| The advanced biological nutrients removal (BNR) processes have been widely used in recent years, resulting in a low organic (volatile solids, VS) content of waste activated sludge (WAS) from these processes. However, most researchers focused on the anaerobic digestion of sludge with high VS content, and not much has been reported on the anaerobic digestion of sludge with a low VS content. And the effect of anaerobic digestion on the dewatering property of sludge is paid more attention for most researchers.The two-phase anaerobic digestion equipments in laboratory and pilot scale were used in this study. The start-up of two equipments was examined. The mesophilic (35℃) two-phase anaerobic digestion of WAS with different VS contents was investigated during the steady operation, and the variation and impact factors of sedimentation and dewatering properties of anaerobic digested sludge were determined. The effects of ultrasonication treatment on the anaerobic digestion of WAS with low VS content were discussed. And finally, the kinetic models for the mesophilic (35℃) two-phase anaerobic digestion of sludge with different VS contents was established, the determined value from pilot-scale test was compared with the fitted curve from laboratory-scale test.The results from laboratory system showed that the stability was not greatly influenced due to the increase of organic loading rate (OLR), however, the VS removal efficiencies decreased with the increase of OLR, and the variation of deactivation efficiency of total coliform was the same as the VS removal efficiencies. The speciation analyses of heavy metals before and after anaerobic digestion showed that Cd, Pb and Cr had better stabilization efficiencies. The stabilization efficiency of Cu was enhanced and reached 98.4% after anaerobic digestion. The speciation of Zn and Ni were relatively average in the feeding sludge, however the stabilization efficiencies of them was lower. In addition, the stabilization efficiencies occurred mainly in the acidogenic phase other than the methanogenic phase.The thickened WAS and mixture of WAS with kitchen garbage were used to examine the mesophilic (35℃) two-phase anaerobic digestion of sludge with different VS contents. The results showed that the acidification efficiency of acidogenic phase was better than the others for the system with addition of kitchen garbage. There existed good stabilization for the methanogenic phases of three systems. The effluent VS and TCOD concentrations increased with the increase of OLR, and the corresponding VS and TCOD removal efficiencies decreased. The specific methanogenic acivity (SMA) of each system increased with the increase of HRT, and the methane content of biogas also increased. The change of sedimentation and dewatering properties was similar for three systems, the sedimentation and dewatering properties became poorer after the treatment of acidogenic phase, and were improved again after the treatment of methanogenic phase. The sedimentation and dewatering properties correlated well with the content of extracellular polymer (ECP) other than the concentrations of Ca2+ and Mg2+. The particle size of digested sludge became smaller, which was also an important influence factor of dewatering property.The combination of physicochemical treatment was used to improve the efficiency of subsequent anaerobic digestion of WAS with low VS content. According to the results from batch tests, the combined treatment of ultrasonication and lime was used as the pretreatment of anaerobic digestion. The effects of combined treatment on the efficiency of subsequent anaerobic digestion were investigated. The results showed that the stabilization of system increased with the increase of OLR. The VS removal efficiency decreased 5% when the HRT decreased from 15 d to 10 d, which suggested that the sufficient HRT must be maintained for the mesophilic anaerobic digestion of WAS with low VS content. The SMA of two phases was in a range of 0.35~0.38 L CH4/gVSrem at different OLRs, and the variations of biogas production and methane content were not apparent. However, the specific INT-ETS of acidogenic phase increased with the increase of OLR, and the value exceeded that of methanogenic phase.The results of pilot-scale test showed that the effluent VS concentration increased with the increase of OLR, and the corresponding VS removal efficiency decreased. The specific methane yield of acidogenic phase was stable under different OLRs, and that of methanogenic phase decreased gradually. The specific INT-ETS activity increased with the increasing of OLR for acidogenic phase, and that of methanogenic phase was stable. However, the dewatering property of digested sludge became poor with the increasing of OLR, and the CST value of acidogenic phase was higher than that of methanogenic phase, the change of dewatering property was similar with the sedimentation property. The analyses of heat balance for pilot-scale system showed that in the cold zone, the heat energy produced from biogas could compensate about 65.5 percent of heat exhausted for the operation of mesophilic TPAD process when the HRT was 10 days and the equipment was kept warm.The Chen-Hashimoto model was used to examine the kinetic characteristics of mesophilic two-phase anaerobic digestion under different influent VS concentrations. The results showed that the kinetic parameters ofμmax and K of substrate II was lower than that of substrate I when the influent VS concentration of substrate II was higher than that of substrate I, which suggested that there existed some inhibitory matter in substrate II. The kinetic parameters ofμmax and K of substrate III were both higher than the others. The comparison of determined value from pilot-scale test and fitted curve of laboratory-scale test showed that the VS removal efficiency and methane yield from pilot-scale test was lower than those of laboratory-scale test, however, the change trends of them were similar.
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