How to prepare sound water: PAGs vs chlorine

World Health Organisation and the United Nations Organisation stress in their reports that the quality of water in open and underground sources used for drinking water supply has considerably decreased in the past 10 15 years, and the contamination level has grown. A large part of the world populace has no access to the safe drinking water and dies from the effects of consuming low-quality water. The issue of drinking water safety is very urgent in the Commonwealth of Independent States. The majority of the water supply systems in these countries intakes water with high level of biological and chemical contamination. Besides that, water purification facilities still use the traditional technology, namely, application of aluminium salts for coagulation, and of chlorine for decontamination of water. The facilities are often so old that they could have seen Lenin alive.

Water supply systems rely mostly on the water from the open natural sources, as well as on subterranean water sources. However, water in rivers and lakes contains suspended solid particles, dissolved substances, and also a variety of microorganisms. These microorganisms cause such dangerous diseases as cholera, enteric fever, and dysentery, to tell nothing about the relatively innocuous skin and respiratory diseases. It was believed before now that water from subterranean sources is absolutely safe and does not require special treatment. In fact, many contaminants, including even faecal waters, find their way into the underground water sources through the soil. Water from such sources can disseminate a number of bacterial and viral infections. Intestinal diseases are the most common among them. Enteroviruses, especially hepatitis, are the most dangerous and difficult suppress during water treatment.

High decontamination potential and a wide spectrum of biocide action allows reliable contamination of water for various applications even in case of high degree of initial biological and chemical contamination.

Chlorination of water is the principal technique of water decontamination since the start of the 20th century, because it allows reducing the concentration of the majority of microorganisms to a safe level. Wide application of chlorine in water preparation technologies is due to its efficiency and its ability to preserve the treated water. Chlorination also reduces water coloration, removes its odour and taste, and allows reducing the coagulant consumption. Chlorine availability and its moderate cost, as well as the vast experience of its application, ensure its leading position among the chemicals used for water preparation. More than 90% of water preparation stations use chlorine for water discoloration and decontamination. The annual world consumption of chlorine reaches two million tons.

However, this ‘number one’ reagent has essential drawbacks. The principal of them is high toxicity of chlorine itself and of its compounds formed in the reactions of chlorine with organic substances contained in water. E.g. chlorine reacting with the products of algae and wood putrefaction produces cancerogenic compounds, as well as mutagenic compounds. In the middle 1970s American researchers identified more than 300 chemical compounds produced in chlorinated water. Such toxic compounds as chloroform, carbon tetrachloride, bromine-dichlorine-methane, dibromine-chlorine-methane, and even dioxin derivatives belonging to the most dangerous poisons.

The variety of the compounds produced via this interaction is due to the difference in chemical and physical properties of water, water sources, and water treatment conditions at the water preparation facilities. Another disadvantage of chlorination is that it often degrades the organoleptic properties of natural water due to the formation of chlorinated indole compounds and chlorophenols.

Water contaminated with chlorine organic compounds was proven to be responsible for up to 75% of human diseases, including pneumonia, gastritis, liver, urinary bladder, and rectum diseases, as well as oncological diseases and various allergic reactions. A number of women suffer sterility due to the regular consumption of chlorinated water. Use of tap water contaminated with chlorine organic compounds has other hazards as well. These compounds can penetrate even undamaged skin during bathing. Moreover, due to the large interaction area in this case, the quantity of cancerogenic compounds introduced into the human organism can be rather large.

Note that chlorine is not an absolute protection; it does not kill all the microorganisms present in water. It acts mostly on the vegetating forms of bacteria, and does no harm to the spores; gram-positive bacteria are more immune to the chlorine than gram-negative ones. Chlorine is not very active against viruses; it does not act on protozoa cysts.

Active chlorine escapes water easily. So concentration of chlorine decreases on the way from the water purification facility to the water tap in an extensive pipeline system. This leads to secondary contamination of water with microflora and to pipeline corrosion. Hyperchlorination of water, however, causes formation of trihalogen methanes, especially in the parts of the water supply system closest to the water preparation facility. These compounds can cause immunity decrease, disrupt metabolism and endocrine system functionality, initiate cancer and mutations, including congenital malformation. On the other hand, several kilometres away from the purification plant the number of bacteria in water might exceed the maximum allowable concentration by two or even three orders of magnitude.

Introduction of groups with biocide properties into the structure of a polymer flocculant opens the perspective for the creation of new generation chemicals. Such new chemicals have both biocide and flocculant functions, which is very important for the water treatment technology.

