PELB(E) 55/10/161 Pt 26
Tel : 2848 2606
Fax : 2530 5264
29 September 1998
Clerk to the LegCo Panel on
3/F, Citibank Plaza
(Attn : Miss Odelia Leung)
Dear Miss Leung,
LegCo Panel on Environmental Affairs
Special Meeting on 5 October 1998
I refer to your letter of 14 September 1998 and enclose the following papers for tabling at the special meeting on 5 October :
Paper 1 : The Planning and Provision of Sewage Infrastructure
Paper 2 : The Four Groups of Acceptable Options for SSDS Stage II : Pros and Cons
Paper 3 : Comparison of Centralized and Distributed Sewage Treatment Systems
The papers will address issues (g), (a) and (d) listed in your letter in that order. Below are our response to the other issues raised, following your same notations :
- The EIA Ordinance (EIAO) was implemented on 1.4.98. In view of the fact that inevitably there would be major projects on-going at the time the EIAO was implemented, the Administration has made transitional arrangements whereby such EIAs would follow the statutory process to the maximum extent possible. Under the transitional arrangements endorsed by the Advisory Council on the Environment (ACE), EIA study briefs for designated projects that were issued by EPD before 1.4.98 have been placed on the EIAO Register established under the Ordinance. The project proponents would continue these EIA studies in accordance with the study briefs and the Technical Memorandum which governs the EIA process. On completion, the EIA reports are submitted for approval under the EIAO, and the ACE and the public are consulted before approval is given.
Having already been commissioned in May 1996, the SSDS EIA was one of the studies covered by the foregoing transitional arrangements. The study brief was placed on the EIAO Register as soon as the EIAO was implemented on 1.4.98. The brief requires the study to be carried out in two phases and a final EIA report covering both phases will be formally submitted in accordance with the provisions of the EIAO. Both ACE and the public will be consulted before any final approval is given. The report will have to meet the requirements specified in the EIAO Technical Memorandum which covers the EIA process.
Although, for the SSDS EIA, we have no statutory obligation to undertake any consultation prior to completion of the final report, in view of the importance of the study and the public interest, we have taken the initiative to consult the LegCo EA Panel and brief interested parties early on the findings of the options assessment. This is partly to assist us in deciding which option to adopt, and also partly so that any comments received can be taken into account in the second phase of the study. The public and the ACE will again have the opportunity to comment on the final EIA report when it is formally submitted for approval under the EIA Ordinance.
- The detailed arrangements of the study had been extensively discussed between the two sides under the Sino-British Joint Liaison Group (JLG) since 1995. Following the return of sovereignty, discussion of sewage disposal arrangements was dropped from the JLG. A new Expert Group on Sewage Disposal was set up in July 1998 to continue the discussion. The Hong Kong side was led by the Secretary for Planning, Environment and Lands. The Mainland side was led by Mr Zhang Liangdong, Director, Hong Kong Economic Affairs Department, Hong Kong and Macau Affairs Office, State Council and its members included :
A Technical Group was also set up under the Expert Group to review the reports submitted by the consultants. Members of the Mainland's Technical Group are :
|Mr Qiao Zhiqi||
Pollution Control Department
National Environmental Protection Agency
| Mr Lu Shouben||
General Management Department
State Bureau of Oceanography
|Mr Luo Yue||
Deputy Secretary General|
Guangdong Provincial People's Government
|Mr Liu Qiang||
Hong Kong Economic Affairs Department
Hong Kong and Macau Affairs OfficeState Council
|Prof. Jing Wenyong||
Consultant of the National Environmental Protection Agency|
Professor of Tsinghua University
|Prof. Zhou Jiayi||
Research FellowInstitute of Environmental Protection|
State Bureau of Oceanography
At its first meeting held in August 1998, the Technical Group agreed that the methodology for the EIA study was correct; that the study has been carried out in a thoroughly professional manner; and that the Consultants' findings were technically sound. The Mainland side have also expressed the view that, having regard to the cost-effectiveness of the different options, it would be sensible at this stage to proceed with an outfall at either East or West Lamma and use CEPT plus disinfection as the treatment process. They urged that the discharge and its impacts should be carefully monitored in the early years to check that the predicted performance is delivered and, if there are any signs that this is not the case, an early decision should be made to upgrade the system.
