King County and project team identified a new technology for treating CSO discharges. The technology (RapidStorm™ manufactured by OVIVO®) uses silicon carbide (SiC) membranes in conjunction with the addition of a chemical coagulant. A pilot project was conducted at the County’s West Point treatment facility.
The RapidStorm™ pilot unit supplied for testing included SiC membranes plates arranged in three stacks installed in a 28-foot-long by 8-1/2-foot-wide by 17-foot-tall steel tank. Ancillary equipment included permeate / backwash pumps, coagulant feed system, a chemical cleaning system, air scour blowers, online instrumentation, and remote communication hardware. The pilot received feed flow from the West Point Treatment Plant primary effluent channel and supplemented with fire hydrant water, as testing required, to simulate lower strength CSO influent.
Process and performance testing objectives for the pilot study included documenting water quality performance and providing a basis of design for full-scale project planning. Water quality was monitored through online instrumentation, grab sampling, and composite sampling during multiple test runs. Process and performance testing was initiated in September 2020 with a total of sixteen test runs completed by the end of November 2020.
The pilot was successfully tested at an average instantaneous flux rate of 100 gallons per square foot per day (gfd), a peak instantaneous flux rate of 200 gfd, and under a simulated CSO hydrograph without exceeding the maximum transmembrane pressures of 10 pounds per square inch (psi). A flux rate of 100 gfd in the pilot was equivalent to approximately 300,000 gallons per day of treatment capacity.
Effluent water quality results were favorable with total suspended solids (TSS) less than 5 mg/l, turbidity less than 0.1 NTU, and fecal coliform count consistently less than 400 MPN / 100 ml without supplemental disinfection.
The development of new innovative technologies such as the one tested in this project have the potential to reduce the receiving water impacts from CSO, SSO and even stormwater discharges.