The potential of municipal yard waste to be denitrification bioreactor fill

The use of denitrification bioreactors to mitigate nitrate in agricultural drainage has recently gained much interest in the US Midwest and in other agricultural regions around the world. However, because this is a new technology, there are still many questions regarding bioreactor design and construction including uncertainty about denitrification carbon source material. The physical, chemical and biological properties of the utilized carbon media effect overall bioreactor performance through factors such as hydraulic conductivity, porosity, carbon: nitrogen ratio, and microbially available carbon. In selecting a carbon source material, there is an important balance between the optimal media properties, practicality, and material cost. The use of free material may help minimize material cost, but may not provide other sufficient media properties. To investigate this, pilot scale bioreactors were used to compare nitrate removal efficiency and media longevity of hardwood chips (summer #1) with free, municipal chipped storm debris (summer #2). Bromide tracer tests allowed comparisons between reactor hydraulics and buried bags of carbon media allowed for pre- and post-operation observations of the media. Initial results were confounded by flooding and operational problems in summer #2, but generally indicated the storm debris provided nitrate removal but also degraded over the period.