5.1. Water disinfection

The larger part of pathogenic microorganisms in drinking water is removed by means of water treatment techniques, such as coagulation, flocculation, settling, and filtration. To increase drinking water safety, disinfection is applied as a final treatment step.

Disinfection is of unquestionable importance in the supply of safe water for drinking purposes. Water disinfection means the removal, deactivation or killing of pathogenic microorganisms. During this process microorganisms are destroyed or deactivated, causing termination of growth and reproduction. When microorganisms are not removed from drinking water, its usage will cause illness in humans. Disinfection is a process related to sterilization and during this process all present microorganisms are killed, both harmful and harmless. It can be performed by means of physical or chemical disinfectants. These disinfectants also remove organic contaminants from water, serving as nutrients or shelters for microorganisms. They should not only kill microorganisms, but must also have a residual effect, which means that they remain active in the water after disinfection. A disinfectant should prevent pathogenic microorganisms from growing in the plumbing after disinfection, causing the water to be decontaminated.

For chemical disinfection of water the following disinfectants can be used:

• Chlorine (Cl₂)
• Chlorine dioxide (ClO₂)
• Hypochlorite (OCl^-)
• Ozone (O₃)
• Halogens: Bromine (Br₂), iodine (I)
• Bromine chloride (BrCl)
• Metals: copper (Cu²⁺), silver (Ag⁺)
• Kaliumpermanganate (KMnO₄)
• Fenols
• Alcohols
• Soaps and detergents
• Kwartair ammonium salts
• Hydrogen peroxide
• Several acids and bases

For physical disinfection of water the following disinfectants can be used:

• Ultraviolet light (UV)
• Electronic radiation
• Gamma rays
• Sounds
• Heat

5.2.The disinfection mode of action

The inactivation of microbiological contamination in natural or untreated water through chemical inactivation, is one of the final stages for reduction of pathogenic microflora in drinking water. Combinations of water purification steps (oxidation, coagulation, settling, disinfection, and filtration) make (drinking) water safe after production. In order to protect the water from microbiological contamination, at the end of the purification process, many countries apply an extra measure. This is a second disinfection step in order to protect the water from microbiological contamination in the water distribution system. During this secondary disinfection process, the kind of disinfectant that is usually used is different than the one used at the early stage of the process. This type of disinfection ensures that microorganisms will not multiply in the water during the distribution in the system. They can remain in the water after the first disinfection step or can end up in the water during back flushing of contaminated water (which can contain groundwater microorganisms due to cracks in the plumbing).

Disinfection commonly takes place because of disruption effect on microbial cell wall, or caused changes in cell permeability, protoplasm or enzyme activity (because of a structural change in enzymes). All these disturbances in cell activity cause reduction or termination of propagation of microorganisms and their elimination from the system. Oxidizing disinfectants also demolish organic matter in the water, causing lack of nutrients.

There are several different disinfectants, which either kill or deactivate pathogenic microorganisms. Examples of disinfectants are chlorine containing substances, peroxide, bromine, silver-copper, ozone, and UV. All disinfectants have benefits and drawbacks and can be used for water disinfection depending on the circumstances.

Besides drinking water disinfection, it may also be applied in swimming pools and cooling towers. Water disinfection is a very important factor for these applications.

Swimming pools contain a large variety of contamination, originating mainly from swimmers. The contamination contains microorganisms, among other things.

To prevent swimmers from getting infected by pathogenic microorganisms, swimming water must be disinfected. Swimming pool water is often circulated. Before the water is returned to the swimming pool, it is purified. The purification includes disinfection.

Cooling towers are used to cool down process water. After that the water can be reused. Within cooling towers circumstances are ideal for growth and multiplication of microorganisms. Biofilm development is a major problem in cooling towers, because this promotes corrosion and blocks the system.

Another problem in cooling towers, as well as in ventilation systems, is the development of Legionella bacteria. These bacteria spread through aerosols and can cause Legionnaires' disease – a very serious disease that resembles pneumonia. Many countries now have legal standards, determining that the development of Legionella bacteria in cooling towers should be prevented by disinfection of cooling water.

In the early 1970s, it was found that chemical disinfectants can form specific by-products. When this was discovered, research started on health effects of these by-products. Today, there are legal standards indicating maximum levels of disinfection by-products in drinking water. Methods to lower the concentration of disinfection byproducts in drinking water have also been researched.

5.3.Factors that influence water disinfection

• CT: A factor for measurement of the contact time between disinfectant and microorganism and the concentration of disinfectant. CT is used for calculating the concentration of disinfectant, which is required to adequately disinfect water. C refers to the final residual concentration of a particular chemical disinfectant in mg/L. T refers to the minimum contact time (minutes) of material that is disinfected with the disinfectant. Therefore, the units of CT are expressed in mg-min/L.