|Mr Qiao Zhiqi||
Director of Pollution Control Department|
National Environmental Protection Agency
|Prof. Jing Wenyong||
Consultant of the National Environmental Protection Agency|
Professor of Tsinghua University
|Prof. Zhou Jiayi||
Research FellowInstitute of Environmental Protection|
State Bureau of Oceanography
|Mr Huao Zhaoming||
Senior Engineer of Guangdong Environmental Protection Bureau
|Mr Lin Xingdao||
Deputy Secretary of Zhuhai Government and Head of Construction Committee
|Mr Liu Qiang ||
Hong Kong Economic Affairs Department
Hong Kong and Macau Affairs OfficeState Council.
- The list of organisations has already been sent to you on 16 September. Briefing sessions for these organisations were held in September.
- The SSDS was selected in 1989 after a two-year intensive consultancy study of many disposal options. The scheme was reviewed in 1994 under the SSDS Stage II Options Review Study in which over 50 different sewage treatment options were considered. An International Review Panel, comprising three internationally renowned experts from the PRC, Denmark and the US, was appointed to oversee the work of the consultants and concluded that the strategy adopted was technically sound (although some modification to the Stage I treatment process was proposed), cost effective and appropriate. Engineering and feasibility studies had been conducted before the construction of Stage I began in 1994. The proposal for Stage I is technically sound and it will be completed by 2000 as planned.
The Stonecutters Island Sewage Treatment Works, a key element of the SSDS Stage I, was commissioned in May 1997 and is treating sewage from north-west Kowloon which represents 25% of the available capacity for this stage. The balance of the flow will be available in 2000 when the construction of the six sewerage tunnels is completed.
The completion contract for the two western tunnels was awarded in July 1997. Excavation for the tunnel from Kwai Chung to Tsing Yi is about 65% complete and that from Tsing Yi to Stonecutters is 20% complete.
Another two contracts for the remaining four eastern tunnels from Tsueng Kwan O and Chaiwan to Stonecutters were awarded in January 1998. The contracts are now in the final stage of preparation for tunnelling works in full swing. Excavation for three tunnels has already commenced, and the remaining one will start soon.
Although we have encountered problems which have caused some impact on the progress of the tunnelling works, they were manageable and will not jeopardise the completion of the project as planned. Our target remains to complete the SSDS Stage I works in 2000.
for Secretary for Planning,
Environment and Lands
LegCo EA Panel - Special Meeting on 5 October 1998 1. Background
to discuss the SSDS EIA
The Planning and Provision of Sewage Infrastructure
2. Implementation of Sewerage Master Plans
||The population of Hong Kong has grown significantly in recent years. Over two million tonnes of sewage is discharged by one route or another into Hong Kong coastal and inshore waters each day. As a result, there has been a continuing decline in water quality.
||In the mid 1980s, the Environmental Protection Department (EPD) planned to tackle the water pollution problem by the implementation of what is now called the Strategic Sewage Disposal Scheme (SSDS), and a series of Sewerage Master Plans (SMPs). Upon the commissioning of the SSDS system, sewage generated in the urban areas around the Victoria Harbour and the Tseung Kwan O New Town will be conveyed to the Stonecutters Island for centralized treatment prior to discharging into the South China Sea. However, it must be noted that the SSDS is just one of the measures to improve water quality.
||Sewerage Master Plans are the blueprints for the infrastructure required to collect sewage and provide treatment on a catchment-by-catchment basis. The entire territory has been divided into 16 areas (Figure 1), and one SMP has been produced for each of them. The recommendations of these SMPs are being implemented progressively since the late 1980s.
