TABLE OF CONTENT

10 HAZARD TO LIFE ASSESSMENT. 10-1

10.1 Introduction. 10-1

10.2 Environmental Legislation, Standards and Criteria. 10-1

10.3 Study Objectives and Methodology. 10-2

10.4 Description of Surroundings. 10-4

10.5 Hazard Identification. 10-7

10.6 Frequency Analysis. 10-12

10.7 Consequence and Impact Analysis. 10-19

10.8 Risk Evaluation. 10-21

10.9 Recommendations. 10-23

10.10 Environmental Monitoring and Audit Requirement 10-24

10.11 Conclusion. 10-24

10.12 Reference. 10-24

List of Tables

Table 10.1 Land and Building Population Considered for Hazard Assessment 10-4

Table 10.2 Traffic Population Considered for Hazard Assessment 10-5

Table 10.3 Time Modes Adopted for Hazard Assessment 10-5

Table 10.4 Daytime Weather Class-Wind Direction Frequencies at Lau Fau Shan Weather Station. 10-6

Table 10.5 Night-time Weather Class-Wind Direction Frequencies at Lau Fau Shan Weather Station. 10-6

Table 10.6 Composition and Properties of Biogas from Anaerobic Digestion Process. 10-9

Table 10.7 Summary of Biogas or Methane Incidents. 10-9

Table 10.8 Possible Hazardous Scenarios and Hazardous Outcomes of the Organic Waste Co-digestion Facility at YLSEPP. 10-12

Table 10.9 Summary of Spontaneous Failures Frequencies. 10-13

Table 10.10 Hong Kong International Airport Civil International Air Transport Movements of Aircraft 10-14

Table 10.11 Calculation for Aircraft Crash Frequency. 10-15

Table 10.12 Accident Involvements of Medium / Heavy Goods Vehicles in Hong Kong. 10-16

Table 10.13 Summary of Base Event Frequencies. 10-17

Table 10.14 Assumptions used in Fault Tree Analysis. 10-18

Table 10.15 Ignition and Explosion Probabilities for Gas Releases. 10-18

Table 10.16 End Point Criteria for Vapour Cloud Explosions. 10-21

List of Diagrams

Diagram 10.1	Societal Risk Guidelines for Acceptable Risk Levels
Diagram 10.2	Schematic Diagram of QRA Process
Diagram 10.3	Aircraft Crash Coordinate System
Diagram 10.4	Individual Risk Contours for YLSEPP
Diagram 10.5	Societal Risk Curve for YLSEPP

List of Appendices

Appendix 10.1	Process Flow Description
Appendix 10.2	Population Data
Appendix 10.3	Review of Historic Incidents Database
Appendix 10.4	Fault Tree Analysis
Appendix 10.5	Event Tree Analysis

ABBREVIATION

Abbreviation	Full title
ACABAS	Advisory Committee on the Appearance of Bridges and Associated Structures
ACE	Advisory Council on the Environment
AMO	Antiquities and Monuments Office
CEDD	Civil Engineering and Development Department
CLP	CLP Power Hong Kong Limited
DC	District Council
DCS	District Cooling System
DEVB	Development Bureau
DLC	District Land Conference
DSD	Drainage Services Department
EMSD	Electrical and Mechanical Services Department
EPD	Environmental Protection Department
ExCo	Executive Council
FC	Finance Committee
GEO	Geotechnical Engineering Office of CEDD
HD	Housing Department
HyD	Highways Department
ITC	Innovation and Technology Commission
LandsD	Lands Department
LCSD	Leisure and Cultural Services Department
LegCo	Legislative Council
PFC	Public Fill Committee of the CEDD
PlanD	Planning Department
PlanD/UD&L	Urban Design and Landscape Section of PlanD
PWSC	Public Works Subcommittee
RC	Rural Committee
SMO	Survey and Mapping Office of LandsD
TD	Transport Department
TPB	Town Planning Board
VCAB	Vetting Committee on Aesthetic Design of Pumping Station Buildings
WSD	Water Supplies Department

10 HAZARD TO LIFE ASSESSMENT

10.1 Introduction

10.1.1.1 This section identifies the hazardous scenarios associated with the generation, storage, utilization, processing and transmission (if applicable) of biogas during operation of the Project, and presents the analysis and findings of the Quantitative Risk Assessment (QRA) undertaken.

10.1.1.2 An organic waste co-digestion facility that processes off-site pre-treated organic wastes (approximately 100 wet tonnes / day) and sewage sludge and handles the associated wastewater and biogas will be installed at YLSEPP. The preliminary site layout of the proposed YLSEPP and the location of its biogas related facilities are shown in Appendix 10.1.

10.1.1.3 In accordance with Section 3.4.11 and Appendix I of the EIA Study Brief (ESB-313/2019), a hazard assessment should be conducted to evaluate (i) the biogas risk to existing, committed and planned off-site population due to operation of the organic waste co-digestion facility; and (ii) the potential risk associated with the transport, storage, manufacture and use of chlorine/other non-fuel gas DGs due to the Project.

10.1.1.4 The potential risks from chlorine and other non-fuel gas DGs are associated with the operation of a proposed Reclaimed Water Facility, which is proposed to be located adjoining the northern boundary of the YLSEPP. There will be no chlorine/other no-fuel gas DGs related hazards due to operation of the proposed YLSEPP, as such, the potential risk associated with the transport, storage, manufacture and use of chlorine/other non-fuel gas DGs will not be considered in this assessment.

10.2 Environmental Legislation, Standards and Criteria

10.2.1.1 The requirements and criteria for assessing hazard to life are outlined in Section 3.4.11 of the EIA Study Brief (ESB-313/2019) and Annex 4 of the Technical Memorandum on Environmental Impact Assessment Process (EIAO-TM), respectively. The estimated risk levels due to biogas-related operations were compared with the individual and societal risk criteria set out in the Hong Kong Risk Guidelines (HKRG) to determine the acceptability of the risk levels estimated in this hazard assessment.

Hong Kong Risk Guidelines (HKRG), EIAO-TM Annex 4

10.2.1.2 Individual risk is the predicted increase in the chance of fatality per year to an individual due to a potential hazard. The individual risk guidelines require that the maximum level of individual risk should not exceed 1 in 100,000 per year i.e. 1×10^-5 per year.

10.2.1.3 Societal risk refers to the risks to the whole population. It is expressed graphically by plotting the cumulative frequency (F) of N or more deaths in the population from incidents at a certain installation against the number of fatalities (N) (Diagram 10.1 refers). Two F-N risk lines are used in the HKRG to denote “Acceptable” or “Unacceptable” societal risks. To avoid major disasters, there is a vertical cut-off line at the 1,000 fatality level extending down to a frequency of 1 in a billion (1x10-9) per year. The intermediate region indicates that the acceptability of societal risk is borderline and that it should be reduced to a level which is “as low as reasonably practicable” (ALARP). It seeks to ensure that all practicable and cost-effective measures that can reduce risk are considered.

Diagram 10.1 Societal Risk Guidelines for Acceptable Risk Levels

10.3 Study Objectives and Methodology

10.3.1 Study Objectives

10.3.1.1 The main objectives of the hazard assessment, as detailed in Appendix I of the EIA Study Brief, are to:

(i) identify hazardous scenarios associated with the generation, storage, utilization, processing and transmission (if applicable) of biogas due to the Project and then determine a set of relevant scenarios to be included in a QRA;

(ii) execute a QRA of the set of hazardous scenarios determined in sub-section (i) above, expressing population risks in both individual and societal terms;

(iii) compare individual and societal risks with the criteria for evaluating hazard to life stipulated in Annex 4 of the EIAO-TM; and

(iv) identify and assess practicable and cost-effective risk mitigation measures.