CT = disinfectant concentration x contact time = C mg/L x T minutes

• In case of application of a particular disinfectant to water, it does not only react with pathogenic microorganisms. It also reacts with other impurities, such as soluble metals, particles of organic matter and other microorganisms. The use of a specific disinfectant for reactions with all these substances gives information for disinfection demand of the water. When these basic disinfection demands are satisfied, a residual disinfectant concentration can be established. The disinfectant concentration that has to be added to water, is made up by the sum of the disinfection demand and the residual disinfectant concentration. Once there is a residual disinfectant concentration, this residual concentration has to be maintained during the required contact time to kill pathogenic microorganisms. To adequately disinfect the water, it is therefore required to supply the water with a higher disinfectant concentration than the concentration required to kill pathogenic microorganisms.
• Usually a dose of 12-20 mg/L chlorine is required for a free chlorine residual concentration of 6-8 mg/L. The time required to deactivate a particular microorganism decreases when the applied disinfectant concentration (mg/L) is increased. Laboratory tests are conducted to find out which contact time is most effective.
• CT is commonly used to determine the effect of a particular disinfectant against a certain microorganism under specified conditions. There is a difference between the relative effects of chemical disinfectants against different microorganisms. Often a certain level is added to CT, for example 99 %. This means that 99 % of the microorganisms are deactivated by the disinfectant. CT can be used to compare the effect of various disinfectants against microorganisms (Table 1).
• According to Table 1, ozone is the most effective disinfectant; the CT value of ozone is very low. Chloramines are least effective and cannot be used against Giardia lambia. Chlorine is effective against E. coli bacteria and Polio virus. The CT value of chlorine used against Giardia lambia is much higher than that of chlorine used against E. coli bacteria and Polio virus.

Table 1. Comparison of CT values for the 99 % inactivation of microorganisms at 5°C

5.4.Important peculiarities for determination of CT

5.4.1. The type of microorganism

Disinfectants can effectively kill pathogenic microorganisms (bacteria, viruses, and parasites). Some microorganisms can be resistant. E. coli bacteria are more resistant to disinfectants than other bacteria and for this reason they are used as indicator organisms. But several viruses are even more resistant than E. coli which absence does not mean that the water is safe. Protozoan parasites like Cryptosporidium and Giardia are very resistant to chlorine.

5.4.2. The age of the microorganism

The effect of the disinfection activity of a particular disinfectant also depends upon the age of the microorganism. The young microbial population is more vulnerable to the killing effect of the disinfectant. In older populations, different reserve metabolites like polysaccharide shell over their cell wall, makes them more resistant to disinfectants. When 2.0 mg/L chlorine is used, the required contact time to deactivate bacteria that are 10 days old is 30 minutes. For bacteria of the same species and of the age of 1 day, 1 minute of contact time is sufficient. Bacterial spores can be very resistant. Most disinfectants are not effective against bacterial spores.

5.4.3. Water nature

The nature of the water that requires treatment has its influence on the disinfection. Materials in the water, for example iron, manganese, hydrogen sulphide and nitrates, often react with disinfectants, which disturbs disinfection. Turbidity of the water also reduces the affectivity of disinfection. Microorganisms are protected against disinfection by turbidity.

5.4.4. Temperature

The temperature also influences the affectivity of disinfection. Increasing temperature usually increases the speed of reactions of disinfection. However, increasing temperature can also decrease disinfection, because the disinfectant falls apart or is volatized.

5.5.Standards for application of >water disinfection

5.5.1. Drinking water disinfection standardization

For decades, chlorine has played an important role in water treatment. Chlorine is the most widely applied disinfectant. The advantage of chlorine is that is can easily be produced and is relatively cheap. Chlorine effectively kills pathogens. It contributes to the reliability of drinking water produced from surface water. Chlorine tablets are used to disinfect water on locations where no collective drinking water treatment takes place. After the discovery of chlorinated by-products, the use of alternative disinfectants has increased.

5.5.2. Standards for drinking water disinfection in the EU

Most European countries applied drinking water disinfection at the end of the nineteenth century and the beginning of the twentieth century. Chlorine was often used for this purpose. The eldest known application of drinking water disinfection in Europe was the addition of chlorinated bleach in Middelkerke (Belgium). In 1905, the London Metropolitan Water Board started applying drinking water disinfection after researching the disinfection mechanism of chlorine in water purification. The organization thought that chlorine disinfection was a suitable alternative for long-term storage of raw water. During storage pathogenic bacteria died out naturally.

