Biofilters & bioretention

Typical design

Biofilters are facilities where water flows vertically through a vegetated filter media. A combination of physical, chemical and biological mechanisms remove pollutants from the stormwater as it passes through the filter material.

On top of the biofilter there is a ponding volume (depth often around 10-30 cm, but up to 1 m can be used in special cases). Here water is stored temporarily since the stormwater inflow commonly exceeds the infiltration into the filter medium. The filter depth usually varies between appr. 60 cm to 90 cm, depending on what type of pollutants is considered most important to remove. At the bottom of the filter material water can either infiltrate further into the underlying soil, or a drainage layer and/or pipe can lead the water into a sewer pipe.

The choice of filter material is critical for the functioning of the biofilter. The filter material is a compromise between different, partly conflicting purposes. A high infiltration capacity increases the amount of water treated and reduces (untreated) overflows. On the other hand, finer materials (with lower infiltration capacities) provide usually higher adsorption capacities. Commonly infiltration capacities around 150-500 mm/hr are recommended. Clogging will, however, reduce the infiltration capacity over time. Often sand-based materials with relatively low organic matter content are chosen. Too high amounts of nutrient-rich organic matter have been shown to cause leaching of phosphorus and nitrogen and should thus be avoided. Thus, the vegetation has to be adapted to that material if treatment is prioritised (and not vice versa). Numerous filter material amendments have been tested with mixed results.

The vegetation is an important part of the biofilter. It enhances treatment mainly indirectly by supporting numerous treatment processes. Also plant uptake can occur but this is not the main removal pathway (and since plants degrade, one would have to harvest the plants if this pathway should be long-lasting). Plants significantly enhance nutrient removal. Further, plants decrease the clogging risk and thus help to maintain the infiltration capacity over time. The vegetation also contributes to aesthetical values and can make some contribution to biodiversity.

To reduce clogging of the filter material, a forebay or a buffer strip is sometimes added where coarser particles can settle before the water flows into the filter.

Since biofilters are mainly aimed at improving stormwater quality they are designed for modest flow rates, e.g. corresponding <1-year rainfall event or rain up to a certain rainfall depth (e.g. 10 or 20 mm). Larger flow rates are bypassed through an overflow to protect the filter and guide the water into the pipe network instead.

Aims in stormwater management

Biofilters are primarily aimed at removing pollutants from stormwater. Therefore they are designed for small rains. For such rains there will be a delay in runoff and decrease in runoff volume. For larger rains the filter is normally designed so the flow bypasses the filter so there will be no effect on runoff rates or volumes.

If stormwater quantity is also important some adjustments can be made in the biofilter, for example a larger ponding volume, but it is often difficult to balance this with the requirements for good treatment at lower flows. Thus, the more common solution in that case is to connect the overflow to downstream (sub-surface) detention facility, for example a coarse gravel filled storage volume.

Implementation in catchments

Generally, it is recommended that biofilters should be around >2-5% of the impervious area of their catchment. Besides this rough estimate, the ponding depth also plays a role (a filter with more ponding storage could be smaller to manage the same amount of water). Biofilters are most commonly built as small units throughout the catchment. In this case they can be integrated into the street design. It is possible, but less common, to build larger biofilters close to the catchment outlet.

Since surface clogging of biofilters is a common risk, maintenance of the forebay is crucial to sustain their function over time. Also, the top layer of the filter material, where clogging occurs and most pollutants are trapped, has to be replaced regularly (intervals ranging from years to decades).

Additional benefits

• Vegetation contributes to biodiversity
• Aesthetic values (depending on the design)