Rheology and dewaterability of sludge during anaerobic digestion

Biogas is one of the most hopeful renewable energy sources in the world. The anaerobic digestion (AD) process has been studied intensively over the last few decades, its application on biomass and solid waste digestion, as well as in wastewater treatment, agriculture. In order to gain a comprehensive insight, the interactive relationship between substrate rheology, physicochemical properties, and biogas production as well sludge dewaterability was investigated. Anaerobic digestion experiments were performed on conventional sludge and low temperature thermal hydrolysis pretreatment sludge. The experiment lasted for 20 days. Rheological indicators and physical and chemical indicators of conventional sludge and low temperature thermal sludge were determined by the rotational viscometer method and standard method during anaerobic digestion. The results showed that the pH value was maintained 6.8 to 7.9, which was within the allowable range of anaerobic digestion. The concentration of Free Ammonia (FAN) is between 10-130 mg/L, which was lower than the inhibition level of anaerobic digestion. The content of VFAs (Volatile Fatty Acid) in low temperature thermal hydrolysis sludge was higher than that in conventional anaerobic digestion sludge. The final Volatile solids removal level (VSr) of low temperature thermal hydrolysis-anaerobic digestion reached was 48.3%, and it was higher than conventional anaerobic digestion (44.6%). The dewaterability of sludge increased with the anaerobic time during the AD, and the low temperature thermal pretreatment increased the dewaterability of digestion by 1.59%, which demonstrated the improved dewaterability of low temperature thermally hydrolysis treated sludge. After the conventional anaerobic digestion and the low temperature thermal digestion, the yield stress of the sludge decreased by 64.51% and 71.47%, respectively. The consistency coefficient decreased by 90.94% and 92.83%, respectively. It implied the sludge fluidity significantly enhanced. During the whole digestion process, the VS/ Total solids (TS) and yield stress (τ0t) showed a logarithmic downward trend with time. Using the linear equation fitting and Pearson correlation statistics analysis, the correlation between rheological (yield stress and yield stress change) and physicochemical parameters (VS/TS and dewaterability) were investigated. The results showed that all the goodness of fit (R2) were greater than 0.94. During the whole conventional anaerobic digestion, the yield stress was positively and strongly correlated with VS/TS value (R=0.975, P<0.01), and the yield stress change was negatively and strongly correlated with dewaterability (R=-0.989, P<0.01). During low temperature thermal hydrolysis anaerobic digestion, the yield stress was positively and strongly correlated with VS/TS value (R=0.990, P<0.01), and the yield stress change was negatively and strongly correlated with dewaterability improved (R=-0.992, P<0.01). It is implied that there is a strong correlation between sludge yield stress and sludge VS/TS value. From the perspective of rheology, it provided the new ideas and theoretical basis for monitoring and optimization in the process of anaerobic digestion. In this study, the theoretical basis and feasibility of rheological control indicators for evaluating and monitoring low temperature thermal hydrolysis and anaerobic digestion processes (performance) were provided. Finally, the future work should focus on investigating the specific effects of specific ingredients (including polysaccharides, proteins, lipids, humic acids, glycoproteins, etc.) in EPS on sludge dehydration, and exploring the different forms of water in the sludge during anaerobic digestion, to reveal the relationship between the change law and sludge rheology, and clarifying the mechanism of sludge rheology and dehydration performance during anaerobic digestion. At the same time, the research on the influence of rheology on the level of chemistry and microorganism also needs to make progress.