A researcher prepares solutions with varying levels of E. coli to test the effectiveness of steel chips at removing the bacteria. Source: South Dakota State University

Steel chips may offer an inexpensive means of countering E. coli contamination in storm runoff, according to South Dakota State University researchers.

Steel chips collected as waste material from a machine shop were tested in sizes of 0.5-8 mm with simulated storm water containing 10-106 colony forming units of E. coli/100 mL. Contact times of 5-20 minutes were evaluated at pH levels of 5, 7 and 9.

About 85-98 percent of the bacteria were removed during small column testing. Chips of 0.5-2 mm size proved most efficient, and a 99 percent removal rate was observed at a 20 minute contact time. Removal rates of at least 90 percent were documented at all pH levels, although lower levels yielded the best results.

The researchers also simulated intermittent storm water events in which water circulated for six hours, after which chips were allowed to dry for six hours followed by six hours of flow. A high efficiency removal rate was also demonstrated during these trials.

The team plans to install a filter for pilot-scale testing at a site where storm water from a 10-acre residential area drains into a retention pond.