Exploring the engineering-scale potential of designer biochar pellets for phosphorus loss reduction from tile-drained agroecosystems

Artificial drainage has led to significant amounts of non-point dissolved reactive phosphorus (DRP) loss from tile- drained agroecosystems, jeopardizing water quality and triggering harmful algal blooms. Designer biochar has shown great promise on the laboratory scale for removing DRP from contaminated water. However, whether its removal performance, stability, and engineering value can be sustained under field conditions over time remains unclear. This study reported the first engineering application of designer biochar pellets used in an intensely tile- drained agroecosystem to reduce DRP losses from drainage water. Two types of designer biochar pellets with different particle sizes (Phase I - biochar pellets size 2-3 cm vs. Phase II - biochar pellets size <1 cm) were manufactured and placed into the specifically designed phosphorus removal structure (i.e., biochar-sorption chamber) to capture DRP from tile drainage water. Field demonstrations revealed that small-sized biochar pellets (<1 cm) were significantly more efficient at capturing DRP than larger pellets (2-3 cm). A comprehensive analysis further indicated that multi-factors could affect the performance of designer biochar pellets in DRP loss reduction, such as influent DRP concentrations, drainage flows, and biochar pellet sizes. Techno-economic analysis and life cycle assessment indicated that the designer biochar pellets have notable economic and environmental benefits. On the pilot scale, the average production cost of designer biochar pellets was $413/ton biochar. The average DRP removal cost was $359±177/kg DRP for tile-drained agroecosystems under wide economic and system design parameters. Furthermore, the utilization of designer biochar pellets to remove DRP from drainage in combination with subsequently using spent biochar as a soil amendment provides environmental benefits to achieve negative global warming potential (-200 to -12 kg CO2 eq/kg DRP removal) and energy production. Overall, this work offers a novel strategy to explore the potential for engineering-scale application of biochar for sustainable water quality protection and helps elucidate the costs and benefits in the context of watershed nutrient loss management.