“Infectious diseases caused by pathogenic bacteria, viruses and parasites (such as protozoans and helminths) represent the most common and widespread health risk associated with drinking water.”
World Health Organization (WHO). “Guidelines for Drinking Water Quality”
Water is truly the source of life for all living things. However, most consumers perceive this statement from a subjective point of view, without thinking that the same water is a perfect environment for living, but not always safe, microbes. Today, there are a number of harmful (pathogenic) microorganisms whose presence in water, according to the WHO, is not allowed. Some of the most prominent are:
- bacteria (cholera vibrio, salmonella, Pseudomonas aeruginosa);
- viruses (hepatitis A, B, E; adenovirus);
- Protozoa (Cryptosporidium).
Fortunately, modern technology of disinfection of drinking water can reliably protect the consumer from all this “micro-artillery”, the main thing is to know how to use them correctly.
Our ancestors knew many centuries ago that it is necessary to disinfect water. In ancient civilizations of Egypt, Greece and Persia they used to boil drinking water and store it in silver vessels before drinking. It is a surprising fact, because microscopes appeared many centuries later, and the different types of microorganisms and their impact on human health have been studied relatively recently. No less surprising and even unexplainable is the fact that the mass epidemics of antiquity have not ceased even in the face of modern scientific and technological progress. Such echoes of the distant past as plague and cholera are already treated, but at the same time, people in underdeveloped countries continue to suffer from massive epidemics of infectious diseases caused by poor quality drinking water and failure to follow basic hygiene rules.
The problem of water safety has recently caused considerable excitement and attracted the attention of various health and environmental organizations around the world. Today, many scientists and research centers are focused on finding and developing alternative methods of water disinfection, which in the current situation would be both affordable and effective.
At the same time, let us slightly narrow the global scope of the problem and consider the situation with water disinfection in our country.
At present, the water supply in Ukraine can be represented as follows:
- 70% centralized water supply (piped water);
- 30% local water supply (wells, wells).
And if in the first case, water purification and control of its compliance with current regulations is carried out through government agencies (water utilities, SES), in the second case, all responsibility falls on the consumer.
Now let’s consider in order, which methods of disinfection are used in both cases.
DISINFECTION OF WATER IN CENTRALIZED WATER SUPPLY SYSTEMS
While all over the world they implement new technologies and reagents for water disinfection, Ukrainian water utilities, as many years ago, use old “good” method of chlorination with liquid chlorine. Chlorine is known to be a toxic and fire hazardous substance, so its transportation, storage and dosing is a big problem for municipal water utilities and poses a serious potential hazard to local residents. At the same time, such a long loyalty to this method is due to a number of significant advantages:
- This is the cheapest method of water disinfection;
- it is well studied and easy to apply;
- Chlorine is active against most microorganisms, including pathogens, and provides a high degree of disinfection;
- Only chlorine and chlorine-containing reagents have an after-effect, which is necessary to maintain microbiological purity of water during its transportation in pipelines.
Because of this effect, chlorination remains an indispensable stage in the process of water disinfection in centralized water supply and, not surprisingly, is universally used in all developed countries. At the same time, in contrast to Ukraine, in the U.S. and European countries instead of liquid chlorine safer chlorine-containing reagents are used – sodium hypochlorite, chloramines, chlorine dioxide. But in this situation it is necessary to understand that the real danger is not the chlorine itself or chlorinated chemicals, but the products which are formed by their interaction with various water impurities, in particular, natural organic substances, typical for surface water sources (see Reactions to chlorine and other chemical reagents). (See the editorial note № 2). As numerous studies have shown, in decontamination processes it is these impurities that are the main precursors of most potentially hazardous to human health byproducts. Therefore, in developed countries, maximum attention is paid to removal of organic impurities before the chlorination stage and the content of by-products in purified water is strictly regulated.
In Ukraine, natural organic impurities are a real disaster for surface water sources and one of the biggest problems for local water utilities. Removal of these impurities in water treatment plants today is carried out using the traditional method of coagulation, followed by sedimentation and filtration of water through mechanical filters with quartz sand. This technology allows you to reduce the organic content by no more than 50-60%. As a result, the normalized values are achieved, but the content of natural organic substances remains quite high, as evidenced by the typical yellowish tap water color. High cost of the latest generation effective coagulants and flocculants, high quality activated carbon, ultrafiltration technologies and other modern methods of removal of natural organics, widely used abroad, make them inaccessible for our water utilities (cf. (See the editorial board’s note No. 3).
Thus, from the official point of view, the microbiological purity of tap water fully complies with regulations and is safe. If you do not like the smell of chlorine, the yellow color of water and the possibility of consuming toxic disinfection products, it is better to purify water at the point of consumption, for example, using household filters.
WATER DISINFECTION IN CONDITIONS OF LOCAL WATER TREATMENT
Based on the above, we can distinguish two main tasks of local water treatment:
the first – purification of tap water from residual chlorine and possible disinfection by-products
The second one is treatment and disinfection of water from wells and other sources of decentralized water supply.
