Denitrifying Bioreactors Research Articles

Denitrification and anammox remove nitrogen in denitrifying bioreactors

Denitrification has been considered the main pathway converting nitrate (NO3–) to dinitrogen gas (N2) in denitrifying bioreactors. Here, the importance of an alternative nitrogen (N) removal process, namely anaerobic ammonium oxidation (anammox), was assessed by monitoring the removal of N species from partially nitrified municipal wastewater passing through fifteen mesocosm scale (∼700 L) bioreactors containing woodchip, coconut husk or gravel during their initial and eighth year of operation. Additionally, lab experiments using a 15N isotope-pairing technique were performed to partition production of N2 to these different microbial processes. The effective removal of both NO3– and ammonium (NH4+) and the formation of hybrid N2 (i.e. 29N2) observed in this study demonstrated that, along with denitrification, anammox was an effective pathway for N removal when both NO3– and NH4+ were present. Anammox removal rates ranged from 0.6 to 3.8 g N per m3 per day, while denitrification rates ranged from 0.7 to 2.6 g N per m3 per day. The contributions of anammox to N removal was dependent on media, with anammox becoming more dominant in bioreactors where denitrification was carbon limited. Designing denitrifying bioreactors to support both denitrification and anammox expands the utility of these passive approaches for improving treatment of wastewater.

Nitrous oxide and methane production from denitrifying woodchip bioreactors at three hydraulic residence times

Denitrifying bioreactors remove nitrate (NO3−) from agricultural drainage and are slated to be an integral part of nitrogen reduction strategies in the Mississippi River Basin. However, incomplete denitrification can result in nitrous oxide (N2O) production and anaerobic conditions within bioreactors may be conducive to methane (CH4) production via methanogenesis. Greenhouse gas production has the potential to trade excess NO3− in surface water with excess greenhouses gases in the atmosphere. Our study examined N2O and CH4 production from pilot scale (6.38 m3) bioreactors across three hydraulic residence times (HRTs), 2, 8, and 16 h. Production was measured from both the surface of the bioreactors and dissolved in the bioreactor effluent. Nitrous oxide and CH4 was produced across all HRTs, with the majority dissolved in the effluent. Nitrous oxide production was significantly greater (P < 0.05) from 2 h HRTs (478.43 mg N2O m−3 day−1) than from 8 (29.95 mg N2O m−3 day−1) and 16 (36.61 mg N2O m−3 day−1) hour HRTs. Methane production was significantly less (P < 0.05) from 2 h HRTs (0.51 g C m3 day−1) compared to 8 (1.50 g C m3 day−1) and 16 (1.69 g C m3 day−1) hour HRTs. The 2 h HRTs had significantly greater (P = 0.05) impacts to climate change compared to 8 and 16 h HRTs. Results from this study suggest managing HRTs between 6 and 8 h in field bioreactors could minimize total greenhouse gas production and maximize NO3− removal.

Application of denitrifying bioreactors for the removal of atrazine in agricultural drainage water

Atrazine and nitrate NO3-N are two agricultural pollutants that occur widely in surface and groundwater. One of the pathways by which these pollutants reach surface water is through subsurface drainage tile lines. Edge-of-field anaerobic denitrifying bioreactors apply organic substrates such as woodchips to stimulate the removal of NO3-N from the subsurface tile waters through denitrification. Here we investigated the co-removal of NO3−N and atrazine by these bioreactors. Laboratory experiments were conducted using 12-L woodchips-containing flow-through bioreactors, with and without the addition of biochar, to treat two concentrations of atrazine (20 and 50 μg L−1) and NO3−N (1.5 and 11.5 mg L−1), operated at four hydraulic retention time, HRT, (4 h, 8 h, 24 h, 72 h). Additionally, we examined the effect of aerating the bioreactors on atrazine removal. Furthermore, we tested atrazine removal by a field woodchip denitrifying bioreactor. The removal of both NO3−N and atrazine increased with increasing HRT in the laboratory bioreactors. At 4 h, the woodchip bioreactors removed 65% of NO3−N and 25% of atrazine but, at 72 h, the bioreactors eliminated all the NO3−N and 53% of atrazine. Biochar-amended bioreactors removed up to 90% of atrazine at 72-h retention time. We concluded that atrazine removal was primarily via adsorption because neither aeration nor NO3−N levels had an effect. At 4-h retention time, the field bioreactors achieved 2.5 times greater atrazine removal than the laboratory bioreactors. Our findings thus highlighted hydraulic retention time and biochar amendments as two important factors that may control the efficiency of atrazine removal by denitrifying bioreactors. In sum, laboratory and field data demonstrated that denitrifying bioreactors have the potential to decrease pesticide transport from agricultural lands to surface waters.

