Researchers at the University of Illinois Urbana-Champaign have found an effective and affordable way for farmers to tackle a big environmental issue: phosphorus runoff. By reusing phosphorus as a slow-release fertilizer, farmers can prevent it from polluting downstream waterways, helping both their crops and the environment.
Schematic showing water flow through a phosphorus removal structure filled with biochar. Image Credit: University of Illinois Urbana-Champaign
Phosphorus removal structures have been developed to capture dissolved phosphorus from tile drainage systems, but current phosphorus sorption materials are either inefficient or they are industrial waste products that aren’t easy to dispose of. This motivated us to develop an eco-friendly and acceptable material to remove phosphorus from tile drainage systems.
Hongxu Zhou, Department of Agricultural and Biological Engineering, University of Illinois Urbana- Champaign
Hongxu Zhou conducted this study as a doctoral student in the Department of Agricultural and Biological Engineering (ABE) at the University of Illinois, which is part of both the College of Agricultural, Consumer, and Environmental Sciences and The Grainger College of Engineering.
Zhou and his co-authors experimented with sawdust and lime sludge, byproducts from milling and drinking water treatment facilities, respectively. By combining these materials, shaping them into pellets, and then slow-burning them in low-oxygen conditions, the team produced a “designer” biochar. This biochar demonstrated a notably higher capacity for phosphorus binding 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, enriching the soil over time.
Focusing on sustainable solutions, the researchers tested these biochar pellets in real field conditions for the first time, assessing their phosphorus removal efficiency in Fulton County, Illinois, over a two-year period.
Like many corn and soybean fields in the Midwest, the test fields were equipped with subsurface drainage systems. This drainage water passed through phosphorus removal structures filled with biochar pellets in two sizes: 2-3 cm pellets were used in the first year, followed by 1 cm pellets in the second year.
Both pellet sizes 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.
Study co-author Wei Zheng, a principal research scientist at the Illinois Sustainable Technology Center (ISTC), part of the Prairie Research Institute at the University of Illinois, was not overly surprised by these findings.
According to Zheng, smaller particles provide more surface area, allowing greater contact time for phosphorus to bind to the designer biochar. His previous lab studies demonstrated that a powdered form of designer biochar is highly efficient for phosphorus removal, though powdered materials are impractical for field applications.
If we put powder-form biochar in the field, it would easily wash away. This is why we have to make pellets. We have to sacrifice some efficiency to ensure the system will work under field conditions.
Hongxu Zhou, Department of Agricultural and Biological Engineering, University of Illinois Urbana- Champaign
After demonstrating the effectiveness of the biochar pellets in field conditions, the research team conducted techno-economic and life-cycle analyses to assess both the economic feasibility for farmers and the overall sustainability of the system.
The cost to produce the designer biochar pellets was estimated at $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 per kilogram removed, with costs varying based on inflation and pellet replacement frequency. A two-year replacement cycle proved to be the most cost-effective.
The life-cycle analysis highlighted the environmental advantages of the system, which could prevent up to 12 to 200 kilograms of carbon dioxide equivalent per kilogram of phosphorus removed by reapplying spent biochar to fields and reducing the need for additional phosphorus inputs. According to Zhou, the system's benefits extend beyond nutrient loss reduction and carbon sequestration; it also supports energy production, reduces eutrophication, and enhances soil quality.
At the moment, there's no regulation that requires farmers to remove phosphorus from drainage water. But we know there are many conservation conscious farmers who want to reduce nitrate and phosphorus losses from their fields. If they’re already installing a woodchip bioreactor to remove nitrate, all they’d have to do is add the pellets to the control structure to remove the phosphorus at the same time. And there’s something very attractive about being able to reuse the pellets on the fields.
Rabin Bhattarai, Study Co-Author and Associate Professor, Department of Agricultural and Biological Engineering, University of Illinois Urbana- Champaign
The study received funding from the US Environmental Protection Agency and the Illinois Nutrient Research and Education Council.
Journal Reference:
Zhou, H., et al. (2024) Exploring the engineering-scale potential of designer biochar pellets for phosphorus loss reduction from tile-drained agroecosystems. Water Research. doi.org/10.1016/j.watres.2024.122500.