SEWERAGE SCHEME FOR NGONG PING DEVELOPMENT BACKGROUND Ngong Ping, at interior part of Lantau and at the relatively high platform at a level of about 440mPD, has always been a tourism hot spot throughout the years. Po Lin Monastery, has already attracted thousands of visitors daily during weekends, especially after the opening of Tin Tan Buddha Statue in 90s. The planned Cable Car will enhance the accessibility, opening up new tourism initiatives such as Ngong Ping Theme Village. Figure 1 Tourism Hotspot at Ngong Ping Po Lin Monastery

Figures 2 & 3 Earlier Sewerage System

The earlier sewage discharge in Ngong Ping relies on distributed septic tanks and soakaway systems, which have severe limitation on the flow to be handled. There are performance, lack of maintenance and odour problems. Worst still, Ngong Ping is located in the environmentally sensitive Water Gathering Ground leading to Shek Pik Reservoir as indicated in Figure 5. Obviously, the earlier sewerage in Ngong Ping is far from satisfactory to cater for the future need. In December 1994, the Environmental Protection Department (EPD) completed the Outlying Islands Sewerage Master Plan (SMP) Study drawing up a SMP for Lantau Island and other outlying islands. One of the conclusions is that the existing treatment and disposal facilities at Ngong Ping should be upgraded to cater for the sewage generated from both existing and committed developments.

Figure 4 Ngong Ping Village

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KEY FEATURES OF THIS PROJECT The following features are quite unique, if not unprecedented. 1. This is the first tertiary treatment works in Hong Kong; 2. The treated effluent is reused for toilet flushing, controlled irrigation and fish pond usage; 3. The work is in full compliance with Environmental Impact Assessment Ordinance (EIAO) with 2 Environmental Permits issued for each of the above (The first Environmental Permit issued under EIAO for reuse of treated effluent); 4. The sewerage system is located within water gathering ground, requiring a careful monitoring of the sewage handling and conveyance; 5. The whole treatment process is fully automatic and unmanned with all operational data relayed by a Monitoring and Control System to Siu Ho Wan Sewage Treatment Works; 6. All the treatment units are housed or built underground, in view of the sensitive tourism location; 7. Stringent odour control is applied to a very high standard; 8. Special landscaping treatment is made to reduce the conventional industrial setting of a sewage treatment works; 9. Sewage Treatment Works completed and commissioned in 2½ years, well ahead of the adjacent Cable Car Terminal which it mainly serves; and 10. The plant is open to public for education purpose with facilities for visitors, namely Ngong Ping Sewage Treatment Information Centre.

KEY ENVIRONMENTAL CONSTRAINTS Whilst the project may apparently be typical of sewerage works, an unconventional approach is adopted due to a number of environmental factors as follows:• The site is located within established tourist attractions. If no proper environmental mitigation measures are taken, a highly conflicting or incompatible eye-sore will be created. Worst still, the site is located inland and access to marine water for disposal is not readily available; • The site is located within WSD’s Water COUNTRY PARK Gathering Ground. It is for this reason that drives the need for tertiary treatment of the effluent such that the impact to NGONG PING Shek Pik Reservoir, in the unlikely event WATER GATHERING GROUND of pipe leakage, is small; • The site is also located in North Lantau Country Park, thus triggering the need to consult Country and Marine Park Committee, apart from the processing under Environmental Impact Assessment Ordinance; Figure 5 Water Gathering Ground leading to Shek Pik Reservoir

• The Ngong Ping Site of Special Scientific Interest is close to a section of the proposed village sewers; and • Part of the proposed village sewers will be laid within the “Conservation Area” zone as defined in the Approved Ngong Ping Development Permission Area Plan. • Apart from being located in an environmental sensitive location, the transport is an issue. Whilst transport to North Lantau has been improved since 1998, the route to Ngong Ping site via Tung Chung Road, South Lantau Road, Keung Shan Road and Sham Wat Road is narrow with sharp bends as these roads were built many years ago. Delivery of heavy and large equipment, such as deodouriser, has to be made by helicopter. In fact, the hilly terrain at nearby Cable Car Tower location has called for the use of donkey or helicopters for the construction materials delivery.