3. Review of Sewerage Master Plans
||The objectives of a Sewerage Master Plan are:
- to upgrade local treatment facilities;
- to replace old sewers which are beyond maintenance or of inadequate capacity;
- to provide sewers for new development needs;
- to divert polluted flows from stormwater drains back into the sewerage system;
- to provide a prioritised programme for these works.
||In parallel with the formulation of the SMPs, the Environmental Protection Department (EPD) bid funds from the CWRF for the implementation of the proposed works. The Drainage Services Department (DSD), being the works agent, was delegated with the task of implementing these works. The detailed design and construction of works commenced in the late 1980s and will span until the middle of the next decade for the whole territory. The total estimated cost is in the order of HK$13 billion.
||Among the 16 SMPs, improvement works in areas such as Hong Kong Island South, Chai Wan and Shau Kei Wan have been completed. Much work is still being implemented in other parts of the Territory. Figure 2 shows the current status of SMP implementation. Figure 3 shows the different treatment works in place or planned around the territory, including SSDS.
||In the formulation of these SMPs, planning data prevailing at the time were used for assessing the future population growth as well as development needs. However, in order to keep up with the rapid pace of development in Hong Kong, the Planning Department responded to the changes by reviewing the Territorial Development Strategy (TDS) which was first prepared in 1984.
As a result of the TDS Review, significant changes have been introduced regarding planning scenarios, particularly in response to the housing initiatives. To account for these changes, EPD has planned to undertake a series of SMP Reviews to identify the necessary sewerage provisions. Each of the SMP Reviews will take 15-24 months to complete according to the following schedule:
|Commencement Year||SMP Reviews
- Central & East Kowloon
- Tuen Mun & Tsing Yi
- Outlying Islands
- Hong Kong Island
- North District and Tolo Harbour
- Tsuen Wan and West Kowloon
Based on the findings of these SMP Reviews, EPD will formulate a prioritised programme of sewerage works for future implementation. In order to protect the Chinese white dolphins on a precautionary principle, it is proposed that chemically enhanced primary treatment plus disinfection be applied for major discharges leading to Northwestern Water Control Zone. This proposal which applies also to the upgrading of Siu Ho Wan, San Wai and Pillar Point sewage treatment works has received full support from the Advisory Council on the Environment and is now at the planning stage.
The existing and future SMP works will complement the SSDS in providing the sewage infrastructure for Hong Kong well into the next century.
LegCo Panel on Environmental Affairs - Special Meeting on 5 October, 1998Introduction
SSDS EIA Study - Information Paper
The Four Groups of Acceptable Options for SSDS Stage II: Pros and Cons
At its meeting on 11 September 1998, during discussion of the results of the first phase of the Environmental Impact Assessment for the Strategic Sewage Disposal Scheme offshore outfall, Members requested a more detailed summary of the pros and cons of the different environmentally acceptable options.
2. A comprehensive account of the assessment of the impacts of the different options is provided in the briefing document circulated with the paper considered on 11 September. The covering paper itself contained a summary of the pros and cons of the different options. This paper attempts a middle road by providing more detail on the environmental and other advantages and disadvantages of the different options.
Levels of Treatment Examined
3. In developing the sewage treatment and disposal options for the Strategic Sewage Disposal Scheme (SSDS), the Consultants have examined a series of treatment levels from the simplest preliminary treatment to biological nutrient removal (BNR, sometimes referred to as "tertiary"). These treatment processes are described below :
- The first stage of sewage treatment is the removal of gross solids by using screens. This process, named as preliminary treatment, will be provided prior to the main tunnel systems in order to protect the downstream sewerage;
- When sewage reaches the treatment works, it first enters a sedimentation tank where 40 - 60% of the suspended solids (SS) and 30 - 40% of the organic matter (measured as biochemical oxygen demand, BOD) would be settled. Addition of chemicals such as ferric chloride at this stage can increase the SS and BOD removal efficiencies to 70 - 80% and 50 - 60%, respectively. These two processes are known as primary treatment and chemically enhanced primary treatment (CEPT), respectively. CEPT is the level adopted at Stonecutters Island and represents the basic building-block for SSDS.
