Hydrodynamic Cavitation for Wastewater Treatment

Introduction

Water pollution has become a global problem due to large amounts of industrial and domestic sewage discharged in the water bodies. This liquid waste coming from various industries like textile, pharmacy, pesticide and petrochemical processing contains large amounts of organic and inorganic compounds like phenols, chlorides, aromatics etc. Also, textile industries use dyes which act as coloured pollutants. These compounds are sometimes non-biodegradable and sometimes toxic to microorganisms. This is the main reason why conventional biochemical processes aren’t able to degrade them.

This is the primary driving force for treatment and recycling of wastewater effluents. The goal of wastewater treatment of organic pollutants is to oxidise them and achieve complete mineralization. Even though, when these polluted effluents are subjected to bio-degradation they undergo only minor structural changes instead of complete being transformed to carbon dioxide and water. Usually aerobic treatment gives effective colour removal. Thus, textile industries are asked to treat their effluents on-site before discharging them in the municipal sewer lines.

Aerobic treatment may provide good results in decolourization of wastewater but aromatic amines and other organic compounds are not degraded in aerobic conditions. These circumstances lead to the need to develop some other technology for treatment of wastewater as every technique discussed above has some drawbacks.

Much work has been done in developing and testing newer techniques and their combinations for wastewater treatment either individually or as a supplementary role to the conventional biological and chemical methods. Cavitation is one such recent technique which has been found substantially beneficial in wastewater treatment.

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Hydrodynamic cavitation occurs when a liquid passes through a constriction structure such as a throttling valve, orifice plate, venturi, etc. At the constriction region, the velocity of the fluid increases at the expense of a pressure loss. If the pressure falls below the vapor pressure corresponding to the liquid temperature, a large number of cavities are formed in the field of flow. At the downstream constriction, the pressure is subjected to a recovery as the cross-sectional area of flow increases gradually or instantaneously. Subsequently, it leads to collapse of cavity, generating extreme conditions and highly reactive free radicals in the effluent water system. Thus, hydrodynamic cavitation has been looked upon as an effective means to treat wastewater.

The collapse of the cavitation bubbles also initiates physio-chemical effects, in addition to the above mechanical effects, resulting in the intensification of physical dispersion processes. Here, the physical effects include production of shear forces and shock waves whereas the chemical effects include generation of free radicals. Violent collapse of the cavities in hydrodynamic cavitation system results in the formation of reactive hydrogen atoms and hydroxyl radicals which recombine to form hydrogen peroxide.

This seminar report covers the basics of hydrodynamic cavitation i.e. its working principle, factors affecting hydrodynamic cavitation, various reactors or assemblies for generating it and their selection criteria. This report also includes the results obtained on treating different wastewaters like dyes, pharmaceuticals, insecticides, pesticides etc. prepared synthetically in a lab or taken directly from a site.

Principle of Hydrodynamic Cavitation

Hydrodynamic cavitation describes the process of vaporization, bubble generation and bubble implosion. When local pressure drops to some point below saturated vapor pressure and recovers or rises above vapor pressure, due to sudden decrease and incise in local pressure cavitation occurs. If the recovery pressure is not above the vapor pressure flashing is said to have occurred. In pipe systems, cavitation typically occurs either due to increase in kinetic energy or an increase in the pipe elevation.

Hydrodynamic cavitation can be produced by mechanical rotation of an object through a liquid or passing a liquid through a constricted channel at a specific flow velocity. Combination of pressure and kinetic energy due to specific (or unique) geometry creates hydrodynamic cavity with high energy cavitation bubbles in downstream of constriction.

Cavitation is damaging when uncontrolled. Cavitation power can be harnessed and used in a non-destructive way by controlling the flow of the cavitation. Controlled cavitation generates free radicals due to disassociation of vapours trapped inside the cavitating bubbles. It can be used to propagate certain unexpected reactions which can lead to degradation or even mineralization of water constituents without any addition of extra chemicals. Extent of cavitation occurring in a system is explained cavitation number and is simply derived from Bernoulli’s theorem expressed by the following equation:

where, P2 is the downstream pressure, PV is the vapor pressure of the liquid and v is the velocity at the constriction where cavitation takes place.

Systems for effecting Hydrodynamic Cavitation

Orifice Plate System

A multiple-hole orifice plate may have different combinations of hole diameters to achieve different degrees of cavitation and cavitation numbers. Some typical arrangements of holes are shown below.

Two parameters which are extensively used to characterize the orifice plate, i.e. α and β, which are given as where, n represents total number holes in the orifice plate dh represents hole diameter dp represents the pipe diameter It has been qualitatively demonstrated that the degradation rate of pollutants increases with increase in α. The degradation rate also increases with decreasing β for a certain flow area. Differences just exist in quantitative aspect, which is due to different cavitation intensities required for the desired chemical transformations. For example, the cavitation intensity required for the decomposition of rhodamine B is much higher than that of KI decomposition. Therefore, varying α and β will lead to a more obvious change in decomposition of the effluent. Compared to other cavitation generating systems, a multiple hole orifice plate offers maximum flexibility over the design parameters. Therefore, this kind of system is exhaustively used.

Cavitation and Jets impingement system

As shown in the figure the reactor contains two orifice plates and two movable flanges supporting orifice plates, which are installed inside the cylindrical vessel. During the experiment, the liquid flows through these two orifice plates from the opposite direction, including hydrodynamic cavitation separately.

Because of this arrangement, both the cavitation jets meet each other at the centre of the vessel and collide with each other and finally flow out from the opening for the outlet. Experimental results showed that this reactor has the capacity to bring more than 50% degradation rate of rhodamine B.

Venturi Tube system

Compared with orifice plate, venturi tube system can generate much lower turbulence intensity since the smooth variation of its cross-sectional flow area leads to a much lesser pressure head loss. A typical venturi type constriction is shown in the figure below. For this system, cavitation intensity is seriously affected by its length and the ratio of throat diameter to pipe diameter.

Cavitation bubbles formed at the throat and gathered downstream of the throat. Subsequently, these bubbles collapse when they come out of the venturi tube. The degradation rate is closely linked with the cavitation pattern. When the number density of cavities become high enough, the entire downstream area becomes filled with cavities. Then, this cavity starts to coalesce with each other and form a cavity cloud, which is called chocked cavitation. Once chocked cavitation occurs, the degradation rate starts decreasing with increase in inlet pressure. The maximum extent of degradation for the organic pollutant Red 120 dye was seen to be 84% under optimized conditions.

Liquid whistle reactor

By using a liquid whistle reactor consisting of an orifice tube and a blade, as shown in Fig. 5, Chakinala et al.63 conducted experiments with real industrial wastewater. Wastewater flows through the orifice tube and generates hydrodynamics cavitation in a high-speed liquid jet. Then, the cavitation liquid jet is projected over the edge of the adjacent blade, creating steady oscillation and pressure fluctuation. The main advantage of this reactor lies in the highly efficient mixing. If combined with Advanced Fenton Process (AFP), this novel hydrodynamic cavitation structure could bring about 60-80% removal of total organic carbon (TOC) under optimized condition.