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December 21, 2005


Cleaning Up Wastewater Magnetically

New process, now in pilot-scale experiments, could cut wastewater treatment costs

Bethany Halford

Cleaning up wastewater is a messy job, but a magnetic powder promises to make the process more efficient and less expensive.

In the widely used activated sludge process, a brown, flocculent culture of microorganisms consumes organic pollutants found in wastewater. As the bacteria eat up the pollutants, they clump into balls called flocs and settle to the bottom of the water treatment tank.


MAGNETIC MAGIC Sakai (right) demonstrates the new wastewater treatment process to a colleague, professor Tajmeri S. A. Islam of Dhaka University, in Bangladesh.

Messy as it sounds, this process is highly effective at purifying wastewater. But it's not without its drawbacks. Sometimes filamentous bacteria in the sludge form mats that prevent sludge from settling. The problem can be so severe that the plant has to be shut down and cleaned. Another major problem with activated sludge is that the bacteria thrive on organic matter, proliferating as they consume the waste. This can result in excess sludge that has to be decontaminated and disposed of at considerable expense.

By adding a little magnetite (Fe3O4) powder to activated sludge, Yasuzo Sakai, a professor of applied chemistry at Japan's Utsunomiya University, and colleagues say they have solved both problems. As bacteria in the sludge consume organic material, they also take up the magnetite, forming what Sakai calls magnetic activated sludge.

This magnetic sludge sticks to a rotating magnetic drum at the top of the treatment tank. "The separation rate is more than 100 times faster than the gravitational separation used in the activated sludge process," Sakai noted. The activated sludge is scraped off the drum and recycled for further use in treating water. The magnetic system also allows Sakai's group to precisely tweak conditions—such as the microbial concentration—so that no excess sludge is produced.

Sakai and colleagues reported that the process works well for cleaning up municipal sewage, swine wastewater, wastewater from the information technology industry, and endocrine disruptors, as well as phosphorus and nitrogen compounds.

On a site adjacent to the municipal sewage treatment plant in Utsunomiya City, Sakai set up a pilot plant based on the magnetic activated sludge process to treat municipal wastewater. From the fall of 2003 to the summer of 2005, the plant continuously treated 16 m3 of sewage in an aeration tank. The process removed organic material as well as the standard activated sludge technique and produced no excess sludge during the 500-day experiment, Sakai said.


Starting small, with magnetic activated sludge in a beaker. Adding magnetite (black) to the beaker on the left makes it easy to remove sludge from the water with a magnet. Moving on, here's the magnetic activated sludge process on the bench scale. The pilot plant at work: The end features a beaker of purified water next to a solution of the magnetic activated sludge.
Chemical & Engineering News
ISSN 0009-2347
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