Ultrasonic treatment for waste-water cleaning

Purification of liquids from undesirable admixtures and malignant bacteria is one of the most important problems of the mankind. It is especially sharply appeared at the cleaning of sewage. At present in the most part of the world (in densely populated territories) there is a lack of pure fresh water. Industrial and domestic waste waters negatively influence on the ecological situation in stretches of fresh water. It is evident, that it is caused by the large quantity of insufficiently cleaned sewage, which constantly comes to rivers and ponds in the connection with functioning of the industrial enterprises and households. The presence of admixtures and malignant bacteria in different liquid food (beverages, acetic seasonings) decreases their consumer properties and can be harmful for people health. So there is no doubt of the development new methods of liquid purification. The article is devoted to the analysis of the problem of purification of domestic and industrial waters from pathogenic microorganisms, undesirable admixtures and malignant bacterium. It is considered existing industrial solutions and their disadvantages. The theoretical research of optimum ultrasonic influence modes providing maximum diffusion coefficient was performed and example of US apparatus was presented.


Introduction
Wastewater treatment technology using ultrasound has a number of advantages that may be important for solving problems of water body pollution.Ultrasonic action promotes physical destruction of cell structures of harmful microorganisms, which can lead to more effective treatment of wastewater from bacteria and other microorganisms.In addition, ultrasound can promote the breakdown of organic compounds, improving the biological treatment process.
Additionally, ultrasonic treatment may be more energy efficient and environmentally friendly than conventional wastewater treatment methods.This can help to reduce the use of chemical reagents and minimize negative environmental impacts.The treatment used, combined with cavitation action, promises to significantly accelerate the wastewater treatment process, as well as significantly reduce the consumption of ozone and added toxic substances.
However, the amount of neutralized contaminants in this treatment method is very limited.This is due to the small diffusion coefficient of the gas mixture (ozone+air) in the liquid, 10000 times less than the diffusion coefficient of ozone in the air, as well as the short time of ozone action.Due to the instability of ozone molecules, the action time is limited to 5 to 7 minutes.
At the same time ozonized purification, which will be accelerated by several orders of magnitude by cavitation action, will reduce the amount of ozone and added toxic and poisonous substances many times, thus contributing to the solution of environmental problems associated with the huge amount of wastewater discharges.The addition of toxic or chemically active substances such as ozone is essential to the ozonized treatment process.Ozone has oxidizing properties and is used to break down organic contaminants, destroys all known microorganisms, acts very quickly -within minutes, removes unpleasant odors, and does not produce toxic by-products, but can be dangerous in large quantities.
The use of ultrasound allows to accelerate the process of transfer of substances inside the liquid due to saturation of the liquid with gas bubbles.The process of accelerating the purification of the liquid is achieved by saturating the liquid with tiny gas bubbles, caused by increasing the area of the of gas bubble ensemble distribution with a developed interfacial surface, which increases the contact surface of gas and liquid, accelerating the cleaning process.
This in turn contributes to solving environmental problems associated with intensive wastewater discharge.There is a shortage of clean fresh water in various parts of the world, especially in densely populated areas.Industrial and domestic wastewater has a negative impact on the ecological situation in freshwater bodies.For example, in Baikal, one of the largest and unique freshwater bodies in the world, there is a decrease in the number of rare fish species such as Siberian sturgeon, sterlet and tugun, which live exclusively in clean water, which indicates the negative impact of wastewater.Apparently, this phenomenon is related to the abundant discharge of insufficiently treated wastewater constantly entering freshwater bodies due to the continuous activities of industrial enterprises and households.In addition, toxic substances used to treat wastewater from microorganisms also pose a significant threat to the environment.So, there is no doubt of the development new methods of liquid purification.

Problem statement
At the development of the apparatuses for the sewage treatment different types of physical and chemical actions are used.But in practice only some of them are applied at the industrial scale.The reagent action is the use of ozone, chlorine dioxide, sodium hypochlorite and hydrogen peroxide, the physical one is the disinfection by ultraviolet or ultrasonic radiation.
Ultrasonic action by high-intensity oscillations in the cavitation mode and ozone action are the most promising methods.
At high intensities ultrasound suppresses and destroys microorganisms.The water treatment by high-intensity ultrasound leads to the disinfection, at that antibacterial effect of ultrasound is mainly concerned with the cavitation [1][2][3][4][5][6][7][8].That is why for the disinfection it is used ultrasonic oscillations with the intensity of more than 2…10 W/cm 2 at the frequency of 20…50 kHz.
The ozone action provides efficient sterilization and disinfection of water.High-concentrated ozone is toxic for the human organism that is why the application of high concentrations of ozone is limited.
The combination of physical and chemical methods of decontamination particularly ultrasound increasing the ozone effect allows essentially improving epidemic safety of sewage and drink waters and minimizing the generation of the by-products due to the reduction of the concentration of used ozone.
Providing of synergetic efficiency increase at combined action of ultrasound and ozone is achieved by the cavitation processes in liquid, which guarantees not only crushing of ozone bubbles into smaller ones to enlarge surface interaction of ozone with water to be cleaned, but increasing interaction effect at the interface: gas-liquid.
Thus, the ability of ultrasonic action to increase ozone effect allows developing the apparatuses of ultrasonic treatment of sewage realizing the considered effects with maximum efficiency for the determined conditions of the process realization.To design such apparatuses it is necessary to reveal optimum conditions of the process realization [1][2].

Theory
At the action of ultrasonic oscillations the insonified medium is a mixture of liquid with gas bubbles separated from liquid by the interface.
To determine equivalent ozone diffusion coefficient in water it is necessary to analyze the equation of microscopic diffusion in the system "liquidgas bubbles".

