Economy of dual water supply systems

Hong Kong is one of the very few coastal cities in the world which utilize “dual water supply systems”. The dual water supply involves two distribution systems, one fresh water distribution system for potable use and another seawater system for toilet flushing and fire fighting purposes. The details of such dual water supply systems have been given in a paper (Tang, 2000) published in the Proceedings of the 26th WEDC Conference. The objective of this paper is not to repeat what has been described in said paper but to report a comparison of the economy of (1) a single distribution system (fresh water supply only) and (2) the dual water supply systems. In other words, the question addressed in this paper is: would it be more economical for Hong Kong to use a single distribution system instead of the existing dual systems?