Cavitation based treatment of industrial wastewater: A critical review focusing on mechanisms, design aspects, operating conditions and application to real effluents

Acoustic cavitation (AC) and hydrodynamic cavitation (HC) coupled with advanced oxidation processes (AOPs) are prominent techniques used for industrial wastewater treatment though most studies have focused on simulated effluents. The present review mainly focuses on the analysis of studies related to real industrial effluent treatment using acoustic and hydrodynamic cavitation operated individually and coupled with H₂O₂, ozone, ultraviolet, Fenton, persulfate and peroxymonosulfate, and other emerging AOPs. The necessity of using optimum loadings of oxidants in the various AOPs for obtaining maximum COD reduction of industrial effluent have been demonstrated. The review also presents critical analysis of designs of various HCRs that have been or can be used for the treatment of industrial effluents. The impact of operating conditions such as dilution, inlet pressure, ultrasonic power, pH, and operating temperature have been also discussed. The economic aspects of the industrial effluent treatment have been analyzed. HC can be considered as cost-efficient approach compared to AC on the basis of the lower operating costs and better transfer efficiencies. Overall, HC combined with AOPs appears to be an effective treatment strategy that can be successfully implemented at industrial-scale of operation.

Strategies to improve biological oxidation of real wastewater using cavitation based pre-treatment approaches

The present work demonstrates the effective application of pretreatment based on cavitation to improve biological oxidation of real municipal and industrial wastewater. The optimum pretreatment conditions based on ultrasonic cavitation for treatment of municipal wastewater were observed as power dissipation of 90 W, a duty cycle of 70% and H₂O₂ dosage of 0.2 g/L resulting in about 24.9% COD reduction. The use of modified sludge and ultrasonic pretreatment for biological oxidation resulted in significant reduction in treatment time (36 h) than the treatment time (60 h) required for biological oxidation using untreated sludge as inoculum. Also, significantly enhanced biodegradability index (BI) from 0.33 to 0.6 was achieved using pretreatment for biological oxidation process. For the treatment of real industrial wastewater, different pretreatment approaches based on hydrodynamic cavitation (HC) in combination with H₂O₂, ozone or Fenton were investigated. The pretreatment using best approach of HC + Fenton resulted in 44.2% of COD reduction in total whereas only 28.1% of COD reduction was achieved for the untreated effluent being applied in the biological oxidation. Overall, the present work demonstrated the effectiveness of the pretreatment based on cavitation for the improved treatment of municipal and industrial wastewaters.