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DEM Approves Stormwater Treatment Device as a Water Quality Best Management Practice in RI

PROVIDENCE – The Rhode Island Department of Environmental Management (DEM) has approved the Filterra® — a proprietary stormwater treatment device that uses regionally acceptable trees or shrubs typically housed in a concrete box to filter and treat stormwater runoff — for use as a water quality best management practice (BMP). The certification issued by the DEM's Office of Water Resources is effective for five years starting June 1, 2021.

The product incorporates an engineered soil media designed with a very high infiltration rate, making it ideal for urban sites with limited amounts of space for stormwater BMPs.

Precipitation in an urban or suburban area that does not evaporate or soak into the ground but instead runs across the land and into the nearest waterway is stormwater runoff. It consists of harmful pollutants like fertilizer, pet waste, chemical contaminants like pesticides, leaking fuel, and motor oil, litter, and sediment such as dirt and sand. This mix of nutrients and contaminants can fuel the growth of harmful algae blooms that reduce oxygen levels and cloud the water, blocking sunlight from reaching underwater vegetation. It can cause beach closures, shellfish closures, and other problems.

"Increased development across the Narragansett Bay watershed has made stormwater runoff the biggest source of pollution to our coastal waters," said DEM Acting Director Terry Gray. "Cleaner beaches and shellfish beds start with better stormwater control. Filterra is a type of green infrastructure that is now approved by RIDEM as a proprietary water quality technology to provide high levels of pollutant removal for stormwater runoff. Small-footprint stormwater treatment systems, like Filterra, are ideal for retrofit applications and meeting water quality goals in urban areas with limited site space or on sites with other constraints such as high water table."

The Filterra® is sold by Contech Engineered Solutions LLC. They have demonstrated via third-party testing conducted in accordance with TAPE (Technology Assessment Protocol – Ecology) that it can remove 85% of total suspended solids, 30% of total nitrogen, 30% of total phosphorus, and 60% of pathogens from stormwater runoff. The models that are approved for use in Rhode Island include the Filterra® Peak Diversion, Offline Filterra®, and the Filterra® Bioscape Boxless. These products are intended for commercial and municipal use.

Proprietary devices that are approved as water quality BMPs in Rhode Island must be designed to treat the first inch of stormwater runoff produced by a site's impervious surfaces (such as roadways, roofs, and parking lots) during a rainfall event. Treating the first inch of runoff from impervious surfaces is essential because it provides stormwater treatment for around 90% of annual rainfall events and ensures that BMPs capture the "first flush" of a storm, where most of the pollutants can be found. Approved proprietary devices also must be designed, built, and maintained to meet the conditions specified in their technology-specific RIDEM certification letter.

The review of the Filterra® application was conducted by members of the RIDEM Office of Water Resource's stormwater engineering program along with members of the RI Department of Transportation and Coastal Resources Management Council. DEM is proud to promote and contribute to the advancement of stormwater treatment technology in hopes of protecting Rhode Island's precious water resources by minimizing the environmental impact caused by land development.

To view the list of RIDEM approved proprietary stormwater technologies and their associated certification letters, click HERE. For additional information on the RIDEM stormwater technology review program, contact Christopher Dill, EIT, at 401-222-4700 ext. 2777404 or

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  • Department or agency: Department of Environmental Management
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  • Release date: 10-07-2021

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