Water Environment Federation (WEF)

Pre-Treatment of Sludges Before Aerobic Digestion to Enhance Sludge Quality

Bench scale experimentation was completed to assess the potential of using a short residence time pre-treatment reactor upstream of aerobic digestion to enhance the destruction of pathogens. The impact of aeration, temperature, hydraulic residence time, solids concentration, feeding frequency and mechanical shearing on the pre-treatment process was investigated. Subsequent testing evaluated the operation of selected pretreatment conditions in a staged configuration with a conventional aerobic digester. Either highly aerobic or highly reducing conditions were observed to be most effective in reducing the concentrations of E. coli. and Salmonella spp. in the pre-treatment reactor. When operated in series with the aerobic digesters, the more highly reducing conditions in pre-treatment were found to enhance die-off of the organisms in the subsequent aerobic digestion.

Aerobic digestion is a common sludge treatment technology that has been employed for solids reduction, vector attraction reduction and pathogen destruction in smaller wastewater treatment plants (typically < 0.2 m3/s) (Metcalf and Eddy, 2003). The performance of traditional aerobic digestion with respect to pathogen reduction may not always be acceptable. A study of full scale plants in Ontario (CG&S, 2000) revealed that aerobic digestion only achieved 0.5 log reductions in fecal coliform concentrations. While aerobic digestion has been employed for a considerable period of time there have been relatively few enhancements to the technology to improve sludge quality. Autothermal thermophilic aerobic digestion (ATAD) processes have been employed at some facilities to produce USEPA Class A biosolids (Kelly, 1990). The implementation of ATAD technology would however require considerable upgrading of many existing aerobic digester facilities in Ontario. The use of staged aerobic digestion has been reported to result in improved solids stabilization and improved pathogen reduction (Daigger and Bailey, 2000). There has however been relatively little implementation of staged aerobic digestion in Ontario. This study examined the use of a short residence time reactor that was employed upstream of aerobic digestion with the objective of reducing the pathogen content of the product biosolids. The underlying premise of the...

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