Purpose This study aims at understanding how the acidogenic fermentation microbial community was impacted by the hydrodynamic cavitation (HC) pre-treatment of the substrates' mixture, constituted by waste-activated sludge and vegetable waste 1:1 on a TVS basis.Methods HC was performed with power = 8 kW, P = 1.4-1.5 bar, Q(mixture) of 25-30 L/min, 1550-1650 rpm, duration: 30 min. Fermentation tests were conducted on cavitated (CAV) and not cavitated (NCAV) mixture at T = 37 degrees C inside 4 L reactors in batch mode, then switched to semi-continuous with an OLR of 8 kg(TVS) m(-3) d(-1). Microbial community was characterized by 16S rRNA sequencing at the beginning and end of the pseudo-steady-state. Ecological diversity and clustering among the samples were determined by beta diversity, Venn diagram, and non-metric multi-dimensional scaling (NMDS) analysis.Results Cavitation was efficient in substrates' hydrolyzation but resulted in a lower microbial diversity of 3.85 (Shannon Index) and VFAs concentration of 12.9 gCOD(VFA) L-1 in the anaerobically fermented cavitated mixture (AF-CAV), respect to 4.54 and 18.2 gCOD(VFA) L-1 in the anaerobically fermented not cavitated mixture (AF-NCAV), respectively. NMDS analysis showed that AF-CAV and AF-NCAV samples formed two different clusters, with VFAs concentration as the only significant factor explaining their difference (R-2 = 1, Pr > r = 0.04167). Functional redundancy among community members probably allowed to maintain a stable VFAs composition despite the microbial community variation observed at the end of the test.Conclusion The insights here provided on the effects of HC confirm the fundamental role played by microbial community in acidogenic fermentation processes and underline its importance in evaluating the effect of substrates' pre-treatment.