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Addressing Digester Foaming by Fixing Activated Sludge System and Gaining Capacity in Both
Author: Patricia Tam, Henryk Melcer, L. Emil Voges, Bill McCarthy; William D. Harlan (City of Richland WA)
Date: 10/07
WEFTEC Conference October 2007

The City of Richland Wastewater Treatment Facility digesters have experienced Nocardiainduced digester foaming problems from the time they went into operation in 1985. Because of the persistent foam, most or all of the thickened waste activated sludge (TWAS) was often not sent to the digesters but directly to the belt filter press to be dewatered separately from the digested sludge. It was observed that Nocardia foam was generated in the two complete-mix aeration basins. During a plant capacity assessment investigation, it was determined that one of the alternatives developed to increase the secondary system capacity coincided with one of the potential solutions previously identified to address digester foaming. One of the aeration basins was converted to plug flow operation with an anoxic selector and the existing turbine sparger aerators were replaced by panel-type fine pore diffusers. The upgrade included a classifying selector to provide physical foam removal from the aeration basin. The upgraded system has been in operation for over a year, and the plant has thus far reaped several benefits from the upgrade, including the ability to operate one aeration basin with adequate aeration, limited digester foaming, significant energy cost reduction, the ability to send all of the TWAS to the digesters, and consequently lower biosolids production rate and cost for disposal.