First Flush Stormwater Capture

First flush stormwater capture intercepts the initial runoff from handling areas, which often contains higher pollutant concentrations than later runoff (Maniquiz-Redillas et al., 2022).

This may include, among others, first flush diversion systems, sedimentation basins or treatment ponds, oil water separators, drainage systems directing runoff to treatment facilities, reuse of treated water for dust suppression, and simple containment pits for smaller ports.

For DMC ports, first flush stormwater capture can prevent contaminated runoff from entering waterways, reduce pollution of marine environments, protect fisheries and coastal ecosystems, support compliance with discharge regulations, and enable water reuse opportunities.

Stormwater runoff from materials handling areas can carry significant pollutant loads, including, among others, suspended sediments, hydrocarbons, heavy metals, cargo residues, and accumulated dust.

The first flush phenomenon, documented in peer reviewed research, refers to the higher pollutant concentration typically found in the initial portion of stormwater runoff after a rainfall event, as accumulated pollutants are mobilized early in the hydrograph (Maniquiz-Redillas et al., 2022).

First flush capture systems are designed to intercept this initial higher pollution portion and divert it for treatment or containment, while allowing cleaner subsequent flows to bypass the treatment system. The volume captured is typically sized based on catchment area, local rainfall patterns, and pollutant buildup rates, with first flush design depths set by regulations often at 6.35 or 12.7 millimeters (0.25 or 0.50 inches) of runoff, although criteria vary by jurisdiction (Froehlich, 2024).

Treatment options for captured first flush include, among others, sedimentation basins that remove suspended solids, oil water separators that remove hydrocarbons, filtration systems (sand, activated carbon, or specialty media), and biological treatment for specific pollutants. Treated water may be discharged to waterways under permit conditions or reused for dust suppression, landscape irrigation, or other non potable applications.

Source control reduces pollutant buildup on surfaces, while first flush capture manages the residual runoff. Gao et al. (2023) review first flush stormwater pollution in urban catchments and the optimization of control measures.

For DMC ports, first flush systems are particularly important given the intense rainfall patterns experienced in many DMCs and the proximity of many ports to sensitive marine environments. The PEMSEA Port Safety, Health and Environmental Management (PSHEM) Code, which applies to all types and sizes of ports, provides a management system framework whose environmental programs cover surface water and groundwater quality (PEMSEA, 2012).