10.3.2 Methodology

10.3.2.1 To ensure that the methodology and approach of this hazard assessment are consistent with those of previous studies with similar issues, the hazard assessments under the approved EIAs for Development of Organic Waste Treatment Facilities (OTWF), Phase 2 (AEIAR-180/2013) [1] and Yuen Long Effluent Polishing Plant (YLEPP) (AEIAR-220/2019) [2] have been reviewed and referenced for this Project.

10.3.2.2 The QRA consists of the following six main tasks:

(i) Data / Information Collection and Update: Relevant data / information necessary for the hazard assessment, including project design and surroundings of the Project were collected;

(ii) Hazard Identification: A set of relevant hazardous scenarios associated with the operations of the organic waste co-digestion facility were identified by reviewing relevant literature and studies with similar installations as well as historical accident database, such as Major Hazard Incident Data Service (MHIDAS);

(iii) Frequency Estimation: Frequencies of each hazardous event leading to fatalities with full justification were estimated by reviewing historical accident data, previous similar projects and using Fault Tree Analysis (FTA) of the identified hazardous scenarios;

(iv) Consequence Analysis: The consequences of the identified hazardous scenarios were analysed by conducting source term modelling and effect modelling.

(v) Risk Integration and Evaluation: The risks associated with the identified hazardous scenarios were evaluated. The evaluated risks were compared with the HKRG in EIAO-TM to determine their acceptability; and

(vi) Identification of Mitigation Measures: Where necessary, practicable and cost-effective risk mitigation measures were identified and assessed to ensure compliance with the ALARP principle in the HKRG. Risks of the mitigated case were re-assessed to determine the level of risk reduction as required.

10.3.2.3 The main tasks of the QRA are shown schematically in Diagram 10.2.

Diagram 10.2 Schematic Diagram of QRA Process

10.3.3 Assessment Year

10.3.3.1 Based on the currently envisaged construction programme, the YLSEPP will be commenced in 2032. However, the Stage 3 works of YLS development is tentatively to be commenced in 2031 and the targeted intake year would be 2038. Year 2038 was proposed as the assessment year for operation of the YLSEPP to take into account population of the YLS development.

10.4 Description of Surroundings

10.4.1.1 Societal risk is a measure of the consequence magnitude and the frequency of the hazardous events. To establish the impact of any release (the number of people likely to be affected) in the future, it is necessary to know the future surrounding population levels. These would include residential population, government and institutional population and transport population but exclude staff of the YLSEPP since they are considered as voluntary risk takers.

10.4.2 On-site Populations

10.4.2.1  The following population is anticipated in YLSEPP during operation phase of the plant:-

(i) Staff: the number of staff of YLSEPP would be less than 100, and they are considered as voluntary risk takers and would not be considered in this assessment.

(ii) Guided visitors: people visited the plant with pre-appointment similar to the current arrangement in other STWs being operated by DSD.  The number of visitors shall be limited to a maximum of 50 per visit. The visitors should be registered at the Administration Building upon arrival to YLSEPP and briefed the potential risks before start of the visits.  During the course of visits, the visitor should be accompanied by plant’s staffs who are familiar with the plant situations and evacuation means. 

10.4.3 Surrounding Populations

10.4.3.1 The site of the proposed YLSEPP is located at the southern part of YLS stage 3 development. The proposed organic waste co-digestion facility is located in the southern corner of the proposed YLSEPP. All population groups included in this assessment are detailed in Appendix 10.2.

Land and Building Population

10.4.3.2 Population covered in the QRA included users of governmental facilities and amenity and also residents of the proposed housings in the YLS Stage 3 development. Estimation of land and building populations was based on the latest information provided by Civil Engineering and Development Department (CEDD) and are summarized in Table 10.1.

Table 10.1 Land and Building Population Considered for Hazard Assessment

ID	Description	Maximum Population in 2038
1	Proposed Reclaimed Water Facility	25
2	Reedbed / Flood Retention Pond	0
3	Site No. A3.3 - Amenity	0
4	Site No. GB3.1 - Green Belt	0
5	Site No. OU3.1 - Hillside River with Scenic Cycle Track (including Reedbed) ^Note 1	103
6	Sie No. DO3.2 - Cycle Park ^Note 1	311
7	Site No. LO3.6 - Local Open Space ^Note 1	96
8	Site No. R3.1e - Residential	804
9	Site No. R3.4a - Residential	1184
10	Site No. R3.4b - Residential	1333
11	Site No. E3.3 - Primary School	821
12	Site No. G3.4 - Government Reserve ^Note 3	0
13	Site No. G3.3 - Government Reserve	10
14	Site No. G3.5 - Government Reserve ^Note 3	0
15	Site No. LO3.7 - Local Open Space ^Note 1	43
16	Site No. ARG3.1 - Agriculture ^Note 2	10
17	Sie No. DO3.1 - District Open Space ^Note 1	114
18	Sie No. VR3.1 - Residential	60
19	Site No. E3.1 - Primary School	821
20	Site No. LO3.5 - Local Open Space ^Note 1	34

Note 1: Population was assumed by using density of 0.01 person per m²

Note 2: Population was assumed based on site survey.

Note 3: The lands were reserved for unmanned facilities.

10.4.3.3 An average of 5% of the residential and school population was taken to be outdoors; while 50% of the population in the proposed Reclaimed Water Facility and sites reserved for governmental uses was taken to be outdoors.

Traffic Population

10.4.3.4 The traffic population considered in this assessment included the population travelling in motor vehicles on Pak Sha Shan Road, Kung Um Road and new roads with the YLS development. With reference to the Traffic Impact Assessment for this Project and the Annual Traffic Census from Transport Department [9], the traffic population was estimated based on the following equation:

10.4.3.5 The traffic population considered in this assessment, which was assumed to be 100% outdoor, is summarized in Table 10.2 and detailed in Appendix 10.2.

Table 10.2 Traffic Population Considered for Hazard Assessment

ID	Description	Maximum Population in 2038
		Daytime	Night-time
R1	Pak Sha Shan Road	7	7
R2	Kung Um Road	13	10
R3	New Road within YLS Development	8	7

Time Modes

10.4.3.6 With reference to previous similar studies [1][2][5][6], four time modes as detailed in Table 10.3 were applied in this hazard assessment to reflect the temporal distribution of population and to address the variation in levels of activities that could lead to a release and the variation in population in the assessment area with time.

Table 10.3 Time Modes Adopted for Hazard Assessment

Day Category	Time Period		Time Mode
Weekday	Daytime	(07:00 to 19:00)	Weekday (Daytime)
	Night	(19:00 to 07:00)	Weekday (Night)
Weekend	Daytime	(07:00 to 19:00)	Weekend (Daytime)
	Night	(19:00 to 07:00)	Weekend (Night)

10.4.4 Meteorological Data

10.4.4.1 Meteorological data is required for consequence modelling and risk calculation. Consequence modelling (dispersion modelling) requires wind speed and stability class to determine the degree of turbulent mixing potential whereas risk calculation requires wind-rose frequencies for each combination of wind speed and stability class.

