The Duck Test…When is a Biofilter not a Biofilter?

Many say that if it looks and quacks like a duck, then it’s probably a duck. When you visualize a biofilter, you might envision bioretention cells, rain gardens, constructed wetlands, bioswales, or vegetated filter strips. What is a visual commonality among these practices? Plants. Following the duck test, if it contains plants and engineered, organic media and functions like a biofilter then it’s probably a biofilter.

However, don’t let a biofilter product name fool you into thinking it’s a biofilter. Nomenclature is important as it implies an expectation; we expect a treatment system with “biofilter” or “wetland” in its name to look and function like a biofilter. However, there are regulatory-approved, proprietary filter systems that contain this nomenclature in their product name, but do not require plants. Some use “soil” without plants and claim the name “biofilter”. In some circumstances, vegetation is sacrificed to meet site constraints for underground “biofiltration” treatment. Other systems may contain vegetation, but the roots make no contact with stormwater flowing through the active treatment zone. To clarify the difference between these types of systems and true biofilters, let’s refer to them as “soil-based filters”. Failing to include plants with roots established in an active treatment zone is not biofiltration, it is media filtration. Vegetation serves as an indicator of biofilter health; thriving plants often mean the system is functioning as designed. You take the plant away and now we have lost our indicator as well as the “bio”.

“Bio” is a crucial part of the word biofilter as it suggests that living organisms work together towards achieving pollutant removal. Most of the biological processes, for which biofilters are named, occur underground in the root zone where the plant root system interacts with the soil environment via root secretions and soil microorganisms. Biofiltration treatment mechanisms rely on biological components such as plants, microorganisms and organic media. When we remove a leg from our balanced stool, plants in this instance, we remove a component of the synergistic community of living organisms within the system. Biofilters containing all of their biological components have more microbial density and diversity and therefore have more ability to transform and uptake pollutants (Hills et al. 2017). The same level of performance cannot be expected when the removal of an important component prevents the biofiltration mechanisms from functioning as intended.

Many MS4 permits define biofiltration as practices using vegetation AND amended soils to detain and treat stormwater runoff. While plants are generally not required to demonstrate TSS performance in accordance with established evaluation protocols, plants play an invaluable role in the field. While a soil-based filter might meet a performance standard, we cannot assume it will function the same as a true biofilter. Is a duck still a duck if it doesn’t look like a duck? Adversaries may believe that if biofilter approvals are based on a sediment performance standard alone, then the biofiltration treatment mechanisms become less important. However, inert silica test sediment does not effectively characterize the complexity of stormwater runoff. What happens when you add organic matter and oil and grease to the inflow? Can we assume the same longevity among vegetated and non-vegetated systems? Load reduction may be impacted due to reduced hydraulic loading rates when the biological mechanisms that support improved infiltration rates are not in place. When soil-based filters are placed underground, potential biofouling will only exacerbate premature hydraulic failure.

While plants are key to sustainability by supporting microbiological activity and maintaining an assimilative capacity over and above systems void of vegetation, there are a multitude of benefits plants offer outside of pollutant reductions including longevity and aesthetics. Sustainability is not just defined by maintaining high pollutant removal, but also by maintaining design hydraulic flow rates over time through root expansion, penetration, exudate production and die-off. When healthy plants are present, media porosity is increased, soil structure is improved, and compaction is reduced, thereby maintaining hydraulic rates.

Many soil-based filter designs meet regulatory pollutant performance standards of conventional biofiltration systems, but operational feasibility is not always comparable and sometimes not even considered when the approval emphasis is on treatment performance. Placing more emphasis on maintenance performance may be a first step in truly understanding performance and longevity. Claiming to be a biofilter but allowing the flexibility of the use of plants to meet specific site conditions assumes equal longevity between different configurations. It cannot be assumed that the maintenance frequency for above ground vegetated systems will be the same as unvegetated soil-based filters that are below ground.

Maintenance is often the forgotten element that is still critical to the design in ensuring hydraulic sustainability. With sometimes little enforcement of inspection and maintenance, our soil-based filter suddenly doesn’t look or act like a biofilter. It is important to match the design maintenance interval with the site application and consider conservative sizing. Installed practices that have not been field-verified to perform and function without vegetation should be inspected regularly and should occur more frequently than the specified maintenance interval during the first year of operation. Soil-based filters void of vegetation may endure increased maintenance costs to the owner, potentially exceeding the upfront capital investment over the life cycle of the system, and unknowingly bypass pollutant loads downstream. Maintenance costs should be estimated on field performance when available. If a system only treats a quarter of a water year before bypass occurs below the design storm, those costs will add up quickly. In these instances, different proprietary filter types should be considered that have been field-verified with longer maintenance frequencies.

Depending on how localities ultimately write their regulations around green infrastructure practices can dictate how interpretations are made around vegetation. A soil-based filter system may meet the green infrastructure requirement through infiltration, but losing the plant makes it a far cry from a biofilter and may create an unknown maintenance burden. When evaluating biofilter options consider the duck test. Let’s keep our biofilters looking and acting like biofilters, and where it’s not feasible, consider alternative field-approved practices.


Hills, M., Allen, V., and Lenhart, J. (2017). "Investigating the Microbiology of Bioretention." World Environmental and Water Resources Congress 2017, 416-430.