Rates, controls and potential adverse effects of nitrate removal in a denitrification bed

Denitrification beds are a simple approach for removing nitrate (NO(3)(-)) from a range of point sources prior to discharge into receiving waters. These beds are large containers filled with woodchips that act as an energy source for microorganisms to convert NO(3)(-) to nitrogen (N) gases (N(2)O, N(2)) through denitrification. This study investigated the biological mechanism of NO(3)(-) removal, its controlling factors and its adverse effects in a large denitrification bed (176 m x 5m x 1.5 m) receiving effluent with a high NO(3)(-) concentration (> 100 g N m(-3)) from a hydroponic glasshouse (Karaka, Auckland, New Zealand). Samples of woodchips and water were collected from 12 sites along the bed every two months for one year, along with measurements of gas fluxes from the bed surface. Denitrifying enzyme activity (DEA), factors limiting denitrification (availability of carbon, dissolved organic carbon (DOC), dissolved oxygen (DO), temperature, pH, and concentrations of NO(3)(-), nitrite (NO(2)(-)) and sulfide (S(2-))), greenhouse gas (GHG) production - as nitrous oxide (N(2)O), methane (CH(4)), carbon dioxide (CO(2)) - and carbon (C) loss were determined. NO(3)(-)-N concentration declined along the bed with total NO(3)(-)-N removal rates of 10.1 kg N d(-1) for the whole bed or 7.6 g N m(-3) d(-1). NO(3)(-)-N removal rates increased with temperature (Q(10) = 2.0). In laboratory incubations, denitrification was always limited by C availability rather than by NO(3)(-). DO levels were above 0.5 mg L(-1) at the inlet but did not limit NO(3)(-)-N removal. pH increased steadily from about 6 to 7 along the length of the bed. Dissolved inorganic carbon (C-CO(2)) increased in average about 27.8 mg L(-1), whereas DOC decreased slightly by about 0.2 mg L(-1) along the length of the bed. The bed surface emitted on average 78.58 mu g m(-2) min(-1) N(2)O-N (reflecting 1% of the removed NO(3)(-)-N), 0.238 mu g m(-2) min(-1) CH(4) and 12.6 mg m(-2) min(-1) CO(2). Dissolved N(2)O-N increased along the length of the bed and the bed released on average 362 g dissolved N(2)O-N per day coupled with N(2)O emission at the surface about 4.3% of the removed NO(3)(-)-N as N(2)O. Mechanisms to reduce the production of this GHG need to be investigated if denitrification beds are commonly used. Dissolved CH(4) concentrations showed no trends along the length of the bed, ranging from 5.28 mu g L(-1) to 34.24 mu g L(-1). Sulfate (SO(4)(2-)) concentrations declined along the length of the bed on three of six samplings; however, declines in SO(4)(2-) did not appear to be due to SO(4)(2-) reduction because S(2-) concentrations were generally undetectable. Ammonium (NH(4)(+)) (range: < 0.0007 mg L(-1) to 2.12 mg L(-1)) and NO(2)(-) concentrations (range: 0.0018 mg L(-1) to 0.95 mg L(-1)) were always very low suggesting that anammox was an unlikely mechanism for NO(3)(-) removal in the bed. C longevity was alculated from surface emission rates of CO(2) and release of dissolved carbon (DC) and suggested that there would be ample C available to support denitrification for up to 39 years. This study showed that denitrification beds can be an efficient tool for reducing high NO(3)(-) concentrations in effluents but did produce some GHGs. Over the course of a year NO(3)(-) removal rates were always limited by C and temperature and not by NO(3)(-) or DO concentration. (C) 2010 Elsevier B.V. All rights reserved.