10.4.4.2 Meteorological data were obtained from Lau Fau Shan Weather Station (2015-2019) where wind speed, stability class, weather class and wind direction are available. This data represented the weather conditions for the area and had already taken into account of seasonal variations, and therefore were considered applicable to this assessment.

10.4.4.3 The data were transformed into an array of weather classes that can be expressed in a combination of wind speed and Pasquill stability classes to represent daytime and night-time meteorological conditions in accordance with the Guidelines for Quantitative Risk Assessment: Purple Book [10]. Pasquill stability classes (A to F) represent the atmospheric turbulence with Class A being the most turbulent class while Class F is the least turbulent class. These combinations are considered adequate to reflect the full range of observed variations in these quantities. It is not necessary or efficient to consider every combination observed. The principle is to group these combinations into representative weather classes, which together would cover all conditions observed. Once the weather classes have been selected, frequencies for each wind direction for each weather class can then be determined. These frequency distributions are given in Table 10.4 and Table 10.5 for the daytime and night-time meteorological conditions respectively.

Table 10.4 Daytime Weather Class-Wind Direction Frequencies at Lau Fau Shan Weather Station

Wind Direction	Frequency (%)
	2B	1D	3D	6D	2E	1F	Total
0	4.55	0.33	2.7	2.31	0.28	0.25	10.42
30	2.09	0.18	3.76	0.96	0.72	0.09	7.8
60	4.47	0.3	10.75	2.36	2.74	0.29	20.91
90	2.35	0.3	4.03	0.97	1.38	0.43	9.46
120	1.35	0.19	2.55	1.29	0.83	0.51	6.72
150	1.33	0.11	3.07	2.2	0.8	0.21	7.72
180	0.98	0.06	1.81	1.11	0.26	0.12	4.34
210	1.06	0.07	2.02	2.51	0.41	0.14	6.21
240	5.77	0.07	3.31	3.29	0.37	0.11	12.92
270	2.43	0.17	0.71	0.29	0.12	0.07	3.79
300	2.05	0.07	0.41	0.41	0.06	0.07	3.07
330	3.51	0.12	1.33	1.47	0.11	0.1	6.64
All	31.94	1.97	36.45	19.17	8.08	2.39	100

Table 10.5 Night-time Weather Class-Wind Direction Frequencies at Lau Fau Shan Weather Station

Wind Direction	Frequency (%)
	4D	6D	2E	1F	Total
0	1.34	1.75	0.67	0.97	4.73
30	4.49	1.41	2.69	0.78	9.37
60	13.96	1.07	12.39	1.59	29.01
90	4.34	0.59	7.19	2.28	14.4
120	2.99	0.73	5.46	2.86	12.04
150	4.22	0.7	7.45	2.16	14.53
180	2.15	0.41	2.46	0.96	5.98
210	1.17	0.46	1.78	0.87	4.28
240	0.34	0.06	0.67	0.61	1.68
270	0.15	0.06	0.18	0.25	0.64
300	0.28	0.2	0.15	0.18	0.81
330	0.99	0.84	0.44	0.26	2.53
All	36.42	8.28	41.53	13.77	100

10.5 Hazard Identification

10.5.1 Introduction

10.5.1.1 A hazard is described as the property of a material or activity with the potential to do harm. Potential hazards associated with generation, transfer, storage and use of biogas in the organic waste co-digestion facility within the proposed YLSEPP were identified. All the operation information and parameters have been confirmed with the engineers. This section outlines the hazards preliminarily identified for the facility.

10.5.1.2 Historical incidents and relevant studies of similar facilities were reviewed to identify the possible hazardous scenarios and to ensure that all the relevant hazardous scenarios were incorporated into this assessment.

10.5.2 Facility Description

10.5.2.1 The organic waste co-digestion facility at the proposed YLSEPP will receive approximately 100 wet tonnes / day of pre-treated organic wastes through pipelines or tankers for co-digestion with sewage sludge and handle the associated wastewater and biogas. Proven biological treatment technologies will be adopted to recover reusable energy, i.e. biogas, from source-separated organic wastes and sewage sludge. Biogas generated will be used onsite heat and power production. The location plan of the facility and the treatment process are illustrated in Appendix 10.1.

Digesters

10.5.2.2 Three duty and one standby cylindrical anaerobic sludge digesters, each of which is 19m (Dia.) × 28m (H) (internal dimension) in size, will be provided to handle the pre-treated organic waste and sludge. The biogas volume of each digester is 330m³. The working temperature and pressure of the digesters will be maintained at 35ºC and 1 bar.

10.5.2.3 The digesters consist of concrete, steel or glass enamel holding tanks, with either gas or top mounted mixing systems. Approximately 100 wet tonnes / day of pre-treated organic waste and 70 wet tonnes / day of sewage sludge will enter the digestion tanks along with additional water to reduce the Dissolved Solid (DS) content from an estimated 15% to 5%. The estimated average residence time of the organic waste / sludge within the digesters is assumed to be 20 days. Digested sludge / organic waste will be dewatered for disposal and the wastewater from the dewatered compost will be transferred to the side-stream treatment facilities / inlet works of YLSEPP for treatment.

10.5.2.4 Heating is required for biomass feeding of the digesters and for heat loss compensation from the digesters. The required heating will be provided via heat recovered from the combined heat and power (CHP) unit, or from a boiler.

10.5.2.5 Pressure relief valves will be installed on the digesters to protect against overpressure (50 mbarg). Overflow pipes will be provided on the digesters for protection against overfilling.

Biogas Holders

10.5.2.6 The biogas generated will be stored in the biogas holders. There will be two cylindrical biogas holders, each of which is 16m (Dia.) × 13.7m (H) in size with a maximum biogas storage of 1,600m³ per tank. The total storage amount of the biogas will be around 3,840 kg. The quantity does not exceed the lower threshold quantity, i.e. 15 tonnes, for Potentially Hazardous Installations (PHIs) for flammable gas and town gas installations in Hong Kong. Therefore, the proposed waste treatment facilities are not classified as a PHI. The biogas storage would be maintained at a temperature 35 ºC and a pressure of 1.03 bar.

10.5.2.7 Dry seal (Wiggins) type biogas holders with steel containment will be used in the proposed facility for evening out variations in biogas production from the digesters. This type of gas holder typically consists of a cylindrical steel shell and a displacement piston, which is allowed to go up and down with the change of volume of gas. The gas tightness is maintained by a seal between the piston and the inside of the shell. There are pressure relief valves on the biogas holder for protection against the exceedance of designed gas storage pressure and overflow pipes for protection against overfilling.

10.5.2.8 A non-return valve will be installed at the inlet pipe to prevent gas from back-flow. Gas is discharged through the outlet pipe by suction blower. There will be emergency shut-off valves at the inlet and outlet pipes of the gas holder. In case of gas holder failure, the emergency shut-off valves can close the inlet and outlet pipes and the release of biogas to the atmosphere can be minimised.

Sulphur Absorption Vessels

10.5.2.9 The stored biogas will go through the sulphur absorption vessels to remove the hydrogen sulphide (H₂S) before passing to the CHP generator to produce electricity and heat for use onsite.

10.5.2.10 Two duty and two standby sulphur absorption vessels, each of which is 3.5m (Dia.) x 3.7m (H) in size, will be provided downstream of the gasholders for the absorption of H₂S in the biogas. The working temperature and pressure of the sulphur absorption vessels will be maintained at 35 ºC and 1.03 bar. The absorption vessels are made of steel and filled with zinc oxide or iron oxide as absorbents. An explosion proof blower will be used to extract the biogas from gasholder to the sulphur absorption vessels at 400 mbarg.

