Abstract
Abstract Achieving microbial compliance during biosolids storage can be complicated by the unpredictable increase of Escherichia coli. Thermal treatment during anaerobic digestion (AD) and the effects of dewatering may be a significant factor contributing to indicator survival. Shear forces present during dewatering may promote cell damage, releasing nutrient for E. coli growth. The effect of cell damage on E. coli survival was assessed in laboratory-scale thermal and physical disruption experiments. E. coli growth curves for disrupted treatments were compared with control conditions and quantified using flow cytometry and membrane filtration techniques. A significant difference (p < 0.05) in the level of damaged cells between control and disrupted conditions was observed. For thermal and physical disruption treatments, the peak of E. coli concentration increased significantly by 1.8 Log and 2.4 Log (CFU (colony forming units) g−1 DS), respectively, compared with control treatments. Research findings contribute to the understanding of bacterial growth and death dynamics in biosolids.
Highlights
Monitoring levels of pathogenic indicators in biosolids validates the effectiveness of treatment processes and ensures the safe recycling of biosolids to agricultural land (European Commission 2015)
Arising green colonies were counted as presumptive E. coli, yellow colonies were counted as coliforms, and pink colonies were recorded as non-coliforms
For samples exposed to a greater degree of thermal disruption, a longer recovery time was necessary for E. coli populations to return back to pre-disrupted levels or above
Summary
Monitoring levels of pathogenic indicators in biosolids validates the effectiveness of treatment processes and ensures the safe recycling of biosolids to agricultural land (European Commission 2015). Escherichia coli indicator concentrations in the digested liquid sludge tend to be (in the case of study sites examined here) approximately 4 Log (CFU (colony forming units) gÀ1 DS) After dewatering operations, in the first 48 h of storage, indicator concentrations have been observed to increase rapidly, reaching levels of approximately 7 Log (CFU gÀ1 DS) in some cases (operational data, Severn Trent plc) This observation is not isolated and has been observed on other treatment sites (Monteleone et al 2004; Higgins et al 2007a; Qi et al 2007; Chen et al 2011). This rapid increase in E. coli levels may indicate the presence of pathogenic bacteria within the biosolids product and is a concern for utility operators
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