Modern methods of water disinfection
and Comparative sanitary-hygienic assessment

Data analysis of drinking water 2000-2006 years, shows that the largest percentage deviation in the centralized water supply systems by bacteriological characteristics were observed in Kharkiv (7.2%), Kirovohrad (7.7%), Ternopyl (8.3%) and the Zakarpatsky(8.7%) regions. Average in Ukraine the density of non-standard samples of drinking water from centralized water systems by bacteriological indices are about 4.7%, particularly in public water supply - 3.5%, institutional - 5%, rural - 7.2%.

To the main nonconformance factors of water sanitation regulations must include: lack of sanitary protection zones (71%) required complex treatment facilities (15%), disinfection units (18%). Moreover, for the last seven years the subsamples of drinking water that does not meet regulatory requirements as well as the reasons of non-standard drinking water quality has not changed.

So, today one of the major problems that need to be solved at the drinking water production is the introduction of progressive methods of treatment, including disinfection of drinking water.

Purification - is water decontamination for sanitary and microbiological quality improvement by reactant or nonreagent methods. The main reactant decontamination method includes: ozonation, chlorination (chlorine treatment and chlorine-containing reagents - chlorine gas, sodium and calcium hypochlorite, bleaching powder, chlorine dioxide, chloramines), processing oxidants (liquid or gaseous), etc.; nonreagent methods - ultraviolet (UV) exposure, etc. (table 1). All decontamination methods have their advantages and disadvantages and need to carry out complex comparative evaluation.

In the early twentieth century it became known about the possibility of effective use of ozone for drinking water disinfection. But ozone turned out to be toxic (1-st class of hazard) and explosive reagent. While the ozonation the aftereffect was absent, in particular the residual ozone concentration of 0.4 mg/dm3 in drinking water decomposes faster than 1 hour. Today it is known that the use of ozone does not exclude the formation of disinfection byproducts. Furthermore, the problem of ozonation products influence on human health remains insufficiently studied until now.

It is known that chlorination of ozonized water leads to formation of more toxic compounds (Goncharuk, VV, 2005; Petrenko NF, 2005; Avchynnykov, AV, 2001; Drahynskyy VL, 1995, Goncharuk Y. I. and others., 1982; Lawrence H., 1980). Thus, lack of aftereffect, insecurity of operation of equipment and formation of disinfection byproducts limits the use of ozone in water supply stations. At present, this method can not replace chlorine completely, but only complement it.

Common use of UV radiation method limits insufficiency of monitoring of decontamination process, with other methods. Evaluation of epidemiological water safety after UV disinfection can be obtained only after 24 hours and determined by the coli-index. Furthermore, reliable disinfection by UV radiation is possible only after effective removal of muddiness and water color. There is clear dependence of cidal effect from muddiness and color of water, type of microorganisms, their quantity, radiation doses, type of unit, so this method can be treated only underground or polished effluents after water filters, especially after the equipment effectiveness test in each case (Goncharuk, V.V., 2005; Avchynnykov, A.V., 2001). So after UV water treatment there is still residual concentrations of disinfectants in the water, which limits application of this method, particularly in practice of centralized drinking water supply. UV can be prospectively applied complex with other methods of decontamination.

Thus, the benefits of chlorine technologies (using chlorine gas, sodium hypochlorite and calcium, chlorine dioxide, chloramines, etc.) are such that have lasting disinfectant effect and method of monitoring by disinfection process, prevent to refuse from them in the practice of centralized drinking water supply. In Ukraine, chlorine gas (98%) is used for water disinfection, hypochlorite sodium (1,1%), small quantities of chlorine dioxide, ozone and other reagents and technologies (0,9%) (Chorunzy P.D., 2004).

But everybody knows the disadvantages chlorination methods: chlorine and its compounds are toxic drugs, so work with them requires strict abidance of safety rules, existing schemes of chlorination have insufficient cleaning effect against enteroviruses, besides in the process of chlorination of drinking water disinfection byproducts are formed, that have carcinogenic, mutagenic and teratogenic properties (Prokopov V.A., Zorina O.V., 2002). It is established that in particular the gastrointestinal and urinary tract are the systems of human body that commonly covered by carcinogenic effects associated with consumption of chlorinated drinking water (Cantor K.G., 1999; Ryabukhin V.G., 1987; Williamson S.J., 1981).

