A Woodchip-Biochar Combo Cuts Both Nitrogen and Phosphorus from Farmland Runoff

Agricultural nutrient runoff is one of the more persistent water quality problems in the U.S. Midwest. Fertilizers applied to fields during the growing season wash through subsurface drainage tiles into streams and rivers, fueling algal blooms and oxygen-depleted dead zones downstream. The Gulf of Mexico hypoxic zone - fed largely by Mississippi River nutrients - covers thousands of square miles each summer.

Woodchip bioreactors, which route drainage water through trenches packed with wood chips that support denitrifying bacteria, have become a recognized approach for removing nitrogen. But they have a gap: they do not remove phosphorus. And when freshly installed, they can actually increase phosphorus concentrations, as phosphorus naturally present in the wood leaches into the water.

A research team at the Illinois Sustainable Technology Center, a division of the Prairie Research Institute at the University of Illinois Urbana-Champaign, has addressed this limitation by adding a second treatment stage. Their one-year field trial, published in the Journal of Water Process Engineering, demonstrates that combining a standard woodchip bioreactor with a custom biochar-sorption channel removes both nutrients simultaneously.

How the Two-Stage System Works

The first stage is familiar technology. Farm drainage flows through a large trench of woodchips, where bacteria break down the wood's carbon content and, under low-oxygen conditions, convert dissolved nitrate into nitrogen gas - a harmless atmospheric component. This denitrification process is well established and accounts for the nitrogen removal.

The second stage is what makes the system novel. Drainage exiting the woodchip bioreactor passes through a channel containing pelletized biochar specifically engineered to capture phosphorus. The "designer" biochar was produced by lead researcher Wei Zheng and postdoctoral researcher Hongxu Zhou using lime sludge - a byproduct of water treatment plant lime processing - mixed with fine sawdust and heated to high temperatures under low-oxygen conditions. The resulting powder was compressed into pellets.

Pelletizing matters: loose biochar powder would wash away in flowing water, making it impractical for field use. The pellet form keeps the material in place while maintaining its chemical reactivity. The biochar captures dissolved phosphorus through reactions that form solid compounds - magnesium phosphate and calcium phosphate - that can later be recovered.

"Dissolved phosphorus is a major concern in farm drainage water," Zheng said. "Like nitrogen, phosphorus promotes algal blooms in rivers or lakes that can produce toxins, block sunlight and deprive aquatic organisms of oxygen."

Field Trial Results

Testing in a one-hectare field trial produced clear results. The bioreactor stage reduced nitrate-nitrogen loads in farm runoff by 58% and ammonium-nitrogen loads by 72%. The biochar-sorption channel reduced dissolved phosphorus concentrations by 3% to 92% and total phosphorus by 20% to 92%, with the range reflecting variation in seasonal flow conditions - performance was lower during high-flow periods when dilution and hydraulic loading increased.

A techno-economic assessment estimated unit removal costs of $90.30 per kilogram of nitrate-nitrogen removed per year and $63.90 per kilogram of dissolved reactive phosphorus removed per year at the one-hectare scale. The researchers note that scaling to a 10-hectare site would substantially reduce these costs through efficiencies of scale - a point relevant to the economics of widespread adoption.

Built-In Cost Recovery

One of the system's practical advantages is what happens to spent biochar. After the pellets are saturated with phosphorus, they can be removed and reapplied to agricultural fields as a slow-release fertilizer. The captured phosphorus has value as a nutrient, partially offsetting system operating costs. Adding biochar to soil may also improve soil health and water retention, and farmers in some regions may qualify for carbon credits through associated sequestration benefits.

"To address this, we wanted to develop a new edge-of-field treatment system capable of capturing multiple nutrients at once," Zheng said.

What Comes Next

The one-year trial at one hectare is an important proof of concept, but commercial-scale adoption requires larger, longer demonstrations. Zheng and colleagues are now evaluating the system at a commercial farm. Seasonal performance variation observed in the trial - phosphorus removal dropped significantly during high-flow periods - also warrants attention in larger-scale design. The system's effectiveness may depend on drainage flow rates that vary considerably with rainfall and soil conditions.

The research was supported by the U.S. Environmental Protection Agency.