This study examined the influence of hydrodynamic cavitation on the biodegradability and composition of brewery spent grain diluted by mechanically pre-treated municipal wastewater. An orifice plate with a conical concentric hole (3/10 mm as an inlet/outlet diameter) was used as the constriction in the cavitation device; the corresponding cavitation number was 0.036. Hydrodynamic cavitation was applied at an inlet pressure of 7 bar, maintaining 30 recirculation passes through the cavitation zone. As a result, the concentration of soluble organic compounds (expressed as soluble chemical oxygen demand), BOD5 value, and BOD5/COD ratio increased. The BOD5/COD ratio was enhanced from 0.074 to 0.091, indicating improved biodegradability of the brewery spent grain. Moreover, solubilization of carbohydrates occurred, as confirmed by the hemicellulose removal with a related monosaccharides release of 87%. On the other hand, the COD value and the concentrations of both total solids and volatile solids decreased significantly, revealing the effective destruction of complex organic compounds via cavitation. Additionally, Fourier transform infrared photoacoustic spectroscopy analysis was conducted to check the findings. It confirmed that hydrodynamic cavitation did not lead to the formation of new compounds that would interfere with the biological treatment of the brewery spent grain diluted by mechanically pre-treated wastewater. (c) 2016 Elsevier B.V. All rights reserved.[Graphys]