Denitrifying Woodchip Bioreactors: A Microbial Solution for Nitrate in Agricultural Wastewater—A Review

Nitrate (NO₃⁻) is highly water-soluble and considered to be the main nitrogen pollutants leached from agricultural soils. Its presence in aquatic ecosystems is reported to cause various environmental and public health problems. Bioreactors containing microbes capable of transforming NO₃⁻ have been proposed as a means to remediate contaminated waters. Woodchip bioreactors (WBRs) are continuous flow, reactor systems located below or above ground. Below ground systems are comprised of a trench filled with woodchips, or other support matrices. The nitrate present in agricultural drainage wastewater passing through the bioreactor is converted to harmless dinitrogen gas (N₂) via the action of several bacteria species. The WBR has been suggested as one of the most cost-effective NO₃⁻-removing strategy among several edge-of-field practices, and has been shown to successfully remove NO₃⁻ in several field studies. NO₃⁻ removal in the WBR primarily occurs via the activity of denitrifying microorganisms via enzymatic reactions sequentially reducing NO₃⁻ to N₂. While previous woodchip bioreactor studies have focused extensively on its engineering and hydrological aspects, relatively fewer studies have dealt with the microorganisms playing key roles in the technology. This review discusses NO₃⁻ pollution cases originating from intensive farming practices and N-cycling microbial metabolisms which is one biological solution to remove NO₃⁻ from agricultural wastewater. Moreover, here we review the current knowledge on the physicochemical and operational factors affecting microbial metabolisms resulting in removal of NO₃⁻ in WBR, and perspectives to enhance WBR performance in the future.