The Stonecutters Island Sewage Treatment Works of HATS Stage 1 uses the chemically enhanced primary treatment (CEPT) process. This is the first time this treatment process is used in Hong Kong.  In this process chemicals (ferric chloride and polymer) are added to the sewage to facilitate faster settlement of suspended solids.  The settled sludge is then removed from the sewage resulting in a clear supernatant for discharge to the sea after disinfection.