Rheology-dependent mass diffusion characteristics of high-solids anaerobic co-digestate of food waste and sewage sludge

Rheology-dependent mass diffusion characteristics of high-solids anaerobic co-digestion (HS-AcoD) systems of food waste (FW) and sewage sludge (SS) are important inputs for system optimization modeling but have not yet been clearly elucidated. Results indicated that the effective diffusion coefficient of digestate (D_eff.) decreased from 8.40 × 10⁻¹⁰ m²·s⁻¹ to 5.99 × 10⁻¹¹ m²·s⁻¹ as the total solids (TS) increased from 10.14% to 18.12%. Higher values of the yield stress, viscosity, consistency index, critical modulus and energy of cohesion but lower values of the flow behavior index were obtained with increasing TS content. A satisfactory correlation was obtained between the rheograms and Herschel–Bulkley model. Higher proportion surface water in the high TS digestate explained the decrease of D_eff. and the deterioration of rheological properties. Finally, the relationship between the rheological parameters and D_eff. could be modeled by empirical equations with forms of power functions. The results of this study can guide the design and operation of HS-AcoD systems to enhance mass transfer during FW and SS disposal processes.