## Abstract

High total suspended solids (TSS) digestion of municipal sewage sludge reduces the required space and volume for digestion plants. However, an important consequence of high TSS is the major influence on sludge rheology. The present case study investigates the rheology of sludge from a 130 m3 high solids digestion pilot plant at Vienna's main wastewater treatment plant (4 M PE120). Raw sludge ranged from 6 to 8% TSS and digested sludge from 3.2 to 4.6%. TSS show an exponential impact on rheological parameters. Increasing raw sludge TSS from 6 to 8% at least doubles the shear stress and increases friction loss by a factor of three. However, under real operating conditions simulated at the pilot plant, there are additional impact factors. The mixing ratio between waste activated and primary sludge influences raw sludge rheology, while solids retention time and loss on ignition affects digested sludge rheology. Nevertheless, friction loss calculations based on a simple power law relationship between shear rate and shear stress proved to be applicable and sufficiently accurate for both raw and digested sludge with high TSS. Altogether, this case study underlines the relevance of comprehensive rheological considerations, measurements and calculations when designing high TSS sludge digestion.