Answer: Sea Water

Fresh water is scarce in Hong Kong thanks to a combination of variables including inconsistent rainfall, few natural lakes and rivers, and a small number of difficult to reach groundwater sources (the few aquifers that Hong Kong has are held beneath very thick and hard granite bedrock).

As a result, the government of Hong Kong has had to be creative in finding ways to preserve their small supply of water including, starting in the 1950s, setting up a dual plumbing system that uses sea water for toilets and reserves fresh water for more important purposes like drinking and bathing.

While there was resistance to the dual plumbing system at first due to the cost and inconvenience, by the early 1970s, the government had completely subsidized the system and adoption rates rose. By the end of the 1990s, the number of citizens using a sea water toilet had risen to 79 percent.