Reactor performance and bacterial pathogen removal in response to sludge retention time in a mesophilic anaerobic digester treating sewage sludge

The effects of sludge retention time (SRT) on reactor performance and bacterial pathogen removal of sludge mesophilic anaerobic digestion (MAD) were investigated in a continuous stirred tank reactor. The average volatile solids removal remained around 20% and the biogas production rate varied from 100 to 132 ml/l d. The MAD process was efficient to remove Salmonella sp. and Escherichia coli with removal efficiencies increased with SRT from 11 d, 16 d to 25 d. However, the Shigella sp. removal was insignificant. The difference in the resistance of the three pathogens to sludge MAD process is helpful to the selection of pathogen indicators in the biosolids. Log reduction of pathogens determined by MPN was much higher than the data by quantitative PCR, suggesting the presence of viable but non-culturable pathogen cells. This study confirms that the control of appropriate SRT for sludge MAD should take both reactor performance and pathogen removal into account.