Inlet / Outlet Piping

10.5.2.11 A total of 100m of aboveground inlet / outlet pipe (150mm Dia.) will be provided to the facility. All other piping will be underground or provided at the basement of concrete buildings. The working temperature and pressure of the inlet and outlet piping will be maintained at 35 ºC and 1.03 bar.

10.5.3 Biogas Properties

10.5.3.1 Biogas is a colourless flammable a combustible mixture of gases at atmospheric conditions that comprises mainly methane (CH₄) and CO₂. Generally, biogas from anaerobic digestion process has a methane content of 55% to 70% by volume. The exact composition of biogas depends on the substance that is being decomposed. If the material consists of mainly carbohydrates, such as glucose and other simple sugars and high-molecular compounds (polymers) such as cellulose and hemicellulose, the methane production is low. However, if the fat content is high, the methane production is likewise high [3]. In general, the physical properties of biogas are also very similar to those of natural gas. While it is non-toxic, in high concentrations it could lead to asphyxiation. A loss of containment can lead to jet fire (if stored/ transferred under sufficient pressure) or to an explosion if the gas accumulates in a confined space. The properties of biogas from Anaerobic Digestion (AD) process are summarized in Table 10.6 [1][2].

Table 10.6 Composition and Properties of Biogas from Anaerobic Digestion Process

Property	Biogas from Anaerobic Digestion
Methane Content	55% – 70%
Carbon Dioxide Content	30% – 45%
Density	1.2 Kg/Nm^3
Lower Caloric Value	23 MJ/Nm^3
Flammability^#	Extremely Flammable
Auto-Ignition Temperature^#	580°C
Flash Points^#	-188°C
Melting Point^#	-182.5°C
Boiling Point^#	-161.4°C
Flammable Limits^#	5% (Lower) – 15% (Upper)
Vapour Density^#	0.59-0.72 (air = 1)

Remark:

# Physical properties of biogas that are similar to natural gas.

10.5.3.2 Given that flammability increases with increase of methane content, and the exact composition of biogas varies with the substance that is being decomposed, it is conservatively assumed that the biogas is 100% methane in the risk model.

10.5.4 Review of Historical Incident Database and Relevant Studies

10.5.4.1 Relevant biogas or methane release scenarios from ORRC and YLEPP (which are facilities of similar nature as the proposed facility at YLSEPP) identified in historical incident databases, such as MHIDAS database, eMARS, FACTS and ARIA, were examined. The recorded hazardous scenarios were mainly associated with leakages from piping, valves and storage vessels and operator error. A total of 11 incidents records related to biogas and methane were identified and these are summarized in Table 10.7 and detailed in Appendix 10.3.

Table 10.7 Summary of Biogas or Methane Incidents

Hazardous Scenario	No. of Cases	Country
Methane Storage Tank Failure	3	Turkey, India, Australia
Methane Pipeline Failure	2	UK, USA
Anaerobic Digestion Plant Failure	6	Italy, France, Germany, India

10.5.4.2 The hazardous scenarios of biogas identified in the relevant studies were reviewed and adopted in this hazard assessment where applicable. Failure events and the respective hazardous scenarios associated with the biogas facilities were identified and assessed in the approved EIA studies for OWTF, Phase 2 (AEIAR-180/2013) [1] and YLEPP (AEIAR-220/2019) [2]. The identified hazardous scenarios were mainly associated with leakages from piping, valves and storage vessels due to undetected material defect.

10.5.5 Spontaneous Failures

Digester Failure

10.5.5.1 Failure of the digesters could be caused by undetected corrosion, fatigue, material or construction defect. Release of biogas could be from various parts of the digesters as well as the associated piping and devices. Possible hazardous outcomes include fireball, jet fire, flash fire and Vapour Cloud Explosion (VCE).

Gasholder Failure

10.5.5.2 Dry seal (Wiggins) type biogas holders will be used for the proposed facility. A dry seal (Wiggins) type gas holder is different from column guided water-sealed gas holder that do not have a gas holder crown. A seal is installed between the piston and the inside of the shell to maintain gas tightness inside the holder and prevent rotation or side movement of the piston. A levelling system consists of wire ropes and balance weights is equipped to prevent tilting of the piston. The seal and the levelling system will be inspected regularly.

10.5.5.3 Failure of the gas holders could be caused by undetected corrosion, fatigue, material or construction defect. Release of biogas could be from various parts of the gas holders or associated piping and devices. Possible hazardous outcomes include fireball, jet fire, flash fire and VCE.

Sulphur Absorption Vessel Failure

10.5.5.4 The absorbents used for removal of H₂S in the sulphur absorption vessels are neither flammable nor explosive that the major hazard will be from the release of biogas. Failure of sulphur absorption vessels could be caused by undetected corrosion, fatigue, material or construction defect. Release of biogas could be from various parts of the process vessels as well as associated piping and devices. Possible hazardous outcomes include fireball, jet fire, flash fire and VCE.

Aboveground Inlet or Outlet Piping Failure

10.5.5.5 Piping will be used to connect process vessels to the gasholder, compressor, and further purification unit and CHP. Failure along the on-site piping may be caused by undetected corrosion, fatigue, material or construction defect, or associated with flange gasket / valve leakage resulting in continuous gas release to the atmosphere. Failures of gaskets and valve leak only tend to give relatively small-scale leakage and will not contribute to any off-site risk. Nonetheless, gasket and valve leak failure were considered and included into pipework failure in this hazard assessment with reference to previous similar studies [1][2]. Possible hazardous outcomes from aboveground / underground piping include flash fire and VCE, while jet fire could also result from failure of aboveground piping.

10.5.6 External Hazards

10.5.6.1 External hazards that are outside the control of the operating personnel could still pose a threat to the organic waste co-digestion facility at the proposed YLSEPP. Such hazards are termed as ‘external hazards’ because they are independent of the operations on-site but can lead to major hazard scenarios. This section discusses the credibility of loss of containment due to the external hazards with respect to Hong Kong’s geographical location.

Aircraft Crash

10.5.6.2 The Project is located around 13 km northeast from the Hong Kong International Airport. The frequency of aircraft crash was estimated using the methodology of the HSE [10] and detailed in Section 10.6.3.

Earthquake

10.5.6.3 In Hong Kong, buildings and infrastructures are designed to withstand earthquakes up to Modified Mercali Intensity (MMI) VII. It was estimated that MMI VIII is required to provide sufficient intensity to result in damage to specially designed structure. It was assumed that failure in earthquake is possible for storage tank rupture, leakage, pipeline rupture and leakage. Details of frequency analysis are given in Section 10.6.3.

Vehicle Impact

10.5.6.4 Only authorised vehicles will be permitted to enter the proposed YLSEPP, and speed will be restricted for vehicle movements within the site. Safety markings and marked crash barriers will be provided to the above ground piping, digesters and gasholders near the internal road. Vehicle impact could cause only leak failure to digesters and gas holders as well as both rupture failure and leak failure to aboveground piping [1][2]. The accident rate was estimated based on statistical data for Vehicle/ Object Crash accident involving medium and heavy goods vehicles in recent years and detailed in Section 10.6.3.