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

Denitrifying bioreactors are dependent on organic matter supply as a substrate for effective NO removal. In this study, the difference in removal efficiency and side effects when using different organic matter sources and dosing strategies was tested in two field experiments. The organic matter sources tested were woodchips and ethanol. The effect of woodchips was tested using woodchip-enveloped drains. Ethanol was supplied to a flow-through reactor by passive dosing by diffusion through silicone tubing. The woodchip-enveloped drains showed a removal efficiency of 80% during the first year of application, but this rate decreased during the second and third years of application, coinciding with a decrease in dissolved organic C and an increase in redox potential. The removal efficiency was higher and remained higher over a longer period of time when the drains were installed more deeply. The flow-through reactor with ethanol could lead to a higher removal efficiency (up to 95%) at higher hydraulic retention time (HRT, 0.1 d) than the woodchip-enveloped drains (HRT = 5 d). Passive dosing of organic substrates is simple, needs little maintenance and no energy, and can be performed independent of electricity. A denitrifying bioreactor with a controlled drainage inlet and outlet is a promising setup for optimizing N removal and minimizing side effects.

Denitrifying bioreactor response during storm events

Denitrifying bioreactors can be an effective management strategy to address excess nitrate from tile-drained fields. However, there has been minimal research on how bioreactors function during storm events, which can account for large portions of annual nitrate load. Storm events cause reduced hydraulic retention time and potentially temperature and flow path fluctuations that may disturb denitrifying microbial communities within the bioreactor. In this study, nitrate concentration and flow rate were monitored during baseflow and storm-induced flow in four denitrifying bioreactors treating agricultural tile drainage to compare removal rate (RR) and removal efficiency (RE) between flow conditions. Removal rate was higher in stormflow than baseflow for all the bioreactors, though this was only significant for two bioreactors. However, RR was lower on average following events compared with pre-storm rates. Lag time, the period between a parcel of water entering the bioreactor and exiting, ranged between 2 and 24 h for the observed storms. Calculations of RR and RE that account for lag time were significantly lower than standard calculations. The model of RR that included a variable of flow variation (coefficient of variation in flow over a 24-hour period) was most effective at describing observed data. This, along with differences in RR between baseflow and stormflow, suggests that bioreactor function in storms should be considered separately from baseflow. Decreased RR following storms also suggests that bioreactors may require additional design considerations to reduce disturbance, especially for fields where much of the annual nitrate runoff load is concentrated in storm-induced events.

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.

Description of “Candidatus Jettenia ecosi” sp. nov., a New Species of Anammox Bacteria

A new species of anammox bacteria, “Candidatus Jettenia ecosi,” was identified in the microbial community of a lab-scale vertical anaerobic upflow bioreactor fed with mineral medium and the biomass immobilized on a brush-shaped carrier. The reactor was inoculated with activated sludge from a denitrifying bioreactor of a municipal wastewater treatment station in the valley of the Mzymta River (Krasnodar krai, Russia). At constant increase of concentrations of the substrates for the anammox process ( $${\text{NH}}_{4}^{ + }$$ and $${\text{NO}}_{2}^{ - }$$ ) in the course of five years, a microbial community containing a new species of anammox bacteria “Candidatus Jettenia ecosi” developed in the bioreactor. Stable activity in a wide range of substrate concentrations (0.02 to 5.6 g N/L), рН (7.2 to 8.8), and under microaerophilic conditions (3% oxygen in the gas phase) were the remarkable features of the new species. Optimal growth temperature was 30°C. Doubling time of physiologically active anammox bacteria was 13 days. Cells of the new bacteria (~1 µm in diameter) exhibited the typical anammox morphology and ultrastructure. The cells had a tendency for attached growth and formation of biofilms. Hopanoids and ladderane lipids, which are among the key markers of this microorganisms, were found in the membranes of the new anammox bacteria. According to the results of the 16S rRNA gene sequencing, the new bacteria belong to the candidate genus “Candidatus Jettenia,” phylum Planctomycetes with the proposed name “Candidatus Jettenia ecosi” sp. nov.

