Disinfection technologies generally comprise 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 undesirable disinfection by-products, land requirement as well as the capital and operating costs.
The first large scale 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 93,000 cubic meters per day (D.W.F.) of secondary effluent. The disinfection target is to achieve E. coli in effluent at a monthly geometric mean of 100 CFU/100 mL or less.
In 2010, a more energy efficient Ultra-violet (UV) irradiation disinfection system was installed at Tai Po Sewage Treatment Works (TPSTW) for treating secondarily treated effluent. The UV system in TPSTW comprises 540 numbers of low pressure high intensity UV lamps with a total land requirement of 306 square meters for treating 100,000 cubic meters per day of secondary effluent. In 2011, another similar Ultra-violet (UV) irradiation disinfection system was installed at Shatin Sewage Treatment Works (STSTW) also for treating secondarily treated effluent. The UV system comprises 1,400 numbers of low pressure high intensity UV lamps with a total land requirement of 460 square meters for treating 340,000 cubic meters per day of secondary effluent. The disinfection target for both STWs is to achieve E. coli in effluent at a monthly geometric mean of 1,000 CFU/100 mL or less.
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-points dosing technique in disinfection was tested at Stanley STW, which is a secondary treatment plant. The results of the trial indicated that the multi-points 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-points dosing system was installed and in operation at the disinfection chamber of Stanley STW.
Under HATS Stage 2A, disinfection treatment was 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 completed in March 2010. For the ADF operation, the effluent, after the chemically enhanced 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. To further enhance the removal to 99.9% of the bacteria in the effluent, final disinfection facilities (FDF) was constructed and completed in January 2016 and the contact time for chlorination of treated effluent is further increased. FDF is in satisfactory operation providing disinfection to the effluent from Stonecutters Island Sewage Treatment Works.