Rheological manipulation of flocculated algal slurries to achieve high solids processing

Minimising the processing volume of downstream unit operations is critical to the scale-up of algal biorefining operations. This requires the ability to process high solids algal biomass, which relies on understanding the rheology of this material. In this work, the characterisation of particle, microstructural and rheological properties of flocculated and centrifuged Nannochloropsis sp. biomass with a solids concentration of ca. 25% w/w solids were undertaken. This suspension was determined to have a cell volume fraction of ca. 0.73. Continuous networks of cell aggregates arising from flocculant cross-linking and compression during centrifugation resulted in a highly viscous, irreversibly shear-thinning material as determined by stress ramp and oscillatory frequency sweep tests. Understanding that the material is highly sensitive to shear presents the opportunity of manipulating its rheology to enable high solids processing. Mechanical shearing was able to decrease the viscosity of flocculated biomass from ca. 1 × 105 Pa s to 1 Pa s as a result of breaking cell aggregate networks. The reduction of viscosity through shear processing is further demonstrated in a cell disruption process through a bench-scale high pressure homogeniser. The rheological methods described here have the potential to be usefully applied to characterising the bulk flow behaviour of microalgal biomass of different species and processing histories. Overall, this paper provides an accurate description of the underlying mechanisms controlling the flow of high solids algal biomass including the link between microstructure and bulk flow behaviour during processing.