Nitrate removal and greenhouse gas production in a stream-bed denitrifying bioreactor

Abstract

Denitrifying bioreactors are currently being tested as an option for treating nitrate (NO 3 −) contamination in groundwater and surface waters. However, a possible side effect of this technology is the production of greenhouse gases (GHG) including nitrous oxide (N 2O) and methane (CH 4). This study examines NO 3 − removal and GHG production in a stream-bed denitrifying bioreactor currently operating in Southern Ontario, Canada. The reactor contains organic carbon material (pine woodchips) intended to promote denitrification. Over a 1 year period, monthly averaged removal of influent (stream water) NO 3 − ranged from 18 to 100% (0.3–2.5 mg N L −1). Concomitantly, reactor dissolved N 2O and CH 4 production, averaged 6.4 μg N L −1 (2.4 mg N m −2 d −1), and 974 μg C L −1 (297 mg C m −2 d −1) respectively, where production is calculated as the difference between inflow and effluent concentrations. Gas bubbles entrapped in sediments overlying the reactor had a composition ranging from 19 to 64% CH 4, 1 to 6% CO 2, and 0.5 to 2 ppmv N 2O; however, gas bubble emission rates were not quantified in this study. Dissolved N 2O production rates from the bioreactor were similar to emission rates reported for some agricultural croplands (e.g. 0.1–15 mg N m −2 d −1) and remained less than the highest rates observed in some N-polluted streams and rivers (e.g. 110 mg N m −2 d −1, Grand R., ON). Dissolved N 2O production represented only a small fraction (0.6%) of the observed NO 3 − removal over the monitoring period. Dissolved CH 4 production during summer months (up to 1236 mg C m −2 d −1), was higher than reported for some rivers and reservoirs (e.g. 6–66 mg C m −2 d −1) but remained lower than rates reported for some wastewater treatment facilities (e.g. sewage treatment plants and constructed wetlands, 19,500–38,000 mg C m −2 d −1).