Few places in the world are facing water issues as severe as China. There isn’t enough of it in the country and much of the water it does have is heavily polluted.
Each person can produce 300 liters of sewage daily, a major contributor to water pollution when not treated properly. To improve its water quality, Chinese authorities are constantly constructing more sewage treatment plants. But more treatment means more sludge coming out of the process, which itself requires proper treatment and disposal.
For every 35 cubic feet of sewage treated, 1.7 pounds of sludge is produced. In a country with a population as big as China’s, that means a lot of sludge.
But the country’s scarce water supplies lead to a difficult question. How do you clean up sludge that was created from cleaning up water? The answer could come from the sea.
Hong Kong has been using seawater to flush toilets for the past five decades, a means of reducing the use of scarce fresh water supplies. Now, Scientists at the Hong Kong University of Science and Technology in partnership with The Netherlands’ Delft University of Technology have come up with a technology that works using Hong Kong’s experience using seawater as part of its sewage system along with bacteria that could eat up the sludge.
The process is called SANI (Sulfate reduction, Autrophic denitrification, and Nitrification Integrated). Researchers have found that combining seawater with SANI could save freshwater and energy resources needed to handle sludge disposal.
A potent mix
Every liter of seawater contains about 2.7 grams of sulfate, a compound that is used to treat sludge, says Delft professor Mark van Loosdrecht, an engineer who uses microbial ecology for process design.
Professor Chen Guanghao at the Hong Kong University of Science of Technology says the team devised a process to use sulfate-reducing bacteria to oxide and eliminate pollutants in the seawater-mixed sludge.
“These are perfectly normal bacteria you can find anywhere in nature, especially in marine environments,” says van Loosdrecht.
The whole idea of using the bacteria, found in ocean, lakes, pipes and sewers, is to reduce sludge production. The microorganism can consume sulfate as part of a process that oxidizes the organic carbon in sludge into CO2.
The conventional process, which used oxygen by bubbling air to the reactors, was energy intensive. Using microbes to digest and degrade organic pollutants in sewage is also unproductive. Only about 40-60 percent of the organic materials will be converted into carbon dioxide and released into the atmosphere because microbes grow so rapidly. The remaining organic matter is converted into biomass, which has to be disposed of as sewage sludge.
Since the sulfate-reducing bacteria grow very slowly, the SANI process can decrease sludge production by 90 percent. Hong Kong is the only place where it works now, Guanghao says, because it has the only seawater toilet-flushing system in the world.
During a pilot project in an area of Hong Kong called Tung Chung, no sludge disposal was required over a period of 225 days. Guanghao says this means that the SANI process can practically minimize the need for sludge disposal, and reduce 50 percent of sewage treatment costs, 50 percent of the space required and 35 percent of carbon dioxide emissions.
He hopes to begin a demonstration trial in China by 2015.
Major benefits if the technology spreads
Van Loosdrecht says that toilet flushing usually accounts for one-third of all human water consumption. Using Hong Kong as a model, many urban areas situated in deltas could replace fresh water with seawater for flushing. He says the SANI process could be used in areas like the Middle East, which has high sulfate levels in the groundwater supply.
In HK, the use of seawater for toilet flushing has contributed to saving about one-quarter of the region’s freshwater consumption. There are problems with using seawater however. The saline sewage appears to have limited some important water reuse options, such as irrigation, which accounts for about one percent of total water demand in highly urbanized cities like Hong Kong.
Top Image: Sedimentation tanks at a sewage treatment plant via Shutterstock.