Landslide

10.5.6.5 The proposed biogas facilities are surrounded by several natural terrains at their southern and western directions. It was conservatively assumed that failure of the aboveground bigas handling facilities including digesters, gas holders, sulphur absorption vessels and pipelines due to natural terrain landslide is possible.

10.5.6.6 In addition, there are two man-made geotechnical features (i.e. 6NW-D/R 16 and 6NW-D/CR 63) located at around 20m away from the eastern boundary of the proposed YLSEPP. These two geotechnical features are located within the YLS development and they will be either demolished or upgraded during the YLS development. As a result, failure of biogas facilities due to the failure of these two man-made features was not further considered in this assessment.

Lightning

10.5.6.7 Lightning sparks could ignite combustible gas in air. The proposed YLSEPP will be equipped with a lightning protection system that can effectively protect the equipment, include the organic waste co-digestion facility, from lightning. Lightning protection installations should be installed following IEC 62305, BS EN 62305, AS/NZS 1768, NFPA 780 or equivalent standards [15]. The installations will be protected with lightning conductors to safely earth direct lightning strikes. The double grounding system will be inspected regularly. Therefore, failures due to lightning strikes are to be covered by generic failure frequencies [1][2].

External Fire

10.5.6.8 External fire means the occurrence of fire event which leads to the failure of the equipment inside the organic waste co-digestion facility. In the proposed YLSEPP, the facilities will be equipped with fire alarm and fire suppression system. In addition, stringent procedures will be implemented to prohibit smoking or naked flames to be used on-site to further lower the probability of initiation due to external fire.

10.5.6.9 However, hill / vegetation fires are relatively common in Hong Kong and could potentially occur near YLSEPP. Details of frequency analysis are given in Section 10.6.3.

Typhoon/ Tsunami

10.5.6.10 Loss of containment due to severe environmental event such as typhoon or tsunami (large scale tidal wave) is not possible as the proposed YLSEPP is designed to withstand wind load for local typhoon while Hong Kong is not threatened by tsunami. Subsidence is usually slow in movement and such movement can be observed and remedial action can be taken in time. Thus, typhoon or tsunami causing a release of biogas was not considered further in this assessment.

10.5.7 Possible Hazardous Scenarios Considered

10.5.7.1 The organic waste co-digestion facility at the proposed YLSEPP will be using organic waste treatment technology, similar to that in the operation of the OWTF Phase 2 and YLEPP, i.e. anaerobic digestion of the organic waste. The sulphur absorption vessels would be structurally similar to anaerobic digestion vessels. Possible hazardous scenarios of the facility are listed in Table 10.8.

Table 10.8 Possible Hazardous Scenarios and Hazardous Outcomes of the Organic Waste Co-digestion Facility at YLSEPP

Potential Sources	Release Type	Hazardous Outcome
Gasholder	Rupture	Fireball; VCE; and Flash fire
	Leak	Jet fire; VCE; and Flash fire
Digester	Rupture	Fireball; VCE; and Flash fire
	Leak	Jet fire; VCE; and Flash fire
Sulphur Absorption Vessel	Rupture	Fireball; VCE; and Flash fire
	Leak	Jet fire; VCE; and Flash fire
Aboveground inlet or outlet piping / pump / non-return valve / flange	Rupture / Leak	Jet fire; VCE; and Flash fire

10.5.7.2 Hazardous outcomes were assessed using PhastRisk 6.7, to determine the risk impact, where the potential risk associated with the operation, layout and facilities threat posed to life and neighbouring property in a hazardous outcome at the Project.

10.6 Frequency Analysis

10.6.1 Introduction

10.6.1.1 Frequencies for each of the identified hazardous scenarios were estimated using the best available failure data or historical accident data in the process and gas industry or failure frequencies of similar installations or events. The frequencies documented in the relevant sources were reviewed and justified as necessary to reflect the specific operation and risk reduction practices evident at the organic waste co-digestion facility.

10.6.2 Spontaneous Failures Frequencies

Digester / Gasholder / Sulphur Absorption Vessel Failure

10.6.2.1 According to Guidelines for Quantitative Risk Assessment: Purple Book, the catastrophic rupture and leak failure frequencies of digester tank / gasholder / sulphur absorption vessel are 1×10^-5 per year and 1×10^-4 per year respectively [10].

Aboveground Piping Failure

10.6.2.2 According to Guidelines for Quantitative Risk Assessment: Purple Book, catastrophic rupture and leak failure frequencies of aboveground piping are 3×10^-7 per metre per year (150 mm dia.) and 2×10^-6 per metre per year (150 mm dia.) respectively [10].

10.6.2.3 A summary of the base event frequencies is shown in Table 10.9.

Table 10.9 Summary of Spontaneous Failures Frequencies

Events	Frequency of Occurrence
	Rupture / Catastrophic Failure	Leak / Partial Failure
Digester	1.00×10^-5 per year	1.00×10^-4 per year
Gasholder	1.00×10^-5 per year	1.00×10^-4 per year
Sulphur Absorption Vessel	1.00×10^-5 per year	1.00×10^-4 per year
Aboveground Inlet or Outlet Piping	3.00×10^-7 per metre per year	2.00×10^-6 per metre per year

10.6.3 External Event Frequencies

Aircraft Crash

10.6.3.1 The model takes into account specific factors such as the target area of the proposed hazard site and its longitudinal (x) and perpendicular (y) distances from the runway threshold (Diagram 10.3 refers).

Diagram 10.3 Aircraft Crash Coordinate System

10.6.3.2 The crash frequency per unit ground area (per km2) is calculated as:

(1)

where N is the number of runway movements per year and R is the probability of an accident per movement (landing or take-off). F(x,y) gives the spatial distribution of crashes and is given by:

Landings

(2)

for .

Take-off

(3)

for .

10.6.3.3 Equations (2) and (3) are valid only for the specified range of x values. If x lies outside this range, the impact probability is zero. This case applies for 07L and 07R runways for arrival and 25L and 25R runways for departure flight path.

10.6.3.4 The probability of an accident per movement R is interpreted from NTSB data for fatal accidents in the U.S. involving scheduled airline flights during the period 1986 – 2010. The 10-year moving average suggested a downward trend with recent years showing a rate of about 2×10^-7 per flight. There were only 13.5% of accidents associated with the approach to landing, 15.8% associated with take-off and 4.2% were related to the climb phase of the flight [17]. The frequency for the approach of landings was therefore taken as 2.7×10^-8 per flight and for take-off was 4.0×10^-8 per flight.

10.6.3.5 The number of runway movement of aircraft N was provided by yearly statistics of the Hong Kong International Airport (HKIA), and the figures in the latest 10 years are presented in Table 10.10 [18]. Due to the social unrest since mid-2019 and the outbreak of COVID-19, the number of runway movement in 2019 and 2020 was considered to be not representative, as such, the number of movements in 2038 was estimated by linear regression of the data from 2011 to 2018.

10.6.3.6 The movement numbers for both landing and take-off adopted in the calculation were divided by 4 to take into account that only a quarter of landing or take-off use a specific runway.

Table 10.10 Hong Kong International Airport Civil International Air Transport Movements of Aircraft

Year	Landing	Take-off	Total
2011	166,919	166,887	333,806
2012	175,861	175,823	351,684
2013	186,048	186,032	372,080
2014	195,520	195,488	391,008
2015	203,043	203,005	406,048
2016	205,793	205,773	411,566
2017	210,339	210,320	420,659
2018	213,899	213,867	427,766
2019 ^[1]	209,904	209,891	419,795
2020 ^[1]	80,330	80,336	160,666
2038	434,442^#	434,429^#	822,998^#

Note:

[1] The data for 2019 and 2020 were not used to calculate the annual growth rate for linear regression due to the social unrest since mid-2019 and the outbreak of COVID-19.

