Technical Note: The Potential of Municipal Yard Waste to be Denitrification Bioreactor Fill

Abstract

The use of denitrification bioreactors to mitigate nitrate in agricultural drainage has recently gained much interest in the Midwestern United States and in similarly drained agricultural regions. However, as the number of bioreactor installations has increased, questions have been raised about the supply and consistency of denitrification carbon source material. In selecting such material, there is an important balance between optimal media properties (e.g., hydraulic properties, chemical composition), practicality, and material cost. The use of free material such as municipal yard waste may help minimize the cost of this voluntary water quality improvement strategy in the Midwestern United States, but may not provide other sufficient media properties. To investigate this, pilot-scale bioreactors were used to compare hardwood chips with free, chipped municipal yard waste in terms of nitrate removal potential and changes in the media. Sampling of bioreactor influent and effluent over a range of retention times showed the yard waste had higher removal efficiencies at a given retention time and higher removal rates than the woodchips. However, buried carbon media bags revealed the yard waste lost weight to a greater extent and more consistently than the woodchips meaning the woodchips had a half-life over two times greater than the yard waste. This, combined with the low carbon-to-nitrogen ratio of the yard waste, indicated yard waste material is not ideal for bioreactor installations that are intended to be low maintenance for at least ten years.