San Francisco's Bayside combined sewer system is allowed one to ten overflows a year. Underground concrete boxes in the City's periphery, which retain and transport water to the Southeast Plant and North Point (NPP) Wet Weather Plant during and after storm events were built
to consistently meet these limits. The capacity of NPP is the limiting factor in the efficiency of this scheme, but expansion of this facility is difficult due to the space constraints. High rate clarification (HRC) processes have been found to be highly competitive for CSO pollutant removal.
Using this technology, the NPP could be upgraded to significantly increase capacity in its current location. Pilot testing of HRC technologies was conducted as part of the NPP Upgrade Project. The two high rate clarification processes tested in this pilot study are well suited for enhanced
primary treatment of combined-sewer wastewater. Suspended solids removal in excess of 80% of influent concentrations can be achieved consistently. COD and BOD5 removal is better than 60%. Optimal treatment is achieved in a matter of minutes when the units are started
full but may be delayed for up to one-half hour if the unit must start empty.
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