- To remove the remaining soluble and colloidal organic matter, the primary or CEPT treated effluent can, if necessary, be transported to an aeration tank where bacteria are cultivated to degrade the organic matter. This process, named as secondary treatment, removes up to 90% of SS and BOD.
- Sometimes it is necessary to add another step to remove the nutrient, nitrogen. Should this be necessary, special types of bacteria can be cultivated in the aeration tanks. This process (known as biological nitrogen removal BNR) not only removes the soluble and colloidal organic matter, but also the nitrogen load in the primary treated effluent.
- Chlorination or ultra-violet irradiation may be provided at the end of the treatment works to disinfect the treated effluent.
4. To maintain an efficient secondary or BNR treatment, primary treatment is a pre-requisite. In terms of pollutant removal, BNR is better than secondary which is in turn better than CEPT or primary treatment alone. However, in terms of costs and land requirement, BNR is higher than secondary which in turn is much higher than CEPT or primary alone.
The Option Evaluation Criteria
5. Marine and onshore environment - Water quality modelling, whole effluent toxicity tests and risk assessments were conducted to determine whether the options can provide adequate sustained protection of beneficial uses of the coastal waters by achieving the proposed water quality criteria. Criteria for air, noise, visual impacts and terrestrial ecology were used to evaluate if any of the sites or options would present unacceptable environmental impacts.
6. Engineering criteria - the evaluation not only covered site constraints, compatibility and how quickly environmental improvements could be achieved, but also, in order to meet sustainability criteria the flexibility of the option strategy to respond to future changes. In particular, projected increases in sewage flows and loads resulting from the latest population projections have also been taken into account. All options have been assessed against a conservative scenario with a projected total population of around 9 million for the year of 2016. This includes future developments such as S.E. Kowloon. The population projections used are as follows :
Comparison of the Four Groups of Acceptable Options:
|Year||SSDS Catchment||Total Population in HK
7. Option 1: CEPT plus disinfection; outfall location at W. Lamma or East Lamma
- All parameters under the Water Quality Criteria would be achieved, with a predicted maximum drop in dissolved oxygen (DO) of 0.2 mg/L and maximum rise in total inorganic nitrogen (TIN) of 0.02 mg/L at Sham Wan and Tung O Wan. After initial dilution (wet season average dilution around 75 times), which would be completed within 100m (or a mixing zone of less than 0.1 km2), the concentration of all toxic substances will be below the level at which marine life may be at risk.
- The advantages are: lowest onshore impacts (in terms of noise, air, terrestrial ecology, sludge quantity and tunnel spoil quantity); lowest land requirement (17 ha); shortest time for completion (7.5 - 8 years); lowest capital cost (HK$11 billion) and lowest recurrent costs (HK$ 0.9 billion). As primary treatment is a pre-requisite for higher levels of treatment, this option also maintains the flexibility for further upgrading to secondary or tertiary treatment in the future.
- The main disadvantage of this option is that the outfall location(s) is not far from a proposed marine park.
8. Option 2: CEPT plus disinfection; outfall location at SE Lamma in the Lema Channel. (Level of treatment similar to Option 1 but with a much longer outfall.)
- All parameters under the Water Quality Criteria would be achieved, with a predicted maximum drop in DO of 0.15 mg/L and no measurable increase in TIN at Sham Wan and Tung O Wan. This means marginally less impact on water quality compared with Option 1. The onshore impacts of this option are comparable with Option 1 although a longer duration and larger tunnel spoil quantity due to the construction of a longer outfall would be expected. It also has the same low land requirement (17 ha) as in Option 1.
- The main advantage is that outfall discharge is further away from the proposed marine park resulting in a higher initial dilution (average wet season dilution up to 100 times).
