Spaghetti filters cleanse water supplies
Chemists package bacteria to eliminate perchlorate.
A tube of bacteria may prove the best way to strip drinking water of nasty chemicals such as nitrates and percholorates, said researchers at the meeting of the American Chemical Society in Washington DC this week.
Perchlorate, an ingredient of rocket fuel and fireworks, has seeped into many drinking-water sources around the world. It is known to inhibit the function of the thyroid gland, and to harm embryonic development. It is not regulated in the United States, where the pollution has received the most attention. But some water treatment plants have started to remove it from waste water on a voluntary basis.
This removal is a tricky business. Most systems produce a concentrate of the chemical that has to be disposed of somehow.
So Bruce Rittmann of the Center for Environmental Biotechnology at Arizona State University, Tempe, turned to bacteria for a different solution. His system uses bacteria that feed on hydrogen gas and perchlorate to produce water and chloride - a compound commonly found in salts and swimming pools.
The bacteria are grown on membranes that are wrapped into spaghetti-like strands, about the width of a human hair. The strands can be filled with hydrogen and bundled into cylinders through which the contaminated water flows.
A 1.5-metre-tall system with 7,000 fibres can clean 4-8 litres a minute, says Rittmann, who has tested it on ground water from California's central valley. But it will take at least two more years, he adds, to boost the efficiency 1,000 times, enough to clean the waste water from a small city.
Other current methods for perchlorate removal involve bacteria fed with ethanol or acetic acid. But this produces salts that contain perchlorate. Another popular process involves ion exchange, which uses resin beads to pick the perchlorate out of the water. But then the beads have to be cleaned or thrown away. Rittmann estimates that his bacterial system, with no waste products, may cost half as much as current systems.
Paul Hatzinger of Shaw Environmental, a Louisiana-based company that sells perchlorate-removal systems, says the jury is still out as to what will be the best option. "It's an emerging technology," he says. "The next step is to build this large scale and see how it compares."