Denitrifying Bioreactors for Nitrate Removal: A Meta-Analysis

The denitrifying woodchip bioreactor is an example of a cost effective and a simple edge-of-field strategy to reduce the nitrogen export from agricultural fields through subsurface drainage systems. Understanding the factors controlling their nitrate-nitrogen removal performance is essential to maximizing the benefits from the bioreactors. This abstract and paper is intended to provide a review of the controlling factors. The major controlling factors are: flow rate through the bioreactors, hydraulic retention time (HRT), carbon source, microbial community, initial nitrate concentration, temperature, pH and concentration of dissolved oxygen (DO) of the inflow water. A primary mechanism impacting the nitrate removal rate is the flow rate. Studies show that, in general, the nitrate load reduction efficiency decreases when the flow rate increases above a baseline flow as a result of insufficient time for the denitrification process to occur as well as increased transport of DO into the reactor. Longer HRT generally removes higher amounts of nitrate from the bioreactor; however, too long residence time causes unwanted chemical reactions such as reduction of sulfate and methylation of mercury. The presence of DO in the water in the reactor negatively affects the denitrifying bacterial activities, causes accumulation of denitrifying intermediates, and accelerates the breakdown of the woodchips. The carbon substrate is important for the denitrification by providing energy to microbes and stimulating the aerobic respiration to reduce the oxygen concentration. The activities of the enzymes which are involved in denitrification process are pH dependent. Managing the factors controlling the performance of the bioreactor greatly improves the bioreactor performance.

Journal of Environment Quality, 2009

Subsurface drainage in agricultural watersheds exports a large quantity of nitrate-nitrogen (NO 3 -N) and concentrations frequently exceed 10 mg L -1 . A laboratory column study was conducted to investigate the ability of a wood chip bioreactor to promote denitrifi cation under mean water fl ow rates of 2.9, 6.6, 8.7 and 13.6 cm d -1 which are representative of fl ows entering subsurface drainage tiles. Columns were packed with wood chips and inoculated with a small amount of oxidized till and incubated at 10°C. Silicone sampling cells at the effl uent ports were used for N 2 O sampling. 15 Nitrate was added to dosing water at 50 mg L -1 and effl uent was collected and analyzed for NO 3 -N, NH 4 -N, and dissolved organic carbon. Mean NO 3 -N concentrations in the effl uent were 0.0, 18.5, 24.2, and 35.3 mg L -1 for the fl ow rates 2.9, 6.6, 8.7, and 13.6 cm d -1 , respectively, which correspond to 100, 64, 52, and 30% effi ciency of removal. Th e NO 3 -N removal rates per gram of wood increased with increasing fl ow rates. Denitrifi cation was found to be the dominant NO 3 -N removal mechanism as immobilization of 15 NO 3 -N was negligible compared with the quantity of 15 NO 3 -N removed. Nitrous oxide production from the columns ranged from 0.003 to 0.028% of the N denitrifi ed, indicating that complete denitrifi cation generally occurred. Based on these observations, wood chip bioreactors may be successful at removing signifi cant quantities of NO 3 -N, and reducing NO 3 -N concentration from water moving to subsurface drainage at fl ow rates observed in central Iowa subsoil.

Agricultural & Environmental Letters, 2020

Woodchip denitrifying bioreactors (WDBR) reduce off‐field tile drainage nitrogen (N) losses from agricultural fields. Limited evaluation exists regarding the influence of flow direction through WDBRs. Changing flow direction could reduce short circuiting. This study evaluated the dependency of nitrate‐N removal and dissolved nitrous oxide (dN2O) production rates on vertical flow direction in triplicate column bioreactors at 12‐h (without carbon dosing) and 2‐h (with carbon dosing) hydraulic residence times. Results presented demonstrate that there was no significant difference in overall N removal rates from these column bioreactors as a function of flow direction. There was the suggestion of lower N2O production in the downflow direction, although this was not statistically significant due to the high variability of the N2O production observed in the upflow direction. Carbon addition led to bioclogging of downflow columns; future work needs to identify dosing rate, placement, and c...

