Increased brine concentration increases nitrate reduction rates in batch woodchip bioreactors treating brine from desalination

This study investigated the effect of brine concentration on nitrate reduction in woodchip bioreactors treating brine from one and two-stage reverse osmosis (RO). An initial field experiment with aboveground, outdoor woodchip bioreactors tested batch experiments using one and two-stage RO brine, and a follow-up lab experiment in temperature-controlled chambers tested the same treatments and a 50:50 mix of one and two-stage brine using aged woodchips (11 months). The effect of brine concentration was greatest in the field experiment in which fresh woodchips were used, with nitrate removal rates of 20–152 g N m⁻³ of saturated woodchip d⁻¹. Brine concentration effect was weaker in the lab experiment with aged woodchips with lower nitrate removal rates of 2–62 g N m⁻³ of saturated woodchip d⁻¹. Results indicate that effect of brine concentration on nitrate reduction was dependent on carbon availability, and that brine concentration may have a smaller effect on nitrate reduction in aged woodchips not exposed to periodic unsaturated conditions. Increased brine concentration increased dissolved organic carbon in the effluent. Sustained nitrate reduction was seen over a wide range of nitrate concentration (40–230 mg N L⁻¹) and salinity (12–43 parts per thousand).