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Sewage could be energy source, scientist says

Ng Kang-chung, SCMP

In as little as 10 years, sewage may become a valued commodity and source of energy rather than a waste-disposal problem.

“That might sound like science fiction. But the basic technology has been there for many decades,” said Professor Herbert Fang, chairman of environmental engineering at the University of Hong Kong.

Bacteria or other micro-organisms are used to break down organic pollutants in biological waste-water treatment. However, this so-called aeration process, adopted in many of Hong Kong’s sewage-treatment plants, produces waste sludge and requires much electricity to power pumps to supply oxygen to keep the organisms active.

But under anaerobic, or no-oxygen, conditions, there are some micro-organisms that can convert sewage first into hydrogen and then methane. Methane can be used as a fuel but it is considered an unwanted greenhouse gas. By controlling treatment conditions, it is possible to produce hydrogen, instead of methane, from waste water, Fang says.

Then, in theory, treatment plants could be designed to take in sewage at one end and send treated water and hydrogen fuel out the other. Production may only be limited by the amount of sewage available.

Fang estimates that treating the 2.9 million cubic metres of sewage produced in the city every day – enough to fill 1,560 standard swimming pools – could account for 1.5 per cent of total electricity consumption. The city uses about 160,000 terajoules of electricity every year, according to government figures.

“While we can treat sewage by the anaerobic process, it can produce hydrogen. It is a sustainable technology that kills two birds with one stone,” said Fang, who was among the world’s first researchers to start such a study 10 years ago. “It has been proved in laboratory-scale reactors. Large-scale application may take just another 10 years or more.”

His study showed that the level of acidity was an important factor affecting the microbial population and, hence, the hydrogen production.

An exhibition on bio-hydrogen production from waste water opens at the Science Museum in Tsim Sha Tsui East today. Fang will be one of the officiating guests

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