Rheological Properties of Manure/Biomass Mixtures and Pumping Strategies to Improve Ingestate Formulation: A Review
Abstract. Research in anaerobic digestion of agricultural by-products has been introducing new and changing heavier fibrous energy-rich feedstocks that, when co-digested with manure, allow the enhancement of the biogas potential per m 3 of volatile solids but also induce significant changes in ingestate rheological properties (in comparison with raw manure). Doubts about the adequate level of amalgamation achievable by these mixtures inevitably arise. Because of the relationship between the total solids content and ingestate apparent viscosity, optimal mixing is not achieved only as consequence of the action of fermenter paddles and other stirring devices, but pumping devices also act positively on the way biomasses and manures are mixed. At up to 8% to 10% total solids, the high shear rates provided by centrifugal pumps (better if equipped with rotating knives) cause the manure-biomass mixture to behave like a Newtonian fluid. When the total solids percentage of the mixture exceeds this threshold, other pumping systems (lobe pumps as well as screw pumps) can provide adequate pressure to drive the mixture to the fermenter; however, in this case, further conditioning devices need to be considered to act positively on the anaerobic influent condition to improve its microbial degradation.