Increasing degradation degree and solubilization of municipal wastewater secondary sludge to anaerobic digester using hydrodynamic cavitation pretreatment

One of the drawbacks of anaerobic digestion technique is the slow rate-limiting hydrolysis of organics. Different pretreatment techniques such as ultrasonic, chemical and thermal were applied to increase secondary sludge disintegration, in order to enhance sludge anaerobic digester performance. Hydrodynamic cavitation (HC) technique has fewer disadvantages compared to other pretreatment methods. Additionally, HC is simple and energy efficient. In this study, the effect of cavitation on sludge properties including soluble chemical oxygen demand (sCOD), turbidity, electrical conductivity, disintegration degree and other parameters downstream of orifice plate was investigated. The geometry of orifice plate had been optimized previously for specific cavitation number (0.2) which is desirable for cell disruption. SCOD rose from 857 mg/L to 5376 mg/L within 60 min pretreatment. By using cavitation pretreatment, degradation of cells occurred in several minutes instead of days. So it can be expected that anaerobic digestion performance will improve due to the release of intracellular substances and availability of enzymes for biological degradation. Experiments performed in this study indicated that cavitation causes the significant increase in sCOD and disintegration degree as a result of breaking up microorganisms’ cell walls and releasing organic substances into soluble phase. The turbidity of soluble phase also rose dramatically from 300 NTU to 5675 NTU within 60 min due to the collapse of cavitation bubbles and other effects of cavitation phenomena. Eventually, a linear relationship was derived for sCOD and turbidity of the soluble phase of pretreated sludge which can be applied to calculate one from another alternatively.