Stochastic multi-objective performance optimization of an in-stream woodchip denitrifying bioreactor

Woodchip denitrifying bioreactors (WDBs) that target filtration of nitrate from farm drainage water are gaining recognition as a tool for tackling the issue of diffuse nitrate pollution from agricultural landscapes. Whilst the hydraulic regime and concentration of nitrate in the drainage water constitute two fundamental environmental variables that determine the size of a denitrifying bioreactor, the issue of over- or under-treatment of water that might otherwise promote undesirable pollution swapping phenomena and construction costs also need to be factored into the overall design process. Conventional methods for optimizing the design of denitrifying bioreactors generally rely on deterministic models, even though many of the design parameters are not known with confidence. In this work we apply an alternative design philosophy and demonstrate how the bioreactor design process can be improved through application of stochastic methods. The design aspect of an ‘in-stream’ WDB planned for installation on a farm in New Zealand is structured as a multi-objective performance optimization problem that is solved in a stochastic framework, using freely available open source tools. Uncertainty considerations regarding values of physical parameters that govern bioreactor performance are incorporated into the assessment, from which a Pareto set of optimal designs was obtained. A 75 m long bioreactor of 1.5 m height was selected as the preferred choice from the optimal set of design solutions. Assuming a 10-year operational life, it is predicted the cost of nitrate removal by the planned denitrifying bioreactor will be NZ$9.70/kg-N (approx.US$6.60/kg-N).

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

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.

Performance of an under-loaded denitrifying bioreactor with biochar amendment

Denitrifying bioreactors are recently-established agricultural best management practices with growing acceptance in the US Midwest but less studied in other agriculturally significant regions, such as the US Mid-Atlantic. A bioreactor was installed in the Virginia Coastal Plain to evaluate performance in this geographically novel region facing challenges managing nutrient pollution. The 25.3 m3 woodchip bed amended with 10% biochar (v/v) intercepted subsurface drainage from 6.5 ha cultivated in soy. Influent and effluent nitrate-nitrogen (NO3–N) and total phosphorus (TP) concentrations and flowrate were monitored intensively during the second year of operation. Bed surface fluxes of greenhouse gases (GHGs) nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2) were measured periodically with the closed dynamic chamber technique. The bioreactor did not have a statistically or environmentally significant effect on TP export. Cumulative NO3–N removal efficiency (9.5%) and average removal rate (0.56 ± 0.25 g m−3 d−1) were low relative to Midwest tile bioreactors, but comparable to installations in the Maryland Coastal Plain. Underperformance was attributed mainly to low NO3–N loading (mean 9.4 ± 4.4 kg ha−1 yr−1), although intermittent flow, periods of low HRT, and low pH (mean 5.3) also likely contributed. N removal rates were correlated with influent NO3–N concentration and temperature, but decreased with hydraulic residence time, indicating that removal was often N-limited. GHG emissions were similar to other bioreactors and constructed wetlands and not considered environmentally concerning. This study suggests that expectations of NO3–N removal efficiency developed from bioreactors receiving moderate to high NO3–N loading with influent concentrations exceeding 10–20 mg L−1 are unlikely to be met by systems where N-limitation becomes significant.

