Under HATS Stage 2A, disinfection treatment is provided at SCISTW to reduce the bacteria level in the chemically treated effluent before discharge. In order to provide as early as possible the disinfection treatment to the existing Stage 1 flow at SCISTW and to facilitate the early re-opening of the closed Tsuen Wan beaches, part of the disinfection facilities under HATS Stage 2A was advanced for early commissioning. The construction of the advance disinfection facilities (ADF) commenced in April 2008 and the works were substantially completed in December 2009. The ADF provids disinfection to the Stage 1 effluent from SCISTW before discharge to the harbour.
An environmental impact assessment (EIA) study has been conducted for the ADF. Under the EIA Study, various disinfection technologies that were available for water and wastewater disinfection, including some emerging technologies, were examined and a careful evaluation exercise was carried out to select the most appropriate disinfection technology for HATS. Local and international disinfection practices were also reviewed. The study concluded that chlorination by the use of sodium hypochlorite solution was the preferred disinfection technology for HATS given the time, land, and planning constraints.
For the ADF operation, the existing effluent culvert system is used as the chlorine contact basin. After the chemically enhanced primary treatment (CEPT process, the effluent undergoes disinfection treatment using sodium hypochlorite solution to remove at least 99% of the bacteria in the effluent and followed by dechlorination to neutralize any residual chlorine before discharge.
A computerized control system is set up to monitor and control the dosing of sodium hypochlorite and sodium bisulphitesolutions based on the measured total residual chlorine (TRO) level in the chlorinated effluent.