- This option has a higher capital cost (HK$13 billion) compared with Option 1 due to the need to construct a longer outfall. Construction time will be longer (8.5 - 10 years) which means a further delay in improving Victoria Harbour's water quality. The other main problem with this option is that there are uncertainties with the geological conditions in the Lema Channel which may ultimately result in further increases in construction time and cost.
9. Option 3: Secondary treatment plus disinfection; outfall location at W. Lamma or East Lamma. (Higher level of treatment compared to Options 1 and 2; outfall location same as Option 1.)
- All parameters under the Water Quality Criteria would be achieved, with a predicted maximum drop in DO of 0.1 mg/L and maximum rise in TIN of 0.02mg/L at Sham Wan and Tung O Wan. This means marginally less impact on water quality than Option 2 (and 1) in terms of DO, but the same nutrient loads. Option 3 requires a relatively short time for completion (7.5 - 8 years - same as Option 1 and significantly shorter than Option 2).
- The main advantage of this option is better effluent quality and hence a marginally lower drop in the DO level in the receiving water body. Moreover there would be some degree of nitrification which would result in a lower ammonia concentration in the effluent and the receiving waterbody compared with Options 1 and 2.
- The main disadvantages are: greater onshore impacts compared with CEPT Options 1 and 2 (higher predicted noise levels due to construction and operation of secondary treatment plant; moderate impact on air quality due to biological treatment and sludge handling; highest sludge quantity and incineration requirements). There are also high land requirements (33 ha), a high capital cost (HK$23 billion) and high recurrent costs (HK$2.0 billion per annum) - almost double those for Options 1 and 2. The outfall is not far from a proposed marine park.
10. Option 4: Tertiary treatment (BNR) plus disinfection; outfall location at W. Lamma or East Lamma. (Highest level of treatment amongst the options; outfall location same as Options 1 and 3.)
- All parameters under the Water Quality Criteria would be achieved, with a predicted maximum drop in DO of 0.1 mg/L and no measurable increase in TIN at Sham Wan and Tung O Wan. This option has the least water quality impact in terms of DO and nutrients. Option 4 requires a longer time for completion (9.5 - 10 years) as compared with Options 1 and 3, as it is likely that reclamation would be needed to meet the large land requirement. (This will add a further 1 - 1.5 years to the construction time as well as increasing the costs.)
- The main disadvantages are: similar to Option 3, there would be greater onshore impacts compared with CEPT Options 1 and 2 (higher predicted noise levels due to construction and operation of tertiary treatment plant; moderate impact on air quality due to biological sludge treatment/handling; moderate sludge quantity and incineration requirements); this option has the highest land requirement (39 ha), highest capital cost (HK$26 billion) and highest recurrent costs (HK$2.3 billion per annum) - more than double those for Options 1 and 2, and higher than Option 3. As for Options 1 and 3, the outfall is not far from a proposed marine park.
11. All the four sets of options can achieve the water quality criteria. In general, the advantages of options with lower levels of treatment and shorter outfalls include shorter construction time, lower cost and land requirement, and flexibility for future upgrading. On the other hand, higher levels of treatment would provide better effluent quality and very marginally better water quality but at the same time incur much higher cost. A detailed chart of comparison is presented in Annex 1. Since all the options will meet the criteria which have been designed to ensure marine life can be sustained over the long term they all meet sustainability requirements.
Key Differences between the Acceptable Options for SSDS Stage II
Special Meeting of LegCo EA Panel on 5 October 1998Introduction
SSDS EIA - Comparison of
Centralized and Distributed Sewage Treatment Systems
At the meeting on 11 September 1998, during discussion of the results of the first phase of the SSDS EIA study, Members of the Panel queried the reasons for proceeding with an EIA of a centralised treatment and disposal network rather than a distributed system. Members asked for a comparison of the pros and cons of a centralised versus distributed system in the context of Hong Kong.
