Design and performance characteristics of an in-stream woodchip denitrifying bioreactor for the treatment of agricultural drainage 1. Design, hydraulics and nitrate removal.

Abstract

Woodchip denitrifying bioreactors (WDBs) are an established edge-of-field practice for mitigating nitrogen (N) loads in agricultural drainage waters. Of the published ‘in-stream’ WDB case studies, many have suffered hydraulic failure due to siltation and clogging. In this study we present a very large (450 m³) in-stream WDB of a modular design that sought to incorporate clogging management features. The WDB was designed to provide long-term treatment of dairy land drain water flowing at 6 L/s containing 6 mg nitrate-N/L on average. The hydraulic function of the in-stream WDB and nitrate removal efficacy were determined from monthly monitoring conducted over the first two years of its operation. A 69 % reduction in the equivalent hydraulic conductivity K̅_x and 25 % reduction in effective porosity were observed over this time, during which the bioreactor was estimated to have retained at least 2.6 t of sediment. A logarithmic relationship between K̅_x and sediment load best described the clogging. The average nitrate removal rate was 3.3 ± 1.0 g N/m³ woodchip/day. Annualised removal rates were 497 and 485 kg N/year, which are on par with performance calculations made during the design phase. Nitrate removal was sensitive to temperature and could be modelled effectively assuming Arrhenius temperature dependence and zero-order reaction. The resulting zero-order nitrate reaction rate for a 20 °C reference temperature was 7.93 ± 4.64 mg N/L/day and the Arrhenius temperature coefficient was 1.06 ± 0.09. Parameter estimates were within the range of values typically associated with WDBs. The need for long-term monitoring of this in-stream WDB is discussed in the context of testing its modular design functionality for increased operational longevity.