To date, the most effective and optimal way to solve the first problem is to use filters containing activated carbon. High sorption capacity of coal and its large specific surface area can remove not only chlorinated substances, but also natural organic compounds. For post-treatment of tap water one can use pitcher-filters, flow-through filters of different modifications and the most effective option of cleaning – the domestic reverse osmosis system. All of these solutions at some stage of treatment contain activated carbon (in the case of reverse osmosis carbon is used as pre-treatment) and fully cope with the task of removing residual chlorine and chlorination by-products, as we have repeatedly written on the pages of WEBT.
The solution of the second problem is more complex. So, in most cases, microbiological contamination of water for wells is not inherent. This is explained by the fact that groundwater lies at a considerable depth and different soil layers (sand, limestone, gravel) serve as a kind of barrier, trapping microorganisms. At the same time, small bacteria and viruses, over time, are still able to penetrate to a considerable depth and infect the underground aquifers. In addition, there is a possibility of contamination of well water during the installation of pumping and water treatment equipment. But this is not the biggest problem, because it can be easily solved by disinfection of the well with sodium hypochlorite solution or granulated calcium hypochlorite (the procedure is recommended at least twice a year). More serious problems with the microbiology of water arise at different stages in the process of its purification.
Thus, in many schemes of local treatment of water from wells and purification of tap water for elimination of undesirable taste and smell of water, activated carbon is used (see “Activated carbon”). above). Most often it is bituminous, coconut shell charcoal, or a combination of these, which allows for very effective removal of both large and small impurities in water.
The most common and commonly used are cartridges containing granulated, powdered or pressed activated carbon (so-called carbon blocks). In addition, cartridge manufacturers add special substances to activated carbon or make mixtures of several types of loads to give additional properties to the adsorption material. For example, silver, as well as other bacteriostatic materials are added to charcoal to prevent bacteria from multiplying (see. “Championship of pitcher filters,” VIVT #4, 2012).
Despite the importance and necessity of this stage, it is often the charcoal filters that are the main source of microbiological contamination of already treated water. What is the problem? In the process of coal filtration various organic impurities are retained, as well as microorganisms, albeit in small amounts, but always present in purified water. With continuous operation of the cleaning system, the constant flow of water prevents microorganisms from firmly attaching to the surface of the coal, so at first there are no problems. If the system does not operate for some time (during weekends or vacations), the entrenched microorganisms acquire all conditions for active life. Normal room temperature, water and nutrient substrate (organic substances sorbed on coal) significantly accelerate the reproduction and development of the microorganisms. On the surface of the carbon a biological film is formed, which, after the resumption of the system, is periodically torn off by the flow of water and contaminate it.
it is possible that after some time, the carbon can still “wash out” of the microbiological contamination, but it is best to immediately replace the cartridge with a new one when the filter is not used for a long time.
Storage containers can be another source of microbiological contamination of water. In essence, the problem is similar to the previous one: when water is stored in the tank for a long time, even with a minimum initial number of microorganisms, their reproduction, development occurs, and the storage tank becomes a permanent source of contamination of purified water. In the local water treatment scheme these are most often tanks for water accumulation after reverse osmosis units. And if the reverse osmosis membrane is really an absolute barrier to all water impurities, including microorganisms, absolute confidence in the sterility of the tank is not. A variety of micro-organisms can be present in the tank initially or get into it at the stage of installation. To prevent or solve this problem, manufacturers recommend periodic treatment of storage tanks with bactericides, such as sodium hypochlorite solution. However, in order to fully guarantee your own safety water after the tank is also better to disinfect.
What methods and techniques of water disinfection can be applied at home? Among the most common are the following:
- UV disinfection
- use of filter media with bactericidal properties.
Boiling is one of the simplest, most affordable, and quite effective ways to disinfect water. This method allows not only to eliminate most microorganisms, including pathogens, but also to get rid of unpleasant water odor caused, among other things, by residual chlorine. However, it is inconvenient to constantly boil drinking water, besides boiling has other disadvantages:
- Residual chlorine in the water can form even more dangerous byproducts when heated;
- some bacteria are quite resistant to heat and die after a long time – up to several hours of continuous boiling.
Thus, boiling can provide a high degree of water disinfection, but for a comprehensive and more lasting effect it is desirable to use additional methods of disinfection.
One of such methods is water disinfection with ultraviolet radiation. Today it is one of the most effective and environmentally friendly methods of fighting the various microorganisms in the water, air and on different surfaces. This physical method has been used for many years and is distinguished not only by high efficiency, but also by the absence of negative effects on the chemical composition of purified water. Until recently, a wide spread of UV significantly hindered the high cost and energy intensity of the method, but today a large range of lamps of different power allows its use in a variety of areas disinfection. For domestic water treatment the UV lamp is just perfect. Compact and ergonomic, they can be easily installed as part of a purification system, ensuring trouble-free operation and effective water disinfection.
