| Anaerobic digestion (AD) plays an important role for its ability to produce energy-rich biogas, to stabilise the waste activated sludge (WAS) and to destroy pathogens. However, AD has several limitations: (a) the treatment efficiency of organic compounds is low, and (b) the processing speed is not high. In this study, a combined microwave/NaOH pretreatment (MNP) process was studied which focused on destruction of floc structures and lysis of microbial cells to release biodegradable organic materials and render them more accessible to AD for improving the anaerobic digestion performances of WAS. The study consisted of three parts: MNP, biochemical methane potential (BMP) tests and semi-continuous anaerobic digestion experiments.Firstly, in the MNP tests, uniform design was successfully applied to determine the relationship of WAS solubilization of COD, SS and VSS to environmental conditions (target temperature, microwave holding time, and NaOH dose) and to establish the quadratic model describing the solubilization ratios to changes in these variables. Within the design boundaries, the predicted maximum solubilization ratio of VSS (85%) occurred at target temperature of 210℃, holding time of 35min, and NaOH dose of 0.2g-NaOH/g-SS. The results showed that the predicted values inosculated with the experimental values very well.Secondly, thermophilic and mesophilic BMP tests were studied in order to investigate the effects of 12 kinds of MNPs on cumulative methane production, cumulative biogas production, relative cumulative methane production and methane production rate. The kinetic parameters achieved from the modified Gompertz equation showed the cumulative methane production potential (P) and the maximum methane production rate (R) of the pretreated WAS were higher than those of the controls. In the thermophilic BMP tests, compared with the control, the peak values of P and R of the pretreated WAS were 333 mL-CH4 @STP·g-1-VSadded (at the standard temperature and pressure (STP) conditions of 0℃and 101.325 kPa) and 73 mL-CH4@STP·g-1-VSa dded·d-1,respectively, which were improved by 41% and 28%, respectively. In the mesophilic BMP tests, compared with the control, the peak values of P and R of the pretreated WAS were 426 mL-CH4 @STP·g-1-VSa ddedå'Œ56-CH4 mL@STP·g-1-VSadded·d-1,respectively, which were improved by 64% and 75%, respectively. After we considered various factors comprehensively, 170℃with 1 min holding time and 0.05 g-NaOH/g-SS were suggested for MNP of WAS.Thirdly,to evaluate anaerobic digestion performances and the dewatering performances of the anaerobic digested sludges, the continuous stirred thermophilic anaerobic digestion process (TADP) and mesophilic anaerobic digestion process (MADP) were operated at HRT (hydraulic retention time)=30 d to treat raw WAS and pretreated WAS (pretreated at 170℃with 1 min holding time and 0.05 g-NaOH/g-SS), respectively. Then the microbial community structures in the thermophilic and mesophilic anaerobic digested sludges were analyzed with 16S rDNA (ribosomal Deoxyribonucleic Acid) cloning techniques. The results showed, compared with the controls, removal efficiencies of VS, TCOD (total chemical oxygen demand) total carbohydrate and total protein were improved in TADP and MADP treating pretreated WAS. The VS removal efficiencies of TADP and MADP treating preteated WAS increased by 27% and 21% respectively as compared with their controls. Compared to the corresponding controls, the SCOD (soluble COD) levels of thermophilic and mesophilic anaerobic digested sludges were enhanced by 51% and 71%, respectively. The methane production rates (L-CH4 @STP /g-CODadded) of TADP and MADP treating preteated WAS improved 13% and 14% repectively when compared to their controls. Both methane contents in the biogas of TADP and MADP treating preteated WAS increased by approximately 8% compared with those of their controls. The analysis results of the microbial community structures analyzed with 16S rDNA cloning techniques showed, the microbial diversity indexes were 53.2 and 138.0 for thermophilic and mesophilic anaerobic digested sludges, respectively. This meant that the microbial community of TADP had less diversity than that of MADP. The results of dehydration experiments showed, the dewaterabilities of digested sludge from TADP and MADP treating preteated WAS were lower than that from the controls. |
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