They were designed when it became apparent that using the Best management practice (BMP) for rainfall runoff was not working.
The practice of removing snow from streets and off-loading it into rivers affects the aquatic ecosystems.
The oil and grit separator causes electrochemical and biological processes, which bind heavy metals and nutrients to the sediment by means of reduction-oxidation reactions.
The runoff then flows into a catch basin, where plants use the salt ions (e.g. K+, P3−, N3−) for their growth.
The runoff is further filtered by the plants, leaving only clean water to go back into streams and rivers.