Plastic carrier polishing chamber reduces pollution swapping from denitrifying woodchip bioreactors

Denitrifying bioreactors with solid organic carbon sources (i.e., “woodchip bioreactors”) have proven to be relatively simple and cost effective treatment systems for nitrate-laden agricultural and aquacultural waters and wastewaters. However, because this technology is still relatively new, design modifications, such as the addition of a post-bioreactor polishing chamber filled with inert media, may offer potential to increase nitrate removal and mitigate unintended bioreactor by-products. Paired-column configurations filled with woodchips followed by plastic biofilm carrier media showed significant nitrate removal within the woodchip bioreactor columns (37, 26, and 88% nitrate removal efficiencies at woodchip column retention times of 7.1, 18, and 52 h), but no significant additional nitrate removal benefit of the post-processing plastic media chamber (41, 22, and 89% nitrate removal efficiencies, respectively). Releases of chemical oxygen demand from the woodchips were likely not sufficient to fuel significant nitrate removal in the polishing chamber. However, the polishing chamber significantly reduced nitrite releases from the bioreactor columns, and provided some mitigation of reduced sulfate during the 52-h retention time testing period (influent, woodchip effluent, and plastic chamber effluent sulfate concentrations of 23.6, 18.8, and 20.7 mg SO42− L−1, respectively). A full-scale post-woodchip polishing chamber filled with inert plastic media generally may not be worth the added cost unless the receiving waters are particularly sensitive to nitrite or hydrogen sulfide.