Baltimore currently uses two new 11,300 m3 (3.0 MG) egg-shaped digesters (ESDs) and six old 4900 m3 (1.3 MG) cylindrical digesters to provide single-phase mesophilic anaerobic digestion of primary and waste activated sludges from its 680,000 m3/d
(180 MGD) Back River wastewater treatment plant. This paper describes studies addressing whether to modify or replace the cylindrical digesters and whether or not modern two-phase digestion is the best technology to employ. Modern, two-phase, anaerobic digestion consists of at least three
separate concepts: acid/gas phase, anaerobic stabilization thermophilic mesophilic digestion (ASTM) and temperature phased anaerobic digestion (TPAD). All of these processes generally claim to have advantages over similar sized single stage mesophilic digesters. The advantages include
greater operational stability, little or no foaming, higher volatile suspended solids (VS) destruction, higher gas production, better quality gas, better dewatering characteristics, clearer filtrate/centrate and the potential to produce a Class A biosolids. In addition, tank volume is
less so the facilities are more economical to build and operate. Each of these concepts are described, discussed and evaluated as they apply to Baltimore, including typical design parameters, typical performance and known full-scale installations.
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