By the results of our experimental studies, including four chlorine-containing reagent sodium hypochlorite has the greatest reactivity on formation of carcinogenic compounds chlororganic (sodium hypochlorite bleaching lime > liquid chlorine or chlorine-gas > chloramines) (Prokopov V.A., Zorina O.V., 2006). Nowadays, hypochlorites are used in low-power stations or water disinfection for a long mine shafts with ceramic cartridges. During the operation in OJSC 'AK Kievvodokanal' of electrolysis units produced by 'Ukrpromtehvod', that produce sodium hypochlorite, also revealed a number of shortcomings of this method. In particular, the need of annual change of metal-oxide coating of titanium anodes and other components. These effects are usually lead to the need of regeneration of anodes by hydrochloric acid.

Sometimes the manufacturers of drinking water are satisfied using commercial hypochlorite for water disinfection. When storing the last, the ability of disinfectant reagent to decrease in time is observed (particularly under the influence of light and heat), it is necessary to consider its use, including special preservatives, that complicates the operation (Chorunzy P.D., 2004). Besides, the leak is possible, which is dangerous for people.

If further to comparison between modern methods of chlorination, we can note the following. The ammonia-chloride of water provides conservation of the residual chlorine in a tap water network and contributes to formation of smaller chlororganic compounds (in comparison to other chlorine containing reagents), but needs to provide long contact with water, has less oxidative and bactericidal effect. Explosive balloon chlorine gas and chlorine dioxide require strict precautions when using. In particular, the initial product of chlorine dioxide manufacture is dangerous hydrochloric acid. The disadvantages of chlorine dioxide are its toxicity (compound 1-st class of hazard) and its transformation products - chlorites and chlorates. In particular, chlorite-ion relates to arbitrary carcinogenic compounds. So, using the chlorine dioxide in water treatment station it is necessary to adapt methods of detect concentrations of oxidant working solution, and by-products content of processing, in particular, that have different levels of LPC in Russia, Europe and the USA (Prokopov V.A., 1997).

Constant growing of demands for drinking water quality and the environment protection in the background of intense contamination of drinking water sources determine the necessity of finding alternative methods of water disinfection, including methods based on the synergy of two or more reagents. Precisely such methods include decontamination of reagent mixture - oxidizing agents. OXI-gas produced by American "OXI" units, which is planed in Ukraine, and liquid oxidant - "Aquachlor" units produced in Russia.

According to Russian and American scientists NSF, water disinfection by oxidants compared to chlorine gas and sodium hypochlorite are more effective, there has been a longer cleansing effect, less contact with oxidant water is needed that substantially reduces the formation of carcinogenic trihalomethanes. These methods are the efficient way of disinfection process monitoring, high efficiency under different physical and chemical conditions of water and combat with biofouling of water supply systems, they can be used to remove iron, manganese and hydrogen sulphide and require less quantity of sulfur dioxide to reduce residual chlorine than in traditional chlorination.

It is interesting to compare two methods of oxidant decontamination together. If you compare composition of two oxidants and their aggregate state, it is possible to come to a conclusion that the OXI-gas is better dissolved in water and has less chlorine in the mixture (70%) and greater content of more active components (30%) that can contribute to more bactericidal effect.

A significant shortcoming of 'Aquachlor' units is a need for hydrochloric acid regeneration of electrodes. Every 40-50 hours of work of 'Aquachlor' units it is necessary to wash the reactor of electrochemical block with toxic solution of hydrochloric acid to remove cathode deposits in the electrodes of reactor by special techniques.

"OXI-units" in comparison to 'Akvahlor' units are safer and easier in operation because they do not need to use hydrochloric acid solution. For their effective work you will need initial safe resources - water and sodium chloride (enough any clean salt "Extra" or stone).

According to American scientists NSF, oxidant gas provides sufficient disinfectant effect (stronger and longer compared to chlorination), if in water that is disinfected, in 10 minutes after oxidant treatment free residual active chlorine is 0.1 mg/dm3. While drinking water disinfection by oxidant gas the temperature, density, total alkalinity, the contents of manganese, iron, ammonia nitrogen, etc. do not change, chlorite and chlorate are not defined.

Thus, the method of OXI-gas disinfection completely devoid of many drawbacks of traditional methods, including chlorination, and it has all its positive qualities, making it a worthy alternative to traditional decontamination methods, particularly in practice of centralized, noncentralized drinking water, service water supply and wastewater.