9.1.1.1 This section presents a summary of the analyses and findings of the Hazard to Life Assessment undertaken for the construction and operation of Sai O Trunk Sewer Sewage Pumping Station (hereafter referred to as the "Project").

9.1.1.2 The Project is to construct and operate a new sewage pumping station, located at the north of Sai O near Nai Chung. The Project consists of Designated Projects (DP) under Item F3(b), Part I, Schedule 2 of the Environmental Impact Assessment Ordinance (EIAO). An application for an Environmental Impact Assessment (EIA) Study Brief under section 5(1)(a) of the EIAO was made to Environmental Protection Department (EPD) and the EIA Study Brief No. ESB-281/2014 (the EIA Study Brief) issued under the EIAO.

9.1.1.3 The proposed sewage pumping station is in the vicinity of two high pressure town gas pipelines and a town gas offtake and pigging station. The proposed sewage pumping station also falls within the 1 km Potentially Hazardous Installation (PHI) Consultation Zone of Ma On Shan Water Treatment Works (MOSWTW). The Study Brief therefore requires a Hazard Assessment (HA) to address the risks associated with the town gas installations and to review the risks from the MOSWTW. The location of the proposed sewage pumping station and the hazardous installations are shown in Figure 9.1.

9.1.1.4 The details of hazard assessments are presented in Appendix 9.1 and Appendix 9.2:

(1) Appendix 9.1 presents the hazard assessment of the high pressure town gas pipelines and the town gas offtake and pigging station; and

(2) Appendix 9.2 presents the review of hazard to life impact from the MOSWTW.

9.1.2 Study Objectives

9.1.2.1 This HA aims to achieve the objectives as set out in Section 3.4.5 of the EIA Study Brief:

“3.4.5 Hazard to Life

3.4.5.1 The Applicant shall follow the criteria for evaluating hazard to life as stated in Annex 4 of the TM.

3.4.5.2 The Applicant shall also note that there are two high pressure town gas transmission pipelines running across the proposed development site as well as a Towngas Offtake and Pigging Station in the vicinity of the proposed Project. A risk assessment shall be carried out to address the risks associated with all the gas installations, having considered the proposed development during construction and operation of the project. The hazard to life assessment for construction and operation phases of the Project shall follow the detailed technical requirements given in Appendix G.

3.4.5.3 The proposed works also falls within the 1 km Potentially Hazardous Installation (PHI) Consultation Zone of Ma On Shan Water Treatment Works (MOSWTW). The Applicant shall conduct a review of the risks from MOSWTW to the Project and assess if risk to life is a key issue with respect to Hong Kong Risk Guidelines given in Annex 4 of the EIAO-TM. Hazard assessment including a Quantitative Risk Assessment (QRA) for MOSWTW shall be conducted if, and only if, risk to life is a key issue·with respect to Hong Kong Risk Guidelines following the requirements in Section 12.1 of EIAO-TM. If a QRA for MOSWTW is required, the detailed technical requirements shall follow Appendix G.”

Appendix G of EIA Study Brief

1. “The Applicant shall investigate methods to eliminate and/or minimize risks from town gas/chlorine. The Applicant shall carry out hazard assessment to evaluate potential hazard to life during construction and operation stages of the Project. The hazard assessment shall include but not limited to the following:

(i) Identify hazardous scenarios associated with town gas/chlorine, and then determine a set of relevant scenarios to be included in a Quantitative Risk Assessment (QRA);

(ii) Execute a QRA of the set of hazardous scenarios determined in (i), 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 TM; and

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

2. The methodology to be used in the hazard assessment should be consistent with previous studies having similar issues.”

9.1.3 Study Approach

9.1.3.1 The overall approach to the hazard assessment is represented in Figure 9.2.

9.1.3.2 The major phases in the HA of the high pressure town gas pipelines and town gas offtake and pigging station are:

i. Hazard Identification: Identify the hazard scenarios associated with the operation of the hazardous installations, and then determine a set of relevant scenarios to be included in a QRA.

ii. Frequency Assessment: Assess the likelihood of occurrence of the identified hazard scenarios.

iii. Consequence Assessment: Assess the consequences and impact to the surrounding population.

iv. Risk Summation and Assessment: Evaluate the risk level, in terms of individual risk and societal risk. The risks will be compared with the criteria stipulated in Annex 4 of the EIAO-TM to determine their acceptability.

v. Identification of Mitigation Measures: Identify and assess practicable and cost-effective risk mitigation measures. The risks of mitigated cases will then be reassessed to determine the level of risk reduction.

9.1.3.3 The risks from the MOSWTW are reviewed through studying the previous hazard assessments of the MOSWTW. As there has not been change to the operation status of the MOSWTW since the previous hazard assessments, the previous hazard assessments’ hazard scenarios, frequency assessment and consequence assessment are directly adopted.

9.1.4 Risk Acceptance Criteria

9.1.4.1 As stipulated in Annex 4 of the EIAO-TM, the Risk Guidelines comprise two measures shown as follows:

i. Individual Risk: the maximum level of off-site individual risk should not exceed 1 x 10^-5 / year, i.e. 1 in 100,000 per year.

ii. Societal Risk: it can be presented graphically as in Figure 9.3. The Societal Risk Guideline is expressed in terms of lines plotting the frequency (F) of N or more fatalities in the population from accidents at the facility of concern. There are three regions as described below:

- Acceptable where the risk is low enough that no action is necessary;

- Unacceptable where the risk is high enough that it should be reduced regardless of the cost or else the project of concern should not proceed; and

- ALARP (As Low As Reasonably Practicable) where the risk associated with the facility of concern should be reduced to a level “as low as reasonably practicable”, in which the priority of measures is established on the basis of practicality and cost to implement versus the risk reduction achieved.

