| Pathogens in sewage sludge represent potential risks to human and animal health. Anaerobic digestion is thought to be effective in reducing pathogen concentration. The main objective of this research was to investigate the E. coli, Salmonella spp. and Shigella spp. contents in the continuous-flow mesophilic anaerobic digestion (MAD) digester at different SRTs, VFA concentrations and pH value by Most probable number (MPN) and qPCR methods. The reactor performance including effluents quality, methane yield and VS reduction were also studied. Results were shown as follows:1. The effects of sludge retention time (SRT) on reactor performance and bacterial pathogen removal of sludge 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 spp. and E. coli with removal efficiencies increased with SRT from 11d, 16d to 25d. However, the Shigella spp. 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.2. In a neutral condition, the effects of VFA concentration on reactor performance and bacterial pathogen removal of sludge MAD were investigated in a continuous stirred tank reactor. The results show that the MAD process was efficient to reduce E. coli and Salmonella spp. contents with the removal levels more than 2 orders of magnitude. The additive VFA concentration of 1000–6000 mg/L had insignificant effect on the inactivation of E. coli. and Salmonella spp.. However, as revealed by the first-order kinetics model, the first-order decay rate coefficient of Salmonella spp. decreased from 0.52 to 0.29 with the VFA concentrations increased. The inactivation of Shigella spp. was inefficient during the MAD process studied.3. In an acid condition, the effects of VFA value on reactor performance and bacterial pathogen removal of sludge MAD were investigated in a continuous stirred tank reactor. At the pH value of 5.0, VS has been accumulated in the reactor, which lead to the decline of pH. Methanogens lost living completely, no evident gas production and VS remove has been found. Reactor operating performances have been destroyed by high VFA concentration in acid conditions. The removal rates of pathogenic bacteria have been improved obviously. Particularly, salmonella spp. has been removed completely and the removal rate achieves the level of Class A.4. The research focus on the effect of pH to the survival of pathogen with high VFA concentration existed. In the study of the control of pH in the feed sludge with the additive VFA concentration of 6000 mg/L, When pH was adjusted above 6.0 (including 7.0), the gas production can still maintain a higher level, but VS removal rate was reduced with the pH. The VS removal rate keep steady at 13%at the pH6.5, but the removal VS at pH6.0 was not very obvious.The romoval rate of pathogens was generally lower (around 1 log MPN/g DS).When pH was adjusted between 5.5 and 5.0, as methanogens lost activity completely, basically no gas production and VS reduction was found. But the reduction rates of three pathogens increased greatly. The removal rate of E.coli was up to 2.78 and 3.30 log MPN/g DS, respectively. Salmonella spp. was removed completely after 20 days. The removal rates of Shigella spp. were 1.48 and 2.03 MPN/g DS, respectively. |
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