The way to clean sewage

GETTING drinking water by immediate purification of sewage, inconceivable sopme years back, is now a reality. An Australian company, Memtec, has successfully used microfiltration technology to purify sewage in a small town in the UK. The town had for the past 30 years been dumping untreated sewage into the sea.

The company has been using this tecnology in a range of applications, from turning sewage into high-quality industrial treatment water to setting up emerging drinking water supplies for flood-ridden communities. In Japan, the technology is used for 'water mining' - lifting sewage locally from the mains, and purifying it to water golf courses and Oriental gardens (Chemical Engineering, Vol 104, No 3)

Microfiltration involves squeezing contaminated water through a special membrane with tiny pores measuring two microns (0.000002 m). This allows only recyclable water to pass through, leaving out all else, including the finest particles, bacteria and viruses. The filtration system is itself cleaned by compressed air, which prevents it from getting clogged with waste.

The report states that Memtec was searching 'for a guinea-pig to try out' its microfiltration technology on a sea beach laden with sewage and Aberporth, a small Welsh tourist town, was selected. It was a radical departure from the conventional sewage treatment technology. Memtec has completed the contract, and Aberporth's beaches are now clean.

Memtec sells similar technology for uses ranging from winemaking to treatment of radioactive waste. In the case of sewage, the extracted waste is recycled into fertiliser. The water obtained can be further purified into drinking water, as is done at Memtec plants in California, where the authorities are concerned about the presence of bacteria resistant to chlorine used in standard water treatment systems. The technology has also been used in industrial processes such as cooling systems at power stations in Australia. Or it may be used to prevent pollution of water bodies like rivers and seas, as in the case of Aberporth, the first such use of the technology.

But the cost of the technology remains prohibitive. Memtec's unit operating costs are three times those of systems that use chemicals or ultraviolet light to clean sewage. Memtec points out that conventional technologies are far less efficient in removing viruses and bacteria. While microfiltration uses more energy than conventional methods, the capital required for setting up a plant using the technology is less than that for a standard plant. Moreover, these plants are much smaller - one- sixth the size of conventional ones.

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