Performance, stability, and cost-effectiveness of a bioreactor-biochar (B²) system for nutrient removal from agricultural drainage

Extensive tile drainage systems in the Midwestern United States are a major source of nutrient pollution, contributing to water quality impairment in downstream watersheds. This study presents an integrated evaluation of an innovative two-stage woodchip bioreactor-biochar (B²) treatment system for reducing nitrogen (N) and phosphorus (P) losses from tile-drained croplands by combining laboratory studies, field trials, and a techno-economic assessment (TEA). Laboratory experiments showed that designer biochar pellets produced from sawdust pretreated with lime sludge significantly enhanced the adsorption capacity for dissolved reactive phosphorus (DRP, water-soluble orthophosphate) compared with that of lime sludge alone. In a one-year field trial, the B² system demonstrated sustained nutrient removal when treating 3, 018 m³ of drainage water. The woodchip bioreactor reduced nitrate‑nitrogen (NO₃-N) concentrations by 58 % with a cumulative load reduction of 1.8 kg. Ammonium‑nitrogen (NH₄-N) loads were reduced from 2.83 to 0.73 kg, with removal efficiency increasing from 64 % to 72 % under the subsequent biochar treatment. Biochar sorption channels reduced DRP by 3–92 % (median 69 %) and total P by 20–94 % (median 55 %), effectively mitigating DRP and TP leaching observed in the woodchip bioreactor effluent. The TEA indicated that the pilot-scale B² system achieved unit removal costs of $90.3/kg NO₃-N/year and $63.9/kg DRP/year. When the system was scaled to treat drainage water from a 10-ha drainage area, the system yielded average removal costs of $4.7 ± 1.9/kg NO₃-N/year and $103.7 ± 153.5/kg DRP/year, with an annualized system cost of $1020.2 ± 80.4 per year. The TEA analysis also suggested that the cost-effectiveness of the B² systems can be further improved through strategic site selection, material sourcing, and flow management. Overall, these results highlight the B² system as a practical, scalable, and cost-effective strategy for improving water quality in tile-drained agricultural landscapes.