Hydrodynamic cavitation pretreatment of waste activated sludge using an optimized pinned-disc rotary generator: Cavitation-collapse pressure pulses, sludge disruption and energetic limits.

Hydrodynamic cavitation (HC) is increasingly considered as a mechanical pretreatment to enhance anaerobic digestion (AD) of waste activated sludge (WAS), but the relationship between cavitation, sludge transformation and energetic benefit remains unclear. In this study, WAS from a municipal wastewater treatment plant was treated using an optimized pinned-disc rotary generator of hydrodynamic cavitation (OPD-RGHC), and the effects of 5, 15, 30 and 100 cavitation passes (Np) were evaluated. While pinned-disc RGHC configurations have previously been investigated, this study advances the concept by applying an OPD-RGHC configuration and linking the number of cavitation passes and measured cavitation-induced pressure pulses to sludge disruption, spectroscopic changes, rheological response and methane-production kinetics. Progressive increases in soluble chemical indicators and disintegration degree confirmed effective sludge disruption and solubilization. Spectroscopic and microscopic analyses indicated microbial cell damage, release of intracellular material and modification of dissolved organic matter. Rheological measurements revealed reduced viscosity and structural integrity, improving flowability but negatively affecting filterability. Biochemical methane potential increased by up to 27 %, while kinetic analysis revealed a non-monotonic response of the hydrolysis rate constant to Np. The highest rates were observed at 15-30 passes, whereas the highest ultimate methane yield occurred at 100 passes. Together with increasing treatment energy demand, these findings demonstrate that OPD-RGHC should be optimized within a constrained process window balancing biochemical performance, rheological behaviour, filterability and energy demand rather than maximizing individual treatment outcomes..