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Recent research provides ample indication that a conventional, single-stage flow train is not the optimal configuration for realizing the full potential for solids stabilization and pathogen reduction in mesophilic anaerobic digestion--and it is now time to put these research findings into practice. Temperature Phased Anaerobic Digestion, two-phase digestion and other advanced digestion process trains may be too ambitious for some, but simple staging of anaerobic digesters would be a good start in improving digestion performance. Staging coupled with draw-and-fill feeding would be even better. Encouragement to do so is provided by a twostage mesophilic digestion system in Rockford, Illinois that has been beating performance expectations in over four years of operation. This paper discusses the Rockford installation, plus a new two-stage digestion system currently in design for the Minneapolis-St. Paul metropolitan area. The anticipated system operation and the design adjustments required to accommodate future operation in a two-stage mode are reviewed.

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With increasing energy costs, wastewater utilities are becoming more interested in reducing purchased energy requirements (natural gas and electricity) or substituting with less expensive sources of energy that may be generated on site. This is particularly true for utilities implementing thermal drying, an energy intensive process. Biogas from anaerobic digestion is used in several installations to offset some of the purchased energy requirements for drying. But is this the best use for the biogas? Would using the gas to generate power and recovering waste heat from that process be a better use? These questions and others related to energy management and alternative energy options will be addressed in this paper.