9.2 Environmental Legislation, Standards and Guidelines

9.2.1.1 The key legislation and guidelines that are considered relevant to the Project are as follows:

· Environmental Impact Assessment Ordinance (EIAO), Chapter 499

- Technical Memorandum on Environmental Impact Assessment (EIAO-TM)

- the Risk Guidelines (RG), EIAO TM Annex 4

9.3 Population Data

9.3.1.1 The population considered in the hazard assessments include those in construction phase (2022). Population data for 2025 was obtained, thus 2025 is taken as the year for operation phase for assessment purpose. Population data within the study areas of the corresponding hazard assessments are collected from:

· desktop study and site survey;

· Census and Statistics Department;

· Planning Department; and

· previous reports of similar studies.

9.3.1.2 The population data adopted in the hazard assessments are detailed in Appendix 9.1 and Appendix 9.2.

9.4 Hazard Identification

9.4.1.1 The hazards associated with the hazardous installations are identified by reviewing the past incidents, existing conditions of the hazardous installations and previous studies. The hazard scenarios include the loss of containment from high pressure town gas pipelines and the toxic release from the chlorine store of a water treatment works, which are detailed in Appendix 9.1 and Appendix 9.2.

9.5 Frequency Assessment

9.5.1.1 With the potential hazards identified, the likelihood of each hazardous scenario will then be determined. The majority of the occurrence frequencies are adopted directly from previous studies and are supplemented by statistics from historical data if necessary. In some cases, event tree analysis is adopted to derive the frequencies of the hazardous scenarios.

9.5.1.2 Appendix 9.1 and Appendix 9.2 provide the details of the frequency assessment in each hazard assessment.

9.6 Consequence Assessment

9.6.1.1 The consequence assessment estimates the impact of each outcome in the area of concern. It includes discharge rate modelling, dispersion modelling and fire and explosion modelling.

9.6.1.2 Major hazards associated with town gas release are mainly fire and explosion, where chlorine mainly poses toxic effects. The details of the consequence assessment are presented in Appendix 9.1 and Appendix 9.2.

9.7 Risk Assessment

9.7.1 Risk Summation

9.7.1.1 By combining the results of frequency estimation and consequence analysis, risk levels of the assessment scenarios are characterised in terms of individual risk (presented in individual risk contours plot) and societal risk (presented in FN curves and Potential Loss of Life).

9.7.2 Individual Risk

9.7.2.1 The individual risks of the high pressure town gas pipelines and offtake and pigging station are presented in details in Appendix 9.1. They are extracted as Figure 9.4, Figure 9.5 and Figure 9.6. For the case of the high pressure town gas pipelines, the highest individual risk is below 1×10^-7 per year. For the case of the offtake and pigging station, the maximum off-site individual risk associated with the station is below 1×10^-5 per year. Therefore, the individual risk levels of both gas installations comply with the individual risk requirements stipulated in the EIAO-TM.

9.7.2.2 As per clause 3.5.4.3 of the Study Brief (Section 9.1.2.1), a review of the risks posed by the MOSWTW to the Project is carried out. Reference is made to a recent hazard assessment study which takes into account the MOSWTW and nearby developments. Since the operating conditions are identical to that mentioned in a previous Hazard Assessment, the individual risk of the MOSWTW do not change and is directly reproduced from the previous Hazard Assessment as Figure 9.7. The individual risk contours can hardly affect the proposed pumping station as shown in the figure.

9.7.3 Societal Risk

9.7.3.1 The societal risks of the high pressure town gas pipelines and offtake and pigging station in the construction stage and operation stage are presented in details in Appendix 9.1. They are extracted as Figure 9.8, Figure 9.9 and Figure 9.10. By taking into account the background population and the future construction staff (Case 2, 2022) and operation staff (Case 3, 2025), the societal risks of both town gas installations are within the Acceptable region of the Risk Guidelines given in Annex 4 of the EIAO-TM. The risk level of Case 2 is lowered than that of Case 3 because of a positive population growth rate in the background population is assumed in between 2022 and 2025. By comparing the risk levels of Case 1 and Case 3, the introduction of the proposed pumping station has an insignificant effect on the societal risk.

9.7.3.2 A review of the risks posed by the MOSWTW to the Project is carried out in Appendix 9.2. The societal risks of the Project in its construction phase and operation phase are extracted from Appendix 9.2 as shown in Figure 9.11 and Figure 9.12. The societal risks (gold line for construction phase and dark orange line for operation case) do not show any significant change when compared to the baseline societal risk of MOSWTW (black dashed line). One should note that the risk is within the ALARP region because of the contribution of risk of the background population instead of the introduction of the Project itself. As such, it is concluded that risk to life from MOSWTW to the Project is not a key issue with respect to the Risk Guidelines given in Annex 4 of the EIAO-TM.

9.7.4 Environmental Monitoring and Audit

9.7.4.1 The individual risks and societal risks of the town gas installations are within the Acceptable region of the Risk Guidelines given in Annex 4 of the EIAO-TM. Mitigation measures and Environmental Monitoring and Audit (EM&A) are not required for the Project. Good site practices are suggested for the construction stage and operation stage with respect to the potential risks posed by the town gas installations in Section 6.5 of Appendix 9.1.

9.8 Conclusion

9.8.1.1 A QRA has been carried out to assess the risk to life associated with the town gas installations during the construction stage and operation stage of the Project. The results showed that both the individual risks and societal risks comply with the Risk Guidelines given in Annex 4 of the EIAO-TM.

9.8.1.2 A review has been conducted to assess the risk to life associated with the MOSWTW during the construction stage and operation stage of the Project. The review showed that the risk to life from the MOSWTW to the Project was insignificant and was therefore not a key issue with respect to the Risk Guidelines given in Annex 4 of the EIAO-TM.