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The Dirty Secret of Primary Sludge Prefermenters
Author: John Bratby, Stephanie Fevig and Jose Jimenez
Date: 10/12
WEFTEC 2012, New Orleans, LA, Sept. 29 Oct. 3, 2012

The impacts of the recycled solids load from primary sludge prefermenters on activated sludge BNR treatment processes are evaluated in this paper. A number of prefermenter modes were evaluated. Modifications to the prefermenter at the OWASA Mason Farm WWTP reduced fermentate TSS from approximately 2,500 mg/L to 280 mg/L. The costs for implementing prefermenters are included, as well as the costs for additional BNR reactor volumes required to accommodate fermenter TSS loads. The costs of purchased carbon sources are also presented. If the specific VFA generation rate is maximized then the cost of prefermenters is comparable to the cost of methanol addition. If the specific VFA generation rate is low then it is more cost effective to add methanol. In this case it would be even more cost effective to add acetic acid. The concept of sustainability related to prefermenters versus chemical addition is recognized, but was not included in the analyses.