Iron- and Biochar-Enhanced Sand Filters

Stormwater conveys bacteria such as E. coli to receiving waters, where contact can be a human health risk. Shingle Creek, the Upper Mississippi River, and many other urban and rural streams in Minnesota periodically exceed the state water quality standard for bacteria, and must identify sources and strategies for reducing bacterial loading into those waters. In urban areas bacteria sources are diffuse –pet and wildlife waste, sanitary overflows and leakages - and options for reducing loads are limited.

The two Commissions have obtained a federal grant to field-trial three applications of a new promising yet simple technology. Biochar – a specially engineered type of charcoal –added to iron-enhanced sand filters has been effective in lab experiments at removing bacteria in synthetic stormwater. The three field trials will test the effectiveness of these filters at treating real-world stormwater runoff.

Minnesota Filters

The St. Anthony Falls Lab (SAFL) at the University of Minnesota refined the design of iron-enhanced benches on stormwater ponds to enhance removal of dissolved phosphorus in stormwater. When rain events cause the pond level to rise, water flows over and through these Minnesota Filters before being discharged downstream. This project will test whether adding biochar to the filter medium can also reduce bacteria in the outflow.

Three existing ponds, one in Champlin, one in Crystal, and one in Minneapolis, will be modified with these enhanced filters. Following installation in May-June 2017, they will be monitored for two years.  Water quality at the pond inflow, outflow, and ambient water quality will be tested for total and dissolved phosphorus, E. coli, and other water quality parameters, and the pond water elevation will be tracked. A fourth pond in the watershed that does not have a filter bench will also be monitored for comparison.

Biobox

The Biobox is a filter to directly gravity treat diverted flow from Shingle Creek by taking advantage of the Webber Park Falls. The Biobox is a four by four by six foot steel box filled with iron-and biochar- enhanced sand placed on the viewing platform next to the falls. A pipe intercepts flow from the creek at the top of the falls, and routes it to the filter, where it will percolate to the bottom and then be piped back to the creek at the bottom of the falls. Of interest is whether the removals from this type of direct treatment can be scaled up to make an impact on concentrations in the Creek.