Holistic Evaluation of Field-Scale Denitrifying Bioreactors as a Basis to Improve Environmental Sustainability

Nitrous oxide and methane production and consumption at five full-size denitrifying bioreactors treating subsurface drainage water

Nitrate (NO3-) removal in denitrifying bioreactors is influenced by flow, water chemistry, and design, but it is not known how these widely varying factors impact the production of nitrous oxide (N2O) or methane (CH4) across sites. Woodchip bioreactors link the hydrosphere and atmosphere in this respect, so five full-size bioreactors in Illinois, USA, were monitored for NO3-, N2O, and CH4 to better document where this water treatment technology resides along the pollution swapping to climate smart spectrum. Both surface fluxes and dissolved forms of N2O and CH4 were measured (n = 7-11 sampling campaigns per site) at bioreactors ranging from <1 to nearly 5 years old and treating subsurface drainage areas from between 6.9 and 29 ha. Across all sites, N2O surface and dissolved volumetric production rates averaged 1.0 ± 1.6 mg N2O-N/m3-d and 24 ± 62 mg dN2O-N/m3-d, respectively, and CH4 production rates averaged 6.0 ± 26 mg CH4-C/m3-d and 310 ± 520 mg dCH4-C/m3-d for surface and dissolved, respectively. However, N2O was consistently consumed at one bioreactor, and only three of the five sites produced notable CH4. Surface fluxes of CH4 were significantly reduced by the presence of a soil cover. Bioreactor denitrification was relatively efficient, with only 0.51 ± 3.5 % of removed nitrate emitted as N2O (n = 48). Modeled indirect N2O emissions factors were significantly lower when a bioreactor was present versus absent (EF5: 0.0055 versus 0.0062 kg N2O-N/kg NO3-N; p = 0.0011). While further greenhouse gas research on bioreactors is recommended, this should not be used as an excuse to slow adoption efforts. Bioreactors provide a practical option for voluntary water quality improvement in the heavily tile-drained US Midwest and elsewhere.

Extreme low-flow conditions in a dual-chamber denitrification bioreactor contribute to pollution swapping with low landscape-scale impact

Denitrification bioreactors are an effective edge-of-field conservation practice for nitrate (NO₃) reduction from subsurface drainage. However, these systems may produce other pollutants and greenhouse gases during NO₃ removal. Here a dual-chamber woodchip bioreactor system experiencing extreme low-flow conditions was monitored for its spatiotemporal NO₃ and total organic carbon dynamics in the drainage water. Near complete removal of NO₃ was observed in both bioreactor chambers in the first two years of monitoring (2019-2020) and in the third year of monitoring in chamber A, with significant (p < 0.01) reduction of the NO₃-N each year in both chambers with 8.6-11.4 mg NO₃-N L^-1 removed on average. Based on the NO₃ removal observed, spatial monitoring of sulfate (SO₄), dissolved methane (CH₄), and dissolved nitrous oxide (N₂O) gases was added in the third year of monitoring (2021). In 2021, chambers A and B had median hydraulic residence times (HRTs) of 64 h and 39 h, respectively, due to varying elevations of the chambers, with drought conditions making the differences more pronounced. In 2021, significant production of dissolved CH₄ was observed at rates of 0.54 g CH₄-C m^-3 d^-1 and 0.07 g CH₄-C m^-3 d^-1 in chambers A and B, respectively. In chamber A, significant removal (p < 0.01) of SO₄ (0.23 g SO₄ m^-3 d^-1) and dissolved N₂O (0.21 mg N₂O-N m^-2 d^-1) were observed, whereas chamber B produced N₂O (0.36 mg N₂O-N m^-2 d^-1). Considering the carbon dioxide equivalents (CO₂e) on an annual basis, chamber A had loads (~12,000 kg CO₂e ha^-1 y^-1) greater than comparable poorly drained agricultural soils; however, the landscape-scale impact was small (<1 % change in CO₂e) when expressed over the drainage area treated by the bioreactor. Under low-flow conditions, pollution swapping in woodchip bioreactors can be reduced at HRTs <50 h and NO₃ concentrations >2 mg N L^-1.

Denitrifying bioreactor microbiome: Understanding pollution swapping and potential for improved performance

Denitrifying woodchip bioreactors are a best management practice to reduce nitrate-nitrogen (NO₃ -N) loading to surface waters from agricultural subsurface drainage. Their effectiveness has been proven in many studies, although variable results with respect to performance indicators have been observed. This paper serves the purpose of synthesizing the current state of the science in terms of the microbial community, its impact on the consistency of bioreactor performance, and its role in the production of potential harmful by-products including greenhouse gases, sulfate reduction, and methylmercury. Microbial processes other than denitrification have been observed in these bioreactor systems, including dissimilatory nitrate reduction to ammonia (DNRA) and anaerobic ammonium oxidation (anammox). Specific gene targets for denitrification, DNRA, anammox, and the production of harmful by-products are identified from bioreactor studies and other environmentally relevant systems for application in bioreactor studies. Lastly, cellulose depletion has been observed over time via increasing ligno-cellulose indices, therefore, the microbial metabolism of cellulose is an important function for bioreactor performance and management. Future work should draw from the knowledge of soil and wetland ecology to inform the study of bioreactor microbiomes.

