Two-phase anaerobic digestion for class a biosolids at moccasin bend WWTP

The Moccasin Bend Wastewater Treatment Plant (WWTP) provides treatment for the City of Chattanooga, Tennessee and the surrounding region. The City has completed upgrades to the solids handling facilities to treat and dewater the primary solids and waste activated sludge (WAS) from the WWTP. This paper summarizes the process to handle the primary solids and the start-up process. Primary solids digestion is designed to handle approximately 47 dry tons per day.

After thickening, the primary solids are digested in a two-phase anaerobic digestion process prior to dewatering through centrifuges. The first phase is thermophilic digestion at temperature of 135º F to 140º F followed by mesophilic digestion at 95º F. Mixing is accomplished by Cannon® gas bubble mixers, six per digester. Liquid-ring gas compressors draw biogas from the digesters and provide mixing via large gas bubbles through the Cannon® mixers. Heating is provided by redundant boilers to provide hot water to heat solids-to-water heat exchangers.  Total cost for the primary solids digestion system was approximately $12.4 million including equipment procurement and installation contracts. The process is designed to produce Class A biosolids as defined by US EPA regulations (EPA 1993).

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