TEMPERATURE-PHASED ANAEROBIC DIGESTER STARTUP

The Waterloo, Iowa municipal wastewater treatment facility has an average day design capacity of approximately 20 MGD. Current average flow is approximately 13 MGD.The facility had six anaerobic digester tanks constructed in 1939. The tanks were modified for temperature-phased anaerobic digestion (TPAD) operation in 2002. Improvements included new covers, mixers, heat exchangers, recirculation pumps, and instrumentation and controls to handle a combination of primary sludge and thickened waste activated sludge. A new hot water system for heating the digesters and gas handling equipment were also included in the project. TPAD process offers higher volatile solids destruction, better dewaterability and the potential for producing a Class A product compared with conventional single stage mesophilic digestion. This paper describes the procedure for bringing the digester system to a temperature-phased anaerobic digester operation whereby two of the digesters receive raw sludge in a thermophilic mode, and forward the digesting biosolids to the remaining four digesters in a mesophilic mode.The process was initially started up totally as a mesophilic system. The daily load of raw sludge is split between two first-stage digesters. The two first-stage digesters feed sludge to the remaining four second-stage digesters. When all equipment, instrumentation, and controls were properly functioning in mesophilic mode, one of the first-stage digesters was elevated to thermophilic mode. The temperature in the digester undergoing conversion from mesophilic to thermophilic mode was raised as fast as possible as required to destroy thermo-resistant mesophilic bacteria. Thermophilic methanogenesis relies more on acetate oxidation at low volatile acid concentration than does mesophilic Methanogenesis. (Ahring, 1994) During the week prior to temperature conversion, the raw sludge loading to the digester undergoing conversion was reduced while the loading to the other first-stage digester was increased. During the conversion, sludge loading to the converting digester were further reduced in reaction to observed changes in pH, volatile acids, and alkalinity, while raw sludge loading to the other digester was further increased. After the converting digester reached thermophilic conditions, the digester parameters eventually returned to normal. Following successful startup of the first thermophilic digester, the second thermophilic digester was converted to thermophilic operation in a similar fashion.Thermophilic digestion has resulted in greater volatile solids destruction and increased gas production.The paper will review detailed startup procedures for converting from mesophilic operation to thermophilic operation, including results of process monitoring and adjustments for the operation.