2. In proceeding with an EIA of a centralised system the Administration has followed up the recommendations of an International Review Panel (IRP) who, in 1994/95, supervised a review of available options and reported to the Panel on their conclusions. At the time there was a general consensus that the recommendations of the IRP should be followed. Nevertheless, in response to Members' concerns, the following paragraphs address the main points of difference between a distributed and a centralised system as they might be applied in Hong Kong, to refresh Members' memories of the earlier debate.
Previous Studies and Conclusions made by the International Review Panel
3. The development of the long term sewage strategy for Hong Kong was first studied in 1987 under the Sewage Strategy Study. The Study evaluated numerous options including the centralized sewage treatment system and distributed treatment systems with multiple discharges. An example indicating the approximate land requirements for a distributed sewage treatment system and the locations of the treatment works is attached in Annex A. The total land requirement for this would be approximately 230 ha. The study concluded that a centralized treatment scheme with deep tunnels and a long outfall (i.e. the Strategic Sewage Disposal Scheme, SSDS) would meet the environmental objectives in a cost-effective manner and should be adopted for Hong Kong.
4. The proposal for a centralized sewage treatment system received the full support of the then Environmental Pollution Advisory Committee. It was set out in the 1989 Government White Paper "Pollution in Hong Kong - A Time to Act" which received broad support from the community.
5. The scheme was then reviewed in 1994 under the SSDS Stage II Options Review Study in which over 50 different sewage treatment options were considered. An International Review Panel (IRP) with three experts from the Mainland, Denmark and the USA were appointed to oversee the work of the consultants. The Study concluded and the IRP confirmed that the original SSDS (a centralized treatment system) was appropriate and discharge of treated effluent within Victoria Harbour should only be regarded as a temporary measure.
6. Stage I of the SSDS is now being built. It follows that a centralised deep tunnel system must be pursued for Kowloon, Chai Wan and Shau Kei Wan, although the possibility still exists of a distributed system on other parts of Hong Kong Island. In deciding whether to pursue this, we would need to consider the following:
Land and Cost Implications
- to ensure these sewage treatment facilities would not affect the surrounding land use, eg. the commercial or residential developments, a large buffer zone of around 200 m would be needed to separate the treatment facilities from the surroundings;
- as it is very difficult to find or create land to put these sewage treatment facilities in the urban area, we would need further reclamation around Victoria Harbour to accommodate these facilities. This would mean further delays and further deterioration of water quality in the harbour; and
- due to the limited assimilative capacity of the harbour, a higher level of treatment (secondary treatment with full nitrification plus disinfection) would be required for all discharges, including Stonecutters Island.
7. In general, more land is required for distributed systems due to the duplication of general facilities such as access roads and administration buildings. Moreover, in the urban area in Hong Kong the resulting treatment works would be incompatible with the surrounding land use, requiring large buffer zones (approximately 200 m wide or 10 ha for each major sewage treatment works).
8. As a distributed system would discharge treated effluent to Victoria Harbour, it would require at least secondary treatment with full nitrification. Even with the latest compact sewage treatment designs, the total land requirement (including the treatment works and the buffer zones) would be at least 120 ha if Stage II and III/IV of SSDS were to be replaced by a distributed system. If the whole of SSDS had been designed as a distributed system, the total land requirement would be around 230 ha. This is around 20 times more than the land occupied by the Stonecutters Island Sewage Treatment Works, or 6 times more than a centralized secondary sewage treatment system utilising sites at Stonecutters Island and Lamma Island. (The projected sizes of the individual sewage treatment works are provided in Annexes A and B.)
