6                    BIOGAS RISK

6.1              Introduction

6.1.1          Organically enriched material will to be left in situ at the WDII reclamation within the western and eastern corners of the Causeway Bay Typhoon Shelter.  As methane gas could be generated under anaerobic conditions, there is a potential for this gas to be released either during construction or after development of the reclaimed area.

6.1.2          In view of the exceedence of the recommended maximum “safe” rate (10 L m-2 per day) of methane emission (derived from Waste Management Paper No. 26A) under the worst case scenario of 100% biodegradable total organic carbon (TOC) and the identification of ‘at risk’ rooms at the proposed developments at the western and eastern corners of the typhoon shelter reclamation, it is recommended that monitoring of gas emission rates should be undertaken following the completion of the reclamation area at the proposed developments.  The review of the gas monitoring results would determine the need for 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.  Typical gas protection measures are described in Section 6.5 for both the ground level and below ground structures at the developments.

6.1.3          With the incorporation of the recommended gas protection measures in the design of the developments, if found to be necessary, together with the implementation of the other recommended precautionary measures, the risk to people and property due to biogas emissions from the WDII reclamation is considered to be low.  The proposed mitigation measures, if necessary, should be examined further at the detailed design stage with regard to the specific design details of individual buildings and the confirmed land use.

6.2              Borehole Monitoring

6.2.1          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.  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.  During the drilling of boreholes, the safety and working procedures described in the Landfill Gas Hazard Assessment Guidance Note (EPD, 1997) should be followed.

6.2.2          Concentrations of methane gas should be measured using portable gas monitoring instruments, as described in Section 6.4.1.  Fluxes should also be measured if the emission velocities are not too low.  It is recommended that monitoring should be undertaken monthly for a period of at least one year prior to the commencement of construction works on the reclamation.  It is also recommended that gas flow rates from the wells should be monitored under different meteorological conditions and to include some occasions when atmospheric pressure is falling quite quickly (for example, immediately preceding a typhoon).

6.3              Monitoring Locations

6.3.1          The proposed locations for borehole monitoring are shown in Figure 6.1.  BH1 is located at the western corner of the existing Causeway Bay Typhoon Shelter where a 5-star hotel is planned.  BH2 is located at the proposed open space area at the west of the Causeway Bay Typhoon Shelter where continuous monitoring throughout the construction period is possible.  BH3 is located at the eastern corner of the existing Causeway Bay Typhoon Shelter where an entertainment complex is planned.

6.3.2          Should more intensive monitoring are required for detailed analysis, it is recommended that further monitoring wells should be located in areas designated for open space as it may be possible to continue monitoring at these locations throughout the construction period.  Monitoring wells should be located away from the areas dredged for the permanent stormwater culvert (as shown in Figure 4.2) and the temporary channel / culvert construction.

6.4              Monitoring Equipment

6.4.1          Monitoring shall be carried out using intrinsically safe, portable gas monitoring instruments. The gas monitoring instrument shall:

·         be capable of continuous monitoring of methane;

·         be capable of continuous barometric pressure and gas pressure measurements;

·         be capable of monitoring temperature of the gas;

·         where possible, comply with BS6020 and be approved by BASEEFA as intrinsically safe, suitable for use in a Zone 2 area to BS5345;

·         normally operate in diffusion mode unless required for spot sampling, when it should be capable of operating by means of an aspirator or pump;

·         display any parameters monitored by clear unambiguous readings given on an alpha numeric display LCD screen with wide angle viewing;

·         have low battery, fault and over range indication incorporated;

·         store monitoring date, and shall be capable of being down-loaded directly to a personal computer;

·         measure in the following ranges:

-     methane                                             0 - 100% Lower Exposure Limit and 0 - 100% v/v

-     barometric pressure                            mBar (absolute)

-     gas pressure (relative to atmospheric)  Pascals atmospheric

-     temperature                                       0 – 100 °C

·         have removable and rechargeable batteries with more than 12 hours continuous operating life;

·         have back-up batteries; and

·         have an oxygen sensor with a life of not less than twelve months and other sensors shall have a life of more than two years before deterioration in performance of the sensor.

6.4.2          To measure the gas flow rates from the open monitoring wells, very sensitive techniques (such as micro-anemometer) will need to be used to measure the anticipated very low flow rates.

6.4.3          The gas monitoring equipment shall be calibrated and maintained in accordance with the manufacturer’s recommendations for calibration and maintenance.

6.5              Mitigation Measures

6.5.1          The mitigation measures recommended in the EIA Report are described below, and comprise precautionary gas protection measures to be incorporated in the design of the developments, precautions to be taken during construction works on the reclamation, and measures to be taken prior to entry of any below ground services or confined space within the reclamation site.  The implementation schedule of the recommended mitigation measures is presented in Appendix A.

Precautionary Gas Protection Measures

General Guidelines

6.5.2          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 Waste Management Paper No. 26A on Landfill Completion is proposed to be adopted as the trigger value.

Scenario 1

6.5.3          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.

6.5.4          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 that 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.

6.5.5          To be conservative, it is proposed to adopt an area of influence of 20 m2 (radius of 2.5m)([1]), which would give:

·            Trigger value of 10 L m-2 per day x 20 m2  =  200 L per day emitted from the borehole

6.5.6          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

6.5.7          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.

6.5.8          The type of gas protection measures would be dependent on the design and use of the particular building.  A possible measure is the incorporation of a low gas permeability membrane in the floor slab of the building.  Further investigation may be required to determine the area of land that 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

6.5.9          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

6.5.10      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.

