For oxidation, disinfection and provision of residual disinfectant (collectively called “chlorination”) in Hong Kong’s drinking water treatment, the Water Supplies Department (WSD) currently uses the following: chlorine gas and/or sodium hypochlorite (NaOCl) solution, which is generated from the OSCG plants installed at large water treatment works (WTWs). In such OSCG plants, the Ion Exchange Membrane (IEM) electrolyser, an advanced electrolysis technology, is adopted—with dissolved sodium chloride as the electrolyte—to generate chlorine gas, which will produce NaOCl solution when further mixed with caustic soda. The NaOCl solution will be consumed on-site or transported to small WTWs not equipped with OSCG plants. Depending on its capacity, each large WTW is installed with one to three OSCG plants, plus an additional standby unit, to ensure a reliable and stable supply of chlorine gas and NaOCl solution for effective chlorination in the treatment process.

Before 2017, WSD procured liquid chlorine directly from the Mainland for drinking water treatment in Hong Kong. However, chlorine is dangerous goods of a kind that requires vigilant and complex operation procedures for safe transportation and storage. It was in this context that WSD introduced OSCG plants in phases—beginning in 2017—to twelve large WTWs, eliminating the need to procure liquid chlorine from Mainland suppliers. Moreover, since WTWs are no longer classified as Potentially Hazardous Installations, certain land with them in the vicinity could be freed for development use.

OSCG Plant

This article is contributed by the Development Bureau of the HKSAR Government.