© 2020 Mechanical heat treatment of municipal solid waste offers a route for the production and separation of a biofiber fraction with high organic content, which is a potential source of sugars for biochemical conversion to chemicals and fuels. This paper describes a study of the enzymatic digestion of this biofiber fraction using a 50L commercial vertical mixer, applying a commercial cellulase. Power measurements, with calibration of mixer constants using standard Newtonian and non-Newtonian fluids were used to determine apparent viscosity of biofiber mixtures, effective shear rate and Reynolds and Power Numbers. Measurements could be easily carried out at high solids contents, >25% dry weight, where initial biofiber mixtures had high viscosity, typically 100–150 Pa S, with strong shear thinning behavior. Microscopy showed that the biofiber contained many individual wood and other plant fibers, which would entangle and so resist shear forces. Partial alignment of these fibers would be responsible for the shear-thinning effect. The fibers were repeatedly cut and reduced in length as a result of enzymatic hydrolysis, responsible for the large fall in viscosity during digestion. The viscosity of the biofiber slurries were monitored up to 48 h digestion time, with analysis of samples to determine sugar monomer content. Digestion kinetics were analyzed using an exponential recovery model, where glucose yields of around 35% were found for digestion of a biofiber derived from standard and also high paper MSW mixtures. Further yield optimization would be required to achieve commercial viability.