Performance of in-line denitrifying woodchip bioreactors for nitrogen reduction from subsurface drained pasture receiving swine lagoon effluent

Abstract

Abstract. Excessive nitrogen losses from subsurface drained lands receiving waste lagoon effluent have become a major public concern. Main purpose of this study was to evaluate performance of four up-flow in-line denitrifying woodchip bioreactors on reducing nitrogen losses from subsurface drained pasture receiving swine lagoon effluent. Influence of factors, such as temperature, initial nitrate concentration, hydraulic retention time (HRT), and flow rate, on effectiveness of bioreactors were also studied. Denitrifying bioreactors were installed at the edge of four subsurface drain lines receiving continuous drainage water flow in the naturally very poorly drained Cape Fear Loam soil located in lower coastal plain of North Carolina. The four drain lines were installed at 1.0 m depth, 12.5 m spacing, managed as two conventional and controlled drainages separately. Nitrate concentrations were significantly reduced by all four bioreactors. Yearly net percent nitrate reduction for controlled drainage-bioreactor (CDB) systems and free drainage- bioreactor (FDB) systems during study period ranged from 25% to 93% and 13% to 59%, respectively. Calculated nitrate removal rates increased with flow rate and initial nitrate concentration, however, it was not always increases with HRT and temperature for every bioreactor. Ratio of water that went through bioreactors and total drainage flow (three out of four) decreased from 2012 to 2014. Up flow in-line bioreactors showed sufficient removal of nitrogen loading from drained pasture lands, however, proper maintenance of anaerobic condition, suitable hydraulic conductivity, proper HRT, and the management of the whole field drainage system were critical on performance of denitrifying bioreactor.