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

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(2)O) 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(2)O and CH(4) production, averaged 6.4 mu g N L(-1) (2.4 mg N m(-2) d(-1)), and 974 mu g 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(2)O; however, gas bubble emission rates were not quantified in this study. Dissolved N(2)O 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(2)O 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)). (C) 2010 Published by Elsevier B.V.