Hong Kong’s saline flush

Residents of Hong Kong flush their toilets using seawater. In the mid-twentieth century, the city state installed saltwater mains in order to help save the drinking water that was in short supply. The seawater was originally intended to be used to clean the streets, but the application in the city’s lavatories followed soon after.

Mark van Loosdrecht, professor of Environmental Biotechnology (faculty of Applied Sciences) argues that flushing with seawater offers specific advantages with regard to water purification. He collaborated with fellow professor
Guang-Hao Chen of the Hong Kong University of Science and Technology (HKUST) on a research programme exploring the benefits of utilising seawater in waste water purification.

Van Loosdrecht met Chen as a result of a response he wrote to one of Chen’s articles. When Chen was appointed professor at HKUST, he asked for Van Loosdrecht’s advice with regard to setting up a new research programme. Instead of elaborating on research underway at TU Delft, the Delft Environmental Biotechnologist advised Chen to conduct research into the specific situation in Hong Kong, namely sewage purification using seawater. The pair have now shown that seawater does indeed offer numerous benefits. Van Loosdrecht reels them off the top of his head: sulphates in seawater leads to less silt production (a waste product), sulphides (created from sulphate oxidation) kill pathogenic bacteria, the chemical bond with sulphides simplifies the recovery of metals, and phosphate from sewage water is easier to recover using magnesium from seawater.

Another associated benefit is the use of seawater for cooling, which is not permitted using drinking water because of potential bacteria growth. All in all, there is sufficient cause to further highlight saltwater sewage treatment under the name SANI. Van Loosdrecht hopes to achieve his objective through a collaboration with UNESCO-IHE, the Institute for Water Education in Delft. Their first project will be at a resort in Cuba, where half of all water used is used for flushing.

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