Ocean fresh toilets

“Using seawater in the toilet, what a great idea,” Professor Mark van Loosdrecht (faculty of Applied Sciences) enthusiastically said when he first heard about this concept ten years ago at a meeting in Hong Kong. His overseas colleagues looked at him with surprise. “Why, do you use drinking water for this?” they astoundingly replied.

In Hong Kong, 90 per cent of toilets are flushed with seawater. And this has been so since the 1960s, which has allowed city to reduce its fresh water usage by 30 percent. Hong Kong is equipped with an elaborate waterworks for this purpose.

So what then  is new about Prof. Van Loosdrecht’s plan? It is the holistic approach. The Professor and his colleagues from Hong Kong University of Science & Technology and KWR Watercycle Research Institute envision energy-efficient climate control in buildings using sea water, the easy recovery of phosphates, and more efficient wastewater treatment. “Our aim is to make use of seawater as an alternative resource of water, energy and fertilizer.”

Thanks to their efforts, Hong Kong International Airport is saving 52 per cent of its freshwater demand and 30,000 MWh of electricity annually. Besides flushing the toilet with salt water, the seawater is used as a coolant in the air-conditioning system.

Next in line is the treatment of saline sewage water, which – according to the researchers – can be done about 30 percent more energy efficiently than the treatment of non-saline sewage water. The researchers want to make use of the sulphate in the seawater, and by doing so they will introduce new biochemical processes into the carbon and nitrogen cycles that produce less sludge.

And by making use of the magnesium ions in seawater, the new technology could  recover phosphorus from urine in the form of magnesium ammonium phosphate, a valuable phosphorus and nitrogen containing fertilizer, by mixing hydrolyzed urine with seawater.

This concept offers an alternative to expensive and energy intensive desalination projects, many of which are occurring in the Middle East. But the project is not only interesting for Hong Kong or the Middle East, but also for the Netherlands, especially for the regions where brackish water is rising to the surface. “We now spend a lot of energy and effort in pumping this brackish water out of our polders,” says the researcher, adding that if it were up to him we would pump this water right into our homes.

Dutch water authorities seem reluctant, however. In the 1990s, there were experiments conducted with second quality water systems in homes. These experiments were stopped because, in some cases, the pipes were not well connected and thus people used the low quality water as drinking water. This could also happen when sea water is introduced in our homes. But would you get sick? “I don’t think so”, says Van Loosdrecht. “The water is so salty you would immediately notice it and spit it out.”