Hydrodynamic cavitation: an advanced oxidation process for the degradation of bio-refractory pollutants

Abstract

In recent years, water pollution has become a major problem for the environment and human health due to the industrial effluents discharged into the water bodies. Day by day, new molecules such as pesticides, dyes, and pharmaceutical drugs are being detected in the water bodies, which are bio-refractory to microorganisms. In the last two decades, scientists have tried different advanced oxidation processes (AOPs) such as Fenton, photocatalytic, hydrodynamic, acoustic cavitation processes, etc. to mineralize such complex molecules. Among these processes, hydrodynamic cavitation (HC) has emerged as a new energy-efficient technology for the treatment of various bio-refractory pollutants present in aqueous effluent. In this review, various geometrical and operating parameters of HC process have been discussed emphasizing the effect and importance of these parameters in the designing of HC reactor. The advantages of combining HC with other oxidants and AOPs such as H 2 O 2 , ozone, Fenton process, and photocatalytic process have been discussed with some recommendation for large-scale operation. It has been observed that the geometry of the HC device and other operating parameters such as operating pressure and cavitation number are the key design parameters that ultimately decide the efficacy and potentiality of HC in degrading bio-refractory pollutants on an industrial scale.

Application of Hydrodynamic Cavitation Reactors for Treatment of Wastewater Containing Organic Pollutants: Intensification Using Hybrid Approaches

“…Under the cavitational effect, dissociation of H 2 O 2 into OH radicals occurs due to extreme conditions generated in the HC reactor, such as local hot spots with high temperature and high pressure. The following chain of reactions occur during the combined treatment of HC and H 2 O 2 [41]. It has been reported that the extent of degradation increases with an increase in H 2 O 2 loading up to an optimum value, beyond which it generally decreases due to scavenging effect, or remains constant [35].…”

Comparing geometric parameters of a hydrodynamic cavitation process treating pesticide effluent

“…It is helpful/ aiding chemical processes, specifically in technology which deals with mortification/degradation of substances which are harmful for environment and human beings [15]. It progresses in the formation, growth and subsequent collapse of micro-bubbles or cavities occurring in great degree of tiny timeframes (milliseconds) with release of large magnitudes of energy [16,17]. Treatment of water due to cavitation is mainly caused due to mechanisms of mechanical (e.g., generation of turbulence, liquid circulation currents and shear stresses), chemical (generation of active free radicals) and heat effects (generation of local hot spots i.e., condition of very high temperature and pressure locally) [16].…”

Degradation of Azodyes in Wastewater by using Hydrodynamic Cavitation Technique

“…Few other studies have reported that low temperature is also favourable to degrade the dye; the 90% degradation of carbamazepine is observed at low temperature of 250 0 c, with further increase in temperature the efficiency decreased [19], which means the temperature depends on the liquid vapour pressure. With increase of temperature, the vapour pressure also increases and thus cavities are formed at low operating pressure and at high temperature [6]. Among all studies, it is concluded that the moderate temperature should be within the range of 250 0 c -500 0 c & is well suited for wastewater treatment.…”

“…Initial concentration is one of the affecting factors in degradation of dyes; different authors have discerned the impact of concentration with different perception. Few authors have estimated the impact of initial concentration on the rate of pollutant degradation, while others have considered the impact of focus on the percent decrease of contamination level in aqueous effluent [6]. The rate of hydroxyl (OH*) radicals utilization will be the greatest as contrasted with low concentration of pollutant.…”

“…In the favourable acidic condition, slower recombination of generated OH* radicals also resist the scavenging of released OH* radicals by other ionic spices present in the solution such as HCO3and CO32- [7,16], where as in basic medium, the dye particles gets ionized and become hydrophilic in nature and subsequently stay in the bulk fluid. Only 10% of created OH* radicals are diffused into bulk solution because of the OH* radicals recombination [6].The temperature of solution is also one of the crucial factors in cavitational yield and degradation efficiency. Different wastewater samples require different optimum temperature to degrade the dye.…”