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Sewerage >Technology Employed >

Disinfection

Disinfection technologies under investigation include ultra-violet irradiation, ozonation, chlorination/de-chlorination and membrane filtration. The choice of technology depends on a number of factors including the capability of the technology in achieving the required disinfection targets taking into account the quality of effluent to be disinfected, possible formation of undersirable disinfection by-products, land requirement as well as the capital and running costs.

A large Ultra-violet (UV) irradiation disinfection system was commissioned in 1999 for the treatment of secondarily treated effluent from the Shek Wu Hui Sewage Treatment Works. The UV system comprises 520 numbers of medium pressure high intensity UV lamps with a total land requirement of 120 square meters for treating 80,000 cubic meters per day of secondary effluent. The disinfection target is to achieve E. Coli in effluent at 100 CFU/100 mL or less at monthly geometric mean.

The use of chlorine for disinfection is also receiving attention. The chlorinated effluent will be further polished by employing de-chlorination in order to reduce or eliminate any trace amount of residual chlorine that may be found after chlorination. This disinfection technology is being widely used in the United States of America. A full-scale plant trial employing multi-point dosing technique in disinfection was tested at Stanley STW, which is a secondary treatment plant, in 1999/2000.  The results of the trial indicated that the multi-point dosing technique could achieve the same disinfection level but with lower chlorine dosage as compared to the single point dosing technique.  The trial was satisfactory and the multi-point dosing system was installed in the disinfection chamber of Stanley STW in 2001.

Under HATS Stage 2A, disinfection treatment will be provided at Stonecutters Island Sewage Treatment Works to reduce bacteria level in the chemically treated effluent before discharge. In order to provide as early as possible disinfection treatment to the existing Stage 1 flow at Stonecutters Island Sewage Treatment Works 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. For the ADF operation, the effluent, after the chemically enhance primary treatment (CEPT) process, will undergo 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. The ADF is now in satisfactory operation providing disinfection to the Stage 1 effluent from Stonecutters Island Sewage Treatment Works before discharge to the harbour. 

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