Water Research, 2011

Denitrification beds are containers filled with wood by-products that serve as a carbon and energy source to denitrifiers, which reduce nitrate () from point source discharges into non-reactive dinitrogen (N 2) gas. This study investigates a range of alternative carbon sources and determines rates, mechanisms and factors controlling NO 3 − removal, denitrifying bacterial community, and the adverse effects of these substrates. Experimental barrels (0.2 m 3) filled with either maize cobs, wheat straw, green waste, sawdust, pine woodchips or eucalyptus woodchips were incubated at 16.8 °C or 27.1 °C (outlet temperature), and received enriched water (14.38 mg N L −1 and 17.15 mg N L −1). After 2.5 years of incubation measurements were made of removal rates, in vitro denitrification rates (DR), factors limiting denitrification (carbon and nitrate availability, dissolved oxygen, temperature, pH, and concentrations of , nitrite and ammonia), copy number of nitrite reductase (nirS and nirK) and nitrous oxide reductase (nosZ) genes, and greenhouse gas production (dissolved nitrous oxide (N 2 O) and methane), and carbon (TOC) loss. Microbial denitrification was the main mechanism for removal. Nitrate-N removal rates ranged from 1.3 (pine woodchips) to 6.2 g N m −3 d −1 (maize cobs), and were predominantly limited by C availability and temperature (Q 10 = 1.2) when outlet concentrations remained above 1 mg L −1. The removal rate did not depend directly on substrate type, but on the quantity of microbially available carbon, which differed between carbon sources. The abundance of denitrifying genes (nirS, nirK and nosZ) was similar in replicate barrels under cold incubation, but varied substantially under warm incubation, and between substrates. Warm incubation enhanced growth of nirS containing bacteria and bacteria that lacked the nosZ gene, potentially explaining the greater N 2 O emission in warmer environments. Maize cob substrate had the highest removal rate, but adverse effects include TOC release, dissolved N 2 O release and substantial carbon consumption by non-denitrifiers. Wood-chips removed less than half of removed by maize cobs, but provided ideal conditions for denitrifying bacteria, and adverse effects were not observed. Therefore we recommend the combination of maize cobs and woodchips to enhance removal while minimizing adverse effects in denitrification beds.

Aquacultural engineering, 2007

This study evaluated wood chips and wheat straw as inexpensive and readily available alternatives to more expensive plastic media for denitrification processes in treating aquaculture wastewaters or other high nitrate waters. Nine 3.8-L laboratory scale reactors (40 cm packed height × 10 cm diameter) were used to compare the performance of wood chips, wheat straw, and Kaldnes plastic media in the removal of nitrate from synthetic aquaculture wastewater. These upflow bioreactors were loaded at a constant flow rate and three influent NO3–N concentrations of 50, 120, and 200 mg/L each for at least 4 weeks, in sequence. These experiments showed that both wood chips and wheat straw produced comparable denitrification rates to the Kaldnes plastic media. As much as 99% of nitrate was removed from the wastewater of 200 mg NO3–N/L influent concentration. Pseudo-steady state denitrification rates for 200 mg NO3–N/L influent concentrations averaged (1360 ± 40) g N/(m3 d) for wood chips, (1360 ± 80) g N/(m3 d) for wheat straw, and (1330 ± 70) g N/(m3 d) for Kaldnes media. These values were not the maximum potential of the reactors as nitrate profiles up through the reactors indicated that nitrate reductions in the lower half of the reactors were more than double the averages for the whole reactor. COD consumption per unit of NO3–N removed was highest with the Kaldnes media (3.41–3.95) compared to wood chips (3.34–3.64) and wheat straw (3.26–3.46). Effluent ammonia concentrations were near zero while nitrites were around 2.0 mg NO2–N/L for all reactor types and loading rates. During the denitrification process, alkalinity and pH increased while the oxidation–reduction potential decreased with nitrate removal. Wood chips and wheat straw lost 16.2% and 37.7% of their masses, respectively, during the 140-day experiment. There were signs of physical degradation that included discoloration and structural transformation. The carbon to nitrogen ratio of the media also decreased. Both wood chips and wheat straw can be used as filter media for biological denitrification, but time limitations for the life of both materials must be considered.

Water Research, 2011

Stable isotopes Acetylene block Natural abundance a b s t r a c t Denitrifying woodchip bioreactors (denitrification beds) are increasingly used to remove excess nitrate (NO À 3 ) from point-sources such as wastewater effluent or subsurface drains from agricultural fields. NO À 3 removal in these beds is assumed to be due to microbial denitrification but direct measurements of denitrification are lacking. Our objective was to test four different approaches for measuring denitrification rates in a denitrification bed that treated effluent discharged from a glasshouse. We compared these denitrification rates with the rate of NO À 3 removal along the length of the bed. The NO À 3 removal rate was 8.73 AE 1.45 g m À3 d À1 . In vitro acetylene inhibition assays resulted in highly variable denitrification rates (DR AI ) along the length of the bed and generally 5 times greater than the measured (NO À 3 dN removal rate. An in situ pushepull test, where enriched 15 NdNO À 3 was injected into 2 locations along the bed, resulted in rates of 23.2 AE 1.43 g N m À3 d À1 and 8.06 AE 1.64 g N m À3 d À1 . The denitrification rate calculated from the increase in dissolved N 2 and N 2 O concentrations (DR N2 ) along the length of the denitrification bed was 6.7 AE 1.61 g N m À3 d À1 . Lastly, denitrification rates calculated from changes in natural abundance measurements of d 15 NeN 2 and d 15 NdNO À 3 along the length of the bed yielded a denitrification rate (DR NA ) of 6.39 AE 2.07 g m À3 d À1 . Based on our experience, DR N2 measurements were the easiest and most efficient approach for determining the denitrification rate and N 2 O production of a denitrification bed. However, the other approaches were useful for testing other hypotheses such as factors limiting denitrification or may be applied to determine denitrification rates in environmental systems different to our study site. DR N2 does require very careful sampling to avoid atmospheric N 2 contamination but could be used to rapidly determine denitrification rates in a variety of aquatic systems with high N 2 production and even water flows. These measurements demonstrated that the majority of NO À 3 removal was due to heterotrophic denitrification. ª