Quantifying treatment system resilience to shock loadings in constructed wetlands and denitrification bioreactors

Wastewater treatment ecotechnologies such as constructed wetlands and denitrifying bioreactors are commonly perceived as robust and resilient to shock loading, but this has proved difficult to quantify, particularly when comparing different systems. This study proposes a method of quantifying and comparing performance resilience in response to a standard disturbance. In a side-by-side study we compare the treatment performance of four different configurations of wetlands and denitrifying bioreactors subjected to hydraulic shock loads of five times the standard inflow rate of primary treated sewage for five days. The systems consist of: horizontal-flow gravel-bed wetlands (HG); single pass vertical-flow sand or gravel media wetlands followed by carbonaceous denitrifying bioreactors (VS + C and VG + C respectively); and a recirculating anoxic attached-growth bioreactor and vertical sand media wetland followed by carbonaceous denitrifying bioreactors (R(A + VS)+C). Resilience was quantified for Total Suspended Solids (TSS), Five-day Biochemical Oxygen Demand (BOD5) and Total Nitrogen (TN) by time integration of Relative Disturbance in Performance relative to pre-shock loading performance (days equivalent Performance Reduction), and by the Recovery Time after shock loading ceased. The quantification method allowed an unbiased comparison of the four different systems. It highlighted important differences in the resilience for different removal mechanisms associated with the configuration of the wetlands/bioreactor systems. Relative Disturbances in Performance were expressed in comparison to percent daily removal under standard loading, and, for the different pollutants were equivalent to loss of between 0.08 and 2.51 days of removal capacity. Average Recovery Times ranged from zero to three days, with all systems exhibiting substantial recovery even during the five-day shock loading period. This study demonstrated that both the horizontal gravel wetland and the vertical flow wetland systems combined with carbonaceous bioreactors tested are generally resilient to shock loading of five times hydraulic and organic loading for periods of up to five days. Standard quantification of performance resilience to shock loadings or other perturbations has potential application across a wide range of technologies and research fields.

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.

Seasonal performance of denitrifying bioreactors in the Northeastern United States: Field trials

Denitrifying bioreactors are increasingly being used for nitrate removal from agricultural drainage water. Filled with carbon substrates, often woodchips, denitrifying bioreactors provide a favorable anaerobic environment for denitrification. Despite performing well in loess soils in the Midwestern United States, field bioreactors have not yet been evaluated in shallow soils over glacial till that are characteristic for the Northeastern United States. This study, therefore, investigates the performance of bioreactors and provides design criteria for shallow soil with flashy discharges.Paired bioreactors, one filled with woodchips and one with a mixture of woodchip and biochar, were installed in tile drained fields in three landscapes in New York State. The bioreactors were monitored for a three-year period during which, the flow rate, temperature, nitrate (NO3−−N), sulfate (SO42−−S) and dissolved organic carbon (DOC) were measured. Results showed that the average NO3−−N removal efficiency during the three years of observations was about 50%. The NO3−−N removal rate ranged from 0 in winter to 72 g d−1 m−3 in summer. We found that biochar was only effective during the first year in enhancing denitrification, due to the ageing. An index for carbon availability related to NO3−−N removal was developed. During winter, availability of the DOC was a limiting factor in bioreactor performance. Finally, to aid in the design of bioreactors, we developed generalizable relationships between the removal efficiency and hydraulic retention time and temperature.

Plastic Biofilm Carrier after Corn Cobs Reduces Nitrate Loading in Laboratory Denitrifying Bioreactors.

Nitrate-nitrogen (nitrate-N) removal rates can be increased substantially in denitrifying bioreactors with a corn ( L.) cob bed medium compared with woodchips; however, additional organic carbon (C) is released into the effluent. This laboratory column experiment was conducted to test the performance of a postbed chamber of inert plastic biofilm carrier (PBC) after corn cobs (CC) to extend the area of biofilm colonization, enhance nitrate-N removal, lower total organic C losses, and reduce nitrous oxide (NO) production at warm (15.5°C) and cold (1.5°C) temperatures. Treatments were CC only and CC plus PBC in series (CC-PBC). Across the two temperatures, nitrate-N load removal was 21% greater with CC-PBC than CC, with 54 and 44% of total nitrate N load, respectively. However, total organic C concentrations and loads were not significantly different between treatments. Colonization of the PBC by denitrifiers occurred, although gene abundance at the outlet (PBC) was less than at the inlet (CC). The PBC chamber increased nitrate-N removal rate and reduced cumulative NO production at 15.5°C, but not at 1.5°C. Across temperatures and treatments, NO production was 0.9% of nitrate-N removed. Including an additional chamber filled with PBC downstream from the CC bioreactor provided benefits in terms nitrate-N removal but did not achieve C removal. The presence of excess C, as well as available nitrate, in the PBC chamber suggests another unidentified limiting factor for nitrate removal.

