When rain falls on a forest or open soil, much of it can infiltrate, evaporate or move slowly through vegetation. In a city, roofs, roads, driveways, car parks and other impervious surfaces interrupt those pathways. Water runs off more quickly and in greater volume, often reaching drains, streams and coastal waters before soil, plants or treatment systems can intercept it.
That runoff can carry sediment and other suspended solids, nitrogen and phosphorus, oil and grease, petroleum hydrocarbons, metals such as copper, zinc and lead, microorganisms including faecal indicator bacteria, litter, pesticides, salts and tyre-related contaminants. The result can be localised flooding, stream-bank erosion, sewer-system stress and declining water quality.
Fungi may eventually contribute to some parts of this challenge. However, they are best understood as potential biological partners within a broader stormwater system—not as a substitute for sound hydraulic design, appropriate planting, pretreatment and regular maintenance.
Established green infrastructure comes first
Green infrastructure manages stormwater close to where it falls by slowing, spreading, storing, infiltrating, evaporating, reusing or biologically treating runoff. Its performance depends on catchment conditions, soil, vegetation, hydraulic loading and maintenance.
Rain gardens and bioretention cells
Rain gardens and engineered bioretention cells are shallow, planted depressions that temporarily receive runoff. Treatment can involve settling, filtration through soil media, adsorption, plant uptake, microbial activity, evapotranspiration and infiltration. Larger or more engineered cells may include an inlet, a pretreatment area, an underdrain and an overflow structure.
The media must balance several competing requirements: it needs to drain quickly enough to prevent prolonged ponding, retain or transform pollutants, support plants and resist clogging. Sediment control at the inlet is therefore important, as is inspection of drawdown time and the condition of the upper media layer.
Bioswales and vegetated swales
Vegetated swales are shallow channels that slow and spread runoff while providing conveyance. Some are primarily drainage features; others are designed with check dams, amended soil or infiltration zones to provide more treatment. Slope, flow depth, vegetation, erosion protection and hydraulic loading strongly influence their results.
Filter strips, detention areas and wetlands
Vegetated filter strips can encourage sediment deposition and infiltration when runoff arrives as shallow, distributed sheet flow. Concentrated flows may simply cut channels through them, so level spreaders or upstream pretreatment may be needed.
Detention basins temporarily store water and release it at a controlled rate. Settling, vegetation and infiltration may improve water quality, but detention should not automatically be treated as complete pollutant removal. Constructed wetlands add wetland plants, soils, water and associated microbial communities. Their treatment may include sedimentation, filtration, adsorption, plant and microbial uptake, and chemical transformations under changing oxygen conditions.
Other established measures include permeable pavement, green roofs, tree trenches, stormwater tree pits, infiltration basins, rainwater tanks, forebays and conventional soil, sand or cartridge filters. These approaches can be combined into treatment trains so that coarse sediment and litter are removed before water reaches finer media or biological treatment zones.
Where fungi already fit
Soil, compost, mulch and planted media are not biologically empty. They contain bacteria, fungi, roots, invertebrates and organic matter that interact with water and pollutants. Saprotrophic fungi decompose dead material such as leaves, straw and wood. Mycorrhizal fungi form associations with plant roots and may influence nutrient uptake, water relations and soil structure.
This means that fungal processes may already contribute to a functioning rain garden or bioretention cell without anyone adding a fungal product. The presence of fungi, however, is not the same as a validated fungal-treatment system. Natural communities vary with climate, feedstock, moisture, pollutants and management.
What mycofiltration means
Mycofiltration generally refers to passing water through fungal mycelium growing in or on a porous substrate such as woodchips, straw, compost, coir or soil. Mycelium is the network of fungal hyphae that makes up the main vegetative body of most filamentous fungi. Mycoremediation is a broader term for using fungi or fungal-associated processes to transform, bind, immobilise or contain pollutants.
A mycofiltration concept is therefore more than “putting mushrooms in a bioswale.” It involves the interaction of fungal biomass, organic media, water flow, soil particles, plants and other microorganisms.
Potential fungal mechanisms
Physical interception
Hyphal networks can add surface area and complexity to a porous medium. In principle, this may help intercept fine particles or some microorganisms. The actual effect also depends on substrate particle size, pore structure, compaction, moisture and flow path. If fungal growth or accumulated fines make the layer too dense, infiltration may decline and water may pond or bypass the treatment zone.
Decomposition of organic media
Fungi break down cellulose, hemicellulose and lignin in woodchips, straw, leaf litter and other organic materials. This decomposition supports wider microbial activity and changes the media over time. Particles may shrink, fine material may accumulate and pore spaces may change. Organic media can also release dissolved carbon, nutrients or other compounds, especially during early wetting or active decomposition.
