The feasibility study on using ozone for the disinfection of CEPT effluent from Stonecutters Island STW

As a continual effort in improving our drainage systems to treat wastewater in an environmentally responsible manner, the Research and Development Team of the Drainage Services Department is considering the use of alternative disinfection methods for disinfecting various types of effluent including effluent from the chemically enhanced primary treatment (CEPT) process and secondary effluent from biological treatment plants with a view to adopting disinfection technologies in a cost effective and environmentally acceptable manner.

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 feasibility study on using ozone for the disinfection of CEPT effluent from Stonecutters Island STW was conducted in 1999. The study concluded that the use of ozone could achieve different disinfection targets at different ozone dose rates and contact times for CEPT effluent with no significant amount of undersirable disinfection by-products.

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 using chlorination/de-chlorination is scheduled for Stanley Sewage Treatment Works in 2000 for evaluation of the technology for secondary effluent in meeting the disinfection target as well as reducing the amount of residual chlorine in the disinfected effluent.