Fungal woodchip filters are a developing option for managing some stormwater pollutants before runoff reaches a receiving waterway. They use woodchips as a filter medium and provide conditions in which fungi can grow through the material. The fungi, together with the woodchip media and other microorganisms, may help retain, transform or break down some contaminants.
This approach should be viewed as one possible treatment step, not a universal replacement for established stormwater infrastructure. Its suitability depends on the pollutants present, the flow pattern, available land, hydraulic design, maintenance capacity and local approval requirements.
What is a fungal woodchip filter?
A fungal woodchip filter is a contained bed, box, trench or similar structure filled with suitably prepared woodchips. Selected fungi may be introduced to the media, or naturally occurring fungi may become established under appropriate conditions. The fungal growth, known as mycelium, forms a network through the woodchips.
As stormwater passes through the system, several processes may occur. Larger particles can be physically trapped, while contaminants may attach to the woodchip surfaces or be acted on by biological communities living in the media. Some fungi produce enzymes that can assist with the breakdown of certain complex organic compounds. However, treatment is not automatic, and performance varies with the contaminant, flow rate, temperature, moisture, oxygen availability, media condition and other site factors.
A practical system commonly includes:
- Upstream treatment or pretreatment, such as a sediment trap, gross-pollutant capture device or vegetated area, to reduce litter and heavy sediment loads.
- A contained woodchip bed with a defined depth, surface area and media specification.
- Inlet and distribution features that spread water across the media rather than allowing short-circuiting through one channel.
- Drainage and overflow arrangements, including an underdrain, outlet and safe bypass for larger storms where appropriate.
- Inspection and maintenance access so staff can check water levels, sediment accumulation, blockages and media condition.
Where might the approach fit?
Fungal woodchip filters may be considered where runoff is relatively localised and can be directed through a treatment unit. Possible settings include selected industrial or commercial drainage areas, car parks, council depots, constructed treatment areas and retrofit locations where a conventional wetland or large basin is not practical.
The approach may be investigated for runoff containing suspended solids and selected organic pollutants, or as a polishing step after primary treatment. It may also be useful where councils want to test a locally available, biological treatment medium within a broader water-sensitive design strategy.
It is less suitable to place an untreated or untested media bed directly in a drainage line carrying large debris loads, very high sediment concentrations or highly variable flood flows. A filter can clog, erode, bypass or become hydraulically overloaded if these conditions are not addressed. It should not be assumed to remove pathogens, dissolved metals, nutrients, hydrocarbons or other pollutants unless the proposed design has relevant evidence and monitoring to support that expectation.
How should a council assess a site?
Before selecting a fungal filter, the project team should understand both the water and the site. A preliminary assessment should consider:
- What pollutants are present, where they come from and whether concentrations change during dry weather and storms.
- How much runoff arrives, including typical flows, peak flows, storm duration and the frequency of overflow conditions.
- Whether there is enough space for pretreatment, the filter, safe bypasses and maintenance access.
- Ground conditions, drainage levels, groundwater, flooding, nearby waterways and the risk of seepage or uncontrolled discharge.
- Whether the filter can be isolated or bypassed during maintenance and emergency events.
- How the media will be sourced, stored, installed and eventually removed.
- Whether land ownership, planning, environmental, public health, workplace safety or waterway approvals apply.
Sampling and basic flow assessment are important where pollutant removal is a project objective. A qualified stormwater, environmental or water-treatment practitioner can help determine whether a fungal filter is technically reasonable and how its performance should be verified.
Installation and day-to-day management
Installation should follow a site-specific design rather than a standard “fill and flow” arrangement. The woodchips need to be contained so they do not wash downstream. Inlet energy should be controlled, and water should be distributed evenly through the media. The design also needs to account for settlement, drainage, access, worker safety and the possibility that the bed will become saturated or blocked.
Pretreatment is often central to practical operation. Removing litter, coarse sediment and oils before water reaches the woodchips can protect the filter and make inspections more manageable. Maintenance may include removing accumulated sediment, clearing inlets and outlets, checking for ponding or bypassing, repairing damaged containment and assessing whether the media remains structurally and biologically suitable.
Inspection frequency should reflect the catchment and local rainfall rather than a fixed assumption. Checks are especially important after large storms, during the early operating period and whenever water remains ponded for longer than the design allows. Staff should have safe access, appropriate personal protective equipment and procedures for potentially contaminated sediment or media.
Woodchips will eventually settle, decompose or lose their useful hydraulic and treatment properties. Replacement planning should be part of the original design. Used media must not automatically be treated as clean garden mulch. Its disposal or reuse should be based on testing where necessary and managed in accordance with applicable local requirements, particularly if it has accumulated contaminants.
Affordability and environmental considerations
Woodchips may be available from local or regional suppliers, which can make them an appealing material to investigate. However, the total cost includes design, sampling, containment, excavation or construction, pretreatment, access, monitoring, maintenance and media replacement. A low-cost material does not guarantee a low-cost treatment system.
Potential environmental advantages may include the use of a renewable biological medium, integration with landscape-based stormwater treatment and reduced reliance on energy-intensive equipment in some applications. These benefits should be considered alongside transport, sourcing, contamination risk, media replacement and the consequences of poor performance. Responsible sourcing and clear end-of-life management are essential.
Limitations and risks
Fungal treatment is not equally effective for every pollutant. Results may be inconsistent if the fungi are poorly matched to the contaminant or if stormwater moves too quickly through the bed. Drying, prolonged saturation, temperature changes, toxic inflows and competition from other organisms may also affect performance.
Potential risks include clogging, uncontrolled overflow, release of fines or degraded media, odour, unwanted standing water and exposure to contaminated material. A fungal filter may not be the best option where land is constrained, flows are extreme, pollutant loads are highly hazardous, reliable treatment standards are required immediately or another proven treatment method can achieve the objective more predictably.
A sensible pathway for a small pilot
Where the concept appears suitable, a council could begin with a contained pilot rather than making a network-wide commitment. The pilot should have a clear treatment objective, baseline information and an agreed decision process.
- Define the target pollutants and the level of treatment the project is seeking to investigate.
- Characterise incoming water and flows before installation, including representative storm events where practicable.
- Design pretreatment, containment, bypasses, access and safety features with qualified assistance.
- Record the woodchip source, media preparation and any fungal inoculation or establishment method.
- Monitor inflow and outflow water quality, flow or residence conditions, ponding, clogging, overflow and media changes.
- Track maintenance time, sediment removal, operational problems and the condition of the media.
- Review results against the original objective before deciding whether to modify, continue, replace or expand the system.
The most responsible use of fungal woodchip filters is evidence-led and site-specific. They may offer councils an additional biological treatment tool, particularly when combined with conventional pretreatment and sound hydraulic design. Their value should be demonstrated through careful planning, monitoring and maintenance—not assumed from the presence of fungi alone.