HATS - Stage 1 Sewage Conveyance System (Completed)

Stage 1 (Completed)

Construction of the Stage I works, which demanded a high degree of engineering expertise, commenced in April 1994. Works were packaged into 15 works contracts to enable participation of both local and overseas contractors with the required specialist skills. The chemically enhanced primary treatment (CEPT) plant at Stonecutters Island was completed and put into operation for treating sewage flows from the North West Kowloon areas in May 1997. It has since shown excellent performance, being recognised as one of the world's most efficient facilities that adopt the CEPT process.

Completion of the HATS Stage I deep tunnel system in December 2001 is not only a major milestone in the HATS project, but also represents one of the most remarkable engineering achievements ever carried out in Hong Kong. These tunnels are the world's deepest sewage tunnels and have a length of about twice that of the Mass Transit Railway line on Hong Kong Island. When the entire Stage I system is fully tested and commissioned in early 2002, sewage from the most densely populated areas around the Harbour will receive proper treatment before disposal. Stage I of the Scheme is designed to serve a population of 3.5 million people.

It was the largest sewerage infrastructure project ever constructed in Hong Kong and consists of a 23.6km deep tunnel conveyance system collecting sewage from Tseung Kwan O, Kwun Tong to Kowloon Bay area, the whole of Kowloon peninsula, Kwai Chung, Tsuen Wan, Tsing Yi, Chai Wan and Shau Kei Wan. It treats about 75% of the sewage (about 1.4 million cubic metres per day) generated on both sides of the harbour. The sewage tunnels transfer the collected sewage to the Stonecutters Island Sewage Treatment Works (SCISTW) for chemically-enhanced primary treatment (CEPT) and subsequent discharge through a submarine outfall to western waters.

  1. Collecting Sewage

Raw sewage from the Stage I catchment, which extends from Tseung Kwan O to Tsing Yi in Kowloon and from Chai Wan to Shau Kei  Wan on Hong Kong Island, is first directed to 7 preliminary treatment works (PTWs) all located along the harbour front. These PTWs, in existence for many years, have been modernised under the HATS to provide a high standard of performance in removing grit and large solids before the sewage enters the deep tunnels. A great deal of effort and coordination was needed during the upgrading process to ensure the proper functioning of these PTWs at all times.

  2. Conveying Sewage to Stonecutters Island

Construction of these tunnels proved to be a challenging task. Exceptional efforts were required in cutting through various sections of faulted ground and traversing under the built-up areas of the Kowloon peninsula, the MTRC underground railways (crossed at six locations), the Eastern Harbour Crossing, and Kwai Chung Container Terminals. All these challenges were successfully overcome and by December 2001 the entire tunnel conveyance system was completed and being progressively tested and commissioned.

  3. Lifting Sewage from Deep Tunnels

When sewage arrives at Stonecutters Island, it is lifted from the deep tunnels to the surface via the Stonecutters Island Main Pumping Station (SCIMPS) which is one of the world's largest underground sewage pumping stations. The pumping station has an internal diameter of 50 metres and is more than 38 metres below ground. Construction of this facility involved deep excavation within a diaphragm wall. Eight massive pumps, each with a capacity of eight cubic metres per second and driven by variable speed drives, are located at the bottom of the pumping station to lift sewage from the tunnels to the inlets of the sewage treatment works. The pump motors and control room are located in the superstructure, which is a circular building with external curtain wall glazing to provide a high quality finish. An automated control system has been installed in the pumping station to control the entire Stage I system, including the seven PTWs, and to operate the pumps in an energy-efficient manner.

  4. Treating Sewage

Sewage entering the system receives treatment at the Stonecutters Island Sewage Treatment Works (SCISTW), which was built on 10.6 hectares of reclaimed land and commissioned in May 1997. SCISTW, with a design capacity for treating 1.7 million cubic metres of sewage per day, is the largest chemically enhanced primary treatment plant in the world. It features space-saving, double-tray sedimentation tanks and employs the latest technology and equipment available. During construction, concurrent activities had to be carried out by up to 10 contractors within the confines of the site, demanding extensive coordination efforts. Since its opening, the plant has been treating sewage flows from the North West Kowloon areas with excellent performance results and is acclaimed by local and international experts as one of the world's most efficient CEPT plants.

  5. Disposal of Sludge

The slurry sludge drawn from the sedimentation tanks is delivered to the adjacent sludge treatment facilities. It is first conditioned with polymer and then pumped into centrifuges where the water content is significantly reduced to produce solid sludge cakes. Upon full commissioning of HATS Stage I, about 800 tonnes of dewatered sludge will be produced and transported to landfill sites every day. Odour control facilities have been incorporated and sealed containers are used to minimise nuisance to the public throughout the treatment, handling and transportation process.

Sludge Cake Silos with Odour Mitigation

  6. Discharging Treated Effluent

Treated effluent from SCISTW is discharged to the waters southwest of Stonecutters Island along a 1.7km long tunnel, which passes under the Northern Fairway at a depth of about 100m. The tunnel is the largest of the Stage I tunnels and was excavated by tunnel boring machine through difficult ground conditions. It is lined with precast concrete segments, to give a finished diameter of 5 m, and is connected to two 3.25m diameter riser pipes and a 1.2km diffuser pipeline laid in a pre-dredged trench on the seabed. To protect the diffuser pipeline from damage by ship anchors, a layer of rock armour was placed around it and each diffuser head was covered by a protective dome. The effluent is dispersed into the tidal stream through 24 diffusers installed along the diffuser pipeline. Since commissioning, the outfall has been operating satisfactorily.

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