Wood chips remove toxins from roadway runoff

According to Thomas Boving, Assistant Professor of Geosciences at the University of Rhode Island, storm water detention ponds in Providence, Rhode Island, designed to filter out pollutants from roads before storm water reaches the local Narragansett Bay, are made ineffective by large volumes of water.

“Most of the contaminants in roadway run-off are attracted to suspended organic material and sediments, which then settle to the bottom of the ponds,” said Boving. “But if the flow rate is too fast, like during a heavy storm, there may not be enough time for the solids to settle before flowing out and into the bay.”

Knowing that contaminants cling to organic material, Boving, whose hobby is woodworking, decided to investigate the effectiveness of wood chips on filtering out pollutants.

During laboratory experiments using water containing the carcinogen pyrene, produced by smokestacks, automobile tailpipes, chimneys and outdoor barbecues, Boving found that shredded wood from aspen trees removed 97% of the pollutant. Over time, he found that the wood chips become less effective, and so may need to be replaced every 30 to 60 days for optimum performance, although they could remain effective for up to a year, Boving told edie. Boving points out that although regularly changing the wood chips might technically be the most efficient option due to the low cost of the material, this could lower the public acceptability of the technique.

“I was very encouraged by what I found with this first test,” said Boving. “It fulfils all of the requirements for a successful technology – it’s non-toxic, cheap, available, and public acceptance of these filters is likely very good since no one is concerned about putting wood in water.”

Boving has calculated that up to 100 pounds of shredded wood would be needed each month for three storm water detention ponds, roughly 400 cubic metres in size. The wood chips will be submerged in the ponds, enclosed in netting. He believes that the spent wood chips are likely to be incinerated.

Following tests using a range of organic contaminants on pine – the cheapest wood available – and other types of wood, Boving intends to carry out field tests next year.