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Proceed with Caution in Advanced Anaerobic Digestion System Design

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From September, 2006 to July, 2008, the Madison Metropolitan Sewerage District operated an advanced digestion system including acid phase digestion followed by thermophilic and mesophilic digestion stages in series. The system configuration was a modification of the original TPAD design and resulted from initial start-up problems in early 2006. Additional problems were encountered during the two years of facility operation.

The original concept for the upgraded digestion system was an operation consisting of batch, fill and draw cycles at 12 hour intervals using three existing reactors operated at the thermophilic temperature of 57 deg C. Heat would be recovered by reducing the temperature in transfer to mesophilic digesters. The goal of the digestion system design was to produce a Class A sludge within the digestion system itself. Thus any portion of the sludge could be dewatered and used to create a dry product for recycle. The District had operated a successful liquid land application program for over 30 years. The need for additional winter storage, and narrowing windows of time for land applying the liquid product were making the production of a dry Class A product within the digestion system a cost justifiable alternative.

Because of grease coating raw sludge heat exchanger tubes and rags plugging the heat exchanger tubes the process was modified to incorporate acid phase digestion as a first phase preceding the thermophilic digesters. This modification met with some success from a volatile solids and pathogen reduction standpoint. Operation of the system, though, was hampered by an assortment of additional problems including struvite (magnesium ammonium phosphate) and vivianite (ferrous phosphate) scaling, acid phase digestion failure, and poor gas quality from the thermophilic digesters resulting in engine failure. The operation has since been returned to single stage mesophilic digestion while solutions for problems encountered are being investigated.
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Keywords: acid phase; anaerobic digestion; grease; mesophilic; rags; struvite; thermophilic; vivianite

Document Type: Research Article

Publication date: 2009-01-01

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