What if farmers could effectively stop excess phosphorus from contaminating nearby water bodies while also recycling that nutrient as a slow-release fertilizer without incurring high costs? Research now indicates that this is both feasible and cost-effective.
What if farmers could effectively stop excess phosphorus from contaminating nearby water bodies while recycling that nutrient as a slow-release fertilizer without incurring high costs? Research now indicates that this is both feasible and cost-effective.
“Phosphorus removal structures have been created to capture soluble phosphorus from tile drainage systems. However, the materials currently used for phosphorus sorption are either not very effective or are industrial waste that is hard to dispose of. This motivated us to find a sustainable and efficient material for phosphorus removal from these drainage systems,” explained study author Hongxu Zhou, a doctoral student in the Department of Agricultural and Biological Engineering (ABE) at the University of Illinois Urbana-Champaign.
Zhou and his collaborators utilized sawdust and lime sludge, which are byproducts of milling and drinking water treatment, respectively. They combined these materials, created pellets from the mixture, and slow-burned them with limited oxygen to produce a specialized biochar that has a much greater capacity for binding phosphorus compared to using lime sludge or biochar alone. Once these pellets have captured all the phosphorus they can, they can be spread over fields where the nutrients are gradually released.
Capitalizing on the various sustainable benefits of the designer biochar, the team conducted the first field tests of these pellets, tracking phosphorus removal in fields in Fulton County, Illinois, over a two-year period. Similar to most Midwestern corn and soybean fields, the trial sites had subsurface drainage systems. The drainage water first flowed through phosphorus removal structures filled with biochar pellets of two different sizes. In the first year, 2-3 centimeter biochar pellets were tested, followed by 1-centimeter pellets in the second year.
Both sizes of pellets successfully removed phosphorus, but the 1-centimeter pellets showed significantly better performance, achieving phosphorus removal efficiencies of 38 to 41%, compared to just 1.3 to 12% for the larger ones.
This outcome didn’t surprise co-author Wei Zheng, who noted that smaller particles allow for increased interaction time for phosphorus to adhere to the biochar. Zheng, a principal research scientist at the Illinois Sustainable Technology Center (ISTC), has previously conducted laboratory studies demonstrating that powdered designer biochar works effectively for phosphorus removal. However, using powdered materials in the field is not practical.
“If we were to use powdered biochar in the field, it could easily wash away,” Zhou stated. “Therefore, pellets were necessary. We had to compromise slightly on efficiency to ensure functional performance in real-world conditions.”
After confirming the effectiveness of the pellets in practical settings, the research team carried out economic and life-cycle analyses to assess the costs for farmers and the sustainability of the overall system.
The estimated production cost for designer biochar pellets was $413 per ton, significantly lower than the market prices for alternatives like granular activated carbon, which ranges from $800 to $2,500 per ton. The team also calculated the overall cost of phosphorus removal using this system, averaging $359 per kilogram removed, a figure that fluctuated based on inflation and how often pellets needed replacement—two years turned out to be the most cost-effective approach.
Life cycle analyses indicated that the system—which involves returning spent biochar pellets to fields and eliminating additional phosphorus inputs—could potentially save between 12 to 200 kilograms of carbon dioxide equivalent for each kilogram of phosphorus removed. According to Zhou, the advantages extend beyond just nutrient loss reduction and carbon sequestration; they include energy production, diminished eutrophication, and improved soil health.
“Currently, there are no regulations compelling farmers to extract phosphorus from drainage water. However, many environmentally conscious farmers are eager to decrease nitrate and phosphorus runoff from their fields,” pointed out co-author Rabin Bhattarai, an associate professor in ABE. “If farmers are already setting up a woodchip bioreactor to tackle nitrate, they would simply need to add these pellets to the structure to simultaneously manage phosphorus. Plus, the ability to reuse the pellets in their fields is quite appealing.”
