Disinfection of drinking water: problems and solutions
Prof. Bakhir V.M., Ph. D., Dr. Sc.
ОАО NPO “Ekran”
Ministry of Health of Russian Federation
The basic criteria of quality of drinking water which were formulated in middle of the twentieth century, consist of the following: drinking water should be safe in epidemic relation, it should be harmless in it’s chemical structure and have favorable organoleptic properties. Now these criteria are accepted all over the world. The normative documents are created in the field of quality of drinking water in the various countries, including in Russia - SanPin 18.104.22.1684-01. The same criteria is a basis for the Manual on quality surveillance of drinking water, issued By the world organization of public health, in 1984 and 1994 . [1, 2].
At estimation of risk to health, the most important role is played by microbiological pollutions. So, the researches of doctor Robert Tardiff [3, 4] (USA) have shown, that danger of diseases caused by microbiological pollution of water is many thousand times higher (up to 100000 times), than at pollution of water by different chemicals.
This estimation is most evidently expressed in existing practice of drinking water disinfection in the majority of developed countries. For example, 98,6 % of drinking water is chlorinated in the USA. Ozonation makes only 0,37 %, other methods - 0,75 % . The reason is, that chlorination is the most economic and effective method of drinking water disinfection in comparison with any other known methods. Chlorination provides microbiologically safe water in any point of the distribution network at any moment due to the “aftereffect”. All other methods of water disinfection, including ozonation and ultra-violet, do not provide a disinfection aftereffect and, hence, require chlorination on one of the stages of water processing. This rule is not an exception for Russia where all ozonation systems of drinking water in municipal water-distribution networks contain the equipment for chlorination.
One of the disadvantages of water chlorination process is the disinfection by-products products (HСP) formation, the most of which are trihalomethanes (ТHМ): chloroform, dichlorobrommethane, dibromochlormethane and bromoform. The trihalomethanes are formed from variety of organic precursors in chlorination-disinfection process. The THM formation reactions are some of the slowest reactions. The THM formation occurs over several hours, and the amount increases after 24 hours. The kinetics of THM formation is depends on many factors such as pH, residual chlorine concentration. Therefore, the application of sodium or calcium hypochlorite for water disinfection as alternative to molecular chlorine does not reduce but considerably increases probability of THM formation. The most rational way for reduction of chlorination by-products formation is the reduction of precursors concentration prior to disinfection stage.
Today, the maximum allowable levels of total chlorination by-products are set from 0,06 to 0,2 mg/l in developed countries and correspond to modern scientific facts on any adverse effects on human health. Scientific discussion on the carcinogenic and mutagenic toxicity of THMs has been lasted for many years in the USA and it has been recognized that the level of THM formation through water disinfection is safe for the above mentioned levels [6-12].
However, the reduction of chlorination by-products, as well as by-products of ozonation which are not less dangerous (see table 1) than chlorination by-products, is one of the reasons to search for new technologies for drinking water disinfection.
Table 1 shows known advantages and disadvantages of basic and alternative methods and technologies of water disinfections.
The characteristics of some water disinfectants
The analysis of the data, presented in table 1, allows to understand, that among known methods there is no ideal one, as well as there is no recipe of “ideal drinking water” with all importance of its parameters for public health effect and safety. It is obvious, that parameters and properties of drinking water are determined by geographical, geological, climatic, hydrological conditions and regional differences in a degree and level of territorial economic development. That is why regulation of drinking water quality in developed countries is based on authentic, scientifically proved specifications of its microbiological (priority parameter) and chemical parameters in respect of safety and harmlessness for the human being and defines the order of drinking water quality control. This order takes into account regional water sources, methods of water processing and its delivery to consumers.
For modern technologies of water disinfection the most important task is the development of the combined methods utilizing the advantages of the known disinfectants (table 1,) and eliminating their disadvantages.
The technology of water disinfection by nascent oxidants generated in AQUACHLOR devices [13, 14] falls into this category.
