Screening transparent model fluids to emulate the rheological properties of high solid anaerobic digestate of food waste

Due to the opacity of high solid anaerobic digestate, optical visualization techniques (e.g., particle image velocimetry) cannot be used directly for measuring the flow field inside high solid anaerobic digestion (HSAD) systems. This study mainly focused on selecting suitable surrogate fluids for optical visualization techniques by comparing the flow curves, thixotropic properties, viscoelastic behavior and creep recovery properties of HSAD digestate with a total solid from 10 % to 15 % (w/w) and transparent model fluids. Results indicated that the Bingham model outperformed the power-law model and Herschel-Bulkley model in describing the flow curves of HSAD digestate, xanthan gum and laponite. The digestate and xanthan gum had a similar thixotropic kinetic coefficient of 0.003 and 0.004, respectively. Besides, the digestate and xanthan gum had a similar modulus transformation trend with a power function whose power exponents were 0.17 and 0.19, respectively. In general, xanthan gum can be a suitable substitute for HSAD digestate with a total solid from 10 % to 15 %. The results can guide the implementation of optical visualization techniques to understand the flow pattern within HSAD systems, and improve the mass and heat transfer efficiency during food waste disposal processes.