#: based on an annual growth rate of +3.6% between 2011 and 2018 estimated by linear regression.

10.6.3.7 Only the aircraft arriving from north-east using either 25R or 25L arrival flight path as well as the aircraft departing towards northeast using either 07R or 07L departure flight path would have potential impact to the proposed YLEPP.

10.6.3.8 For the aircraft arriving from south-west using either 07R or 07L arrival flight path, the longitudinal distance from the runway is around -12km, which is much smaller than -3.275km and thus the potential impact is considered to be zero. Likewise, for the aircraft departing towards south-west using either 25R or 25L departure flight path, the longitudinal distance from the runway is around -12km, which is much smaller than -0.6km and thus the potential impact is considered to be zero.

10.6.3.9 The aircraft crash frequency was obtained by multiplying g(x,y) to the target area which was estimated to be 4.6×10^-2 km² as tabulated in Table 10.11. The total crash frequency was calculated to be 9.8×10^-10 per year.

Table 10.11 Calculation for Aircraft Crash Frequency

Year	Runway	x (km)	y (km)	F(x,y)	N (per year)	R (per flight)	Crash frequency (per unit area)	Target area (km^2)	Crash Frequency (per year)
2038	25R Landing	8.4	9.9	3.9E-06	108610	2.7E-08	1.1E-08	4.60E-02	5.2E-10
2038	25L Landing	7.9	11.5	3.4E-06	108610	2.7E-08	1.0E-08	4.60E-02	4.6E-10
2038	07R Landing	-12.4	13.0	0	108610	2.7E-08	0.0E+00	4.60E-02	0.0E+00
2038	07 L Landing	-12.6	11.4	0	108610	2.7E-08	0.0E+00	4.60E-02	0.0E+00
2038	07L Take-off	8.4	9.9	1.4E-08	108607	4.0E-08	5.9E-11	4.60E-02	2.7E-12
2038	07R Take-off	7.9	11.5	3.7E-09	108607	4.0E-08	1.6E-11	4.60E-02	7.3E-13
2038	25L Take-off	-12.4	13.0	0	108607	4.0E-08	0.0E+00	4.60E-02	0.0E+00
2038	25R Take-off	-12.6	11.4	0	108607	4.0E-08	0.0E+00	4.60E-02	0.0E+00

Earthquake

10.6.3.10 As Hong Kong is situated in a region of low seismicity [12][13] and located rather far away from Circum-Pacific Seismic Belt that runs through Japan, Taiwan and the Philippines [14], the probability of earthquake occurrence at MMI VIII and higher is very low comparing with other places and is estimated to be 1.0×10^-5 per year [1]. It was assumed that failure in earthquake was possible for storage tank rupture, leakage, pipeline rupture and leakage and the probability of failure in earthquake was assumed to be 0.01 [1][11].

Vehicle Impact

10.6.3.11 The overall numbers of accident involvements of Medium/ Heavy Goods Vehicles (M/HGVs) [19] in Hong Kong are tabulated in Table 10.12. The overall accident involvement rate of M/HGVs have been quite steady in recent years. The statistics indicate the overall high and medium impact accident involvement rate per million vehicle kilometre for MGV/HGVs is 0.14. The vehicle crash frequency was therefore estimated to be 1.4×10^-7 per vehicle kilometre per year.

Table 10.12 Accident Involvements of Medium / Heavy Goods Vehicles in Hong Kong

Serious and Fatal Vehicle involvements of M/HGVs	2005	2006	2007	2008	2009	2010	2011	2012	2013	2014	2015	2016	2017	2018	2019	Average
Invol rate: per million veh-km	0.89	0.86	0.82	0.80	0.76	0.83	0.91	0.89	0.87	0.93	0.86	0.96	0.94	0.90	0.95	0.88
Total involvements	1180	1155	1081	1045	907	1031	1 141	1 105	1 085	1 125	1 063	1 167	1 162	1 083	1 093	1 095
Fatal involvements	27	25	21	17	27	16	21	17	25	23	23	18	26	19	22	22
Serious injury involvements	257	212	188	176	147	163	196	175	193	170	250	171	146	134	137	181
Fatal vehicle involvements ratio	2.3%	2.2%	1.9%	1.6%	3.0%	1.6%	1.9%	1.5%	2.3%	2.0%	2.2%	1.5%	2.2%	1.8%	2.0%	2%
Serious injury involvements ratio	21.8%	18.4%	17.4%	16.8%	16.2%	15.8%	17.2%	15.8%	17.8%	15.1%	23.5%	14.7%	12.6%	12.4%	12.5%	17%
High impact accident involvement rate per million vehicle km	0.02	0.02	0.02	0.01	0.02	0.01	0.02	0.01	0.02	0.02	0.02	0.01	0.02	0.02	0.02	0.02
Medium impact accident involvement rate per million vehicle km	0.19	0.16	0.14	0.13	0.12	0.13	0.16	0.14	0.15	0.14	0.20	0.14	0.12	0.11	0.12	0.14

Landslide

10.6.3.12 With reference to GEO Report No. 138 [24], the notional return period quoted for the design event is 1 in 100 years. With the implementation of the mitigation measures envisaged, the notional return period will be in the order of 1000 years, where a conditional probability of occurrence of 0.1 is applied to account for use of back-analysed rheological parameters for mobile failures in the assessment of debris mobility under the runout modelling (Design Event Approach). Thus, 1×10^-3 per year is adopted for the occurrence of natural terrain landslides.

10.6.3.13 A probability of 0.005 was adopted for catastrophic failure of the aboveground vessels and pipelines, while a probability of 0.01 was adopted for partial failure of the facilities [16].

External Fire

10.6.3.14 The YLSEPP is surrounded by vegetation at its southern and western boundary that potential impact due to hill fire was considered. According to the statistics from Agriculture, Fisheries and Conservation Department, the average number of hill fires was 23 per year during the recent 10 years 2011-2020 (range: 13 to 42). Since the total area of country parks in Hong Kong was 43,467 ha as in 2017, the frequency of hill fire in Hong Kong was taken as 5.29×10^-8 per m² per year.

10.6.3.15 At the thermal radiation intensity of 37.5 kW/m², damage to process equipment can happen [20]. From the literature, for a heat flux of 37.5 kW/m², the corresponding flame-to-structure distance is 25m caused by burning in tree canopy producing persistent flames [4]. In line with ORRC, Phase 2 (AEIAR-180/2013) [1], 50m is adopted in this study to account for uncertainty (e.g. spreading of hill fire). The resulting total area used in the frequency calculation was thus the total area of vegetation extending 50m beyond the process area, which is 24,000 m².

10.6.3.16 In the YLSEPP, the facilities will be equipped with fire alarm and fire suppression system (including fire alarms, fire detectors, sprinkler extinguishing system and fire pumps) to protect the facilities against external fire. It was considered that damage to gas holders, vessels and piping happens when there is hill fire as well as failure of fire protection system. By taking into account the failure rate of fire protection system of 2.20×10^-2 per year [20], the overall frequency of damage to the biogas facilities is 2.8×10^-5 per year. It was assumed that damage to the process equipment results in rupture failure and leak failure in equal probability. Hence, the catastrophic rupture and leak failure frequencies of the gas holders / digesters / sulphur absorption vessels / aboveground pipelines are 1.4×10^-5 per year respectively.

