Recommendations for Filtration Practices

A.J. Erickson, J.S. Gulliver, R.M. Hozalski, P.T. Weiss


The required frequency of inspection and maintenance is dependent on the watershed land use (e.g., urban, rural, farm, etc.) and rainfall amounts and intensity, however, it is recommended that visual inspection be performed at least twice per year.

Synthetic runoff testing is recommended for assessment of hydraulic conductivity for small surface filters or underground filters, if an adequate water supply is available. For assessment of hydraulic conductivity on sites too large for synthetic runoff testing, capacity testing is recommended. Pollutant removal performance of surface and underground filtration practices is well established and therefore visual inspection, capacity testing, or synthetic runoff testing is recommended for assessment of pollutant removal performance. Hybrid filtration practices vary in design and installation and therefore can be assessed by visual inspection and either capacity testing, synthetic runoff testing, or monitoring. Monitoring is recommended when capacity testing or synthetic runoff testing do not meet the goals of the assessment program.


The required frequency of maintenance is dependent on the watershed land use (e.g. urban, rural, farm, etc.), construction activities in the watershed and rainfall amounts and intensity. A pretreatment system such as a sediment fore-bay, however, can significantly reduce the frequency and extent of maintenance by removing settleable solids before the filtration practice. Also, maintenance of sediment fore-bays is easier than maintenance of a filtration practice. For guidance on maintenance and sediment fore-bay design, see the Minnesota Stormwater Manual.

Even with a pretreatment system, retained solids will eventually need to be removed from the filter. If the retained solids are at or near the surface of the filter media, the practice can often be repaired by removing the top 2 to 5 inches (5.08–12.7 cm) of filter media, roto-tilling the surface, and replacing the removed media with similar or approved alternative media (Minnesota Stormwater Manual). If this procedure does not resolve the problem, the entire filter bed may need to be replaced to restore functionality.

Macropores such as wormholes can cause short-circuiting of the filtration practice and subsequently result in reduced solids retention efficiency and less peak flow reduction. Macropore problems can be resolved by mixing the media bed or replacing it entirely.

For detailed recommendations on maintenance activities and when such activities are warranted, see the Maintenance section.

Continue to Infiltration Practices or click here for a list of references for the previous sections.