Mechanism analysis to improve sludge dewaterability during anaerobic digestion based on moisture distribution

Previous studies have demonstrated that anaerobic digestion (AD) enhances sludge dewaterability. However, the mechanism of AD influence on digested sludge dewaterability is still not well understood. In this study, moisture distribution and bond energy were used to evaluate the influence of AD on sludge dewaterability. The change from free moisture (FM) to mechanically bound moisture (MBM) was analysed, along with degradation of organic fractions in extracellular polymeric substances (EPSs) of the sludge during AD. Results indicated that the AD process, with a 41.9% reduction in volatile solids (VS) and cumulative methane production of 283.2 mL/g VS, improved sludge dewaterability by disintegrating macro-molecular organic matter into micro-molecular matter. MBM reduction reached 7% in 40 days. Correlations between FM/MBM and extracellular protein (extra-PN) were significant (R = -0.861, p < 0.01; R = 0.869, p < 0.01). Therefore, it can be concluded that the degradation of protein in the extra-microcolony polymer (EMPS) layer results in the destruction of the bond between organic fractions and moisture. As a result, the release of mechanically bound moisture is accelerated. In addition, the moisture content, as well as the extra-PN, continued to vary when AD entered the stabilisation stage, that is, the destruction of protein in EMPS stimulated mechanically bound moisture release during AD. The results of this work provide a better understanding on the effect of AD on sludge reduction.