Ultrasound treatment of digested sludge: process parameters interaction and comparative assessment of batch and flow-through configurations

Ultrasound (US) treatment represents a promising strategy to enhance energy recovery and waste minimization in anaerobic digestion (AD) systems. However, significant knowledge gaps hinder the effective application of US into full-scale processes. Particularly, the treatment of anaerobically digested sludge (ADS) has received limited attention in the literature, despite its potential to enhance the disintegration of recalcitrant organic fractions and improve the overall energy balance of the AD process. Furthermore, existing research typically investigates the effect of US parameters individually, often neglecting interactions between variables. In this study, the US treatment of ADS was investigated through a multivariable experimental approach. A response surface methodology, specifically a Box–Behnken design, was employed to assess the effects of three key variables, namely the total solids content, the US amplitude, and the treatment duration, on four response factors indicative of organic matter disintegration and solubilization, i.e. disintegration degree, soluble proteins, soluble carbohydrates and volatile fatty acids. Notably, the simultaneous effect of the variables was investigated for the first time, demonstrating significant interactions among the selected variables. A high degree of statistical significance was obtained from the regression-based model fitting. In addition, the study compared US treatment in both static and flow-through configurations, under identical operating conditions, revealing the critical influence of the mixing regime on the overall process performance. The presented findings provide valuable insights for the design and optimisation of US reactors for sludge treatment applications, offering environmentally relevant strategies to improve the valorisation of digestate in wastewater management systems.