Nitrate-nitrogen exports from agricultural fields through subsurface (tile) drainage systems contribute to the eutrophication of natural aquatic ecosystems. Additionally, elevated concentrations of nitrate-nitrogen in drinking water area public health concern. The woodchip denitrifying bioreactor is one type of a cost effective, simple edge-of-field practice to reduce nitrate-nitrogen exports through agricultural subsurface drainage systems. The objective of this study is to evaluate the performance of three bioreactors by determining the nitrate removal rate and cost per pound of nitrate removed by the bioreactors. During 2012, we installed two bioreactors near Baltic, SD and near Montrose, SD. In 2013, we installed another bioreactor near Arlington, SD. Water was sampled twice per week at the inlet and the outlet of the bioreactors. The water samples were analyzed for nitrate-nitrogen concentration using a spectrophotometer. The average concentration-based nitrate removal at the Baltic and Montrose bioreactors during the subsurface drain flow season (April-July) in 2012 were 81% and 51% respectively. We have not had any water flow through the Arlington bioreactor at this time and we experienced periodic monitoring equipment failure at the Baltic site. At the Montrose bioreactor, the maximum, minimum, and average removal rate of nitrate-nitrogen in unit volume of woodchips were found to be3.14, 0.03, and 0.98 g N /m 3 volume of bioreactor per day respectively. The nitrate-nitrogen removal rate in unit active flow volume for the Montrose bioreactor was estimated as 12.58 g N/m 3 /d. The average annual cost for the bioreactor installation and maintenance, on a per hectare of effective drained area basis and assuming a 20 years lifetime of the woodchips, were $60, $70 and $130for the Baltic, Montrose and Arlington bioreactors. The cost to remove one unit mass of nitrate-nitrogen was calculated for the Montrose bioreactor. It was approximately $8.68 per Kg N removed ($ 3.94 per lb N). These costs are comparable with results from other bioreactor studies and are competitive compared to the costs of other nitrate-nitrogen reduction practices.

Ecological Engineering, 2010

Loss of nitrate in subsurface drainage water from agricultural fields is an important problem in the Midwestern United States and elsewhere. One possible strategy for reducing nitrate export is the use of denitrification bioreactors. A variety of experimental bioreactor designs have been shown to reduce nitrate losses in drainage water for periods up to several years. This research reports on the denitrification activity of a wood chip-based bioreactor operating in the field for over 9 years. Potential denitrification activity was sustained over the 9-year period, which was consistent with nitrate removal from drainage water in the field. Denitrification potentials ranged from 8.2 to 34 mg N kg −1 wood during the last 5 years of bioreactor operation. Populations of denitrifying bacteria were greater in the wood chips than in adjacent subsoil. Loss of wood through decomposition reached 75% at the 90-100 cm depth with a wood half-life of 4.6 years. However, wood loss was less than 20% at 155-170 cm depth and the half-life of this wood was 36.6 years. The differential wood loss at these two depths appears to result from sustained anaerobic conditions below the tile drainage line at 120 cm depth. Pore space concentrations of oxygen and methane support this conjecture. Nitrous oxide exported in tile water from the wood chip bioreactor plots was not significantly higher than N 2 O exports in tile water from the untreated control plots, and loss of N 2 O from tile water exiting the bioreactor accounted for 0.0062 kg N 2 O-N kg −1 NO 3 -N.

Agricultural & Environmental Letters, 2019

A previously reported experiment showed weekly drying-rewetting (DRW) cycles increase nitrate removal rates in woodchip-based denitrifying bioreactors. A follow-up experiment determined the effect of duration of unsaturated conditions on nitrate removal after rewetting. Three different levels of DRW duration were tested in a 105-d column experiment (n = 2), with woodchips left unsaturated once a week for either 2 h, 8 h, or 24 h. Increasing duration of unsaturated conditions significantly increased nitrate removal rates. The longest DRW duration of 24 h resulted in the greatest increase in nitrate removal rates, relative to constantly saturated woodchips, with mean rate increases reaching 172% by the end of the experiment. Results suggest nitrate removal in denitrifying bioreactors is carbon limited, with labile carbon made available during aerobic periods of DRW cycles the most likely cause of observed rate increases. Both studies show DRW cycles dramatically increase the nitrate removal efficiency of denitrifying bioreactors.

Transactions of the ASABE, 2021

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.. 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 technolo...