Drainage Services Department

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FAQ

1.What’s HATS?

Harbour Area Treatment Scheme (HATS) is a major Government infrastructure project in Hong Kong. It is being implemented in stages to combat water pollution caused by urban development around Victoria Harbour. HATS Stage 1 was commissioned in December 2001 providing treatment to about 75% of sewage from urban areas around the harbour, significantly improving water quality to the eastern and central parts of our harbour. HATS Stage 2 will be implemented in two phases, Stage 2A and Stage 2B. Stage 2A will provide treatment to the remaining 25% of sewage from the northern and southwestern parts of Hong Kong Island. In addition, a disinfection facility will be installed to further improve the quality of the harbour waters, making it possible to re-open the Tsuen Wan beaches. Stage 2B will further provide biological treatment to the sewage. A review is being carried out to determine the time table of implementation of HATS Stage 2B. The full commissioning of HATS can ensure the long term sustainable development of the harbour area.

2.What is HATS Stage 1?

HATS Stage 1 provides treatment to the sewage collected from the eight Preliminary Treatment Works (PTWs) located at Tseung Kwan O, Kowloon, Kwai Tsing and the north-eastern part of Hong Kong Island and conveyed by deep tunnel to Stonecutters Island Sewage Treatment Works (SCISTW) for chemically enhanced primary treatment (CEPT). Commissioned in December 2001, the Stage 1 System treats 75% of the sewage generated from the Harbour area and can serve a population of 4.5 million.

3.What is HATS Stage 2?

HATS Stage 2 will be implemented in two phases, Stage 2A and Stage 2B. Stage 2A will provide treatment to the remaining 25% of sewage generated from the northern and southwestern sides of Hong Kong Island. In addition, disinfection facility will be installed to improve the quality of the effluent and hence the harbour waters, making it possible to re-open the Tsuen Wan beaches. Stage 2B will provide biological treatment to all the effluent, thus improving water quality further. After the completion of Stage 2, the whole HATS system can treat all the sewage generated from the Harbour area and can ultimately serve a total population of 5.7 million.

4.What improvements have been achieved by HATS Stage 1?

With the full commissioning of Stage 1 in 2001, about 600 tonnes of sewage sludge are prevented from entering Victoria Harbour every day and it has brought about substantial improvements to the water quality of the Harbour including: An average increase of about 10% of dissolved oxygen (vital for marine life) in the harbour waters, bringing benefits to marine ecology. The levels of key pollutants in the harbour waters have generally decreased : Ammonia (toxic to marine life) has declined by 25%; Total inorganic nitrogen and phosphorus (nutrients, which in rich supply can increase the likelihood of red tides) have reduced by 16% and 36% respectively; and The overall E.coli level (an indicator of disease-causing micro-organisms) has reduced by some 50%.

5.There have been a lot of problems with the Stage 1 tunnelling work. Is the concept of deep sewage transfer tunnels in the whole HATS project workable?

The use of a tunnel system at an average depth of 100 metres to transfer sewage helps avoid opening up many kilometers of roads on the surface which would otherwise have caused serious traffic and social disruptions. It also minimizes disturbance to buildings and structures along the tunnel alignments.

Apart from the unilateral suspension of the tunnelling work in late 1996 by the original contractor which led to major delay of the project, the obstacles encountered are not insurmountable. In fact, dealing with the variable nature of rock is part and parcel of the tunnelling operation itself. The tunnelling works have been completed satisfactorily and the HATS Stage 1 successfully commissioned and put into full operation since December 2001.

6.Does sewage require treatment before entering the deep tunnels?

Sewage must be preliminarily treated to remove large solids and grits to avoid deposition in the deep tunnels and to protect downstream facilities from damage or blockage. The existing eight PTWs at the northern and southwestern parts of Hong Kong Island will be upgraded to cater for the technical requirements of HATS Stage 2A as well as population growth and development around the Island. In addition, architects and landscaping specialists will be engaged to improve the aesthetics of the PTWs. Also, engineers will be deployed to work out state of the art systems that will ensure an odour free environment. Both will bring lasting benefits to nearby residents.

