The Harbour Area Treatment Scheme (HATS, formerly known as Strategic Sewage Disposal Scheme) is to improve water quality in Victoria Harbour by intercepting and treating sewage from developments on both sides of the harbour. The HATS comprises two stages: 1 and 2.

The HATS Stage 1 has, since its full commissioning in December 2001, been preventing 600 tonnes of sludge from entering the Victoria Harbour each day and has brought about general water quality improvements in the harbour. However, the sewage generated from the most densely populated parts of Hong Kong Island, which currently accounts for about 25% of the sewage from the planned catchment of HATS, is only subject to preliminary, screening before discharge. This coupled with the effluent discharged from the Stonecutters Island Sewage Treatment Works (SCISTW) without disinfection, is the source of water quality problems in the western parts of the harbour. Moreover, with the anticipated growth in population and business activities in the harbour area, water quality would resume a deteriorating trend in future unless the sewage from Hong Kong Island is intercepted and treated, and the overall treatment level at Stonecutters Island is raised by completing HATS Stage 2.

After taking into account the public's views sought in a five-month consultation conducted in 2004, the Government announced in 2005 the programme for HATS Stage 2. The programme is to implement HATS Stage 2 in two phases, namely Stage 2A and Stage 2B. Under Stage 2A, deep tunnels will be built to bring sewage from the northern and south western areas of Hong Kong Island to SCISTW. The treatment works will be expanded to provide centralized chemically enhanced primary treatment and disinfection for all sewage from the whole of the HATS catchment. Part of the Stage 2A disinfection facilities may be fast-tracked for completion in 2009. Under Stage 2B, a new biological treatment plant on a site adjacent to SCISTW is proposed.

Since the full commissioning of HATS Stage 1, although the overall bacteria (E. coli) levels in the harbour have been reduced by some 50%, the levels in the western harbour and the beaches along the Tsuen Wan coast have increased due to the discharge of large volume of treated but un-disinfected effluent off Stonecutters Island, resulting in closure of four gazetted beaches along the Tsuen Wan coast in 2003. In response, Government is implementing a project now to advance the provision of part of the permanent disinfection facilities under HATS Stage 2A to improve the water quality in the western harbour area for completion in 2009 to disinfect the treated effluent at SCISTW before discharge so that water quality along the Tsuen Wan coast can be improved. The purpose of the "Harbour Area Treatment Scheme - Provision of Disinfection Facilities at Stonecutters Island Sewage Treatment Works" (hereinafter referred to as "the Project") is to implement the above proposal.

The key elements of the disinfection facilities, also termed the advance disinfection facilities (ADF) include:
a) Chlorination system - provision of a sodium hypochlorite solution storage farm and associated dosing system; and
b) Dechlorination system - provision of a sodium bisulphite storage and associated dosing system; and
c) Other ancillary works.

In order to ensure compliance with appropriate standards and requirements during operation of the Project, environmental monitoring shall be conducted to verify the performance of the new disinfection facilities and to confirm the acceptability of the impact of the chlorinated/dechlorinated discharge on the receiving marine environment.