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Water treatment using the cavitation effect

S. A. Melnikov, eng.  
                                                                                                                           (DonNTU)                                                                                                                       
                                                                                                              V. L. Debelyy, Cand.Sc.                                                                                                           
(SOJSC Trust Donetskshakhtostroymontazh)

Ecologists refer to many engineering procedures factors destroying nature. But they agree in their opinion that the most negative exposure of engineering procedures affects the water resources of our planet. Lack of drinking water is registered in all regions of the world, including Ukraine. It is known that over 1 billion people (about one fifth part of the earth population) suffer from lack of water. 

The main source of water pollution in Donbass are mines. Underground and surface waters flow, first of all, to the underground workings, go through cracks and pores of rocks, are filled with different substances, which may be in suspension, colloidal state and solution. Due to it the specific density of mine waters at the temperature of 15 is greater than the one of the clean water and varies from 1015 to 1025 kg/m3. At mines of the basin, solid impurities content in waters achieve 3 g/l. In colloidal state there are found mainly earth silicon 82, alumina 12, ferrous oxide 203, organic matters. The mine waters may also contain oxygen 2, carbon dioxide 2, nitrogen N2, methane 4, hydrogen sulphide 28 and hydrogen 2.

Water acidity is unstable and it usually increases in spring, when along with growing inflow PH reduces in 2-3 times.  The highly acid water (pH < 3) has significant aggressive properties relating to metal. That is why special measures are required to provide for reliable operation of dewatering plants.

At mines of Donbass the water is acid, with hydrogen ion concentration factor of 0,8-5, at some of them it has alcaline reaction with pH > 8... 12.

Mine water is usually highly mineralized and hard. The measuring unit of water hardness is assumed to be 1 mg equivalent of calcium and magnesium ions contained in 1 l of water, 1 mg hardness equiv. corresponds to content of  20,04 mg/l ++ or 12,16 mg/l g++. The above mentioned indicates that using of mine water without pretreatment is practically impossible, and its discharge into water basins causes pollution and a threat for flora and fauna.

There is a series of methods for water treatment and decontamination. The water is effectively purified from solid impurities mainly through filters. For biological treatment with the purpose of decontamination, the low pressure UV lamps are traditionally applied. They perform their functions partially; however, they are inefficient for destruction of spore-forming bacteria, viruses, fungi, algae and mold. Effective radiation levels for decontamination of spore forms and fungi amounts to 100-300, and lamps may provide only 16 J/cm2. Low efficiency of this method is connected with necessity in frequent maintenance of plants (cleaning of lamps protection cover).

The proposed method gives an opportunity to avoid disadvantages associated with the method of using the UV radiation. The method of biological water treatment consists of application of hydrodynamic self-vibrating effect by means of the device enabling transformation of flow energy generated by turbulent immersed fluid jet into hydrodynamic self-vibrations  producing cavitation effect.

The device is a DC hydrodynamic radiator consisting of vibration exciter and vibrating system creating cavitation. The cavitation area is formed inside the fluid flow and does not contact with surfaces liming it, thus eliminating their wear. As a radiator, an elastic plate is applied being exposed by the fluid flow inducing bending vibrations therein, the main eigenfrequency where of


where - is a coefficient of proportionality depending on the way how the plate is fixed;
t - width of the plate;
l - length of the plate;
- elastic modulus;             
p - material density of the plate

It should be noted that mass of the plate involved into the process of cavitation area formation somewhat reduces the eigenfrequency. In the fluid jet acting upon the plate there appear vibrations with the frequency of 

fc=kv/h

where  is a coefficient of proportionality depending on the speed and distance to the plate;
v - jet speed;
h - distance between the nozzle and the plate.

The most efficient operation of hydrodynamic radiator is in case of matching fc and  f.

With fluid motion speed of 2030 m/s and excess pressure of 0,51 MPa in the DC hydrodynamic radiator, vibrations with the frequency of  25 kHz are formed, which generate caviation - aperture of fluid flow discontinuity with formation of bubbles, wherein the pressure is far exceeding than the one in the flow. 

When bubbles collapse due to emission of a great quantity of energy, the microflora available in the fluid is absolutely destroyed. In case if this method is applied, such mold spores as Asergollus are destroyed (this kind of spores is not destroyed by the UV radiation).

The processes taking place in the so-called cavitation area are subject to detailed study.

According to researches, with change of vibration frequency of hydrodynamic radiator within the range of 15-20 kHz one may observe destruction of chemical compounds and elements available in the contaminated water, at the molecular level. It will enable efficient treatment of highly contaminated waters, which in most cases include mine waters.

The cavitation water treatment method with using the hydrodynamic radiator is efficient and simple in realization.

 

 



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