10.6.3.17 A summary of the base event frequencies is presented in Table 10.13.

Table 10.13 Summary of Base Event Frequencies

Events	Frequency of Occurrence
Aircraft Crash	9.8×10^-10per year^#
Earthquake	1.0×10^-5 per year
Vehicle Impact	1.4×10^-7 per vehicle-km per year
Landslide	1.0×10^-3 per year
External Fire	5.29×10^-8 per m^2 per year

10.6.4 Fault Tree Analysis

10.6.4.1 Fault Tree Analysis (FTA) was conducted to evaluate the frequencies of the identified biogas release scenarios. FTA is the use of a combination of simple logic gates, “AND” and “OR” gates, to synthesise a failure model of the biogas facilities. Fault Tree Analyses are shown in Appendix 10.4. The assumptions used in FTA are summarised in the following Table 10.14.

Table 10.14 Assumptions used in Fault Tree Analysis

Items	Assumed Value	Justification
Probability of rupture failure in aircraft crash	1	On conservative approach
Length of internal road close to biogas facilities	0.67 km	Measured from the site plan (Appendix 10.2 refers).
No. vehicle movements per hour	30	Included tankers, sludge collectors and staff vehicles
Probability of vehicle running into gasholder / digesters / absorption vessels / pipelines	0.5	With reference to approved EIA report of the OWTF Phase 2 [1], and based on the fact that concerned process vessels are only at one side of the road.
Probability of vehicle causing damage to gasholder / digesters / absorption vessels / pipelines	0.5	With reference to approved EIA report of the OWTF Phase 2 [1].
Probability pipeline rupture failure in car crash	0.1	With reference to approved EIA report of the OWTF Phase 2 [1].
Probability pipeline leak failure in car crash	0.9	With reference to approved EIA report of the OWTF Phase 2 [1].

10.6.5 Ignition and Explosion Probability

10.6.5.1 In general, the probability of immediate or delayed ignitions depends on the scale of release, the presence and location of ignition sources, and the weather conditions.

10.6.5.2 Possible ignition sources include hot surfaces, static electricity, flame and hot particles from external fire etc. [20]. The ignition probabilities are further split between immediate ignition and delayed ignition in equal proportions [1]. Immediate ignition of biogas could lead to a fireball or jet fire, whereas delayed ignition could cause a flash fire or vapour cloud explosion. Table 10.15 shows the total ignition probabilities and explosion probabilities according to gas release size [20].

Table 10.15 Ignition and Explosion Probabilities for Gas Releases

Release Size	Ignition Probability	Explosion Probability
Minor (< 1 kg/s)	0.01	0.04
Major (1 – 50 kg/s)	0.07	0.12
Massive (> 50 kg/s)	0.3	0.3

10.6.5.3 Event Tree Analysis (ETA) was developed to determine the possible hazard event outcomes from the identified hazardous events and to estimate the hazard event frequencies from the initiating release frequency. Event Tree Analyses are shown in Appendix 10.5.

10.6.6 Estimating Generic Frequencies

10.6.6.1 Generic frequency was estimated based on the historical incidents review identified the accidents involving generation, transfer, storage and use of biogas or methane, anaerobic digesters or facilities of similar nature. The generic accident frequency can be estimated through the information of the number of biogas plants works involved, the operating period and the total number of accidents occurred within the operating period. The objective of the generic frequency estimation is to confirm the appropriateness of adopting generic failure frequencies for this hazard assessment.

10.6.6.2 The generic frequencies estimated based on European experience were 1.73×10^-4 incident per plant-year [1], whilst the overall failure frequency for organic waste co-digestion facility was 2.57 ×10^-3 (according to FTA shown in Appendix 10.4), which was greater than the estimated value from the European historical incidents. The failure frequencies adopted for the facility in this hazard assessment were therefore considered reasonably conservative.

10.7 Consequence and Impact Analysis

10.7.1 Introduction

10.7.1.1 Consequence and impact analysis were conducted to provide a quantitative estimate of the likelihood and number of deaths associated with the range of possible outcomes (i.e. fireball, jet fire, flash fire etc.) which were resulted from failure cases identified. The consequence assessment consists of two major parts, including:

· Source term modelling – to determine the appropriate discharge models to be used for calculation of the release rate, duration and quantity of the release; and

· Effect modelling – to determine dispersion modelling, fire modelling and explosion modelling from the input of source term modelling.

10.7.1.2 Releases from hazardous sources and their consequences were modelled using PhastRisk 6.7.

10.7.2 Source Term Modelling

10.7.2.1 For instantaneous failure, the whole content release of a tank is modelled. In case of continuous release, release parameters such as release rate and exit velocity are calculated by a discharge model according to storage conditions. Release duration is based on capacity of the storage tank [1]. For piping connecting to the storage tank, release duration is based on the time to empty the whole tank gas content for anaerobic digesters and the response time to completely isolate the gasholder. Release parameters together with release duration are then fed into the dispersion model to calculate the effect. Process vessel, piping and storage vessel would be the major release sources.

10.7.3 Potential Hazardous Outcomes and Effect Modelling

10.7.3.1 This section gives a brief description of the physical effects models used in the study to assess the effects zones for the following hazardous outcomes in case of loss of containment at the food waste/sewage sludge co-digestion facility

Gas Dispersion / Flash Fire

10.7.3.2 Since biogas is lighter than air, its releases will tend to rise rapidly due to the buoyancy nature of the gas under atmospheric conditions. They will propagate and be diluted as a result of air entrainment with the influence of wind. The Unified Dispersion Model (UDM) model is used for the dispersion calculation of biogas for non-immediate ignition scenarios. The model takes into account various transition phases, from dense cloud dispersion to buoyant passive gas dispersion, in both instantaneous and continuous releases.

10.7.3.3 The principal hazard arising from a cloud of dispersing biogas is the delayed ignition of the flammable cloud that cause a flame to flash back to the release location and develop into a stable jet or crater fire. Large scale experiments on the dispersion and ignition of flammable gas clouds show that ignition is unlikely when the average concentration is below the Lower Flammable Limit (LFL) or above the Upper Flammable Limit (UFL).

10.7.3.4 Major hazards from flash fire are thermal radiation and direct flame contact. It is considered that there is no scope for escape within the LFL of a flammable cloud in a flash fire. Therefore, a fatality probability of 100% of persons present within the flammable cloud is assumed for flash fires.

Fireball

10.7.3.5 The release rate following a rupture, if ignition was immediate, would be too high to give a stable flame, and the initial ‘quasi instantaneous’ release is characterised as a fireball. The fireball is limited to a maximum duration of 30 seconds. The combustion would develop into a stable jet fire once the instantaneous release has been burnt and the release rate has become sufficiently steady for a flame to stabilise as stated by Bilo and Kinsman [21]. A release from a hole, if ignited, gives a stable flame close to the hole and produces a jet fire.

