Stage 1 (Commissioned)
Stage 1 was commissioned in December 2001 providing 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 lsland Sewage Treatment Works (SCISTW) for chemically enhanced primary treatment (CEPT). The Stage 1 system treats 75% of the sewage generated from the Harbour area and can serve a population of 4.5 million.
Stage 2 (Commissioned)
Stage 2 was implemented in two phases, Stage 2A and Stage 2B. Stage 2A was commissioned in December 2015 providing treatment to the remaining 25% of sewage generated from the northern and southwestern sides of the Hong Kong lsland. In addition, disinfection facility was installed to improve the quality of the effluent and hence 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 2A, 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.