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WATER DISINFECTION WITH ULTRA-VIOLET RAYS
Ye. Voronova, ass. prof., T. Korchinova master of KHNADU
Abstract. In the article it is written about the method of water disinfection by means of ultra-violet radiation. The main advantages and disadvantages of this method are discussed here.
Key words. Ultra-violet rays, bacteria, chlorating, ozonation, radiation, microorganisms, pathogenic microflora
Introduction
The problem of water disinfection arose in the 80th of the XIX century after ascertainment of the true reason of aquatic infections appearance. Nowadays it is well known that water can be a source of distribution of such serious diseases as cholera, ammoebiasis, enteric fever, paratyphoids, dysentery, gastroenteritis, tuberculosis, diarrhea and others like these.
Pathogenic microorganisms can't be fully withdrawn neither at defending nor at the artificial biological sewage treatment. At the complex of artificial biological sewage treatment from 91 to 98% such bacterias are removed. Therefore it is necessary to carry out the disinfection of water after a mechanical and artificial biological sewage treatment.
Advantages of the method
The Russian researcher Maklanov was the first who studied bactericidal actions of short-wave ultraviolet rays at the end of XIX century.
The intensive application of ultra-violet disinfection of water in medium and large volumes is connected with the following advantages of this technology:
- energy consumption in industrial ultraviolet plants constitutes 50-100 Vt-h/m 3 of any sewage;
- the stage of ultra-violet disinfection isn't linear but grows up exponentially with the increase of ultra-violet radiation dose that is why an insignificant increase of ultra-
violet power at the set assigned consumption of the processed liquid in several times increases the level of disinfection. In the case of overdosing there aren't negative effects of oxidizing technologies;
modern ultra-violet complexes based on arc lamps of low pressure operate on 110, 220, 380 V and the ozonizing complexes consume from 8 to 30 kW that leads to higher requirements to power safety and qualification of serving personnel; modern ultra-violet complexes provide a high level of reliability and simplicity in their operation.
the automation of large systems, which requires the adjustment of electric parameters is more simple, more reliable, than for chloride;
the absence of gaseous (or liquid) technologically dangerous ingredients (ozone, chloride) at ultra-violet disinfection of water provides more safety and failure of such systems;
ultra-violet complexes and their periphery units aren't inferior in compactness, but even in a number of cases exceed the systems of chlorinating and ozonization; the usage of ultra-violet disinfection does not alter the oxidizing descriptions of water; radio-biological and chemical researches show the absence of unwanted consequences in doses of ultra-violet radiations considerably exceeding doses
practically necessary for disinfection in it means the absence of by-effects;
- the minimum of basic parameters determining the calculations of options are:
- the maximum expense of the processed
water;
- the rate of transmission of ultra-violet
radiation through the water;
- the set level of the disinfection.
- minimum continuance of process (seconds);
- in comparison with ozonization ultra-violet disinfection demands two times less investments and five times less investments for exploitation. It is connected with the insignificant expenses of electric power, the absence of necessity in expensive reagents, the simplicity of exploitation, the absence of the need in special serving personnel and the lack of technical safety measures. In addition, the application of ultra-violet complexes into sewage treatment complexes is provided without any construction and assembling and does not require to construct separate buildings;
- this method excludes the introduction of such toxic and ecologically dangerous matters as chlorine, chlororganics and chloramines into rivers;
- decomposition of dioxins at the action of ultra-violet rays.
In general the disinfection effect of bactericidal ultraviolet rays is caused by photochemical reactions and as a result the irreversible damages of deoxyribonucleic acid (DNA) take place. Except DNA ultra-violet rays influence on other structures of a cell, especially on ribonucleic acid (RNA) and cellular membranas. The ultraviolet rays with the wave-length of 200 to280 nm have the most influence on bacterias. The maximum bactericidal action occurs when wavelength is about 250-260 nm.
The process of bacteria destroying is described by the equation:
p=poe-Ek, (1)
where p - is the number of bacteria per unit volume which stay alive after the action of ultraviolet radiation
po - the initial number of bacteria per unit volume;
E - the intensity of the ultra-violet rays' flow; t - the duration of exposure;
k - (to 2500) the coefficient of bacterias' resistance.
Microorganisms that are in water have the different resistance level to the action of bactericidal rays and the K coefficient depends on the type of bacteria. The coefficient of resistance of different kinds of vegetative and pathogenic bacterias practically does not exceed the coefficient of resistance of coliform bacterias which approximately equals to 2500, and it is taken in calculating of the required number of bactericidal energy for disinfection. To determine the necessary quantity of bactericidal energy the water absorbing by means the ultra-violet radiation should be taken.
By stability to ultraviolet microorganisms are sorted in the following order:
vegetative bacterias > viruses > sporing bacterias > cysts of protozoa.
It is marked more higher resistance of yeast and fungi to the influence of ultra-violet rays than in bacterias and viruses. Often the dose of irradiation varies from 16 to 40 mDzh /sec2 and depends on quality of the processed water, its application equipment and other factors.
The dose control of ultra-violet radiation is necessary with usage of such method of water disinfection. The main reasons of irradiation dose increase in ultra-violet reactors are:
- lamp damage (it is controlled according to tension in one or in a group of lamps);
- lamp intensity decreases due to deterioration;
- modern ultra-violet sources have a stable time of work (~ 1 year) and their operating life can be determined by a meter of lifelength;
- contamination of quartz covers or sharp worsening of water quality are determined by the indexes of ultraviolet selective sensor.
- turbidity and chrominance of water which diminish the radiation intensity in the water layer have a great influence on the effect of disinfection of water with ultra-violet rays. The degree of this influence can be assessed by water absorbing coefficient in ultraviolet areas. The influence of water hardness, pH and temperatures is insignificant.
Disadvantages of the method
There are disadvantages limiting substantially the application of this method, namely:
1) Water treatment with high indexes of turbidity, chrominance, content of iron can be harmful for health. Microorganisms, especially in the surface water may be connected with the components of suspension to be located in conglomerates protecting them of ultra-violet rays.
2) Suspension in water is heterogeneous its different parts take in variously, beat back, reflect ultra-violet rays. The presence of suspension can prevent interaction of the necessary dose of ultra-violet radiation with microorganisms resulting in insufficient efficiency of disinfection.
3) The reduction of viable bacteria growth is possible in some cases of application of ultra-violet irradiation usage.
4) It is known that during the process of transportation of water through water pipes some indexes of its quality change depending not only on properties of initial water.
To solve these problems water must be undergo the preliminary treatment and provide the aftereffect with the help of reagents, for example, chlorine (to 1 mg/l).
Conclusion
The creation of powerful sources of radiation, new constructive decisions of ultra-violet plants supplied by sensitive sensors which allow to measure and to control the intensity of radiation in the processed water and to provide the automatic control of intensity depending on quality of the processed water, the absence of harmful by-products and high efficiency to pathogenic microflora have made this method competitive and comparable with the method of chlorinating because of its coast.
Литература
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Рецензент Душкин С.С., профессор, д.т.н., Харьковская академия городского хозяйства.
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