Effectiveness of Denitrifying Bioreactors on Water Pollutant Reduction from Agricultural Areas

Abstract

HighlightsDenitrifying woodchip bioreactors treat nitrate-N in a variety of applications and geographies.This review focuses on subsurface drainage bioreactors and bed-style designs (including in-ditch).Monitoring and reporting recommendations are provided to advance bioreactor science and engineering.Abstract. Denitrifying bioreactors enhance the natural process of denitrification in a practical way to treat nitrate-nitrogen (N) in a variety of N-laden water matrices. The design and construction of bioreactors for treatment of subsurface drainage in the U.S. is guided by USDA-NRCS Conservation Practice Standard 605. This review consolidates the state of the science for denitrifying bioreactors using case studies from across the globe with an emphasis on full-size bioreactor nitrate-N removal and cost-effectiveness. The focus is on bed-style bioreactors (including in-ditch modifications), although there is mention of denitrifying walls, which broaden the applicability of bioreactor technology in some areas. Subsurface drainage denitrifying bioreactors have been assessed as removing 20% to 40% of annual nitrate-N loss in the Midwest, and an evaluation across the peer-reviewed literature published over the past three years showed that bioreactors around the world have been generally consistent with that (N load reduction median: 46%; mean ±SD: 40% ±26%; n = 15). Reported N removal rates were on the order of 5.1 g N m-3 d-1 (median; mean ±SD: 7.2 ±9.6 g N m-3 d-1; n = 27). Subsurface drainage bioreactor installation costs have ranged from less than $5,000 to $27,000, with estimated cost efficiencies ranging from less than $2.50 kg-1 N year-1 to roughly $20 kg-1 N year-1 (although they can be as high as $48 kg-1 N year-1). A suggested monitoring setup is described primarily for the context of conservation practitioners and watershed groups for assessing annual nitrate-N load removal performance of subsurface drainage denitrifying bioreactors. Recommended minimum reporting measures for assessing and comparing annual N removal performance include: bioreactor dimensions and installation date; fill media size, porosity, and type; nitrate-N concentrations and water temperatures; bioreactor flow treatment details; basic drainage system and bioreactor design characteristics; and N removal rate and efficiency. Keywords: Groundwater, Nitrate, Nonpoint-source pollution, Subsurface drainage, Tile.

Comparison of microbial communities in replicated woodchip bioreactors

“…Since the early 2010s, literature describing the performance of these systems has increased exponentially, with the primary focus being engineering design and optimizing nitrate removal characteristics. Denitrifying bioreactors installed globally have been shown to consistently and efficiently reduce non‐point source N loads by 40 ± 26% (mass‐basis) (Christianson et al., 2021).…”

Carbon supplementation and bioaugmentation to improve denitrifying woodchip bioreactor performance under cold conditions

“…In an earlier review of bioreactor studies, Schipper et al (2010) reported a range of NRR of 2 to 22 g N m -3 d -1 from temperatures ranging from 2 to 20°C, with greater rates corresponding to higher temperatures. The most recent review of peerreviewed bioreactor studies since Addy et al (2016) reports a median of 5.1 g N m -3 d -1 , with 95% of NRRs <15 g N m -3 d -1 (Christianson et al, 2021). Dosing the woodchip bed with C improved the NRR yet did not increase outflow DOC concentrations over the Control or BioAug treatments.…”

“…Many treatment designs use denitrification, conversion of dissolved nitrate to dinitrogen gas via microbial activity, to accomplish reductions. Designs include constructed/treatment wetlands (Bachand and Horne, 2000;Crumpton, 2001), denitrification walls (Manca et al, 2020), and woodchip denitrification beds (Schipper et al, 2010;Addy et al, 2016;Christianson et al, 2021).…”

Phosphorus removal in denitrifying woodchip bioreactors varies by wood type and water chemistry

“…Using the range of observed mass removals of 0.4 to 13 mg DRP/kg and assuming an average bioreactor size of 100 m 3 (following Christianson et al 2021 ) and bulk density of 200 kg/m 3 (e.g., Goodwin et al 2015 ) would result in DRP removals of 8.0 to 260 g at the field scale. Assuming an average drainage area of 20 ha for this hypothetical bioreactor would result in P loss reductions 0.40 to 13 g DRP/ha, albeit this would be on a one-time basis and the ultimate fate of this P is still unclear.…”