Enzymatic transformation
Some fungi produce extracellular enzymes, including laccases and peroxidases, that can alter complex organic molecules. White-rot fungi have attracted attention because their lignin-degrading enzymes may also act on some petroleum compounds, pesticides, dyes and pharmaceutical residues.
Transformation is not automatically complete detoxification. Products of partial degradation may remain, and performance depends on fungal species, temperature, oxygen, moisture, nutrient availability, pollutant chemistry and contact time. Rapidly passing roadway runoff through a shallow layer may not provide enough time for these reactions.
Adsorption, biosorption and accumulation
Fungal cell walls contain chitin, glucans, proteins and other compounds that can bind some metals and organic contaminants. This passive binding is called biosorption. Living fungi may also take up certain substances through bioaccumulation.
These processes can move contaminants from water into fungal biomass, sediment or organic media, but they do not destroy elements such as metals. A medium that captures copper or lead may become a contaminated material requiring careful handling later.
Interactions with plants and soil organisms
Mycorrhizal fungi may help some plants obtain water and nutrients, while fungal and bacterial communities can influence soil aggregation and root-zone activity. These effects could support vegetation in green infrastructure, particularly where plants face intermittent drought or poor soils. They do not guarantee improved removal of a particular pollutant, and outcomes depend on the plant, fungal association, soil and water regime.
What the evidence shows—and does not show
Research is promising but not yet sufficient to treat mycofiltration as a standard municipal stormwater practice. A 2015 laboratory study examined Stropharia rugoso-annulata mycelium grown on alder woodchips and woodchip–straw media using synthetic stormwater. Results suggested some reduction in E. coli and other indicator organisms under the reported conditions. They also showed that uninoculated wood and straw media could themselves release bacteria. Laboratory results should not be applied directly to all urban runoff or full-scale systems.
A 2023 review prepared for the Massachusetts Department of Transportation identified encouraging laboratory and case-study findings involving pathogens, erosion, metals and organic pollutants, but found limited field evidence, few long-duration studies and no established design parameters or predictable performance outcomes for routine transportation stormwater use. Possible applications such as colonised compost socks, coir logs, organic blankets, woodchip overlays and amended bioretention media remain research or pilot concepts in many settings.
Natural colonisation versus deliberate inoculation
Mulch, compost, woodchips, leaf litter and soil naturally acquire diverse fungal communities. Allowing those communities to develop may be simpler and more resilient than trying to establish one selected strain. It is also variable, so it should not be presented as a standardised treatment process.
Deliberate inoculation may involve fungal spawn, colonised woodchips, mycelial mats, treated compost or plant-root inoculants. Practical questions include how much inoculant is needed, whether it survives installation, how it competes with native organisms and how it responds to drought, salt, heat and pollutants. Detecting a fungus after installation would not necessarily demonstrate that it remains active or improves pollutant removal.
Hydraulic design and maintenance remain decisive
Fungal processes cannot be separated from hydraulics. High flows can scour mulch, damage fungal growth and carry water around the media. Very rapid flow reduces contact time for adsorption and biological transformation. Conversely, stagnant water may cause prolonged ponding, odours or undesirable low-oxygen conditions.
- Contact time: Physical settling may occur quickly, while enzymatic treatment generally needs longer interaction. Residence times discussed for related bioreactor concepts are not universal mycofiltration standards.
- Peak storms: Overflow structures and bypasses may be necessary to protect a treatment cell. During major storms, some water may receive little or no contact with fungal media.
- Sediment and clogging: Inlets, forebays and upper media layers need inspection and cleaning. Accumulated sediment, leaves and organic fines can reduce hydraulic conductivity.
- Media renewal: Mulch and other organic layers may need replacement. A fungal component should be treated as part of a maintainable media system, not as a maintenance-free filter.
Planning for spent media
Woodchips, compost, sediment and fungal biomass may accumulate hydrocarbons, metals or other pollutants. Before spent material is reused as ordinary mulch, compost amendment or landscaping soil, testing may be prudent where contamination is plausible. Disposal and reuse decisions depend on the contaminants, concentrations, local regulations and proposed land use.
Metals are not eliminated by fungi, and organic pollutants may be only partly transformed. Councils and community groups considering a pilot should decide in advance who will inspect the system, measure performance, remove sediment, replace media and document the destination of spent material.
A realistic role for fungi
Fungi could add biological activity to organic media, support plants and soil communities, intercept some particles, and potentially help transform selected organic pollutants under suitable conditions. The most responsible applications are likely to be monitored, site-specific pilots integrated with established green infrastructure.
For councils, landscape designers and community organisations, the practical starting point is still good source control, pretreatment, planting, flow distribution, overflow protection and maintenance. Fungi may become a useful component of those systems, but adding fungal-rich material does not automatically turn a rain garden or bioswale into a reliable pollutant-treatment facility. Evidence-based hydraulic design should remain the foundation while fungal applications are tested and evaluated.