Substantial population growth in the past decades around Victoria Harbour resulted in water quality deterioration from the discharge of preliminary treated sewage into the Harbour.The Harbour Area Treatment Scheme (HATS) is a centralised sewage collection and treatment scheme for areas on both sides of Victoria Harbour.Construction of this world - class sewage treatment infrastructure is being carried out in several stages,viz Stage 1,Stage 2A and Stage 2B.Stage 1 of HATS involved the construction of a 23.6 km - long system of tunnels deep underground,carrying currently 1.4 million tonnes of sewage each day from Kowloon and the northeastern part of Hong Kong Island to a sewage treatment plant at Stonecutters Island for chemically enhanced primary treatment before disposal.Stage 1 of HATS has brought about noticeable water quality improvement to the Victoria Harbour since its full commissioning in 2001 by treating 75％ of the harbour sewage,but more needs to be done to restore the Harbour to a healthier condition.Stage 2A of HATS will upgrade the preliminary treatment works around the northern and southwestern shore of Hong Kong Island and construct a 21 km- long deep tunnel system to convey the sewage from Hong Kong Island to Stonecutters Island Sewage Treatment Works (SCISTW),where the existing chemical treatment facilities will be expanded and new disinfection facilities added.Detailed planning and design for this phase is well underway with the aim of commissioning Stage 2A in 2014.Stage 2B of HATS will involve the provision of biological treatment at the SCISTW to further improve the effluent quality.The timing of the Stage 2B implementation will be decided upon a review of water quality trends,population increases and sewage flow build up to be undertaken in 2010 - 11.Improved harbour water quality will benefit society in many different ways,including for example,enhanced recreational opportunities,scenic quality,aesthetics,public health and safety as well as ecological and fishery resources.