The necessity to transport and store large amounts of liquid chlorine within the city limits is very dangerous, especially due to the growing number of man-caused catastrophes and terrorist attack threats. Consider the following recent example. The consequences of the crash of the roof of the “Transvaal Park” entertainment centre might have been far graver, had the chlorine containers in the basement been broken. Chlorine escaping from these containers might have killed much more people both inside and outside that ill-fated building. Discharge of chorine and its compounds into environment are very dangerous. Increased chlorine concentration can be observed in the vicinity of the majority of water preparation facilities. Besides that, chlorine irritates skin and the mucous tunics of eyes and respiratory tract. It discolours materials, causes corrosion of metals. Equipment and devices at the water preparation facilities literally “burn” in chlorine vapours.

Attempts to modify water chlorination technology did not much improve the situation. Chlorine dioxide and chloramine were used instead of chlorine for some time. However, decontaminating potential of chloramine is lower than that of the active chlorine by an order of magnitude. This chemical is absolutely ineffective against viruses and protozoa; moreover, its activity rapidly falls off as the temperature decreases. Attempts to introduce chlorine in smaller doses repeated twice or thrice were not successful, either. Non-volatile chlorine-organic compounds were produced in this case instead of volatile compounds. The former compounds are even more dangerous for humans, and they are more difficult to remove. Techniques for chlorine-organic compounds removal from water (venting it with a large volume of air, application of activated carbon filters, etc.) were found inefficient and not economically feasible.

Substitution of chlorine in water treatment technologies with ozone has been widely discusses recently. High biocide activity of ozone, especially against chlorine-resistant bacteria, spores, viruses, and protozoa cysts, guarantees high degree of water decontamination. High oxidising potential of ozone allows also to reduce water coloration and concentration of iron and manganese, and to remove odours and taste. Compact size of ozonation equipment is also attractive, and so is the possibility to automate the water treatment process. However, even this technique has its drawbacks. Ozone itself is even more toxic than chlorine, and the oxidation products of water ozonation are often more toxic than the initial water contaminants (especially when the initial concentration of organic admixtures is high).

Besides, water ozonation at the water treatment facilities produces biologically unstable water that intensifies microorganism growth in the water supply pipelines. Ozone decomposes rapidly in water and does not have a prolonged after-effect therefore chlorine needs to be introduced into reservoirs with purified water. Treatment of ozonised water with chlorine, in turn, produces still more toxic chlorine-organic compounds. All these deficiencies, as well as high power consumption of this technology, high equipment costs, and high corrosion activity of ozone are the reasons why ozonation still is not widely applied even in economically developed countries.

Thus, application of oxidising chemicals in the technologies of natural water purification leads to the contamination of drinking water with toxic chemicals produced via oxidation and chlorination of the organic admixtures. Search for non-oxidising chemicals for treatment and decontamination of drinking water in local and centralised water supply systems is a very urgent problem. The more so, since the quality of natural water degrades rapidly. The efficiency of the chemical against various microorganisms and its low toxicity for humans are the chief concerns. However, the choice of chemicals complying with these requirements is not wide.

One of the alternative techniques for water purification and decontamination is application of new generation biocides developed by the Institute of Ecotechnologies. These water-soluble chemicals based on polyalkylene guanidines (PAGs) have both disinfecting and flocculating properties. Biopag (polyhexamethylene guanidine chloride, PHMG chloride) and Phosphopag (polyhexamethylene guanidine phosphate, PHMG phosphate) are the most perspective chemicals of this family. In order to understand the role of PAGs in water treatment technology we discuss water treatment process in more detail below.

This process includes the three basic stages: coagulation, flocculation, and decontamination. Aluminium salts (sulphate, oxysulphate, oxychloride), iron salts or their mixtures are most often used as coagulants. These salts are hydrolysed in water and form insoluble hydroxide in the form of colloid particles with a large surface. The colloid particle surface adsorbs suspended particles and the greater part of the soluble admixtures. This leads to colloid particles becoming larger, heavier, and precipitating together with the adsorbed admixtures. Thus water is purified and clarified. However, only colloid particles with a well-developed surface produce the desired result, because they effectively adsorb the admixtures and precipitate completely. Otherwise small colloid particles remain suspended in water, pass through the filters and get into the water supply system. The treated water in the latter case is turbid, coloured, and contains an unsafe residue of the coagulant (aluminium) that is very hazardous for the human organism.

Flocculants are auxiliary compounds introduced into water to enhance the coagulation process. Flocculants aggregate the colloid particles, increasing their density and weight, thus enhancing sedimentation and separation of water during the filtration stage. Inorganic compounds (silicic acid) or water-soluble polymers are used as flocculants. Cationic surfactants like polyacrylamide, poly-N,N’-dimthyl-N,N’-diallylammonium chloride (ÂÏÊ-402), etc. belong to the latter group. However, various microorganisms remain in water purified and clarified with coagulants and flocculants. To prevent propagation of these microorganisms, water is treated with biocides, mostly with oxidants discussed above.

Polyguanidine chemicals suppress biocorrosion of equipment and hydroengineering structures of the water treatment facilities.