9. It has sometimes been suggested that a distributed system would be cheaper than a centralised system because (a) it would avoid the capital costs of constructing the deep tunnels and (b) maintenance costs would be less than with a deep tunnel system. This argument is wholly without foundation. A distributed system with discharges into the harbour would require a high level of treatment. The costs of building the treatment works, and of forming the land, would be considerably more than the proposed centralised system. Similarly recurrent costs would be much higher. A detailed comparison is given in the following table (assuming biological nutrient removal is provided in each case). The middle column represents a hybrid with three treatment plants on Hong Kong Island and deep tunnels to transfer the flows. If Stage III/IV is not constructed on Hong Kong Island, then another two treatment works would be required at Central, and Wanchai East (see Annex A).
||Distributed system with few STWs and some deep tunnels (With SSDS Stage I, Annex B)
||Distributed system with no deep tunnel (No SSDS, Annex A)
|Location of Treatment Works
||Stonecutters Island and Lamma Island
||Stonecutters Island, Aberdeen, Green Island and North Point
||12 treatment works at various locations along the Victoria Harbour
- Deep tunnel
|Recurrent cost (per year)
- Deep tunnel
- STW & outfalls
|Total Land Requirement
Water Quality Implications
||1. assumes existing outfalls could be re-used.|
2. costs include operation of screening plants.
3. biological nitrogen removal is assumed for all systems
10. As most of the land in the urban area around Victoria Harbour has been developed, adopting distributed systems would no doubt require further reclamation in the harbour, which would require additional environmental impact assessments. There is no guarantee that these would be favourable. Even if they are, it would take around 8 years more to form the land, which would mean no upgraded sewage treatment would be available until 2014 at the earliest.
11. The sewage borne bacteria, measured as E.coli, in Victoria Harbour has already increased three fold (from 3000 to 9000 counts per 100 mL) during 1988 - 1997. With further delays in providing the sewage treatment facilities, the E.coli level in Victoria Harbour could rise to around 20,000 counts per 100 mL by 2016. On the other hand, there would be a continuing failure to meet the water quality objectives for dissolved oxygen and ammonia; Victoria Harbour will be rather like a sewage oxidation pond.
12. Conversely, under existing plans, construction of SSDS Stage III/IV will bring an early improvement in the water quality in Victoria Harbour.
Onshore Environmental Impacts
13. Both the construction and operation phases of a distributed system could have significant impacts on nearby residents. There would likely be very strong local opposition, especially in view of the additional traffic impacts arising from the need to transport sludge from the works.
Risks of System Failure
14. The relative risks associated with the failure of a centralized system can be separated into tunnels and sewage treatment works. Deep tunnels were adopted in order to avoid the MTR, utilities and piled foundations, and to reduce to a minimum disruption to traffic during construction and maintenance. Deep sewage tunnels have been used successfully elsewhere such as Boston and Sydney. (Deep tunnels are used for our own MTR and no-one has called into question their technical feasibility or suggested that they carry high risks of failure.) Endorsement of the Scheme by the IRP indicates that it is technically sound. Although the first contractor claimed he could not fulfil the contract, Stage I tunnels are being built now. The problems so far are not uncommon or unusual in engineering terms.
15. To ensure a smooth construction of the Stage II tunnels, we will carry out additional geological surveys. As more information becomes available, we will be in a better position to specify suitable construction equipment and necessary mitigation measures to ensure smooth progress of the work. Minor adjustment of the routing can also be made, if necessary. Thus construction risk would be minimal.
16. As far as operational risk is concerned, with preliminary treatment upstream and adequate flow velocity designed (to prevent sedimentation), the tunnels will be free from routine inspection and maintenance and the operational risk would also be minimal.
17. It has been argued that a distributed system must be more reliable than a centralised system because the risk of failure is also distributed. This argument is unsound for two reasons viz:
- a large centralised system provides greater stability of flow quantity and quality and is much less likely to be disrupted by sudden surges in flow or high concentrations of toxic substances;
- given that a distributed system would necessarily entail a number of biological treatment works, if it is accepted that CEPT provides an adequate treatment level for the proposed centralised system then a distributed system is actually more risky. CEPT is essentially a physico-chemical process which is unlikely to be disrupted by sudden changes in flow or sewage quality. Even if such systems are disrupted they can quickly be brought back into normal operation. Biological systems however are famously susceptible to changes in sewage quality and, once disrupted, take a long time to re-establish. So not only is the risk of failure greater but so too is the risk of prolonged unacceptable impact.
