Applied denitrifying bioreactor cost efficiencies based on empirical construction costs and nitrate removal.

Adoption of edge-of-field conservation practices, such as denitrifying bioreactors, may be intrinsically linked to barriers associated with cost. However, most previous bioreactor cost efficiency assessments assumed values for either costs and/or nitrate removal. The objective of this work was to use actual construction costs as well as monitored nitrate removal to develop empirical cost efficiencies for eight full-size bioreactors in Illinois, USA. Capital construction costs were obtained via invoices or personal communications. A cash-flow discounting procedure was used to develop an equal annualized cost for each bioreactor assuming two media recharges over a 24-y planning horizon. These costs were combined with monitored nitrate removal based on one to six years of monitoring per site. Construction costs averaged $12,250 ± $7520 across the eight sites (or, $16,020 ± $9960 in 2023 price levels) but considering one of the sites was a paired bioreactor system, costs averaged $10,890 per bioreactor unit. Drainage treatment area-based cost averaged $132/ha-y and treatment area was strongly correlated with capital costs (R2 = 0.90; p = 0.001). The bioreactors averaged $108/m3 of woodchips and available federal government conservation programs could have offset an average of 70% of this cost. Monitored nitrate removal across 27 site-years resulted in a median of $33/kg N-y removed. This mass-based cost efficiency was higher than most previous assessments because the monitored nitrate removal for the study sites was lower than has been previously assumed or modeled. Future reporting about bioreactor recharge timing and cost will help guide assessment and planning. Water quality planning efforts should also consider the increasingly important engineering design costs, which were not included here. Suggested research and outreach to improve bioreactor cost efficiencies involves scaling the physical capacity of this technology for larger treatment areas, revisiting the use of low-cost non-standard fill media, and providing practical construction training.

Paired denitrifying bioreactors with wide orientation for increased drainage flow capacity.

Denitrifying bioreactors are a conservation drainage practice for reducing nitrate loads in subsurface agricultural drainage. Bioreactor hydraulic capacity is limited by cross-sectional area perpendicular to flow through the woodchip bed, with excess bypass flow untreated. Paired bioreactors with wide orientations were built in 2017 in Illinois, USA, to treat drainage from a relatively large 29 ha field. The paired design consisted of: a larger, Main bioreactor (LWD: 6.1 × 18.3 × 0.9 m) for treating base flow, and 2) a smaller, Booster bioreactor (7.8 × 13.1 × 0.9 m) receiving bypass flow from the Main bioreactor during periods of high flow. Over three years of monitoring, the paired bioreactor captured 84-92% of the annual drainage discharge which demonstrated an expanded cross-sectional area could improve bioreactor flow capture, even for a large drainage area. However, the paired bioreactors removed 6-28% of the annual N load leaving the field (1.8-5.6 kg N ha-1 removed; 52-161 kg N), which was not a notable improvement compared to bioreactors treating smaller drainage areas. The design operated as intended at low annual flow-weighted hydraulic retention times (HRTs) of usually ≤2 h, but these short HRTs ultimately limited bioreactor nitrate removal efficiency. Daily HRTs of <2 h often resulted in nitrate flushing. The Main bioreactor had higher hydraulic loading as intended and was responsible for the majority of flow captured in each year although not always the most nitrate mass removal. The Booster bioreactor provided better nitrate removal than the Main at HRTs of 3.0-11.9 h, possibly due to its drying cycles which may have liberated more available carbon. This new design approach tested at the field-scale illustrated tradeoffs between greater flow capacity (via increased bioreactor width) and longer HRT (via increased length), given a consistent bioreactor surface footprint.

Woodchip bioreactors provide sustained denitrification of brine from groundwater desalination plants.

