Elevated phosphorus levels in agricultural runoff are a major contributor to water quality degradation. A powdered biochar-based material has been engineered to efficiently remove dissolved phosphorus from drainage waters and subsequently function as a nutrient-rich soil amendment.

The remedial material was composed of sawdust and lime sludge, byproducts from milling and drinking water treatment facilities. These ingredients were formed into pellets and slow-burned under low-oxygen conditions to yield a biochar with a greater capacity for phosphorus capture than either lime sludge or biochar alone. Once the pellets have absorbed their maximum phosphorus load, they can be applied to fields, where the bound nutrient is gradually released to enrich the soil.

Tests were conducted in fields equipped with subsurface drainage systems configured so that drainageSource: University of Illinois water passed through phosphorus removal structures filled with biochar pellets in two sizes. Pellets of 2 cm to 3 cm size were evaluated in the first year while 1 cm pellets were emplaced in the second year. All of the pellets successfully removed phosphorus, but the smaller 1 cm pellets proved significantly more effective, achieving a phosphorus removal efficiency of 38% to 41 %, compared to 1.3% to 12 % for the larger pellets.

Life-cycle analysis estimates that the cost to produce these biochar pellets is $413 per ton, less than half the market price of alternatives like granular activated carbon, which ranges from $800 to $2,500 per ton. For phosphorus removal, the system's average cost was calculated at $359/kg removed, with a two-year replacement cycle indicated as the most cost-effective.

The approach described in Water Research can prevent up to 200 kg of carbon dioxide equivalent per kg of phosphorus removed by reapplying spent biochar to fields and reducing the need for additional phosphorus inputs.

Scientists from University of Illinois, Kansas State University and the Metropolitan Water Reclamation District of Greater Chicago contributed to this research.