensuring that any gas derived from the ground does not enter a building. Typically these involve the incorporation of some kind of low gas permeability membrane in the design of the floor and any below ground walls of any ‘at risk’ rooms. In addition, measures must be taken to avoid or seal any openings in the floor (e.g. at service 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.

6.14.92 There are various proprietary products which may be used and the specific details of their application will depend very much on the individual building design.

6.14.93 In all cases, extreme care is needed in the installation of the membrane and the other protection measures to ensure that they meet the design requirements and to avoid damage during installation and subsequent construction works. Adequate QA/QC procedures are therefore essential to ensure that such measures are effective.

6.14.94 Ventilation within Buildings: As an additional or alternative measure for the protection of specific rooms, ventilation (passive or mechanical depending on the circumstances) may be provided to ensure that if any gas enters the room it is dispersed and cannot accumulate in dangerous volumes. 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 permeability membrane whereas for low sensitivity areas ventilation may be used on its own.

6.14.95 Protection of Below Ground Services: As they are installed below ground, conduits in which services (e.g. electricity and other cables) are installed are particularly prone to the ingress and accumulation of gas derived from the ground. It is therefore important to prevent such conduits acting as easy routes by which gas may enter buildings by avoiding, as far as possible, any penetration of floor slabs by such services or, as a minimum, ensuring that any unavoidable penetrations are carefully sealed using an appropriate low permeability sealant.

6.14.96 In addition, accumulation of gas within any associated manholes or access chambers can present a risk to the staff of the utility companies. All such companies and organisations which may have cause to work on such below ground infrastructure should be warned of the potential dangers and advised to take appropriate precautions. Clear warning notices together with contact details for further information and advice should be provided on any access points to below ground chambers.

Precautions During Construction

6.14.97 Similarly, 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 any 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.

6.14.98 The exact requirements for precautionary measures during the construction phase would need to be specified following assessment of the results obtained from the monitoring recommended above and the details would depend on the depth of excavation or nature of the voids/structures involved.

6.14.99 Further information and advice, including the precautions required for the drilling of the monitoring boreholes, may be found in the Hong Government’s advisory document Landfill Gas Hazard Assessment Guidance Note prepared by the Environmental Protection Department.