Inactivation Of Pathogen Indicators In Surplus Sludge During Pot Experiment And Thermophilic Anaerobic Digestion

With the constant improvement of sewage pipe network and wastewater treatmentplant construction, the capacity of wastewater treatment increased, resulting in largeamounts of surplus sewage sludge. Because surplus sewage sludge contains highdensity of pathogens, it has to be treated to reduce pathogens before being disposed offor land application. The purpose of this paper was to investigate the inactivationbehavior of pathogens during pot experiment and thermophilic anaerobic digestion.The main conclusions were obtained as follows:(1) Firstly, we investigated the inactivation of pathogen indicators in dewateredsludge during short-term pot experiment in clay soil. The results shows that pH,ammonia nitrogen and available phosphorus in the mixed topsoil sludge variedsignificantly. The concentrations of ammonia nitrogen and available phosphorus in thesurface soil were always low compared to grass potted. Total coliform and fecalcoliform significantly increased in the surface soil when adding surplus sewage sludge.The grass in the soil can effectively kill pathogen indicators. Meanwhile, the totalnumber of total coliforms and fecal coliform in the soil with grassing was lower than thesoil without grassing, no matter adding the surplus sewage sludge or not. Salmonellawas not detected during in the pot experiment without adding surplus sewage sludge,but was detected in the pot experiment with adding surplus sewage sludge, indicatingthat, a short-term pot experiment can not effectively inactivated Salmonella.(2) Secondly, we investigated the effect of substrate concentration on pathogeninactivation during thermophilic anaerobic digestion. The results show that, with theincrease of substrate concentration, VFAs and cumulative methane production increased.The density of total coliforms in the suspension liquid has a2.0-3.0logs decline and offecal coliforms has1.8-3.3logs decline after28days thermophilic anaerobic digestionat substrate concentration of28-84g·L-1and55oC. More than99%of total coliformsand fecal coliforms have been killed after28days digestion. Salmonella spp. was notdetected in the suspension and solid after anaerobic digestion. When substrateconcentration was higher than45g·L-1, the inactivation of total coliforms and fecalcoliforms declined.(3) Thirdly, we investigated the influence of thermophilic anaerobic digestion ofsurplus sludge at optimal C/N ratio on methane production and pathogenic indicatorsinactivation. The results showed that thermophilic anaerobic digestion not only converted organic matter into biogas, but also effectively inactivated total coliforms andfecal coliforms in the supernatant and sludge. After28days thermophilic anaerobicdigestion, the density of total coliforms and fecal coliforms has a2.6-2.9logs of declinein the supernatant, and the density of total coliforms has a3.4-3.6logs of decline and offecal coliforms has a3.8-4.3logs of decline in the digested sludge. Salmonella spp. wasnot detected after thermophilic anaerobic digestion in the supernatant and the digestedsludge. Thermophilic anaerobic digestion can effectively inactivated pathogenindicators of suspension and solid at optimal C/N ratio.(4) Lastly, we investigated the influence of VFA and ammonia-nitrogen on theinactivation of pathogen indicators during thermophilic anaerobic digestion. Theresults showed that VFA is an important factor for the inactivation of pathogenindicators. After8days’ thermophilic anaerobic digestion, more than99%of totalcoliform and fecal coliform were inactivated. The initial concentration of ammonia haslittle effect on the inactivation of pathogen indicators during thermophilic anaerobicsludge digestion. While after28days’ thermophilic anaerobic digestion, more than99%of the total coliform and fecal coliform in the sludge was inactivated.