Performance of Denitrifying Bioreactors at Reducing Agricultural Nitrogen Pollution in a Humid Subtropical Coastal Plain Climate

Denitrifying bioreactors are an agricultural best management practice developed in the midwestern United States to treat agricultural drainage water enriched with nitrate‐nitrogen (NO3N). The practice is spreading rapidly to agricultural regions with poor water quality due to nutrient enrichment. This makes it imperative to track bioreactor NO3‐N reduction efficiency as this practice gets deployed to new regions. This study evaluated the application and performance of denitrifying bioreactors in the humid subtropical coastal plain environment of the Chesapeake Bay catchment to provide data about regionally specific NO3‐N reduction efficiencies. NO3‐N samples were taken before and after treatment at three denitrifying bioreactors, in addition to other nutrients (orthophosphate‐phosphorus, PO4‐P; ammonium‐nitrogen, NH4‐N; total nitrogen, TN; total phosphorus, TP) and water quality parameters (dissolved oxygen, DO; oxidation reduction potential, ORP; pH; specific conductance, SPC). Total removal ranged drastically between bioreactors from 10 to 133 kg N, with removal efficiencies of 9.0% to 62% and N removal rates of 0.21 to 5.36 g N removed per m3 of bioreactor per day. As the first bioreactor study in the humid subtropical coastal plain, this data provides positive proof of concept that denitrifying bioreactor is another tool for reducing N loads in agricultural tile drainage in this region.

Effect of Biochar on Nitrate Removal in a Pilot-Scale Denitrifying Bioreactor.

Denitrifying bioreactors (DNBRs) harness the natural capacity of microorganisms to convert bioavailable nitrogen (N) into inert nitrogen gas (N) by providing a suitable anaerobic habitat and an organic carbon energy source. Woodchip systems are reported to remove 2 to 22 g N m d, but the potential to enhance denitrification with alternative substrates holds promise. The objective of this study was to determine the effect of adding biochar, an organic carbon pyrolysis product, to an in-field, pilot-scale woodchip DNBR. Two 25-m DNBRs, one with woodchips and the other with woodchips and a 10% by volume addition of biochar, were installed on the Delmarva Peninsula, Virginia. Performance was assessed using flood-and-drain batch experiments. An initial release of N was observed during the establishment of both DNBRs, reflecting a start-up phenomenon observed in previous studies. Nitrate (NO-N) removal rates observed during nine batch experiments 4 to 22 mo after installation were 0.25 to 6.06 g N m d. The presence of biochar, temperature, and influent NO-N concentration were found to have significant effects on NO-N removal rates using a linear mixed effects model. The model predicts that biochar increases the rate of N removal when influent concentrations are above approximately 5 to 10 mg L NO-N but that woodchip DNBRs outperform biochar-amended DNBRs when influent concentrations are lower, possibly reflecting the release of N temporarily stored in the biochar matrix. These results indicate that in high N-yielding systems the addition of biochar to standard woodchip DNBRs has the potential to significantly increase N removal.

