For more than 50 years seawater has been used for toilet and urinal flushing in Hong Kong, saving significantly quantity of fresh water. If you are near the coast, the process is straightforward, sea water is filtered, treated and then pumped to purpose built service reservoirs ready for use in residual and commercial buildings. Eighty percent of Hong Kong buildings are served by the sea water infrastructure. In the year 2011, the potable water savings were 740 tonnes per day, or more than 270,000 tonnes per year.
All new buildings by regulation mustbe provided with two separate water systems, potable and flushing water. The flushing water infrastructure must be designed to handle sea water. in buildings uPVC is the material of choice for pipework serving all sanitary appliances. WC cisterns are designed without any metal components exposed to the corrosive seawater.
Underground the pipework distribution utilises cement lined piping for sea water to the service reservoirs and buildings as shown in the above diagram (courtesy of Hong Kong Water Supplies Dept. http://www.wsd.gov.hk).
Having separate potable and sea water infrastructure has another advantage, in the event that one water service is shut-down for maintenance, that doesn’t stop all the water services. Furthermore, since the sea water provides part of the buildings water demand, the potable infrastructure is smaller and lower cost.
One of the major objections against using sea water is a concern that potable water and seawater piping could be cross-connected, in reality that’s an unlikely occurrence because the materials are different, its physically difficult to connect a cement lined pipe to PE piping.
Its obvious really, in coastal areas, why flush our precious potable fresh water down the toilet? The use of sea water is a low-tech solution to lower and conserve potable water usage, and very cost effective.
Update: John Herbert was appointed to the Hong Kong Green Building Council Faculty June 2012 and chairs the Water Aspects technical group.