In a degree thesis for Novia University of Applied Sciences, author Vinodani Weerasekara Ekanayake Mudiyanselage investigated the effectiveness of a sustainable filtration method at Västankvarn Farm in Finland. The Baltic Sea faces constant threats from eutrophication, a process driven by nitrogen and phosphorus runoff that triggers harmful algal blooms and oxygen depletion. Because agricultural activity accounts for over half of the human-made nutrients entering the sea from Finland, finding low-cost solutions like biochar is a high priority for environmental researchers. Biochar theoretically traps pollutants through a process called adsorption. However, applying this technology to real-world agricultural streams presents significant physical and chemical challenges that can hinder its performance.

The findings from this case study indicate that the two-stage biochar filtration system did not achieve a measurable reduction in nutrient concentrations. Statistical analysis showed no significant difference between the water entering the system and the water leaving it for either total nitrogen or total phosphorus. In some instances, the concentration of nutrients leaving the filters was actually higher than the levels measured at the start. These disappointing results were largely driven by the sheer scale of the pollution in the stream. Total nitrogen levels were found to be exceptionally high, fluctuating between seven thousand one hundred and one hundred twenty thousand micrograms per liter. Total phosphorus followed a similar trend, peaking at twenty-nine thousand micrograms per liter in late August. These figures represent a level of pollution that likely exceeded the physical capacity of the biochar to trap and hold the nutrients.

Beyond the chemical limitations, the physical environment of the stream played a major role in the system’s failure. Agricultural streams are often heavy with suspended solids and mud, especially after rainfall. Within four months, the biochar filters became heavily clogged with sediment, causing water to overflow or channel around the filter media instead of passing through it. This lack of contact time meant the biochar could not interact with the dissolved nutrients. Furthermore, the study used unmodified wood biochar with large particle sizes between twenty and fifty millimeters. While larger pieces are easier to handle in a stream, they have a smaller surface-area-to-volume ratio, which limits the number of available sites where chemical adsorption can occur.

The research also explored the relationship between streamflow and nutrient transport. A moderate correlation was found between the water flow rate and the concentration of pollutants, suggesting that high-flow events wash significant amounts of fertilizer and waste from the surrounding farmland into the water. However, high concentrations were also observed during low-flow periods, likely due to continuous inputs from nearby cow barns. This complexity suggests that the stream is influenced by both immediate runoff and steady point-source pollution. The study concludes that while biochar is a sustainable and eco-friendly material, it is not a standalone solution for severe agricultural contamination. Future efforts must focus on integrated approaches, such as combining biochar with constructed wetlands or settling basins to remove sediment before it reaches the filters.

Source: Weerasekara Ekanayake Mudiyanselage, V. (2025). Evaluating the nutrient removal efficiency of biochar in a small stream- A case study of Västankvarn farm (Degree thesis). Novia University of Applied Sciences, Raseborg, Finland.