Question: The county engineer suggested we revise the drainage around our plant to separate our storm water from our process water. Why would I want to do this, and how would this affect my storm water permit?

Answer: Storm water refers to rain and melted snow that falls onto, and discharges off, plant property. When the precipitation falls onto non-plant-affected areas of the site, or those areas that do not contain source materials (materials which could conceivably contribute pollutants to storm water), many regulatory agencies do not require any specialized management or treatment the runoff produced. When runoff flows across plant areas or contacts source materials, producers are usually required by their storm water permit to manage via "best management practices" (BMPs) or otherwise treat the runoff. Should the runoff become commingled with process water generated at the plant prior to discharge, that runoff must then be treated as process water, which usually has considerably stricter discharge requirements. Keeping storm water separate from process water can reduce the severity of the discharge requirements. Should your permit require retention or detention of process water prior to discharge, separating storm water from process water can also reduce the size and volume of retention or detention areas. (Retention areas are impoundments that retain storm water for more than 24 hours. Detention areas do not have any permanent standing water, as they temporarily detain a portion of storm water runoff for up to 24 hours after a storm.) Under most storm water permits, producers are required to implement BMPs for their Storm Water Pollution Prevention Plans. The separation of storm water runoff from process water flow is a recognized BMP. Drainage ditches, curbs, swales, and kneewalls are structures that keep runoff water away from process areas.Many plants construct process drainage that keeps storm water runoff from stockpiles, scales, and haulway areas separate from concrete-processing areas. Since most of this water may be high in total suspended solids, remediation may only require a settlement area. This water often becomes a source of mixing water.Water from the concrete production area may have a pH level near 12.0 due to the concrete rinse water. Prior to discharge, the pH must be lowered. Limiting the amount of water in this treatment area helps reduce operating costs. Examples of good storm water management practices were submitted by CAMAS Colorado Inc.'s Flanagan Ready Mix plant in Broomfield, Colo., as part of an entry in the National Ready Mixed Concrete Association's Environmental Excellence Award 1996 contest.