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Bio-filters for Edge-of-Field Water Quality Management

Richard Cooke
Assistant professor
Department of Agricultural Engineering
(217) 333-0944; r-cooke@illinois.edu
Richard Cooke

Vegetative filter strips have been installed along the banks of many rivers and streams in Illinois as an edge-of-field water treatment method. Ostensibly, nutrients and pesticides in runoff are retained or transformed in these filters, thereby reducing contaminant loads in receiving waters. However, outside of extreme flood events, most of the water leaving agricultural fields in flat tile-drained watersheds is from tile flow. In such watersheds, the efficacy of filter strips is low, since much of the water that enters streams and rivers flows through subsurface tile drains, never coming into contact with the filters on the soil surface. It is necessary, therefore, to find a subsurface solution to a subsurface transport problem.

Schematic of subsurface bio-filter

Figure 1. Schematic of subsurface bio-filter.

Water table management has been shown to be effective in reducing nitrate concentrations in tile effluent. However, this practice might not be suitable for most existing drainage systems. In addition, in many cases where water table management is applicable, there is some resistance to adopting it, as the installation of control structures complicates farming operations and management practices.

This research project is designed to test the efficacy of subsurface bio-filters, a practice that combines the benefits of vegetative filter strips and water table management systems without their drawbacks. Bio-filters are subsurface structures, so they interact with tile water. They do not necessitate any changes in farming operations. Unlike other edge-of-field treatment systems, such as vegetative filter strips, ponds, or wetlands, bio-filters do not require that land be taken out of production.

Six cells were constructed to serve as laboratory-scale bio-filters. These cells were filled with different materials, mostly agricultural byproducts. Water with nitrate concentrations typically found in tile outflow was passed through them. The flow rates were adjusted to represented retention times ranging from two to 20 hours.

Effect of filter material on bio-filter efficacy

Figure 2. Effect of filter material on bio-filter efficacy.

We have determined that the best material to use in these subsurface bio-filters is a mixture of corncobs and gravel. The gravel ensures steady flow conditions, while the corncobs serve as a food source for the bacteria that break down the nitrates. We have also determined that if water with nitrate concentrations of up to 30 mg/L is retained in these filters for six or more hours, the concentration of nitrate in the outflow will be less than 10 mg/L-the limit set by the Environmental Protection Agency for drinking water.

Later this year, a field-scale bio-reactor will be installed at the outlet of a tile that empties into a river or stream. Flow rates into and out of the bio-reactor will be monitored, and water samples will be collected and analyzed.

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