Understanding Bio-Filtration

Bio-filtration is a method of capturing a site’s pollutants carried by stormwater runoff.  It was first developed by Prince George’s County (PGC), Maryland, Department of Environmental Resources in the early 1990’s as an alternative to traditional BMP structures. The EPA, Environmental Protection Agency, is the biggest proponent of BMPs. Bio-filtration is a practice that manages and treats stormwater runoff using a conditioned planting soil bed and planting materials to filter runoff. The method combines physical filtering and adsorption with biological processes. The system consists of a flow regulation structure, pretreatment filter strip or grass channel, sand bed, pea gravel overflow curtain drain, shallow ponding area, surface organic layer of mulch, a planting soil bed, plant material, a gravel underdrain system, and an overflow system.

In recent years, bio-filtration has been employed in parking lots, along highways, in shopping centers, and all having mixed results, some being very successful.

Areas that use bio-filtration don’t necessarily use the best growing medium. More and more bio-filtration areas are using very sandy soil which lack nutrient rich elements necessary for successful plant growth. This is why these areas are back-of-house on large sites, roads and highways, and areas that quiet frankly don’t need dressing up.

Another method of runoff capture

Besides capturing runoff, bio-filtration seeks to accomplish the following, via the Department of Environmental Quality of the State of Oregon:

“Bioswales and constructed wetlands are being used more and more to address pollutants in storm water runoff. Many installations of these BMPs have failed or have not been as successful as was hoped when their use was first contemplated. Most of the limited success or failures can be attributed to insufficient information being available or to bad or no expert input into the design, construction, vegetating, or maintenance of the bioswale or constructed wetland.”

As naturalized bio-filtration areas continue to be popularized, their benefits grow. Revegetating and reconstructing the environment by re-introducing native plants back into nature, those plants clean the runoff. Collecting runoff is one thing, but to clean it before it enters streams, lakes, and tributaries is the penultimate goal. Let’s continue to clean and collect the water that we so dearly need to retain.

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