I never really thought about it before, but stormwater can have contaminants that will eventually get into the water system by traveling through the storm drain system and be emptied into bodies of water. The storm sewer does not transport water into a special holding area where it is treated; it simply takes it into a creek, river or other naturally occurring water system. As a result of this, there are concerns about what washes into the storm sewers system in the form of pollutants that can contaminate rain water as it washes over the ground and picks up chemicals that it picks up along the way. These pollutants on their own might be in small, insignificant amounts that do not have much of an impact on the environment in and of themselves, but the combination of vast array of pollutants entering the water system simultaneously and over time the cumulative effect of these pollutants can have damaging effects on the environment, which is why the use of stormwater filters may be suggested.
Stormwater filters can help to alleviate issues with pollutants by removing them to some degree from the water as it passes through the drain and into the storm system. The installation of stormwater filters into sewer inlets can be limited to areas where a higher pollution quotient would be expected, such as around an industrial facility or an automotive garage where fluids often leak from defective vehicles. Some areas have laws that require stormwater filters in areas flagged as higher risk. These laws show regard for the environment and may inflict penalties for modifying or circumventing stormwater filters installed in the drains. As with any filtration system, stormwater filters will require periodic maintenance or replacement when the filters reach their intended capacity, but the need for care certainly does not outweigh the benefits stormwater filters represent to the environment.