PAGs are water-soluble polymers that are cationic surfactants like polyacrylamide and ÂÏÊ-402, therefore they also act as flocculants. On the other hand, they are highly effective disinfectants with a wide spectrum of biocide activity. The latter is due to the guanidine groups attached to the polymer chain. Such groups determine the action of many natural and synthetic antiseptics and pharmaceutical substances. PAGs in water solutions effectively suppress various types of microorganisms: gram-positive and gram-negative bacteria (including tuberculosis mycobacteria), viruses, fungi of various types, yeast, mould, algae. PAGs are active over a wide temperature (0-30? C) and pH (6-9) range. In contrast to chlorine and ozone, PAGs are not oxidants, and the mechanism of their action on microorganisms is quite different.

The physical properties of PAGs make them very convenient for various applications. PAGs are water-soluble solids stable and safe in long-term storage (test duration exceeds 15 years), and have neither colour nor smell. PAGs have no taste when diluted to the concentration required for water purification, non-toxic for humans, animals, and hydrobionts. They are environment-friendly, because they precipitate to the bottom layers of natural water basins, and activated sludge decomposes them into simple non-toxic compounds.

PAGs are compatible with other chemicals used in water treatment technologies. Therefore they can be applied in the existing technological processes without an essential reconstruction of equipment. PAGs do not cause metal corrosion and do not require complicated special equipment for their handling.

Large-scale experiments with PAGs (PHMG chloride, PHMG phosphate, and their mixtures) have been performed for many years in Ukraine in the research laboratories and at the water preparation facilities in Kiev, Zhitomir, Vinnitsa, etc. These experiments have shown that PAGs purify and decontaminate water effectively both in combination with coagulants and without them. The success of PAGs application depends on the quality of natural water (that can vary over the year), choice of the chemical’s dose, and exposure duration which is inversely proportional to the applied dose.

The experiments performed at the Desnyanskaya water treatment facility have shown that the correct choice of treatment regime allows obtaining high-quality water, because PAGs help to remove humus compounds and metabolites of the microorganisms living in natural water sources. PAGs also form insoluble complexes with heavy metal salts, thus facilitating their removal.

High flocculating ability of PAGs allows reducing water turbidity, residual aluminium concentration in the purified water, and coagulant consumption in water treatment processes.

Water treated with this technology is very similar in organoleptic tests to the spring water, and complies with all the requirements for drinking water: total microbe number is 2-4 CFU/ml (the norm is below 100 units), residual iron content is 0.16 mg/l (the norm is 0.48 mg/l). After a year of uninterrupted functioning of the experimental facility for river water treatment, no traces of slime or biofouling were to be found, and no chemical corrosion of the equipment was observed.

Residual PAG concentration does not exceed the maximum allowable level determined for this chemical, whereas the initial biocide concentration was much greater than the maximum allowable value. The matter is that PAGs are effective complexing agents that interact via intermolecular forces with organic and non-organic compounds contained in natural water. Therefore the greater part of the chemical reacts with the flocculating admixtures and is detained by the filters. Residual PAG concentration in water is sufficient for the conservation of purified water.

Chemicals developed by the Institute for Ecotechnologies (Biopag, Phosphopag) are patented as biocide substances, registered with the Health Ministry of Russian Federation, and licensed for decontamination of drinking and technological water. These biocides also decontaminate effectively swimming pool and sanatory pool water. They suppress pyocyanic pathogens that are immune to chlorine. On the other hand, PAGs in contrast to the chlorine do not cause allergic reactions, do not irritate skin, and, on the contrary, facilitate healing of scratches and wounds.

Low toxicity, high stability of PAGs and their working solutions ensures ecological safety for the environment and in the working zone of the water treatment facilities, as well as during storage and transportation.

The Institute of Ecotechnologies has developed waterproof substances based on the water-soluble biocidal polymers: organosoluble varnish Septopag and organomineral sorbent Ceopag. Septopag varnish contains Biopag chemically bound to the film-forming base. This varnish forms waterproof polymer film on the surface of the treated objects. The film has the biocide properties of PAGs, yet it does not liberate biocide into water. Such coating ensures long-term prevention of biofouling of surfaces used in contact with water and also decontaminates water inside the treated vessel. According to the preliminary data, the biocide activity of such coating lasts at least 10-12 months. The varnish is recommended for long-term protection of equipment, vessels for drinking water storage and transport, and also for long-term water preservation (including drinking-quality water). Organomineral sorbent Ceopag is loose material (zeolite) having biocide, cation exchange and anion exchange properties. Water passing through a Ceopag layer is decontaminated and demineralised. The researchers of the Institute for ecotechnologies continue development and study of new biocides with valuable properties and various features.

Konstantin EFIMOV
Director of the Institute
of Ecotechnologies
Irina VOINTSEVA
Dr.Sci. (chemistry)

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