Using unique carbon source combinations to increase nitrate and phosphate removal in bioreactors

University of Minnesota M.S. thesis. June 2016. Major: Land and Atmospheric Science. Advisors: Gary Feyereisen, David Mulla. 1 computer file (PDF); ix, 95 pages.Nitrogen (N) and phosphorus (P) losses from croplands contribute to impairment of water bodies. This study was conducted to test candidate denitrifying bioreactor media for nitrate-N and dissolved reactive P (DRP) removal from agricultural effluent in drainage ditches. The nitrate-N and DRP removal performance of carbon materials widely available in the Midwest, wood chips (WC) and corn cobs (CC), were compared to treatments of mixed materials: wood chips and hardwood biochar (WC+BC), wood chips and sodium acetate (WC+A), corn cobs and modified coconut coir (CC+MC), and corn cobs, modified coconut coir, and modified macadamia biochar (CC+MC+MBC). Water with a nitrate-N concentration of 20 mg N L-1 and a DRP concentration of 0.3 mg P L-1 was pumped through PVC columns packed with treatment media. The flow rate was adjusted to match the rise and decay of a typical drainage hydrograph. Effluent was sampled after hydraulic residence times (HRT) of 1.5, 8, 12, and 24 h. The laboratory experiment was conducted at 15°C for 14 weeks, 5°C for 13 weeks, and 15°C again for 7 weeks in a temperature controlled chamber, designated the warm run, cold run and rewarm run, respectively. Nitrate-N load reductions ranged from 24% to 96% in the warm and rewarm runs and from 4% to 80% in the cold run. Nitrate-N load reduction performance at all temperatures was in the order of: WC+A > CC+MC > CC > CC+MC+MBC > WC > WC+BC. The nitrate removal rate (NRR) was highest at the 1.5h HRT for the WC+A treatment at all temperatures. Cumulative DRP load reductions in the warm and rewarm runs were statistically higher in the CC, CC+MC, and CC+MC+MBC treatments, with DRP load reductions of 74%, 81%, and 67%, respectively. The WC+A treatment had the highest DRP load reduction in the cold run, with a 45% reduction. The CC, CC+MC, and CC+MC+MBC treatments had both high NRR and high DRP percent concentration removal in the warm and rewarm runs, but the WC+A treatment had higher removal of both nutrients in the cold run and specifically at lower HRTs. For both nitrate-N and DRP load reductions during high flows and cold temperatures, WC+A would be the recommended treatment. Future work should focus on the addition of carbon such as sodium acetate to enhance bioreactor performance during high drainage and cold temperature conditions