To improve the water quality of the Victoria Harbour, the Harbour Area Treatment Scheme (HATS) formerly known as Strategic Sewage Disposal Scheme was committed in late eighties to be implemented in stages to provide treatment for the sewage collected from the urban areas on both sides of the Harbour. The HATS Stage 1 comprised the Stonecutters lsland Sewage Treatment Works and 23.6km of deep tunnels for treating sewage collected from urban Kowloon, Kwai Tsing, Tseung Kwan O and north-eastern Hong Hong Island by chemically enhanced primary treatment before discharge into the western harbour through a submarine outfall. Since its commissioning in late 2001, the HATs Stage 1 system has brought substantial and widespread improvements in the general water quality of the Harbour.
In late nineties, the Environmental Protection Department undertook a series of studies on the development of further stages of the HATS. In April 2000, an International Review Panel (IRP) was invited to review the way forward for the further stages of the Scheme (now called HATS Stage 2). In its report released in November 2000, the IRP proposed four treatment and discharge options for HATS Stage 2.
All the four options proposed by the IRP are environmentally acceptable and technically feasible. Among the options, the centralized option performs the best overall in terms of environmental, engineering, cost, social and land aspects and is selected as the preferred option for the development of HATS Stage 2.