A project to expand the City of Garland Duck Creek Wastewater Treatment Plant from 20- to 40-mgd (average daily flow) was initiated in 2002 to provide for future growth and planned diversion of flow from the City's other treatment plant. The selected expansion configuration provides
a new stand-alone activated sludge system (aeration basins and final clarifiers) that will operate in parallel with the existing trickling filter/solids contact (TF/SC) system and treat 50 percent of the 40 mgd design flow. Three design features were incorporated into the design of
the new facilities to provide the requisite capacity while reducing the capital and/or operating costs of the activated sludge system. These features included 1) chemically-enhanced primary treatment (CEPT), 2) step-feed activated sludge (SFAS), and 3) anoxic zones in the bioreactor. This
paper describes each design feature, its impacts, and its economic benefits. An economic evaluation indicated that CEPT significantly reduced capital costs, but increased operating costs associated with chemical addition and added solids production. Step feeding resulted in a significant
reduction in required bioreactor volume, more than sufficient to cover the cost of the step feed facilities. Anoxic zones increased construction cost, but produced offsetting savings through reduced aeration costs and reduced solids production. Each feature was found to be cost-effective when
both capital and operating costs are considered.
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