Cavitation-based pre-treatment of wastewater and waste sludge for improvement in the performance of biological processes: A review

[Display omitted] •Analysis of geometric and operating conditions in terms of evaluation matrices.•Increased biodegradability and COD reduction are observed for wastewater.•Waste sludge pre-treatment yields higher methane/energy in anaerobic digestion.•The system is observed to be cost effective and energy positive. Long treatment times, large quantity of sludge generation, inhibition of micro-organisms and inability to degrade refractory pollutants are common disadvantages of biological treatment processes. Cavitation-based pre-treatment processes can enhance the treatment efficiency of biological treatment including aerobic oxidation and anaerobic digestion. This work presents a critical review on cavitation-based pre-treatment for subsequent biological oxidation process as well as for the treatment and modification of waste sludge for subsequent anaerobic digestion. For wastewater pre-treatment, important metrics to be assessed are COD reduction, and biodegradability index enhancement. In several studies, a BI improvement up to 50–60 % has been observed with cavitation. For sludge pre-treatment, particle size reduction, soluble COD and degree of disintegration (DDCOD) increase, and enhancement of biomethane production potential as the performance metrics have been reviewed. The effect of several process parameters like ultrasound power, hydrodynamic cavitation pressure and geometry, time, and pH are critically reviewed and compared for various studies. Improvements in treatment times, higher enzymatic digestibility, removal of refractory pollutants, and lower inhibition in the biological processes were observed as the key advantages due to the use of cavitation. Optimum cavitation numbers for efficient pre-treatment using hydrodynamic cavitatio lie between 0.05 and 0.15. It is observed that low hydrodynamic pressures are the most advantageous for sludge disintegration and also the process is highly time dependent. Cavitation, especially the hydrodynamic mode, is demonstrated as an economically feasible advanced oxidation-pretreatment for sludge modification and biological oxidation processes leading ultimately to an ‘energy-positive system’. Future studies in this context should mainly focus on the development of continuous flow-pilot scale systems that can be subsequently considered applicable commercially.