10.7.3.6 Due to the large size and intensity of a fireball, its effects are not significantly influenced by weather or wind direction. The principal hazard of fireball arises from thermal radiation. The thermal radiation from a fireball at given distances from the fireball centre are estimated using the PhastRisk’s built-in fireball modelling suite in which TNO model and HSE model are adopted. The modelling suite is set such that it decides the most appropriate one in the effect modelling. Sizes, height, shape, duration, heat flux and radiation are determined in the consequence analysis. A 100% fatality is assumed for anyone within the fireball radius.

Jet Fire

10.7.3.7 A jet fire occurs following the ignition and combustion of a pressurised flammable gas, which burns close to the release source. The jet fire which follows the fire ball is assumed to be directed vertically upwards out of the crater. The jet fire shape is the frustum of a cone and the location and orientation of the frustum are dependent on a number of factors such as release rate and wind speed.

10.7.3.8 Combustion in a jet fire occurs in the form of a strong turbulent diffusion flame that is strongly influenced by the initial momentum of the release. The principal hazards from a jet fire are thermal radiation and the potential for knock-on effects. Jet fires also dissipate thermal radiation and causes casualty and damage to the population and property nearby. The thermal effect to adjacent population is quantified in the consequence model.

Vapour Cloud Explosion

10.7.3.9 A vapour cloud explosion can occur when a flammable vapour is ignited in a confined or partially confined situation. When there is a large amount of pressurised gas rapidly releasing to the atmosphere from a pressurised tank, a vapour cloud could be formed, dispersed and mixed with the surrounding air. If the vapour cloud is passing through a confined / semi-confined environment and gets ignited, the confinement could limit the degree of expansion of the burning cloud and create an overpressure and explosion.

10.7.3.10 The risk model will be accounted for the VCE hazard according to probabilities for delayed ignition in consequence modelling. The program models the delayed ignition effect by considering the flammable cloud area and location of ignition sources at each time step. Potential damage from a VCE is caused by overpressure.

Thermal Radiation

10.7.3.11 Hazardous consequences, such as jet fire, flash fire, etc. were assessed using PHAST’s consequence models. Fatality probabilities of various hazardous event outcomes were evaluated at a number of end-point criteria in each type of hazard outcome. The estimation of the fatality/ injury caused by a physical effect such as thermal radiation or overpressure requires the use of probit equations, which describe the probability of fatality as a function of some physical effect. The probit is an alternative way of expressing the probability of fatality and is derived from a statistical transformation of the probability of fatality.

10.7.3.12 The probability of fatality, Pr, due to exposure to heat radiation, i.e. jet fire and fireball is given by the following probit relationship by Eisenberg et al. which provides one of the more conservative estimates [22]:

Where,

Pr is the probit associated with the probability of fatality;

Q is the heat radiation intensity (kW/m²);

t is the exposure time (s).

Overpressure

10.7.3.13 The probability of fatality due to overpressure is taken from CIA guidelines [23] as shown in Table 10.16. The indoors fatality probability is higher taken into account the increased risk from flying debris such as breaking windows [9].

Table 10.16 End Point Criteria for Vapour Cloud Explosions

Overpressure (psi)	Fatality Probability (Outdoors)	Fatality Probability (Indoors)
5	0.09	0.55
3	0.02	0.15
1	0.00	0.01

10.8 Risk Evaluation

10.8.1 Introduction

10.8.1.1 By combining the population data, meteorological data, results of frequency estimation and consequence analysis, risk levels due to the operation of the organic waste co-digestion facility at the proposed YLSEPP are assessed and evaluated in terms of both individual and societal risks.

10.8.1.2 Individual risk is a measure of the risk to a chosen individual at a particular location. As such, this is evaluated by summing the contributions to that risk across a spectrum of incidents which could occur at a particular location.

10.8.1.3 Societal risk is a measure of the overall impact of an activity upon the surrounding community. As such, the likelihoods and consequences of the range of incidents postulated for that particular activity are combined to create a cumulative picture of the spectrum of the possible consequences and their frequencies. This is usually presented as an F-N curve and the acceptability of the results can be judged against the societal risk criterion under the risk guidelines.

10.8.2 Individual Risk

10.8.2.1 The individual risk (IR) contours associated with the organic waste co-digestion facility at the proposed YLSEPP are shown in Diagram 10.4 . The risk levels were estimated based on 100% occupancy with no allowance made for shelter or escape, which can be referred from the user manual of PhastRisk.

10.8.2.2 Individual risk contours down to the level 1×10^-9 per year are shown in the diagram. The level 1×10^-5 per year individual risk contour is confined entirely within the boundary of the YLSEPP. The maximum individual risk remains below 1×10^-5 per year at the site boundary and meets the HKRG requirements.

Diagram 10.4 Individual Risk Contours for YLSEPP

10.8.3 Societal Risk

10.8.3.1 The societal risk results for the proposed YLSEPP are presented in Diagram 10.5 in form of F-N curves for comparison with the HKRG.

Diagram 10.5 Societal Risk Curve for YLSEPP

10.8.3.2 The societal risk associated with operation of the biogas facilities in YLSEPP falls within the “Acceptable” region. The potential loss of life (PLL) for the facility was estimated to be 1.79×10^-5 per year. The affected individuals are mainly the users of the open space adjoining the eastern boundary of YLSEPP (i.e. population ID# 7) and road population on Pak Sha Shan Road and Kung Um Road.

10.9 Recommendations

10.9.1.1 While the risks associated with organic waste co-digestion facility are within the acceptable region and no mitigation measures are required, it is still advisable for the following good safety practices and recommended design measures to be followed for the design and operation of the facility as far as practicable:

· the process plant building should be provided with adequate number of gas detectors distributed over various areas of potential leak sources to provide adequate coverage;

· all electrical equipment inside the building should be classified in accordance with the electrical area classification requirements. No unclassified electrical equipment should be used during operations or maintenance;

· all safety valves should be designed to discharge the released fluid to a safe location and stop misdirection of fluid flows in order to avoid hazardous outcome;

· safety markings and crash barriers should be provided to the aboveground piping, digesters and gas holders near the entrance;

· fixed crash barriers should be provided in areas where process equipment is adjacent to the internal roadway to protect against vehicle collision. Adequate warning signage and lighting should also be provided and maximum speed limit should also be in place; and

· lightning protection installations should be installed following IEC 62305, BS EN 62305, AS/NZS 1768, NFPA 780 or equivalent standards;

· suitable fire extinguishers should be provided within the site. An External Water Spray System (EWSS) should be installed in appropriate areas, such as around the gasholders, digester and sulphur removal vessels. The facilities should also be equipped with fire and gas detection system and fire suppression system; and

· stringent procedures should be implemented to prohibit smoking or naked flames to be used on-site.

10.10 Environmental Monitoring and Audit Requirement

10.10.1.1 The EIA study concluded that no unacceptable risk is anticipated during the operation phase of the Project, no mitigation measures would be required. Good safety practices and recommended design measures are recommended to further manage and minimize the potential risks during operation phase of the Project. No environmental monitoring and audit requirements would be required.

10.11 Conclusion

10.11.1.1 A quantitative hazard assessment was conducted to evaluate the biogas risk to existing, committed and planned off-site population due to operation of the organic waste co-digestion facility at the proposed YLSEPP in accordance with Section 3.4.11 and Appendix I of the EIA Study Brief (ESB-313/2019).

10.11.1.2 Both the individual and societal risk levels were found to meet relevant requirements stipulated in the HKRG, i.e. the off-site individual risk level is far below 1×10^-5 per year and the societal risk falls into the “Acceptable” region, no mitigation measure is required.

10.12 Reference

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