Mycofiltration is an emerging approach to managing contaminated water with the help of fungi growing through an organic filter medium. In an agricultural setting, that medium may include woodchips, bark or other woody materials colonised by fungal mycelium. The resulting bed can intercept runoff before it reaches a ditch, drain, wetland or stream.
It is best understood as one component of a broader runoff-management system—not as a stand-alone purifier. A fungal filter may help retain sediment, support microbial treatment and interact with some dissolved pollutants, but performance depends on the contaminant, flow conditions, media, temperature and design. A bed that reduces sediment or one indicator bacterium should not be assumed to remove every pesticide, nutrient, pathogen or unknown contaminant.
What agricultural runoff can carry
Rainfall, snowmelt and irrigation can move water across fields and through drainage systems. The pollutants carried by that water depend on soil type, slope, ground cover, fertilizer and manure management, livestock access and the location of drains or ditches.
Runoff may contain:
- Suspended soil and sediment
- Particulate phosphorus attached to soil particles
- Dissolved nitrate, phosphorus and other nutrients
- Crop residues and dissolved organic matter
- Pesticides and herbicides, either dissolved or attached to particles
- Bacteria, viruses and other microorganisms associated with manure or livestock areas
- Site-specific contaminants such as veterinary chemicals or other emerging pollutants
Infiltration and subsurface drainage can also transport dissolved contaminants toward groundwater, ditches and streams. This distinction matters because sediment-bound pollutants may respond to settling and physical filtration, while dissolved pollutants move with the water and may require sorption, plant uptake or biological transformation.
How mycofiltration works
An organic filter bed provides a porous structure through which water flows. Woodchips and similar materials can slow the water, trap particles and provide surfaces for bacteria, fungi and other microorganisms to colonise. Organic carbon may also support microbial processes.
When fungi grow through the media, their fine hyphae can contribute to particle interception and create additional biological surface area. Fungal biomass may bind some compounds, a process known as biosorption. Certain fungi also produce enzymes capable of transforming particular organic molecules. These mechanisms are scientifically plausible, but they are not universal. Fungal species, substrate, pH, oxygen availability, moisture, temperature, hydraulic loading and contaminant chemistry all influence the result.
Physical filtration and biological treatment are different
Physical processes include sedimentation, straining, interception and adsorption to the media. They are most relevant to soil particles, crop residues and some particle-associated phosphorus or pesticides.
Biological processes include microbial degradation, fungal biosorption and enzymatic transformation. In a suitable low-oxygen, carbon-rich zone, bacteria may also carry out denitrification, converting nitrate into nitrogen gases. That is the principal purpose of a conventional denitrifying woodchip bioreactor, which is a more established practice for subsurface nitrate drainage than fungal mycofiltration specifically.
Vegetation adds further functions. Plants slow water, encourage infiltration, trap sediment and provide root zones where nutrient cycling can occur. A treatment system works best when these different processes are deliberately combined rather than attributed solely to visible fungal growth.
Intercept runoff before it reaches a waterway
The strongest strategy usually begins by reducing pollution at its source. Crop cover, cover crops, conservation tillage, erosion control and nutrient applications based on crop demand and soil testing reduce the load reaching any treatment bed. Livestock and manure storage should be kept away from direct drainage routes, and contaminated yard or barn water should be contained rather than simply diverted toward a field filter.
Next, runoff should be slowed and spread. Grass filter strips, contour practices, grassed waterways, level spreaders and settling areas can reduce velocity and remove much of the incoming sediment. Vegetated filter strips perform best when water reaches them as relatively even sheet flow. A concentrated gully or pipe discharge can cut channels through the strip, bypass the media and cause erosion.
A possible treatment sequence is:
- Source control and erosion reduction
- Vegetated buffer or level-spreading area
- Sediment trap or settling zone
- Woodchip or fungal organic filter bed
- Constructed wetland, saturated buffer or other polishing stage
- Stable, monitored outlet and emergency overflow
Where an organic filter bed might fit
Potential locations include the outlet of a tile-drain system, downstream of a sediment trap, beside a farm ditch or at the inlet to a constructed wetland. A bed may also be considered between a livestock-related runoff source and a larger treatment area, provided the source is not high-strength effluent that exceeds the bed’s capacity.
Tile-drain bioreactors generally use an enclosed media chamber, inlet and outlet pipes, flow-control structures and a bypass or overflow route. Lining or other protection may be needed to prevent surrounding soil from entering the media. Standards for denitrifying woodchip bioreactors should not automatically be applied to surface drainage or to a fungal system; open-ditch flows are often more variable and difficult to control.
Flow rate and contact time
Water moving too quickly has limited contact with the media and may receive little treatment. Excessively slow flow can cause ponding, hydraulic backup, odours or unintended bypass. Storm peaks can overwhelm a small bed, so the system needs a safe route for exceptional flows.
Contact time must be matched to the treatment objective. For context, USDA Natural Resources Conservation Service guidance for a specific denitrifying-bioreactor design uses a minimum hydraulic residence time of three hours at peak treatment capacity. That is not a universal mycofiltration specification. A fungal bed intended to reduce sediment, nitrate, bacteria or a pesticide may require different conditions, and field performance should be verified rather than assumed.
Maintenance is part of the design
Inspect the inlet, distribution system, media surface, outlet and overflow after major storms and at regular intervals. Sediment accumulation can clog the pores, create ponding and force water around the bed. Pretreatment is especially important where erosion or manure-related solids are substantial.
Woodchips settle and decompose. Fine material can reduce porosity, while the available carbon is gradually consumed. Media may eventually need to be replenished or replaced. A ten-year media life is used in the design context of some denitrifying bioreactors, but it is not a guaranteed lifespan for every organic or fungal filter.
Do not judge performance only by whether mushrooms are visible. Fungal activity can decline if the bed dries, receives damaging chemical concentrations, becomes unsuitable for the selected organism or is overwhelmed by competing microorganisms. Where pollution reduction matters, compare inlet and outlet flow and water quality. Depending on the risk, useful measurements may include turbidity, suspended solids, nitrate-nitrogen, total and dissolved phosphorus, pH, temperature, conductivity, indicator bacteria and targeted pesticides.
Important limitations
Research on mycofiltration includes laboratory columns and small experimental systems. Some studies have found reductions in indicator bacteria or particular organic contaminants, while also showing that unconditioned organic media can release microorganisms. These findings support further investigation, not a guarantee of pathogen removal.
Pesticide treatment is highly compound-specific. A fungus that transforms one pesticide may have little effect on another, and transformation products may still require assessment. Organic filters should not replace pesticide-label requirements or legally required runoff controls.
A small mycofiltration bed is also not a substitute for designed treatment of dairy or feedlot wastewater, manure-storage leakage, silage leachate, slaughter wastewater or other concentrated effluent. High nutrient loads, strong organic waste, pathogens, salts and unknown contaminants may require containment, storage, engineered treatment, disinfection, regulatory approval and professional environmental advice.
A practical decision framework
Before considering mycofiltration, identify where the water comes from, how much arrives during normal and peak events, and which pollutants are present. Ask whether source-control measures can reduce the load first and whether a buffer or settling area can protect the organic bed. Plan for extreme storms, sediment removal, media replacement and a stable bypass.
Mycofiltration may be a useful experimental or site-specific addition to a treatment train, particularly where a farm can control flow and monitor results. It should be selected for a defined purpose and evaluated against alternatives such as vegetated buffers, saturated buffers, denitrifying woodchip bioreactors, controlled drainage and constructed wetlands. The goal is not to make one fungal filter handle every contaminant, but to place an appropriately designed biological step where it can complement sound farm water management.