Novel Design and Field Performance of Phosphorus-Sorbing and Denitrifying Bioreactors.

A novel bioreactor for removing nitrate-nitrogen (NO3-N) and total phosphorus (TP) in subsurface drainage flow was designed and tested. The prototype consisted of a 1000-L reinforced cubic tank filled with layers of woodchips, corn cobs, and P-sorbing materials. Three materials were selected for the P removal media including steel slag, crushed recycled concrete and limestone. The field experiment consisted of three replications of three experimental treatments and a water distribution system which was designed to divert water from a subsurface drain outlet to each of the nine bioreactors. Potassium acetate (CH3CO2K) was used as an external carbon source to enhance denitrification. The experimental period lasted between May and December 2016 at the Southwest Research and Outreach Center near Lamberton, MN Precipitation during this period was 41% greater than normal. The experiment was divided into two phases: no acetate addition (two periods) and plus acetate addition (four periods). The target flow rate of subsurface drainage water delivered to each bioreactor was 4.0 L per minute Mean discharge rate was 3.5, 3.0, and 2.7 m3 d-1 for the crushed concrete, limestone. and steel slag, respectively. The hydraulic residence time ranged between 3.4 and 4.3 hour. Mean NO3-N concentration ranged from 13.1 to 13.8 mg/L at the cube outlets while that of the untreated source water averaged 19.2 mg/L. Nitrate-nitrogen load reduction ranged between 31 and 37%. Mean TP concentration ranged from 79.3 to 113.2 ug/L at the cube outlets while that of the untreated source water averaged 228.5 ug/L. Total phosphorus load reduction ranged between 25 and 41%. This design for bioreactors is meant to be installed directly under a drainage outlet if the ditch geometry allows. Based on the current results, it has the potential to be used as an alternative design to classical denitrifying bioreactor beds for subsurface drainage water treatment.