A Review of the factors controlling the performance of denitrifying woodchip bioreactors

Abstract. The denitrifying woodchip bioreactor is an example of a cost effective and a simple edge-of-field strategy to reduce the nitrogen export from agricultural fields through subsurface drainage systems. Understanding the factors controlling their nitrate-nitrogen removal performance is essential to maximizing the benefits from the bioreactors. This abstract and paper is intended to provide a review of the controlling factors. The major controlling factors are: flow rate through the bioreactors, hydraulic retention time (HRT), carbon source, microbial community, initial nitrate concentration, temperature, pH and concentration of dissolved oxygen (DO) of the inflow water. A primary mechanism impacting the nitrate removal rate is the flow rate. Studies show that, in general, the nitrate load reduction efficiency decreases when the flow rate increases above a baseline flow as a result of insufficient time for the denitrification process to occur as well as increased transport of DO into the reactor. Longer HRT generally removes higher amounts of nitrate from the bioreactor; however, too long residence time causes unwanted chemical reactions such as reduction of sulfate and methylation of mercury. The presence of DO in the water in the reactor negatively affects the denitrifying bacterial activities, causes accumulation of denitrifying intermediates, and accelerates the breakdown of the woodchips. The carbon substrate is important for the denitrification by providing energy to microbes and stimulating the aerobic respiration to reduce the oxygen concentration. The activities of the enzymes which are involved in denitrification process are pH dependent. Managing the factors controlling the performance of the bioreactor greatly improves the bioreactor performance.