Carbon Dosing Increases Nitrate Removal Rates in Denitrifying Bioreactors at Low‐Temperature High‐Flow Conditions

Abstract

Nitrogen losses from croplands contribute to impairment of water bodies. This laboratory experiment evaluated various C sources for use in a denitrifying bioreactor, a conservation practice designed to reduce N losses. The nitrate removal efficiency of candidate treatments (corn cobs [CC], corn cobs with modified coconut coir [CC+MC], corn cobs with modified coconut coir and modified macadamia shell biochar [CC+MC+MBC], wood chips [WC], wood chips with hardwood biochar [WC+BC], and wood chips with continuous sodium acetate addition [WC+A]) were tested with up-flow direction. Effluent was sampled after a repeated weekly flow regime with hydraulic residence times of 1.5, 8, 12, and 24 h. Column temperatures were 15°C for 14 wk (warm), 5°C for 13 wk (cold), and again 15°C for 7 wk (rewarm). Cumulative nitrate N load reduction was greatest for WC+A (80, 80, and 97% during the warm, cold, and rewarm runs, respectively). Corn cob treatments (CC, CC+MC, and CC+MC+MBC) had the second greatest cumulative load reductions for all three temperature experiments, and WC and WC+BC had the lowest performance under these conditions. The nitrate removal rate was optimum at the 1.5-h hydraulic residence time for the WC+A treatment: 43, 30, and 121 g N m d for the warm, cold, and rewarm runs, respectively. Furthermore, acetate addition greatly improved wood chip performance and could be used to enhance nitrate N removal under the cold and high-flow-rate conditions of springtime drainage for the north-central United States.

Increased Duration of Drying–Rewetting Cycles Increases Nitrate Removal in Woodchip Bioreactors

“…Slopes of the linear regression across treatment groups in both DRW 2018 and DRW 2017 studies were 7.0 to 8.6. These results show that increased DRW duration led to greater DOC export, supporting the hypothesis that these aerobic periods release more labile DOC and help overcome the C limitation of denitrification believed to occur in woodchip bioreactors with recalcitrant, lignin-heavy media (Saliling et al, 2007;Cameron and Schipper, 2010;Feyereisen et al, 2016;Roser et al, 2018). Degradation of lignin by anaerobic respiration is negligible (Zeikus et al, 1982;Holt and Jones, 1983;Odier and Monties, 1983), with lignin breakdown primarily accomplished through aerobic microbes or fungi (Kirk and Farrell, 1987).…”

“…To the authors' knowledge, a method yielding such a dramatic increase in NO 3 removal rates in woodchip bioreactors without requiring additional energy or chemical inputs (e.g., C dosing, heating, electrical stimulation, etc.) has not been reported (Cameron and Schipper, 2011;Law et al, 2018;Roser et al, 2018). Further research is required to quantify total C loss from woodchips since DRW cycles will decrease woodchip lifespan.…”

Passive Dosing of Organic Substrates for Nitrate‐Removing Bioreactors Applied in Field Margins

“…This could stimulate N removal in winter, when C availability from wood chips might be lower due to low temperature. An example of this using acetate instead of ethanol as additional substrate was described recently (Roser et al, 2018).…”

Soil and water management: opportunities to mitigate nutrient losses to surface waters in the Northern Great Plains

“…Woodchip bioreactors are the most tested technology, where simple structures intercept drainage waters and facilitate nutrient removal through addition of a carbon source to stimulate biological denitrification that converts nitrate to nitrogen gas. Physico-chemical treatments placed after bioreactors have also been proposed to improve nutrient removal using a wide variety of materials (Jaynes et al 2008;Buda et al 2012;Hussain et al 2014;Hua et al 2016;Roser et al 2018). For example, Salo et al (2015) demonstrated that a woodchip bioreactor system with iron media as post-treatment removed nitrate and phosphate from subsurface drainage.…”