The AQUACHLOR devices generate a combination of oxidants, which are by disinfection efficacy come as chlorine, chlorine dioxide and ozone, and at the same time significantly reduce formation of chlorination and ozonation by-products. The AQUACHLOR devices are alternative and safe - in - operation on-site chlorine generators which can be used as a replacement for cylinders and containers with liquid chlorine for municipal water disinfection, industrial waste water treatment, swimming pools water disinfection.
Fig. 1. Flow chart diagram for Aquachlor device
The main principle of AQUACHLOR devices is an electrochemical synthesis of gaseous mixture of oxidants - chlorine, chlorine dioxide, ozone from saturated solution of Sodium Chloride (200 - 250 g/l) under pressure in diaphragm modular electrochemical elements FEM-7, each of which is a separate electrochemical reactor.
The block diagram of AQUACHLOR device is presented on Fig. 1.
The Sodium Chloride solution is delivered into the anode chambers of electrochemical reactor of the device from the pressurized tank. Due to the features of design of elements FEM-7 and pressure differential on the diaphragm (from 0,5 up to 1,0 kgs/sm2), an electro-diffusion of sodium ions and water through the ceramic diaphragm is carried out. This resulted in a complete division of sodium chloride solution into gaseous products, removed from the anode chamber and a sodium hydroxide (concentration 120 - 150 g/l) formed in the cathode chamber. The gaseous oxidants produced in the anode chamber together with micro-droplets of water containing hydroperoxides – singlet oxygen, peroxide and super-oxide of hydrogen are delivered by the injection pump into the part of water to be processed resulting in concentrated (0,5 up to 2,0 g/l - average about 1 g/l) solution of oxidants. Along with Sodium Hydroxide solution, the hydrogen is generated at the rate of 1,4 g for every 100 g of gaseous oxidants in the cathode chambers of electrochemical elements FEM-7. For production of 1 kg of oxidants in AQUACHLOR devices it takes approximately 1,7-2,0 kg of dry Sodium Chloride and about 2 KW-h of electric power.
The basic reaction in the electrochemical reactor of AQUACHLOR device is the formation of molecular chlorine and sodium hydroxide:
NaCl + H2O – e ® NaOH + 0,5 H2 + 0,5 Cl2
Simultaneously with a lower current efficiency the reactions of chlorine dioxide formation directly from acidified in anode chamber (Cl2 + H2O « HClO + HCl) are taking place:
2NaCl + 6H2O – 10e ® 2ClO2 + 2NaOH + 5 H2 ;
HCl + 2H2O - 5e ® ClO2 + 5 H+ .
In addition, the reaction of free oxygen/ozone formation by anode oxidation and direct decomposition of water is taking place:
3H2O - 6e ® O3 + 6H+ ;
2H2O - 4e ® 4H+ + O2; Þ O2 + Н2О - 2e ® O3 + 2 Н+ .
The reactions of active oxygen/peroxo-radicals formation are taking place with very low current efficiency:
H2O - 2e ® 2H+ + O· ; Н2О - е ® HO· + Н+ ; 2H2O - 3e ® HO2 + 3H+ .
In contrast to traditional chlorine production technologies such as mercury, a diaphragm electrolysis and ion-selective membrane electrolysis, the AQUACHLOR technology for gaseous mixture of oxidants production does not require acidification of feeding Sodium Chloride solution, does not require additional expenditure on water and chemical reagents, allows to divide the chloride solution into targeted products in one operation cycle by electrochemical reactor, and thus is essentially new.
The targeted product coming out of AQUACHLOR device is 0.1% aqueous solution of oxidants’ mixture (chlorine, chlorine dioxide, ozone) for municipal water disinfection, industrial and household wastewater and swimming pools water.
The aqueous solution of oxidants is a colorless transparent liquid with рН = 2,5 + 0,5 and a smell of chlorine. Gaseous mixture of oxidants synthesized in AQUACHLOR device, consists of molecular chlorine (90 - 95 %), chlorine dioxide (3 - 7 %) and small quantity of ozone (0,5 - 3,0 %). This gaseous mixture of oxidants also contains approximately 0,5 - 1,5 % of extremely active oxidants - singlet oxygen and micro-droplets of moisture containing hydroperoxide and oxy-chlorine products of electrochemical reactions, generated in the anode chamber of the electrochemical modular elements under excess pressure when Sodium ions separated by the ion-selectivity process through the ceramic diaphragm.