7.What does the Sewage Conveyance System of HATS Stage 2A comprise?

The sewage conveyance system (SCS) comprises a network of interconnected sewage tunnels and vertical shafts. The vertical shafts collect sewage from the preliminary treatment works in North Point, Wan Chai East, Central, Sandy Bay, Cyberport, Wah Fu, Aberdeen and Ap Lei Chau. The collected sewage is conveyed to the Stonecutters Island Sewage Treatment Works via a total of 21km of deep tunnels with depths varying from 70m to 160m below sea level. The system essentially operates as a siphon.

8.How are the sewage tunnels built?

Drill-and-blast method is chosen for constructing the sewage tunnels. In addition to providing more working space for temporary support installation; yielding a more controllable programme; and resulting in higher value of excavated rocks for re-use, this method is more versatile in controlling groundwater during construction, especially for deep tunnels under sea. The smaller diameter tunnel from Ap Lei Chau to Aberdeen will be constructed using horizontal directional drilling method. The tunnel drilling equipment has a remote directional steering device to control the alignment of the tunnel during construction.

9.What treatment process is adopted at SCISTW?

The Stonecutters Island Sewage Treatment Works (SCISTW) is a chemically enhanced primary treatment (CEPT) plant and adopts a space-saving double-tray sedimentation tank design to reduce its footprint. It currently has a capacity for treating 1.7 million cubic metres of sewage each day and will be expanded under HATS Stage 2A to increase its capacity to 2.45 million cubic metres per day to cope with the increased flow from Hong Kong Island and future developments in the HATS catchment, serving a population of 5.7 million people ultimately. Occupying only 10.6 hectares of reclaimed land, SCISTW is one of the most compact treatment plants of its kind in the world.

10.How is sewage lifted to the surface when arriving at SCISTW?

When sewage arrives at SCISTW, it will be lifted from the deep tunnels to the surface via two very large main pumping stations, one serving the Stage 1 flow and the other one for the Stage 2 flow respectively. The two pumping stations are interconnected by a tunnel to allow flow to be diverted if needed to enhance the reliability of the operation of these main pumping stations. Both pumping stations have an internal diameter of over 50 metres and are about 40 metres below ground, involving deep excavation within a diaphragm wall. Massive pumps driven by variable speed drives are installed at the bottom of the pumping station to lift sewage from the tunnels to the sedimentation tanks.

11.How is treated effluent discharged?

Treated effluent from SCISTW is discharged to the waters southwest of Stonecutters Island via a 5m diameter, 1.7km long tunnel which passes under the Northern Fairway at a depth of about 100m. The outfall tunnel is connected to a 1.2km diffuser pipeline laid in a pre-dredged trench on the seabed through two riser pipes. The effluent is dispersed into the tidal stream through 24 diffusers installed along the diffuser pipeline.

12.How is odour from the treatment plant controlled?

All potentially odorous facilities at SCISTW, including the pumping stations, sludge dewatering building, sludge cake silos, sedimentation tanks, etc. will be enclosed and controlled to provide an odour-free environment. The foul gas inside the enclosures will be extracted and ducted to designated deodourization facilities to undergo treatment using various technologies such as bio-trickling filters before it is released into the open air.

13.What benefits will be brought by HATS Stage 2A?

With the completion of Stage 2A we can :

Treat wastewater flowing to the harbour;
Maintain a healthy marine environment whilst meeting future development needs;
See more species of marine life;
Re-open the Tsuen Wan beaches;
Remove the sight of sewage plumes from the harbour; and
Conduct cross-harbour swimming race.

14.When will HATS Stage 2B be implemented?

To facilitate the sustainable development of the harbour area in the long term, HATS Stage 2B will be constructed near to SCISTW and will provide biological treatment to all the effluent, thus improving water quality further. A review is being carried out to determine the time table for the implementation of HATS Stage 2B.