6.5.11      Depending on the monitoring results, it may be necessary to incorporate a number of gas protection measures into the design of the proposed development.  Specific details cannot be provided until the results of the monitoring are available, and the proposed landuse and building design are known and confirmed.  A combination of different measures may be used for protecting both the ground level and underground structures at the development against possible risks due to biogas emissions.  Discussions would need to be held with the developer and architects to determine the protection measures which are the most appropriate and feasible.  Typical gas protection measures that may be adopted are described below.

Measures to Prevent Ingress of Gas into ‘At Risk’ Rooms

6.5.12      To prevent the ingress of methane gas into a building, a low gas permeability membrane may be incorporated in the design of the floor and any below ground walls of identified ‘at risk’ rooms (for examples, rooms housing electrical equipment, pumps or switchgear).  In addition, measures should be taken to avoid or seal any openings in the floor (for example, at services entry points).  Such techniques are commonly used where there is a risk of landfill gas entering a building and have been employed on a number of developments in Hong Kong.

6.5.13      There are various proprietary products available in the market and the specific details of their application will depend on the detailed design of the ‘at risk’ rooms.  Possible measures include gas-resistant polymeric membranes that can be incorporated into the floor or wall construction as a continuous sealed layer.  Membranes should be able to demonstrate low gas permeability and resistance to possible chemical attack.  Other building materials such as dense well-compacted concrete or steel shuttering also enhance resistance to gas permeation.  In all cases, extreme care is needed during the installation of the membrane and subsequent construction works to avoid damage to the membrane. 

Ventilation within ‘At Risk’ Rooms

6.5.14      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. 

6.5.15      The basement car park proposed at the development would be susceptible to ingress and accumulation of any biogas emissions from the reclamation.  The basement car park ventilation system will be designed to ensure that the car park air quality guidelines given in ProPECC PN 2/96 Control of Air Pollution in Car Parks are achieved.  The minimum ventilation rate for a basement car park is 5 to 6 air changes per hour in order to comply with the EPD requirement on carbon monoxide concentrations within car parks.

6.5.16      It is recommended that several ventilation systems should be installed and evenly distributed within the basement car park.  Therefore, even during equipment failure, it is unlikely that the entire exhaust system would break down.  To cater for the situation of power failure, it is recommended that a back-up power supply shall be provided for the ventilation system so that certain designated exhaust systems would still operate.  Under normal conditions, the power failure should be rectified within a few hours.

Protection of Utilities or Below Ground Services

6.5.17      Below ground ducts or trenches for the installation of utilities or services (for example, 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

6.5.18      Special care must be taken during the first two years of construction activities on the reclamation.  Sub-surface excavations into the mud layers might encounter gas occasionally, but not at levels likely to be dangerous provided that the gas vents freely to atmosphere.  Emission rates are unlikely to be sufficient to sustain a flame.  These gas bubbles will only occur for short periods, and therefore, as a precaution, smoking and naked flames in the vicinity of drilling activities and excavations of 1 m depth or more should be prohibited.

6.5.19      Precautions may be required to ensure that there is no risk due to the accumulation of gas within any temporary structures, such as site offices, during construction works on the reclamation area.  It may be necessary, for example, to raise such structures slightly off the ground so that any gas emitted from the ground beneath the structure may disperse to atmosphere rather than entering the structure.  A minimum clear separation distance of 500 mm, as measured from the highest point on the ground surface to the underside of the lowest floor joist, is recommended in the Landfill Gas Hazard Assessment Guidance Note, EPD (1997).

Precautions Prior to Entry of Below Ground Services

6.5.20      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.

6.5.21      The working practices should follow the Landfill Gas Hazard Assessment Guidance Note, EPD (1997) guidelines as follows:

·         Any chamber, manhole or culvert that is large enough to permit access to personnel should be subject to entry safety procedures.  Such work in confined spaces is controlled by the Factories and Industrial Undertakings (Confined Spaces) Regulations of the Factories and Industrial Undertakings Ordinance.  Following the Safety Guide to Working in Confined Spaces ensures compliance with the above regulations.

·         The entry or access point should be clearly marked with a warning notice (in English and Chinese) which states that there is the possibility of flammable and asphyxiating gases accumulated within.

·         The warning notice should also give the telephone number of an appropriate competent person who can advise on the safety precautions to be followed before entry and during occupation of the manhole.

·         Personnel should be made aware of the dangers of entering confined spaces potentially containing hazardous gases and, where appropriate, should be trained in the use of gas detection equipment.

·         Prior to entry, the atmosphere within the chamber should be checked for oxygen, methane and carbon dioxide concentrations.  The chamber may then only be entered if oxygen is greater than 18% by volume, methane is less than 10% of the Lower Explosive Limit, which is equivalent to 0.5% by volume (approximately), and carbon dioxide is less than 0.5% by volume.

·         If either carbon dioxide or methane is higher, or oxygen lower than the values given above, then entry to the chamber should be prohibited and expert advice sought.

·         Even if conditions are safe for entry, no worker should be permitted to enter the chamber without having another worker present at the surface.  The worker who enters the chamber should wear an appropriate safety / recovery harness and, preferably, should carry a portable methane, carbon dioxide and oxygen meter.

6.5.22      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.



([1])   ERM Hong Kong Ltd. (March 2000).  Agreement No. CE 70/97, Green Island Development Engineering Investigation and Planning Review.  Environmental Impact Assessment Review - Biogas Assessment (Final).