High-Rate Operation of Primary Sedimentation Tanks to Provide Additional Wet-Weather Treatment Capacity

As many regions of the country with combined sewers face the challenges of handling wetweather flows, maximizing peak flows through existing treatment works is a common goal. High-rate operation of existing primary sedimentation tanks provides one opportunity to increase treatment capacity during wet-weather. This paper reports the results of full-scale stress testing conducted at the Woods Run wastewater treatment plant owned by the Allegheny County Sanitary Authority (ALCOSAN) located in Pittsburgh, Pennsylvania.Surface overflow rates (SOR) ranged from 1,000 to 3,100 gallons/day/square foot (gpd/ft2) and performance was monitored by sampling and analyses of primary influent and effluent total suspended solids (TSS) and biochemical oxygen demand (BOD). TSS and BOD removals were affected by influent concentrations and not by SOR. This study concludes that SOR guidelines for capacity of primary sedimentation tanks should not be cause to overlook high-rate operation when site-specific testing may reveal additional treatment capacity.