When rain water runs down dirty city streets and into storm sewers, it can be carrying a lot of filth with it – filth such as E. coli bacteria, which may end up polluting rivers. There could be an inexpensive and efficient new way of ridding the water of that bacteria, however, using chips of waste steel.

Building upon the findings of a previous research project, South Dakota State University grad student Peng Dai started by obtaining discarded carbon steel chips from a machine shop in the city of Sioux Falls. Those chips were then placed in a glass column, and E. coli-tainted water was run through them. Dai experimented with various chip sizes, E. coli concentrations (ranging from low to extremely high), pH levels, and contact times of five to 20 minutes.

He found that by using chips 0.5 to 2 millimeters in diameter, longer contact times, and lower pH, he was able to kill almost 99 percent of the bacteria in all concentrations. Even at higher pH levels, at least 90 percent of the E. coli was removed.

"The surface of E. coli is negatively charged, and the steel chip is positively charged," Dai explained to us. "E. coli can be absorbed on the steel chip surface because of electrostatic attraction." From there, ferrous and ferric ions from the steel oxidize the E. coli's cell membrane, causing it to die.

Plans now call for another grad student to field-test a larger version of the system in a real-world setting, in which storm water from a 10-acre (4-hectare) residential area drains into a retention pond. At the inlet to the pond, the storm water will run through a structure resembling an open-top box, which will be filled with steel chips.

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