Mycofiltration for Road and Car-Park Runoff

Roads, car parks and other paved urban surfaces are efficient at moving rainwater—and at carrying whatever has accumulated between rainfall events into drains, waterways and receiving soils. Mycofiltration, which uses fungal-rich organic media such as woodchips or straw, is being explored as one possible biological layer in the management of this runoff. Its potential is real, but the technology remains emerging for transportation stormwater and should be approached as part of a carefully designed treatment train, not as a stand-alone cure.

What can paved-surface runoff contain?

Runoff quality varies substantially from one site to another. A busy highway, a residential street, a shopping-centre car park and a lightly used paved area will not necessarily produce the same pollutant mixture. Traffic volume, vehicle types, pavement materials, nearby land use, rainfall intensity, the length of the preceding dry period, maintenance activities and winter deicing all influence what is washed away.

Depending on the site, runoff may contain:

sediment, fine road dust and pavement-wear particles;
tyre and brake-wear particles, including microplastic and rubber-associated material;
oil, grease, hydrocarbons and polycyclic aromatic hydrocarbons (PAHs);
metals such as copper, lead and zinc;
nitrogen and phosphorus from atmospheric deposition, fertiliser, vegetation and nearby activities;
litter, leaves and other gross solids;
deicing salts and other dissolved ions; and
bacteria, pesticides or other chemicals associated with particular land uses.

Sediment is important in its own right, but it can also carry pollutants attached to its particles. Physical removal of sediment may therefore reduce particle-bound metals and hydrocarbons. It does not, however, demonstrate that dissolved pollutants have been removed. Tyre-related contaminants are also an evolving area of research. Tyre wear can contribute rubber particles, metals and hydrocarbons, while chemicals such as 6PPD-quinone have raised concern because of their toxicity to some aquatic species. A woodchip or fungal filter should not automatically be assumed to remove all tyre-derived chemicals.

Start with a conventional stormwater treatment train

Before considering a fungal-rich filter, runoff should be managed through ordinary source-control and hydraulic measures. The aim is to prevent excessive pollutant loading, slow the water and protect downstream treatment media from clogging.

A site-specific treatment train might include:

street sweeping, spill prevention, litter control and careful application of deicing salt;
kerbs, channels, catch basins, flow spreaders and conveyance controls;
gross-solids baskets, sediment traps, forebays or settling chambers;
grass filter strips, swales, vegetated areas, detention features or bioretention cells; and
engineered sand, soil, cartridge or sorbent systems designed for particular pollutants.

Pretreatment is especially important. Incoming sediment and debris can form a low-permeability layer, clog pores and cause water to bypass a filter before its chemical treatment capacity has been used. A sediment forebay or accessible catch-basin sump can allow concentrated material to be removed without excavating the entire biological filter.

Where could mycofiltration contribute?

Mycofiltration generally refers to water passing through a substrate colonised by fungal mycelium, often using lignocellulosic materials such as woodchips, straw, bark or similar organic media. These materials can create a tortuous flow path that slows water, encourages settling and provides surfaces for fungal hyphae and other microbial biofilms.

Several treatment mechanisms may operate at the same time:

Physical capture: particles may be screened, intercepted or settled as flow velocity decreases.
Sorption: organic matter and lignocellulosic surfaces may retain some metals and hydrophobic organic compounds. Sorption includes surface attachment and partitioning into a material; adsorption more specifically refers to accumulation at a surface.
Biosorption: fungal cell-wall chemistry can bind some metal ions. This may occur with living or dead biomass.
Immobilisation: a contaminant may become less mobile or less bioavailable without being destroyed.
Biotransformation and degradation: selected fungi, particularly some wood-decay fungi, produce enzymes capable of transforming certain organic compounds.

These mechanisms are not interchangeable. Capturing a pollutant in the media is different from degrading it. A compound may be retained temporarily, transformed into another compound or broken down more completely. Disappearance of a parent chemical from water is not, by itself, proof that toxicity has been eliminated.

What fungi cannot be assumed to do

Fungi cannot destroy metals. Metals may be captured with sediment, adsorbed by wood or organic matter, bound to fungal biomass, precipitated or otherwise immobilised—but the elements remain in the captured material. Retention can weaken as media becomes saturated or as pH, salinity, redox conditions and dissolved organic matter change.

Nor should a fungal filter be described as a universal treatment for synthetic contaminants. Fungal enzymes act on some chemical structures under particular conditions. Their effectiveness depends on the species or strain, moisture, temperature, oxygen, pH, pollutant concentration, contact time and competition from other microorganisms. Laboratory studies with selected fungi and individual compounds do not prove that an outdoor filter will treat the complex mixture found in road runoff.

Nutrient removal also requires verification. Microbial activity may support nitrogen transformations under suitable conditions, but intermittent stormwater flows are not equivalent to controlled woodchip bioreactors used in other settings. Organic media can release dissolved organic carbon and, in some circumstances, phosphorus. Influent and effluent monitoring is therefore essential.

Hydraulic design and performance

Road and car-park runoff often arrives in short, high-flow pulses. A biological filter must be designed for both water-quality treatment and safe conveyance of larger storms. Useful features may include a sediment forebay, flow-distribution structures, a defined media depth and particle-size range, an underdrain or controlled outlet, an overflow route and safe access for inspection.

Finer media may capture more small particles but clog more quickly. Coarser woodchips generally preserve permeability but may provide less fine-particle filtration. Organic media can settle, decompose and change porosity as they become wet and biologically active. Higher flow rates also reduce contact time and may increase pollutant breakthrough or particle resuspension.

Performance should be demonstrated rather than inferred from the presence of fungal growth. A pilot or operating system may need to track flow, drawdown time, water levels, bypass frequency, suspended solids, dissolved and particulate metals, hydrocarbons or PAHs, nutrients, chloride, dissolved organic carbon and any site-specific tyre-related compounds.

Maintenance and spent media

Inspect the system after construction, major storms and at a regular site-appropriate interval. Look for sediment accumulation, standing water, slow drainage, surface sealing, erosion, preferential flow paths, blocked outlets, media settlement, damaged containment and signs that the bypass is activating.

Sediment should be removed before it forms a cap over the media. Depending on pollutant loading and design, the upper layer of woodchips may eventually need loosening, removal or replacement. Degraded media, underdrains, flow controls and distribution structures may also require renewal. Re-inoculating fungi should not be treated as a substitute for restoring hydraulic function or replacing exhausted media.

Spent material requires particular care. Woodchips or other media that have accumulated metals, PAHs, hydrocarbons, tyre particles or persistent chemicals are not automatically garden mulch or compost feedstock. Composting changes organic matter, but it does not destroy metals and may not eliminate persistent organic contaminants. Contaminated media should be contained, protected from leaching and dust release, and characterised through appropriate laboratory testing where required. Disposal or treatment must follow applicable local and national waste regulations. Land application should only be considered where testing and regulatory requirements demonstrate that it is safe and permitted.

A promising layer, not a replacement system

The strongest case for mycofiltration is as a contained, monitored biological layer after gross-solids removal, sediment settling and flow attenuation. Woodchips and fungal communities may add useful physical, sorptive and biological functions, particularly for selected organic compounds and particle-associated pollutants.

However, peer-reviewed evidence for routine deployment on transportation stormwater remains limited. Mycofiltration should therefore complement—not replace—source control, conventional drainage design, engineered filtration, monitoring, maintenance and responsible waste management. Its value will depend on the actual pollutant profile, hydraulic conditions and long-term behaviour of the media at the site being treated.