Microaerobic pretreatment of extended air sewage sludge for the enhanced destruction of pathogenic bacteria in aerobic digestion
Creators: name: Sherif, Ranya
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2007
Subjects: subject: Engineering, Sanitary and Municipal.
subject: Engineering, Environmental.

description: The inactivation of pathogens in sewage sludge reduces the risks of infection through contaminant pathways associated with handling and disposal. Domestic sludge sourced from a rural treatment plant was found to contain high levels of the indicator microoganisms E. coli and fecal coliforms and pathogenic bacteria Salmonella spp., Shigella spp., and C. perfringens. An effective and simple approach to enhance pathogen removal in a rural treatment setting was desired. Existing literature suggested that draw/fill staged configurations tended to yield better inactivation rates. Other literature suggested that the build-up of inhibitory compounds such as VFAs were detrimental to pathogens and that VFAs could be accumulated in higher temperature microaerobic conditions. The investigation of microaerobic pretreatment was trialed as a novel approach to staged digestion for improved pathogen removal. Microaerobic pretreatment of aerobically digested sludge improved inactivation of aerobic bacteria but the inactivation of persistent spores of C. perfringens were inconclusive. Microaerobic pretreatment alone was investigated in three phases of the experiments and did not result in inactivation greater than one log reduction for any bacteria monitored. In Phase I where feed solids concentration was varied across four reactors, the lowest solids loading of 1.1% TS showed the best removal rates of pathogens. In Phase II, contact time was evaluated in terms of feeding frequency and residence time. It was found that less frequent feeding and longer residence times were more effective in removing pathogens as expected from the reactor kinetics and suggested by the literature. The impact after digestion was found to be significant in Phase III for fecal coliforms, E. coli, Salmonella spp. and Shigella spp. It appeared that changes to the sludge matrix in microaerobic pretreatment improved digester performance in terms of pathogen removal. Operating variables were monitored to gain an understanding of the factors impacting performance. Statistical analyses were performed at the 90% confidence interval to determine which if any factors differed significantly between systems and stages. The major findings were that mesophilic pretreatment (35°C) with air supplied at 0.06vvm yielded significantly higher ammonia levels after pretreatment than did pretreatment at ambient temperatures. This was hypothesized to account for the significantly greater extent of nitrification observed in downstream aerobic digestion over the effluents from digestion without pretreatment and those that were pretreated under ambient microaerobic conditions. Accordingly, the pH in those digesters was significantly lower than after digestion without pretreatment and after digestion with ambient microaerobic pretreatment. This enhanced depression in pH was hypothesized to account for the significantly enhanced inactivation in pathogens.