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FORMULATION OF SEWERAGE INFRASTRUCTURE Overall Strategy Based on the above environmental constraints, the overall strategy is formulated. The complete sewerage infrastructure contains a sewage collection network to nearby Ngong Ping village, sewage treatment facilities and effluent export and disposal facilities. For sewage treatment strategy, 2 scenarios are considered as follows: • Conveying sewage to outside Water Gathering Ground for treatment at an existing or a planned sewage treatment works(e.g. Tai O Sewage Treatment Works); or • On spot treatment at Ngong Ping and convey the effluent to marine waters for disposal. The quality of the treated effluent in the export pipeline when passing Water Gathering Ground must be high. In view of the environmentally sensitivity, the first scenario is not preferred as any leakage of sewage along the sewage export pipe, in the unlikely event of pipe leakage, will inevitably contaminate Water Gathering Ground leading to the Shek Pik Reservoir. Hence the second option is adopted with the effluent from the sewage treatment works treated to meet Water Quality Objectives of the receiving water body, Southern Water Control Zone. The following minimum effluent quality standards are required:Water Quality Parameter Minimum Effluent Standard BOD5 10mg/l (95%ile) Suspended Solid 10mg/l (95%ile) Ammonia Nitrogen 1mg/l annual average Total Nitrogen 6mg/l annual average E. coli. 100 count/100ml geometric mean Table 1 Minimum effluent quality standards

Effluent Export and Disposal Route A number of possible routings have been considered in the planning stage, as indicated in Figure 6. • Sham Wat Route – Potential impact to Sham Wat Stream and Valley which are of high ecological value. Cable Car Route • Cable Car Route – Potential impact to woodland. Sham Wat Route • Tung Chung Route – Passing through stream course as well as requiring access road for the pipeline maintenance. STP Tung Chung Route • Tai O Route – This routing was proposed in the Tai O Route earlier feasibility study but experienced strong objection from Tai O residents during Tung Wan Route consultation. • Tung Wan (Downstream of Skek Pik Figure 6 Possible Effluent Disposal Routes Reservoir) Route is finally adopted due to the least environmental impact as most of the pipeline sections were laid under the public roads, with no objections raised. Sewage NGONG PING 北 N

TUNG CHUNG

Treatment TAI O NGONG PING ROAD SHAM WAT ROAD

The final effluent export pipeline configuration is shown in Figure 7:-

5.7 km EFFLUENT EXPORT PIPELINE

LANTAU PEAK KEUNG SHAN ROAD SHEK PIK RESERVOIR

TUNG WAN

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Figure 7 Final Effluent Export Pipeline Configuration

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SEWAGE TREATMENT WORKS Sewage Treatment As illustrated in the process schematic diagram, the following are the typical components:• Incoming sewage to the treatment works undergoes preliminary treatment in Inlet Works, secondary treatment in the Sequencing Batch Reactor (SBR) basin, tertiary treatment in Tertiary Filters and disinfection in UV disinfection system.

Figure 8 Process schematic diagram of Ngong Ping Sewage Treatment Works

• Inlet Works provide fine screening, grit removal and grease separation for preliminary treatment prior to the biological treatment downstream. - Fine Screen – in the form of screen with bar spacing at 6mm. Figure 9 Fine screen with bar spacing 6mm

- Grit Trap – For grit removal with 95% of grit of size exceeding 0.25mm removed.

Figure 10 Vortex Grit Trap

- Grease Separator – to facilitate the separation of oil from the sewage, relying on the floating action induced by the air generated from the coarse bubble diffusers.

Figure 11 Grease Separator

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Baffle Wall

Pre-React Chamber

Main React Chamber

Figure 12 SBR Basin Configuration

Decanter

Effluent

• Biological Treatment - Sequencing Batch Reactor (SBR) is adopted for the following reasons:- complete treatment in a single basin which is compact in size; - biological nutrient removal; - filamentous growth controlled by creating an anoxic/anaerobic condition during filling; and - more toleration to hydraulics and organic shock load. In Ngong Ping Sewage Treatment Works, ICEAS, a variation of traditional SBR, is adopted with the additional advantage to allow continuous filling of sewage to the SBR basin. The aeration system is of fine bubble diffuser type to increase the oxygen transfer efficiency. With the creation of aerobic zone and anoxic zone, nitrification and denitrification processes can take place within a tank.

