a method and system for treating stormwater runoff containing impurities includes collecting the runoff in a basin and allowing the runoff to settle in the basin for a predetermined time before allowing the filtration step to be initiated . the time delay is controlled by a controller sensitive to rain fall , turbidity , or other variables selected by the user . during the filtration step a separator member is positioned floatingly between a filter element and impurities floating on the surface of the unfiltered water .