Performance of Agricultural Residue Media in Laboratory Denitrifying Bioreactors at Low Temperatures

Denitrifying bioreactors can be effective for removing nitrate from agricultural tile drainage; however, questions about cold springtime performance persist. The objective of this study was to improve the nitrate removal rate (NRR) of denitrifying bioreactors at warm and cold temperatures using agriculturally derived media rather than wood chips (WC). Corn ( L.) cobs (CC), corn stover (CS), barley ( L.) straw (BS), WC, and CC followed by a compartment of WC (CC+WC) were tested in laboratory columns for 5 mo at a 12-h hydraulic residence time in separate experiments at 15.5 and 1.5°C. Nitrate-N removal rates ranged from 35 to 1.4 at 15.5°C and from 7.4 to 1.6 g N m d at 1.5°C, respectively; NRRs were ranked CC > CC+WC > BS = CS > WC and CC ≥ CC+WC = CS ≥ BS > WC for 15.5 and 1.5°C, respectively. Although NRRs for CC were increased relative to WC, CC released greater amounts of carbon. Greater abundance of nitrous oxide (NO) reductase gene () was supported by crop residues than WC at 15.5°C, and CS and BS supported greater abundance than WC at 1.5°C. Production of NO relative to nitrate removal (NO) was consistently greater at 1.5°C (7.5% of nitrate removed) than at 15.5°C (1.9%). The NO was lowest in CC (1.1%) and CC-WC (0.9%) and greatest in WC (9.7%). Using a compartment of agricultural residue media in series before wood chips has the potential to improve denitrifying bioreactor nitrate removal rates, but field-scale verification is needed.

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."

Controls Influencing the Treatment of Excess Agricultural Nitrate with Denitrifying Bioreactors.

Denitrifying bioreactors have been suggested as effective best management practices to reduce nitrate and nitrite (NO) in large-scale agricultural tile drainage. This study combines experiments in flow-through laboratory reactors with in situ continuous monitoring and experiments in a pair of field reactors to determine the effectiveness of reactors for small-scale agriculture in New York. It also compares the use of a typical woodchip media with a woodchip and biochar mixture. Laboratory results showed linear increase in NO removal with both increased inflow concentration and increased residence time. Average removal of NO in weekly monitoring of field reactors over the course of two growing seasons was 3.23 and 4.00 g N m d for woodchip and woodchip/biochar reactors, respectively. Removal of NO during two field experimental runs was similar to in situ monitoring and did not correlate with laboratory experiments. Factors that are uncontrollable at the field scale, such as temperature and inflow water chemistry, may result in more complex and resilient microbial communities that are less specialized for denitrification. Further study of other controlling variables, other field sites, and other parameters, including microbial communities and trace gas emissions, will help elucidate function and applicability of denitrifying bioreactors.

Fecal Bacteria, Bacteriophage, and Nutrient Reductions in a Full-Scale Denitrifying Woodchip Bioreactor.

Denitrifying bioreactors using woodchips or other slow-release carbon sources can be an effective method for removing nitrate (NO) from wastewater and tile drainage. However, the ability of these systems to remove fecal microbes from wastewater has been largely uninvestigated. In this study, reductions in fecal indicator bacteria () and viruses (F-specific RNA bacteriophage [FRNA phage]) were analyzed by monthly sampling along a longitudinal transect within a full-scale denitrifying woodchip bioreactor receiving secondary-treated septic tank effluent. Nitrogen, phosphorus, 5-d carbonaceous biochemical oxygen demand (CBOD), and total suspended solids (TSS) reduction were also assessed. The bioreactor demonstrated consistent and substantial reduction of (2.9 log reduction) and FRNA phage (3.9 log reduction) despite receiving highly fluctuating inflow concentrations [up to 3.5 × 10 MPN (100 mL) and 1.1 × 10 plaque-forming units (100 mL) , respectively]. Most of the removal of fecal microbial contaminants occurred within the first meter of the system (1.4 log reduction for ; 1.8 log reduction for FRNA phage). The system was also efficient at removing NO (>99.9% reduction) and TSS (89% reduction). There was no evidence of consistent removal of ammonium, organic nitrogen, or phosphorus. Leaching of CBOD occurred during initial operation but decreased and stabilized at lower values (14 g O m) after 9 mo. We present strong evidence for reliable microbial contaminant removal in denitrifying bioreactors, demonstrating their broader versatility for wastewater treatment. Research on the removal mechanisms of microbial contaminants in these systems, together with the assessment of longevity of removal, is warranted.