SIDE-BY-SIDE COMPARISON OF STEP FEED AND PLUG FLOW ACTIVATED SLUDGE PROCESS PERFORMANCE AT THE ROCK CREEK ADVANCED WASTEWATER TREATMENT FACILITY

Driven by continuing rapid growth, potential site space constraints, and pressures to reduce cost, Clean Water Services (formerly Unified Sewerage Agency of Washington County, Oregon) decided to evaluate the performance of the step feed activated sludge process on a demonstration basis at its Rock Creek Advanced Wastewater Treatment Facility. Construction and startup of a new 1.7 million gallon three-pass step feed demonstration aeration basin was completed in June 2001. Performance of the step feed basin was compared with that of an existing plug flow aeration basin of identical size.Testing was conducted in two modes. Mode 1 involved demonstration of comparable SRT and performance with higher loadings to step feed, while Mode 2 involved higher step feed SRT and comparable performance under similar loading conditions.Testing results demonstrate that the step feed process may be flexibly used to either achieve higher capacity (flow) at comparable sludge ages, or to achieve a higher level of treatment (higher sludge age) at comparable flows, while maintaining comparable solids loading rates to the secondary clarifiers. The results also show that the step feed system achieved a better and more reliable effluent quality as measured in this case by lower effluent ammonia-N concentrations. Results of this testing support the recommendation for future plant-wide conversion from plug flow to step feed at the Rock Creek facility.