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“Better Treatment Focusing on SRT Not Solids Inventory"
Author: Eric J. Wahlberg, Barbara Browne, Noble Fulcher, Biju George, Donald Linn, Larry Scanlan, Dan Siler, Wendy Anderson, Mike Daniels, Steve Rogowski and Steve Walker.
Date: 7/209
WE&T, Vol. 21, No. 3, March 2009, pp. 30-39

The activated sludge process is widely used because of its performance, versatility, and controllability. Unfortunately, its controllability also may be the source of operating problems, including poor solids settleability and noncompliant effluent quality. Even when properly designed, operated, and maintained, an activated sludge process is expensive and will only become more so as power costs increase. given this expense and biosolids disposal challenges, treatment plant owners and operators must shift their focus. rather than simply “making permit” (meeting the national Pollutant Discharge Elimination System [NPDES] permit requirements that regulate effluent quality), owners and operators should strive to “make permit and produce biosolids as cost-effectively as possible.” This goal better accounts for the environmental, economic, and energy pressures today’s owners and operators face. To meet this goal, owners and operators must optimize their wastewater management systems. optimization often is defined as “doing more with less.” Historically, municipal treatment plant personnel have assumed this meant “minimize operating expenses” and spent much time and effort attempting to do so. Unfortunately, some cost reductions did not lower overall treatment costs. Inspired by The Goal: A Process of Ongoing Improvement (E.M. Goldratt and J. Cox, 2004), staff at the metropolitan Sewer District of Greater Cincinnati (MSD) and the Denver Metro Wastewater Reclamation District (MWRD) used a different approach to optimize their activated sludge processes. They had different reasons for doing so. MSD wanted to optimize four treatment plants — mill creek, Little Miami, Polk run, and Sycamore — to reduce operating costs while maintaining effluent quality. MWRD, on the other hand, focused on maintaining effluent compliance during permit-complying upgrades and lengthy construction activities involving prolonged process and equipment outages.

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