Contaminant Fractionation by Particle Size: An Effective Tool for Improving Filtration Effectiveness for Compliance With Stringent NPDES Permits for Stormwater Discharges

The current NPDES permit of an industrial aerospace complex located on Ventura
County, California requires to provide treatment to any stormwater overflows originated
at the complex typically during storm events. These overflows occur when the individual
capacities of a series of storage ponds located at different points of the site are exceeded.
A stormwater treatment system (STS) is required to avoid exceedances of certain
constituents of concern (COCs), especially certain heavy metals (iron, manganese,
copper, mercury, lead) and organics (dioxins and volatile organic compounds – VOCs),
that may washout from the soil during storm events. Coarse filtration (sand and
multimedia filters) was the basis of the STS, but did not achieve the required removal of
COCs to meet the permit. Additional analyses were required to increase filtration
effectiveness. Fractionation of the solids present in the water revealed the presence of
large fractions of silts and clays, which are too small to be removed by coarse filtration.
These small solids also had a significant fraction of the total COCs attached to them,
which prompted the exceedances of the permit limits. This analysis highlighted the need
for coagulation chemicals to clump these small particles together and form flocs that may
be further removed by coarse filtration while meeting permit limits. This analysis also
allowed to better characterize the raw water from different storage ponds and select the
location with the best water quality to feed the STS.