Reducing adverse side effects by seasonally lowering nitrate removal in subsurface flow constructed wetlands

Subsurface flow constructed wetlands with wood chips (SSF-CWs) have proven to effectively reduce the loss of nitrogen (N) from agricultural fields to surface water, however in some cases production of negative side effects such as methane and phosphate occur. We examined if these side effects can be avoided by decreasing the hydraulic retention time (HRT) from on average 82 h to 11 h during summer to autumn in two pilot SSF-CWs. Furthermore, we investigated the potential of the SSF-CWs to reduce phosphorus (P) loss from agricultural drainage systems. The influent and effluent concentration of total N (TN), nitrate-N, total P, phosphate-P, suspended sediment, and sulphate were monitored for five years (2013-2017). Methane concentrations were measured during two periods in 2014 and 2017. Flow was measured continuously by electromagnetic flow-meters. The nitrate-N removal was reduced from 98-100% to 27-32% and the sulphate reduction from 32-53% to 1-2% when decreasing HRT. Concurrently this resulted in a considerable decrease in the difference between the effluent and influent concentration of phosphate-P and methane concentration compared to similar periods in the preceding years. The SSF-CWs retained 67-85% of the annual loading of particulate P, but acted as both a sink and source of phosphate-P, thus further initiatives are therefore required to prevent phosphate-P release from SSF-CWs. Although during the entire monitoring period the SSF-CWs retained 29-33% of the total P loading. In summary, this study stresses how important a holistic approach is when implementing and designing new N mitigation measures.

Biochar fails to enhance nutrient removal in woodchip bioreactor columns following saturation

Denitrifying bioreactors are edge-of-field structures that remove excess nitrogen (N) from intercepted agricultural drainage by supporting the activity of denitrifying microorganisms with a saturated organic carbon substrate. Although these bioreactors successfully mitigate N export, the typical woodchip systems have little effect on phosphorus (P), which is also often present in environmentally harmful quantities in drainage waters. Currently, the evidence that amending woodchip bioreactors with biochar enhances both N and P removal rates is mixed, but more work is required to test this hypothesis under controlled conditions. To determine the effect of biochar amendment on nitrate (NO₃-N) and phosphate (PO₄-P) removal in woodchip bioreactors, three media types-aged woodchips (W), 10% (B₁₀) and 30% (B₃₀) biochar by volume-were tested under different operational conditions during five-day laboratory trials with horizontal, flow-through columns. Nutrient removal was observed under different flow rates yielding hydraulic residence times of 3, 6, and 12 hours with four formulations of simulated agricultural drainage, all combination of 16.1 or 4.5 mg L^-1 NO₃-N and 1.9 or 0.6 mg L^-1 PO₄-P. Each unique treatment with respect to media type, HRT, and influent formulation was tested in triplicate using independent columns. All treatments successfully removed NO₃-N, but PO₄-P removal was inconsistent. Cumulative NO₃-N removal efficiencies ranged 15-98% with an average removal rate of 11.0 g m^-3 d^-1; biochar amendment enhanced removal only in response to sufficiently high loading rates. Cumulative PO₄-P removal efficiencies ranged from 66% removal to 170% export of the influent load; biochar addition was associated with increased export. These results indicate that pine-feedstock biochar poses a substantial increase to PO₄-P leaching risk and only modestly enhances NO₃-N removal given sufficiently high loading.

Moving Denitrifying Bioreactors beyond Proof of Concept: Introduction to the Special Section

Denitrifying bioreactors are organic carbon-filled excavations designed to enhance the natural process of denitrification for the simple, passive treatment of nitrate-nitrogen. Research on and installation of these bioreactors has accelerated within the past 10 years, particularly in watersheds concerned about high nonpoint-source nitrate loads and also for tertiary wastewater treatment. This special section, inspired by the meeting of the Managing Denitrification in Agronomic Systems Community at the 2014 Annual Meeting of the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, aims to firmly establish that denitrifying bioreactors for treatment of nitrate in drainage waters, groundwater, and some wastewaters have moved beyond the proof of concept. This collection of 14 papers expands the peer-reviewed literature of denitrifying bioreactors into new locations, applications, and environmental conditions. There is momentum behind the pairing of wood-based bioreactors with other media (biochar, corn cobs) and in novel designs (e.g., use within treatment trains or use of baffles) to broaden applicability into new kinds of waters and pollutants and to improve performance under challenging field conditions such as cool early season agricultural drainage. Concerns about negative bioreactor by-products (nitrous oxide and hydrogen sulfide emissions, start-up nutrient flushing) are ongoing, but this translates into a significant research opportunity to develop more advanced designs and to fine tune management strategies. Future research must think more broadly to address bioreactor impacts on holistic watershed health and greenhouse gas balances and to facilitate collaborations that allow investigation of mechanisms within the bioreactor "black box."