New Water Treatment Process Could Help Cities Cut Sludge Disposal Costs

The new technique, called the activated magnetic sludge process, was described today to scientists gathered in New Orleans for the weeklong 225th national meeting of the American Chemical Society, the world's largest scientific society.

The new twist in the technique is the introduction of magnetic separation, according to environmental engineer Yasuzo Sakai, Ph.D., of Utsunomiya University in Japan, who presented the research.

Normal activated sludge treatment — in common use throughout the world — involves the use of bacteria that help purify the water by eating harmful organic pollutants. However, this can leave behind tons of decontaminated sludge that then must be disposed of, often in landfills. Some of the "excess sludge," as it's called, is biomass waste, which is created by the growth of the pollutant-eating bacteria.

A sewage treatment plant for a population of 100,000 people typically generates 200-500 tons of sludge every day, including 2-5 tons of biomass waste, says Sakai.

The addition of magnetic separation results in "zero emission of excess sludge," he claims.

In laboratory testing, the scientists added powdered magnetite, a mineral or iron ore with a strong magnetic force, to a tank full of activated sludge. The tank also contained a rotating magnetic drum. As the bacteria eat the pollutants in the sludge, they take up the magnetite and then stick to the drum where they can be scraped off and used again to treat more wastewater rather than needing to be discarded as excess sludge. In addition, since the bacteria are ultimately separated from the sludge, their growth is minimized.

During the yearlong test, the researchers say they treated 80 liters — approximately 21 gallons — of sewage every day "without excess sludge production." Conventional treatment would have produced about three - six liters of excess sludge each day, they claim.

Besides reducing excess sludge, the researchers believe magnetic separation has another benefit.

Conventional activated sludge treatments rely on gravity to work. As the bacteria consume the pollutants in the wastewater, they clump together in balls called flocs, which then settle to the bottom of the tank, allowing clean water to flow. Sometimes though, long strands of bacteria called filamentous bacteria can grow too large and form mats that prevent the flocs from settling — a problem known as sludge bulking. In severe cases of sludge bulking, the treated water comes out dirty and contains solid material.

Since the magnetite causes the flocs to stick to the magnetic drum, rather than depending on gravity to settle them to the bottom of the tank, the treatment can help avoid the sludge bulking problem and improve the clarity of the water, say the researchers.