By Dr. Bob Andoh

For years municipalities around the globe have invested in stormwater treatment systems to remove pollutants from runoff. Whether they are used for stand-alone treatment or as pretreatment devices to keep our infiltration bays from clogging, these “sedimentation devices” have an important role in stormwater treatment. Yet a troubling problem has emerged and become more apparent: many proprietary treatment systems don’t hold what they catch. When flows increase, previously captured pollutants can re-suspend and discharge right out of the system. Moreover, many communities are not aware of this phenomenon — known as “washout” — leaving them with a false sense of security about the ability to keep their waterways clean.

By Prof. Bob Andoh

Many developers and municipalities have invested in filtering systems to meet the US EPA mandate for reducing these storm water runoff problems. Their experiences reveal a fundamental flaw in the design of conventional “down flow” filtration. The capital investment in these immense systems — sometimes spanning whole football fields in size — is tremendous and regular maintenance is cost-prohibitive.