18. Compared with a distributed system, a centralized system is more cost effective both in terms of capital and recurrent costs, is less vulnerable to influent change if CEPT plus disinfection is adopted, has least impact on the community and requires less space. On the other hand, a distributed system would bring many disadvantages without delivering any compensatory environmental or other benefits. A summary comparison of distributed and centralized sewage treatment schemes is provided in Annex C.
Annex C. Summary Comparison between Distributed and Centralized Sewage Treatment Schemes
|Distributed System including some deep tunnels
||Stonecutters Island, North Point, Green Island, Aberdeen
||Stonecutters Island, Lamma Island quarry (if secondary/BNR treatment is required)
|| Greater amount of land required than the centralized system. New land reclamation likely to be required to accommodate sewage treatment plants which will include BNR. Rough estimate of additional land required (to be reclaimed) is about 110 ha. Estimated total land requirement is as follows:
SCI STW: 65ha (Pop: 4.4m)
NP STW: 28ha (Pop: 0.4m)
GI STW: 18ha (Pop: 0.4m)
Abd STW: 9ha (Pop: 0.3m)
|Formed land available (provided Lamma Quarry site is allocated for the biological STW). Higher treatment level BNR may require additional land requirement, but significantly less than the distributed system. Rough estimate of additional land required to be reclaimed if BNR is to be provided is about 5-6 ha. Estimated total land requirement for a total population of 5.5m is as follows:
SCI STW: 11ha (CEPT only)
LI STW: 28ha (BNR only)
|Time for Construction
||An additional 8 years is required for land formation.
||Moderate time required for deep tunnels and STW constructions (and a smaller degree of reclamation if BNR is required).
||Within Victoria Harbour, western approaches to Victoria Harbour (Western Fairway), East Lamma Channel
||Southern Waters south of Lamma Island
||Secondary treatment will be required.
||CEPT is more stable and easier to operate than secondary treatment works (for Options 1, 2)
||Potential sewage transfer system and STWs close to populated areas. Large buffer zones needed.
||Potential site at quarry relatively buffered from populated areas (sea - south & east, steep slopes - north and most of west sides)
||Construction noise likely to have significant impacts on nearby residents.||Construction noise likely to have much less impact on residents.
||Potential air pollution from the construction works very likely to impact on residents in the vicinity. Odour from STW's during operation also possible (but measures could be applied to mitigate this concern).
||Construction works (and operations) will be away from densely populated areas; therefore, air related impacts not as significant.
||Disruptions to land-based traffic likely to be more significant than centralized system due to construction of sewage transfer and STW works in vicinity of urban areas. During operational phase, sludge transportation by either land or marine disposal method have a higher impact due to a higher number of sites (4).
||Minimal impacts from land or marine based traffic due to construction of deep tunnel and STW (which is away from urban areas). During operational phase, sludge to be disposed of by marine transportation from only 2 sites.
Treatment Plant Failure
||Failure at a particular STW location not likely to affect the performance of STW's at other locations. In any case, redundancies in treatment capacities, power supplies, mechanical & electrical equipment and control systems would need to be provided at each treatment works.
||The Stage I design and construction have incorporated the following philosophies in the works:
- modernisation of the PTW's with the provision of additional screening and grit removal facilities at each site so that the full design flow can be treated even when one unit in each treatment stage is down for maintenance.
- Provision of standby tanks at the SCI STW to allow for full treatment of the peak design flow even with tanks down for maintenance.
- provision of standby mechanical and electrical equipment for all plant.
- dual incoming power supplies to all major plants including the Stonecutters Island STW to enhance reliability.
- provision of 100% redundancy of the scheme's computerised monitoring and control systems.
||Moderate risk of process failure for the secondary treatment works due to high dependent of the organisms on various external factors including temperature, pH, and toxicity levels of influent sewage.
||Low risk of process failure for the primary treatment works due to its being predominantly a physical process. If secondary treatment is required, similar process failure risk as discussed for the distributed system.