( ) ( ) (
) where Dmicro is the microscopic diffusion coefficient, m 2 /s, depending on the type of the phase in the observed point r and equal to Dl, if the phase is a liquid or Dg, if the phase is gas being inside the bubble; C is concentration of gas, mol/m 3 ; u is velocity of liquid flow, m/s.
As the ozone diffusion coefficient in a liquid phase Dl exceeds in more than 1000 times the ozone diffusion coefficient in a gas phase Dg, it is assumed, that the ozone concentration is constant in all internal volume of the single cavitation cavity.
The liquid density as the free length of the molecules slightly changes with the pressure growth.It can be thought, that the pressure drops generated in a liquid do not influence on the diffusion coefficient.
The diffusion coefficient in a gas phase is assumed to be infinitely large in comparison with its value in a liquid.That is why, only the sizes and the amount of the cavitation bubbles influence on the equivalent ozone diffusion coefficient in the cavitating medium.
Taking into account the change of the bubble radius in a period of time and the presence of determined concentration of the bubbles by the averaging the equation ( 1) it can be applied the expression for the equivalent diffusion coefficient in the cavitating liquid (2) [1-2]: where δl is the volume content of the bubbles (the cavitation index); Dl0 is the true diffusion coefficient in a liquid phase; T is ultrasonic oscillations period, s; t is instant value of time, s.
On the base of given expression it was obtained following values of the equivalent diffusion coefficient at different frequencies at various viscosity of the absorbing liquid (Fig. 1-5).The breakage of dependences corresponds to degenerated cavitation (long-lived bubbles without collapse).As it follows from the graphs (Fig. 1-5) at the rise of the ultrasonic pressure the equivalent diffusion coefficient increases due to the growth of the cavitation index.
The analysis of the dependence allows determining, that there is an optimum frequency of the ultrasonic radiator operation, at which the value of the equivalent diffusion coefficient is maximum.The optimum frequency does not depend on the viscosity of liquid-absorbent and the intensity of oscillations and it is about 30 kHz.
At the optimum frequency the increase of the equivalent diffusion coefficient can achieve 120 %.The presence of the optimum frequency is concerned with the fact, that at frequencies lower than the optimum it is generated less cavitation bubbles and at the frequencies higher than the optimum one it is smaller critical radius of the bubble, at which it degenerates into long-lived one.
The influence of ultrasonic oscillations on the phase equilibrium constant (solubility limit of gas) is determined first of all by the pressure drops in the medium, as the constant essentially depends on the pressure.
As the medium is cavitating, the value of the pressure drops can be defined by the following expression: where psh is the relative amplitude of shock wave pressure (the ratio of the pressure in the observation point to the pressure in the cavitation bubble nucleus) determined by the specific power of the shock waves, Pa; r1 is the distance between the observation point and the cavitation bubble nucleus, m; nbub is the concentration of the cavitation bubbles, m -3 ; K is the transformation ratio of the initial ultrasonic oscillations into the energy of the cavitation bubble collapse, m -1 .
The preliminary estimation of the averaged value of the pressure drops (over time and volume) indicates its exceeding in more than 4 times from the atmosphere pressure.It allows stating, that ultrasonic action lets increasing the phase equilibrium constant from 0.3…0.5 to 0.8…0.9.

Results of experiments
To prove the efficiency of high-intensity ultrasonic oscillations at the treatment of sewage it was designed test facility.During the process it was used the ultrasonic technological apparatus of "Volna-M" series with the special aeration nozzle.
The photos of the process realization are shown in Fig. 6.Fig. 6a shows single air bubbles leaving the nozzle on the radiating surface.Fig. 6b demonstrates that ultrasonic action fully breaks air coming from the nozzle.It is appeared additional bubbles inside the volume.
Carried out theoretical and experimental studies proved the efficiency of the application of ultrasonic aeration and allowed determining of preliminary conditions for further design of the industrial constructions of the apparatuses for the sewage treatment.

Conclusion
In proposed apparatus of ultrasonic treatment of sewage the task of the efficiency increase of ultrasonic treatment was solved due to essential growth of the intensity of ultrasonic action on liquid medium with contamination particles and providing even action in all zones of the treatment for considerable increase of water purification.
Moreover, the efficiency of water purification increases due to the growth of water saturation by ozone or ozone-containing gas.It is achieved by the cavitation solution of supplied ozone-containing gas near the oscillating surface of tool and the efficiency increase of interaction at the interface "gasliquid".
Practical realization in the laboratory conditions allows proving, that proposed apparatus provides the treatment of sewage up to the normative values in more than 3 times.

Figure 1 .
Figure 1.Dependences of the relative rise of the diffusion coefficient on the pressure amplitude of oscillations at different viscosities (the frequency is 20 kHz).

Figure 2 .
Figure 2. Dependences of the relative rise of the diffusion coefficient on the pressure amplitude of oscillations at different viscosities (the frequency is 25 kHz).

Figure 3 .
Figure 3. Dependences of the relative rise of the diffusion coefficient on the pressure amplitude of oscillations at different viscosities (the frequency is 30 kHz).

Figure 4 .
Figure 4. Dependences of the relative rise of the diffusion coefficient on the pressure amplitude of oscillations at different viscosities (the frequency is 40 kHz).

Figure 5 .
Figure 5. Dependences of the relative rise of the diffusion coefficient on the pressure amplitude of oscillations at different viscosities (the frequency is 60 kHz).

Figure 6 .
Figure 6.Air flow without ultrasonic action (a) and with ultrasonic action (b) (air flow additional pressure more than 0,5 atm).