Disinfection of drinking water: problems and solutions

By materials of article of Prof. Bakhir V.M., Ph. D., Dr. Sc.
"Disinfection of drinking water: problems and solutions",
issued in Russian journal "Drinking water", №1, 2003. Water

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 drinking water quality in the various countries, including in Russia - SanPin 2.1.4.1074-01. The same criteria are a basis for the Manual on quality surveillance of drinking water, issued by the world organization of public health, in 1984 and 1994.

At estimation of risk to health, the most important role is played by microbiological pollutions. So, the researches of Dr. Robert Tardiff (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 (HCP) formation, the most of which are trihalomethanes (THM): 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 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.

However, the reduction of chlorination by-products, as well as by-products of ozonation which are not less dangerous (see table) than chlorination by-products, is one of the reasons to search for new technologies for drinking water disinfection.

Table shows known advantages and disadvantages of basic and alternative methods and technologies of water disinfections.

The characteristics of some water disinfectants

The name and characteristics	Advantages	Disadvantages
Basic-type disinfectants
Chlorine Is applied in a gaseous form and requires strictest safety measures	efficient oxidant and disinfectant effectively eliminates unpleasant taste and odors featured with aftereffect prevents and controls growth of algae, biological slimes and microbes decomposes organic contaminants (phenols, etc.) oxidizes iron and magnesium decomposes hydrogen sulfide, cyanides, ammonium and other nitrogen compounds	strict requirements for transportation and storage potential risk to health in case of leakage formation of disinfection by-products, such as trihalomethanes. Formation of bromates and brom-organic disinfection by-products at presence of bromides
Sodium hypochlorite Is applied in a liquid form (trade concentration - 10 -12 %), can be obtained on-site through electrochemical generation.	effective against most of pathogenic microorganisms relatively safe during storage and use when produced on site does not require transportation and storage of hazardous chemicals	ineffective against cysts (Giardia, Cryptosporidium) looses its activity during long-term storage potential danger of gaseous chlorine emission during storage produces disinfection by-products, such as trihalomethanes, including bromates and brominated by-products in presence of bromides generated on-site requires immediate use, or in case of storage, special measures to purify water and salt heavy metals ions generated on-site with concentration of free available chlorine above 450 mg/l and рН >9 accumulates chlorates over time
Chlorine dioxide On-site generation only. Commonly excepted as the most effective disinfectant among other chlorine containing agents for water treatment at alkaline pH levels	works in small dozes does not react with ammonia nitrogen does not react with oxidizable compounds to form trihalomethanes; destroys some THM precursors destroys phenols that cause - source of unpleasant taste and odor effective oxidant and disinfectant for all types of microorganisms, including cysts (Giardia, Cryptosporidium) and viruses does not react with bromides to form bromates or brominated by-products Improves removal of iron and manganese by rapid oxidation and settling of oxidized compounds	Requires on-site generation equipment requires transportation and storage of inflammable chemicals forms chlorates and chlorites in contact with some organic materials and compounds poses unique odor and taste
Chloramine Is formed by mixing of ammonia with free available chlorine.	commonly used as a disinfectant with prolonged action persistent residual minimize unpleasant taste and odor reduces level of trihalomethanes and haloacetic acid formation prevents biofilms formation in distribution systems	provides weaker oxidation and disinfection then free chlorine is inefficient against viruses and cysts (Giardia, Cryptosporidium) requires increased doses and contact time for disinfection presents danger to individuals on dialysis machines, since it can pass through membranes in dialysis machines and induce oxidant damage to erythrocytes produces disinfection by-products, including nitrogen-based compounds and chloral hydrate
Alternative disinfectants
Ozone Has been used for several decades for taste and odor control, color removal and disinfection	strong disinfectant and oxidant very effective against viruses most effective against Giardia, Cryptosporidium, and other known pathogens enhances turbidity removal under certain conditions controls taste and odor does not form chlorinated by-products	produces disinfection by-products: aldehydes, ketones, carboxylic acids, brom -containing thrihallomethanes (including bromoform), brominated by-products; brominated acetic acids; peroxides; quinones necessity to use biologically active filters to remove by-products does not provide residual disinfection effect requires high initial expenses for equipment significant expenses for operators training and installation support reacting with organic compounds, ozone disintegrates the into smaller molecules , which become s a feeding media for microorganisms in water distribution systems
Ultraviolet Exposing water to UV radiation effectively inactivates various microorganisms	does not require storage and transportation of chemicals does not form disinfection by-products effective against cysts (Giardia, Cryptosporidium)	no residual action high maintenance requirements high initial capital cost high operating (energy) cost disinfecting activity depends on water turbidity, hardness, biofouling of UV lamps, wavelength of UV radiation or power failure does not provides express method for measuring the efficiency of water disinfections

The analysis of the data, presented in table, 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) and eliminating their disadvantages.

In Russia disinfection of water for household and drinking purposes oxidants solution is carried out in accordance with the requirements of SanPiN 2.1.4.1074-01.