Combining Denitrification and Phosphorus Uptake in Edge-of-Field Bioreactors.

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 tile drainage effluent. The nitrate-N and DRP removal performance of candidate media treatments – woodchips (WC), corn cobs (CC), woodchips with biochar (WC+BC), woodchips with continuous sodium acetate injection (WC+A), corn cobs with modified coconut coir (CC+MC), and corn cobs with modified coconut coir and biochar (CC+MC+MBC) – were tested by pumping water with 20 mg/L nitrate and 0.3 mg/L phosphate through PVC columns packed with 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. Preliminary results show that nitrate-N reduction performance at 15°C during all flow regimes was: WC+A > CC+MC > CC > CC+MC+MBC > WC > WC+BC. Nitrate-N concentration reductions ranged from 99.7 to 54% for 24 h HRT and from 25.4 to 0.4% for 1.5 h HRT. The nitrate-N removal rate for WC+A was 50 g N m^-3 d^-1 at 1.5 h HRT. Averaged across all HRTs, DRP reduction performance was: CC+MC > CC > CC+MC+MBC > WC+A > WC > WC+BC, ranging from 83 to 12%. In conclusion, denitrification can be coupled with phosphorus uptake in bioreactors. Future work is needed to determine the mechanism of phosphorus uptake.