Design and performance of an in-stream woodchip denitrifying bioreactor treating agricultural drainage 2. Co-benefits and greenhouse gas emissions

Abstract

Woodchip denitrifying bioreactors (WDBs) are an established edge-of-field practice for mitigating nitrogen losses in agricultural watersheds. WDBs also have potential to remove other aquatic contaminants associated with farming. We examined the efficacy of a large (450 m³) in-stream WDB at removing phosphorus (P) and microbial pathogens, present in farm drainage water, as secondary treatment co-benefits. We provide the first examination of Campylobacter removal effected by a WDB. Greenhouse gas emissions were also evaluated as a pollution-swapping phenomenon and related to the physio-chemical state of the bioreactor. Occasionally, small quantities of P were exported from the WDB, but overall it acted as a net sink for P with approximately 17.5 kg of total P and 8.1 kg dissolved reactive P removed over the 25-month study period. The WDB showed moderate capacity for removal of microbial contaminants. The median reduction of Escherichia coli was 0.98-log_, which was less than the median reduction of 1.23-log determined for Campylobacter. The in-stream WDB acted as a source of greenhouse gases and production was related to hydraulic retention times and temperature. Production and export of methane was distinctly seasonal and showed a strong positive correlation with temperature, whereas nitrous oxide emission was influenced by cooler temperatures. On average, 4 % of the nitrate-N denitrified by the bioreactor was exported as nitrous oxide. Our results indicate that methane export could be increasing as the hydraulic function of the WDB declines.