Denitrification Bioreactor Performance: Effects of Media and Temperature on Nitrate Removal.

Denitrification bioreactors are a promising technology for mitigation of nitrate-nitrogen (NO₃-N) losses in subsurface drainage water.  Bioreactors are constructed with carbon substrates, typically wood chips, to provide a substrate for denitrifying microorganisms. Columns were packed with wood chips (WC), barley straw (BS), corn cobs (CC), corn stover (CS), a combination of corn cobs followed by wood chips (CC-WC), and corn cobs followed by a plastic (Kaldnes^TM) biofilm support (CC-K). A synthetic water solution (50 mg L^-1 of NO₃-N) was pumped through the columns at a 12-h hydraulic residence time.  Bioreactors were operated at 16°C (experiment 1) and 1.7°C (experiment 2).  At 16°C, NO₃-N removal was greatest for the CC and the CC-K reactors (35-40 g N m^-3 d^-1), and least for WC (1.4 -5.6 g N m^-3 d^-1).  The BS, CS and CC-WC materials were intermediate in NO₃-N removal. At the lower temperature, the same trend was observed with respect to the relative NO₃-N removal by the different materials, but removal was decreased in all treatments (< 10 g N m^-3 d^-1).  The abundance of denitrifying bacteria was assessed by qPCR analysis of the nosZ gene.  Gene abundance followed a similar trend as NO₃-N removal with CC supporting 3.76 x 10¹² gene copies and WC supporting 2.18 x 10⁸ copies g^-1. Gene abundance was not reduced in the cold temperature experiment. Although the Kaldnes did not provide the denitrifiers with a carbon source, a biofilm developed that contained (> 10⁹ copies g^-1), suggesting that soluble C sources were utilized.