The main working anti-microbial agent in the solution of oxidants is a hypochlorous acid, which is formed during dissolving of chlorine in water along with chlorine dioxide. These agents take more than 98% of all oxidants contained in a solution with concentration equal to 1 g/l of the free chlorine equivalent. The advantages and disadvantages of solution of oxidants produced by AQUACHLOR device are presented in table 2.
The productivity of AQUACHLOR device can be adjusted by adjusting the direct current through the electrochemical reactor. The opportunity of an instant stop of process and its instant start is stipulated.
The AQUACHLOR devices are certified in Russian Federation as well as the solution of oxidants produced by these devices has a sanitary certificate of the State Service of Epidemic Control of the Russian Federation. The application of a solution produced by AQUACHLOR device for water disinfection (drinking water supply, household and industrial waste water and water of swimming pools), is regulated by the Instruction authorized by the State Service of Epidemic Control of the Russian Federation. The AQUACHLOR devices are manufactured in two basic models: AQUACHLOR-100 and AQUACHLOR-500 with productivity 100 and 500 grams of oxidants per hour correspondingly (TU 3614-702-05834388-02, ОКP 36 1469). The electrochemical reactor of AQUACHLOR device - 500 is a modular one. This feature allows to adjust oxidants productivity by connecting the necessary amount of FEM-7elements into a united hydraulic system.
The productivity of AQUACHLOR -100 and AQUACHLOR-500 is correspondingly 100 and 500 liters per hour.
Safe operation of AQUACHLOR devices at no risk of a poisoning of the operators and environment by uncontrollable emission of chlorine are guaranteed by small volumes of gaseous oxidants (less than 200 ml), which under small pressure (about 1 kgs/sm2) proceed through the pipeline inside the device, through a pressure regulator and through an injecting pump to be dissolved in small volume of processed water, thus converting into analogue of chlorinated water.
Thus, AQUACHLOR device has obvious advantages by safety factor, ecologically friendliness and profitability for water disinfection at minimal risk in comparison with the existing chlorination technologies.
The solution of oxidants produced by AQUACHLOR devices is mixed with water to be disinfected in the proportion ensuring the initial given level of oxidants according to the technology of water treatment by free (gaseous or liquid) chlorine. The hydro peroxides, ozone and chlorine dioxide enter into reaction with water’s admixtures and disappeared after 5 – 10 minutes.
The main disinfectant substance ensuring the oxidants residual is the hypochlorous acid (HClO), the presence of which guarantees water disinfection in complete conformity with the known technological processes of liquid or gaseous chlorine. The presence of ozone and hydro-peroxides in the solution of oxidants provides absence of by-products formation. This fact is confirmed by a number of experimental research works during operation of AQUACHLOR devices at water processing stations (drinking water stations, waste water treatment stations).
The solution of Sodium Hydroxide can be used as a coagulant or as a washing-up liquid (dissolution is necessary).
The concentration of oxidants in solution produced by AQUACHLOR devices can e determined by standard methods used for water chlorination.
The quantity of oxidants generated can be calculated by the amount of direct current passed through the electrochemical reactor of the AQUACHLOR device.
The AQUACHLOR devices are recommended to be installed and in ventilated rooms. Their overall dimensions of the Aquachlor devices are comparable to the sizes of facilities used for storage and distribution of liquid chlorine of the equivalent chlorine productivity. Hydrogen generated during the electrolysis is removed through a separate line to an atmosphere.
The characteristics of the new alternative water disinfectant - solution of oxidants from AQUACHLOR installation
Water disinfection by an aqueous mixture of oxidants produced by AQUACHLOR devices, conforms to the requirements of SanPin 22.214.171.1244-01 (Standard of Russian Federation).
All known disinfectants for portable water treatment have both advantages and disadvantages. The most effective disinfectant produced by electrochemical device Aquachlor is combining advantages of existing disinfectants in the solution of oxidants and is free from their disadvantages.
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