In large capacity water treatment systems, UV lamps are most often installed after the carbon filter before the domestic reverse osmosis unit and the domestic hot water outlet (see “UV lamps for domestic hot water”). Figure. 1). Given the high susceptibility of carbon filters to microbiological contamination, this placement of the UV lamp prevents the following possible problems
- Biological fouling of heating elements of household equipment (boilers, boilers) and reducing their efficiency. Very often in the operation of boilers, water over time, gets an unpleasant smell of hydrogen sulfide, which is also caused by the growth of microorganisms in the boiler;
- biological fouling of reverse osmosis membranes, reducing productivity and quality of purification;
- The possibility of pathogens entering the human body when bathing;
In addition, to prevent the possibility of contamination of the water after the storage tank, it is also recommended to install the UV lamp in the treated drinking water supply line.
Another physical method of water disinfection, which can be applied in the local water treatment – ultrafiltration. The essence of disinfection with ultrafiltration is that the passage of water through a semi-permeable membrane with a small pore size detains various impurities: colloids, organic substances, algae and most microorganisms (bacteria and viruses). Characteristically, until recently this method was mainly used to remove colloidal impurities and suspended solids on an industrial scale. Nowadays the interest in its use for removal of microorganisms has significantly increased, in particular in domestic water treatment.
Today, various companies produce compact, easy-to-use Ultrafiltration cartridges, which are as effective disinfection as UV lamps. The cartridges of this type are also recommended for use after the storage tank for additional treatment of water from possible microbiological contamination.
What to do, if you use filters jug or flow type, where the use of ultraviolet and ultrafiltration is impossible or impractical? First of all, the water after these types of filters can be boiled. It no longer contains chlorine, partially purified from natural organic impurities, so boiling in this case is very effective and safer. At the same time, an optimal and more reliable solution to this problem is the use of loads with bactericidal properties in household filters. Most often for this purpose use standard loads (carbon, ion exchange resins), containing silver ions. The bactericidal properties of this noble metal have long been known, but perhaps not everyone knows that silver also has toxic properties (see editorial note 4). Editor’s note #4). Therefore, the main requirement for sorbents based on it is as follows: metal ions must be firmly fixed on the material to avoid the possibility of its ingress into drinking water. For example, activated carbon containing silver ions has been used for quite a long time. Unfortunately this sorbent was not widely used as it did not only fail to provide a proper disinfection of water but also considerably increased its microbiological composition. An effective sorbent devoid of these disadvantages was developed by scientific-production association “Ecosoft” and is actively used today in drinking water filters of TM “Nasha Voda”. The sorbent is a highly porous polymeric material containing silver nanoparticles. Since the particles remain in a carefully fixed state, they are not washed out and do not get into the purified water. At the same time, microorganisms present in the water contact with the silver on the surface of the polymeric material and die. Thus this biocide sorbent simultaneously provides both proper disinfection of water and its compliance with quality standards of silver content.
In addition to all of the above, one of the most reliable and convenient solutions in terms of local water treatment is the use of integrated water treatment systems. The latest representatives of this direction is P’URE line of products manufactured by “Ecosoft” Ltd. Depending on modification, P’URE system may include up to 9 stages of purification, which allows to get ultrapure water with improved taste and, most importantly, safe bacteriostable. The latter is achieved in the final stage of treatment where an ultrafiltration membrane is used to disinfect water after the storage tank.
the eternal battle
Microorganisms in water are no less dangerous to human health than other contaminants. Therefore, the effective disinfection of drinking water is a very important and indispensable stage of its purification. Today, disinfection of drinking water is a major problem for India and many African countries, where microbiological contamination of water is the main cause of frequent mass epidemics of infectious diseases among the population. The use of well-known and effective methods of disinfection, as well as comprehensive water treatment in this situation is difficult and often impossible. However, especially for these difficult cases, scientists have developed an affordable and sufficiently effective method of disinfecting water in ordinary PET (polyethylene terephthalate) bottles under the influence of direct sunlight. The technology is called SODIS and is intensively used today in over 30 countries (Indonesia, Bolivia, Philippines, Uzbekistan, Vietnam and others).).
For developed countries, armed with a full arsenal of effective technologies, water disinfection, it would seem, is not a problem. Nevertheless, as recent studies have shown, the problem does exist. For example, many species of microorganisms get used to disinfectants and even UV radiation over time. Resistant (resistant) strains of microbes are formed, for the removal of which it is necessary to increase the dosage of reagents (in the case of UV intensity), which is only a temporary solution.
In addition, water disinfection plays an important role not only in drinking water supply but also in industrial water treatment (WVT No. 3, 2012). Since, various microorganisms are a threat not only to human health, but also to the effective operation and functioning of various equipment and processes.
Thus, today the search for new alternative methods and ways of water disinfection is important and urgent. The editorial staff of the VIVT magazine will continue to inform you of new developments in this area.