Nutrient removal in modular bioreactors
by Jeff Strock, Andry Ranaivoson, Gary Feyereisen, Kurt Spokas, David Mulla and Marta Roser
Ditches convey surface runoff water and subsurface tile drainage from artificially drained agricultural lands and are important to the agricultural economy of Minnesota and other Midwestern states. However, traditional methods of surface and subsurface drainage often result in degraded water quality. There has been increased interest in developing Best Management Practices (BMPs) for mitigating the effects of subsurface drainage. Ideally, a successful BMP would mitigate the negative impact of subsurface drainage while limiting its negative consequences on crop production practices and crops. A potentially successful BMP would be the design of a bioreactor which can mitigate both nitrogen (N) and phosphorus (P) efficiently under a wide range of flow and environmental conditions. Additionally, the bioreactor would be easily accessible for replacing and recycling the P sorbing and N denitrifying constituents. The effectiveness of a novel bioreactor design that could be placed into or adjacent to agricultural drainage ditches for the removal of N and P was the primary focus of this study.
Field experiments were conducted at the University of Minnesota Southwest Research and Outreach Center (SWROC) in Lamberton, Minnesota to experimentally assess the impact of a novel two phase bioreactor design for removing N and P from agricultural subsurface drainage water. Modular bioreactors were constructed using mixed woodchips plus corn cobs for facilitating denitrification plus either crushed concrete, steel slag or limestone fragments for P sorption. Experimental bioreactors were installed adjacent to an existing drainage ditch/waterway.
Nitrate removal was tied to the retention time in the bioreactor coupled with the addition of acetate. Longer retention time resulted in a greater removal of nutrients however, acetate improved nitrogen removal efficiency. Results also indicate that reduced conditions within the bioreactors but only consistently when acetate was added to the subsurface drainage water. All three P sorbing materials performed adequately for removing P from drainage water. Toward the end of the field experiment, as temperatures decreased, the P removal efficiency of the materials declined. During this time some of the materials acted as a source of P rather than a sink for P removal.