Figure 13 Diffuser Aeration System of SBR System

Figure 14 UV disinfection system

• Tertiary Filtration - After biological treatment, the effluent is diverted to the tertiary filter for polishing with the suspended solid of the effluent after filtration lowered to 10mg/l. The adopted tertiary filter is a granular dual filtration media type with anthracite and silicon sand as the media. A more detailed discussion is made in the Section on Tertiary Treatment. • Disinfection - After filtration, effluent is diverted to UV disinfection system, based on low pressure high intensive type UV lamps, for disinfection prior to the disposal/further treatment for reuse.

Sludge Treatment It is designed to minimise the sludge produced through sludge digestion process. In addition, as the sludge has been stabilised in the digestion process, the odour generation from the sludge will be minimised. The treatment locations of various processes are indicated in Figure 15.

Figure 15 Sludge Stream of Ngong Ping Sewage Treatment Works

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Waste sludge in the SBR basin will be regularly pumped to the sludge thickeners. The solid content of the waste sludge will increase from about 0.5% to about 1.5% after thickening. The sludge is then pumped to sludge digester for digestion and stabilisation. Finally, the digested sludge is dewatered to at least 30% dry solid content in the sludge dewatering facilities. • Sludge Thickening - by gravity type with the advantages of simple and energy saving.

Figure 16 Gravity Sludge Thickener

• Sludge Digestion - The sludge thus thickened is diverted to the sludge digesters with aerobic type adopted to actively control the digester environment through aeration.

Figure 17 Sludge Digester

• Sludge Dewatering - by membrane filter press system.

Figure 18 Filter Press System

TERTIARY SEWAGE TREATMENT As a preliminary classification, the effluent water qualities from various treatment levels are listed as follows: Type Nature of Process Effect on Water Quality Preliminary Coarse and Fine Screen, Grit and No significant improvement of water quality apart Grease Removal from removal of unwanted materials physically Primary Sedimentation 60% of suspended solid and 20% BOD5 removed Secondary Biological Treatment < 20mg/l in BOD5 and < 30mg/l in suspended solid Tertiary Effluent polishing < 10mg/l in BOD5 and < 10mg/l in suspended solid Table 2 Effect on Water Quality by Different Treatment Levels

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Driven by the location of the treatment plant falling within the Water Gathering Ground, the sewage treatment plant was designed to tertiary level incorporating: • nutrient removal processes including nitrification and denitrification processes; as well as • effluent polishing via tertiary filtration system in the form of dual media filters as shown in Figures 19 and 20. TERTIARY FILTER

EFFLUENT FROM SBR

PIPE GALLERY

FILTERED EFFLUENT FILTER CELL BACKWASH TANK

Figure 19 Tertiary Filter System

Figure 20 Section of Tertiary Filter System

Figure 21 Announcement for the First Tertiary Plant by Director of Drainage Services

EFFLUENT REUSE Various quality standards for reuse of treated effluent are listed in the following table:Type of reuse Urban Use E.g. Irrigation in parks Industrial Use E.g. Cooling system Agricultural Irrigation Recreational Use E.g. Fishing Facilities Environmental Use E.g. Wetlands Indirect Potable Use E.g. Groundwater Recharge

BOD5 (mg/l)

Treated Effluent Quality SS (mg/l) E. Coli (count/100ml)

≤ 10

-

Not detectable

≤ 30

≤ 30

≤ 200

≤ 10

-

Not detectable

≤ 10

-

Not detectable

≤ 30

≤ 30

≤ 200

Depends on Site environment & Local portable water quality standard Table 3 Requirement for Effluent Reuse

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The quality of the tertiary treated effluent has already reached a level very close to the limiting standard of effluent reuse, with the exception of residual chlorine, as shown in the following table. Parameters Units Limiting Standard Colour < 20 unit 9 Ammonia N mg/l 2 9 BOD5 mg/l 10 Total Organic mg/l 9 Total SS mg/l < 10 Turbidity NTU < 10 9 E. coli count/100ml 100 8 Residual Chlorine mg/l > 0.5 Synthetic Detergents mg/l 2 1 6-8.5 >2