With respect to effluent…

With respect to effluent targets (TSS and Oil) there has been a comparison to construction dewatering activities. Construction dewatering and treatment is done with pumped systems. A very large dewatering system at 400 gpm is only treating 25 l/s. Stormwater treatment often requires treatment flows in the range of 100 l/s+ to treat the 90th percentile storm. To consistently achieve a 25 mg/l effluent for all flows, a pumped system would be required with filtration down to the 3-5um size range. This is a result of the unknown and wide variability of suspended solids in stormwater. Further, a pumped system might operate at a nominal pressure of 10-15 psi. In ft of head this is over 40ft. Stormwater treatment systems operate on gravity with weirs typically ranging up to 3ft (1.3 psi). A comparative operating head is simply not available. While current stormwater treatment units can demonstrate TSS effluent less than 25 mg/l, the performance will vary by site. For example, a system achieving 80% TSS removal on a site with influent TSS concentration of 200 mg/l would have an effluent concentration of 40 mg/l.

Oil-Grit Separators (that are designed for oil capture) are an effective means to capture oil spills. Low concentrations of oil will have an oil droplet size distribution in the micron size range. To guarantee capture of these small droplets down to 15 mg/l a coalescing plate settler (or other coalescing media) is required.

Given the variability of stormwater, the historical solution of pointing to laboratory and field testing standards would seem the most practical. Further consideration of documented filter longevity during testing could help to develop guidelines for sizing to a minimum maintenance cycle to ensure performance.