Hydraulic evaluation of a denitrifying bioreactor with baffles
Denitrifying ‘woodchip’ bioreactors are an effective conservation practice to reduce nitrate-nitrogen (NO3-N) loss from tile drained agricultural areas. They enhance the naturally occurring denitrification process via the addition of woodchips and maintenance of anoxic conditions. Bioreactors tend to be one of the most cost-effective options for treatment of tile drainage NO3-N, but considering the scale of water quality goals, new approaches to bioreactors are needed to provide the most practical benefit, while limiting the amount of land taken out of production. Current bioreactor design considerations include a bypass flow pipe to prevent significant reduction of drainage capacity in the field. This practical need for a bypass pipe results in a portion of the annual flow volume being untreated which limits a bioreactor’s overall N removal performance. Bioreactors designed to be wider would potentially have greater flow capacity, minimizing this untreated water. To maintain a consistent surface area footprint and not encroach on cropped areas, a wider and shorter bioreactor could use baffles to elongate the flow path, forcing more effective reactor volume utilization. To test this, a new bioreactor (LWD: 16.8 x 10.7 x .91 m; drainage treatment area: 14.2 ha) which included two flow-routing baffles was installed at the University of Illinois Dudley Smith Research Farm (Christian County, IL, USA) in October 2016. A series of potassium bromide conservative tracer tests were performed on this new design during 2018 as well as at three conventionally designed bioreactors to evaluate how the baffles impacted bioreactor hydraulic functioning. This new bioreactor had greater effective volume, lower dispersion, and less short-circuiting compared to the conventionally designed bioreactors. However, this did not necessarily translate into improved NO3-N removal. Overall N load reductions of 23-24% at the edge of the field were similar to many other published studies for bioreactors without baffles. There was 62-64% N removal for water treated in the bioreactor which translated into removal rates of 1.30-1.25 g N m-3 d-1 which were also similar to other studies. Additionally, while this bioreactor was relatively wide to maximize the percentage of flow treated, only 40-41% of the annual flow volume was treated. Although bioreactors are meant for N removal, there was unexpected dissolved P removal (23-24%), mechanisms of which should be investigated further. While the baffles did not lead to increased N removal compared to conventionally designed bioreactors, they did improve bioreactor volume utilization, and thus, the idea of bioreactors with baffles is an idea meriting further exploration at additional sites.
bioreactor, denitrifying bioreactor, baffles, nitrate removal, N removal, water quality, edge of field
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign