Woodchip Bioreactors for Improved Water Quality
Woodchip bioreactors are a promising strategy for removing NO3-N from drainage water. However, field bioreactor performance varies greatly and is influenced by temperature, influent nitrate concentration, and hydraulic retention time (HRT). More research is needed on how to optimize the size of a bioreactor, while achieving adequate nitrate removal. Also, recent questions have emerged regarding ‘pollution swapping’ in bioreactors wherein nitrate is converted to alternate end products instead of being lost as N2 gas through complete denitrification.
The overall goal of this study is to evaluate NO3-N fate in woodchip bioreactors over a range of water retention times, while gaining knowledge about improved bioreactor design for field implementation.
Previous funding provided for the design and installation of nine experimental bioreactors at the Iowa State University Agricultural Engineering Research Farm west of Ames. The reactors are designed to allow for differing hydraulic retention times, influent nutrient concentrations, and fill materials. Sampling ports are located at two locations to provide access to water and fill materials, and at the effluent location. Experiments will begin in spring 2017 as soon as flow is available in the county tile line to provide water to the pilot systems, and will run as long as flow is available. Water samples will be collected weekly, and experiments will be conducted in triplicate over a range of three HRTs - two hours, eight hours and 16 hours.
This project led to the design and installation of nine pilot scale bioreactors. This pilot scale system for testing bioreactors is unlike any other in the world. Retention time, fill material, and influent water quality parameters can be varied. This resource is being used to answer relevant questions regarding bioreactor design and performance, specifically related to nitrate removal and flow. In addition, greenhouse gasses above the bioreactors and in the water discharging from the bioreactors are being monitored. Researchers have found the nitrate mass removal was greatest from the bioreactors with a short retention time, but the concentration reduction was greatest from the bioreactors with the longest retention time. The pilot bioreactors are used frequently for outreach activities to state agencies, K-12 and college students, extension agents and farmers.