5.5.3. The European Drinking Water Directive 98/83/EC

The European Union has a drinking water policy of over 30 years. In 1998, it issued a Directive (98/83/EC) that established the minimum standards for water intended for human consumption. The Directive includes disinfectants and disinfecting by-products limits similar to those recommended by WHO. This Directive ensures that water intended for human use is safe and harmless. The Directive aims to:

• Ensure the control of drinking water quality through applying the latest scientific achievements.
• Provide an effective monitoring and assessment of drinking water quality.
• Deliver adequate and correct information in time to the consumers.
• Contribute to the broader EU water and health policy.

The EU Drinking Water Directive (98/83/EC) applies to:

• All distribution systems serving more than 50 people or supplying more than 10 cubic meters per day.
• Drinking water from tankers.
• Drinking water in bottles or containers.
• Water used in the food processing industry.

However, the Drinking Water Directive does not apply to:

• natural mineral waters; and
• waters which are medicinal products.

In the Directive, a total of 48 microbiological, chemical and indicator parameters are encompassed and are subjected to regular monitoring and testing. When implementing the Drinking Water Directive into their own national legislations, Member States of the European Union can include additional requirements, e.g., they may regulate additional substances that are relevant within their territory or set higher standards. Member States are not allowed, nevertheless, to set lower standards. In respect to the prescriptions of the Drinking Water Directive in Europe, most drinking water production companies use chlorine as a disinfectant. It is added to water as chlorine gas, calcium hypochlorite or sodium hypochlorite. Ozone is added for flavor and odor control. For drinking water preparation from surface water, chlorine is used as a primary disinfectant in most cases. For groundwater treatment, which is a simpler treatment process, chlorine is often the only proper disinfectant. Countries in Europe use alternative disinfectants for drinking water disinfection, as well (Fig. 1). France, for example, mainly uses ozone. As early as 1906, ozone was introduced for drinking water disinfection. Italy and Germany use ozone or chlorine dioxide as a primary oxidant and disinfectant. Chlorine is added for residual disinfection. United Kingdom is one of few European countries that use chloramines for residual disinfection in the distribution network and for removal of disinfection by-products. Finland, Spain and Sweden use chloramines for disinfection occasionally.

Fig. 1. Disinfection applications in some EU Members States (Lenntech: http://www.lenntech.com/applications/drinking/drinking_water.htm)

5.5.4. Biocidal products guidelines

In 1998, the Biocidal Products Directive was also implemented. Furthermore, on 22 May 2012 the Biocidal Products Regulation (BPR, Regulation (EU) 528/2012) was adopted, which repealed the Biocidal Products Directive (Directive 98/8/EC). The last one concerns the placing on the market and use of biocidal products, which are used to protect humans, animals, materials or articles against harmful organisms like pests or bacteria, through the action of the active substances contained in the biocidal product. This regulation aims to improve the functioning of the biocidal products market in the EU, while ensuring a high level of protection for humans and the environment. The BPR aims to harmonize the market at EU level, and simplify the approval of active substances and authorization of biocidal products. According to the BPR, a biocidal product is an active substance or a preparation that contains an active substance, which is intended to kill or deactivate harmful or unwanted microorganisms, by means of biological or chemical resources. Chemical disinfectants for water disinfection are also rated as biocidal products. When a biocidal product is used incorrectly, it may cause damage to human, animal or plant health, or to the environment. The countries of the European Union determine whether a substance can be used for certain purposes. When a company needs permission to apply a certain biocidal product, it must be requested from the government of the country. A demand must also be sent to the EU government. The governments of countries mainly decide whether a substance is permitted. This may cause a substance to be permitted by a certain European country, but restricted by the European Union and vice versa.

References:

1. Council Directive 98/83/EC of 3 November 1998 on the Quality of water intended for human consumption. OJ L 330, 5.12.1998, p. 32–54
2. Synthesis Report on the Quality of Drinking Water in the EU examining the Member States' reports for the period 2008-2010 under Directive 98/83/EC. REPORT FROM THE COMMISSION COM(2014).
3. The consolidated version of the Regulation (EU) No 528/2012 of the European Parliament and of the Council of 22 May 2012 concerning the making available on the market and use of biocidal products (BPR), http://eur-lex.europa.eu/legal-content/
4. NSF Guide to the European Standards for Drinking Water Treatment Units, 2009 (http://www.nsf.org/newsroom_pdf/)
5. EU's drinking water standards, LENNTECH,http://www.lenntech.com/
6. Design of Water Resources Systems. P. Purcell (ed.). Thomas Telford Limited, Bristol 2003.
7. Guidelines for Drinking Water Quality. 2011. World Health Organization http://apps.who.int/iris/bitstream/