Woodchip bioreactors are widely known as a best management practice to reduce excess nitrate loads that are discharged with agricultural leachates. The aim of this study was to evaluate the performance of citrus woodchip bioreactors for denitrification of brine (electrical conductivity ≈ 17 mS cm-1) from groundwater desalination plants with high nitrate content (NO3--N ≈ 48 mg L-1) in the Campo de Cartagena agricultural watershed, one of the main providers of horticultural products in Europe. The performance was evaluated relative to seasonal changes in temperature, dissolved organic carbon (DOC) provided by woodchips, hydraulic residence time (HRT) and woodchip aging. Bioreactors (capacity 1 m3) operated for 2.5 years (121 weeks) in batch mode (24 h HRT) with three batches per week. Denitrification efficiency was modulated by DOC concentration, temperature, hydraulic residence time and the drying-rewetting cycles. High salinity of brine did not prevent nitrate removal from occurring. The high DOC availability (>25 mg C L-1) during the first ≈48 weeks resulted in high nitrate removal rate (>75%) and nitrate removal efficiency (until ≈ 25 g N m-3 d-1) regardless of temperature. Moreover, the high DOC contents in the effluents during this period may present environmental drawbacks. Denitrification was still high after 2.5 years (reaching ≈9.3 g N m-3 d-1 in week 121), but dependence on warm temperature became more apparent with woodchips aging from week ≈49 onwards. Nitrate removal efficiency was highest on the first weekly batch, immediately after woodchips had been unsaturated for four days. It was attributable to a flush of DOC produced by aerobic microbial metabolism during drying that stimulated denitrification following re-saturation. Hence, alternance of drying-rewetting cycles is an operation practice that increase bioreactors nitrate removal performance.

High-frequency, in situ sampling of field woodchip bioreactors reveals sources of sampling error and hydraulic inefficiencies.

Woodchip bioreactors are a practical, low-cost technology for reducing nitrate (NO3) loads discharged from agriculture. Traditional methods of quantifying their performance in the field mostly rely on low-frequency, time-based (weekly to monthly sampling interval) or flow-weighted sample collection at the inlet and outlet, creating uncertainty in their performance and design by providing incomplete information on flow and water chemistry. To address this uncertainty, two field bioreactors were monitored in the US and New Zealand using high-frequency, multipoint sampling for in situ monitoring of NO3-N concentrations. High-frequency monitoring (sub hourly interval) at the inlet and outlet of both bioreactors revealed significant variability in volumetric removal rates and percent reduction, with percent reduction varying by up to 25 percentage points within a single flow event. Time series of inlet and outlet NO3 showed significant lag in peak concentrations of 1-3 days due to high hydraulic residence time, where calculations from instantaneous measurements produced erroneous estimates of performance and misleading relationships between residence time and removal. Internal porewater sampling wells showed differences in NO3 concentration between shallow and deep zones, and "hot spot" zones where peak NO3 removal co-occurred with dissolved oxygen depletion and dissolved organic carbon production. Tracking NO3 movement through the profile showed preferential flow occurring with slower flow in deeper woodchips, and slower flow further from the most direct flowpath from inlet to outlet. High-frequency, in situ data on inlet and outlet time series and internal porewater solute profiles of this initial work highlight several key areas for future research.

Drying-Rewetting Cycles Affect Nitrate Removal Rates in Woodchip Bioreactors.

Woodchip bioreactors are widely used to control nitrogen export from agriculture using denitrification. There is abundant evidence that drying-rewetting (DRW) cycles can promote enhanced metabolic rates in soils. A 287-d experiment investigated the effects of weekly DRW cycles on nitrate (NO) removal in woodchip columns in the laboratory receiving constant flow of nitrated water. Columns were exposed to continuous saturation (SAT) or to weekly, 8-h drying-rewetting (8 h of aerobiosis followed by saturation) cycles (DRW). Nitrate concentrations were measured at the column outlets every 2 h using novel multiplexed sampling methods coupled to spectrophotometric analysis. Drying-rewetting columns showed greater export of total and dissolved organic carbon and increased NO removal rates. Nitrate removal rates in DRW columns increased by up to 80%, relative to SAT columns, although DRW removal rates decreased quickly within 3 d after rewetting. Increased NO removal in DRW columns continued even after 39 DRW cycles, with ∼33% higher total NO mass removed over each weekly DRW cycle. Data collected in this experiment provide strong evidence that DRW cycles can dramatically improve NO removal in woodchip bioreactors, with carbon availability being a likely driver of improved efficiency. These results have implications for hydraulic management of woodchip bioreactors and other denitrification practices.