Rheological evolution of straw-cattle manure (SCM) treated by dry anaerobic digestion in batch and in continuous pilot reactors

Knowledge of rheological evolution of biomass during dry anaerobic digestion (D-AD) is important in the engineering design, modeling, and operation of D-AD reactors. In this work, two methods of rheological analysis, the slump test and the shear-box, were used to measure the evolution of the yield stress, cohesion and friction angle of the straw-cattle manure (SCM) during the D-AD. Firstly, four 60 L batch leach-bed reactors (LBR) were started in parallel and stopped at different stages of the D-AD process on days 0, 10, 21 and 31. Secondly, a 500 L and 2 m length plug flow reactor (PFR) was operated with 40 days of solid retention time and samples were recovered at different positions. The solid degradation during D-AD process was monitored by analysis of the degradation of volatile solids, the fiber content and the Flash BMP. Similar degradation patterns of SCM and rheological evolution were observed in both reactors type. VS content decreased of 10.7% and 10.2% in 30 days in PFR and LBR respectively. VS degradation in both cases was well explained by hemicellulose and cellulose consuming in D-AD process. Considering the rheological analysis, the results showed that D-AD induced a reduction of the yield stress of 28.1 and 24.2% in 30 days in PFR and LBR respectively. Moreover, a similar evolution of cohesion and friction angle value for samples from both reactors was observed. This study demonstrates the close relationship between the state of degradation of the solid biomass and its rheological properties.