Solids contents, particle size, and chemical composition as drivers of viscosity in agricultural digestates and separated liquid fractions.

A comprehensive understanding of digestate viscosity can benefit anaerobic digestion by aiding the selection of pre- or post-treatment methods and supporting the prediction of energy consumption in biogas plants and the impacts of digestate post-processing. Viscosities and chemical composition of 43 full-scale agricultural digestate samples and 12 samples of the liquid fraction of the separated digestate were examined, with eight manure samples included for comparison. The effects of full-scale agricultural digestate separation were evaluated for the first time. Viscosity was described using the power-law model, including the consistency coefficient and flow behaviour index. Shear-thinning behaviour was found in all except three samples. Viscosities of digestates were similar to those of cattle manure, whereas liquid fractions resembled pig manure. Total solids content and content of solids larger than 2 mm correlated (p < 0.001) with the consistency coefficient (r of 0.57 and 0.70) and flow behaviour index (r of -0.56 and -0.69), indicating that the content of solids larger than 2 mm is a better predictor of viscosity. This was further confirmed by substantial viscosity reductions in the liquid fraction of digestate once larger particles were removed, even at inefficient overall total solids removal. Organic components correlated (p < 0.001) positively with the consistency coefficient and negatively with the flow behaviour index; however, they also correlated among themselves and with total solids content, impeding the isolation of effects. Conductivity, pH, density, and some metals did not influence viscosity (p > 0.05), while some other metals did (p < 0.01).