Chlorine DioxideWe are the manufacturer of 99.99% pure Liquid Chlorine Dioxide with trade name Chlorine Dioxide, it is two component base system, and it does not require any generator or chlorine to generate Chlorine Dioxide. It is almost ready to use ClO2 Solution. Chlorine Dioxide has lots of applications for industrial & Human use and has batter result in comparison to other chemicals for the same application (For Proper comparison Please check the attachment) Industrial Market:• Cooling Towers• Pulp & Paper• Irrigation Water• Waste Water• Water Parks & Resorts Human Use:• water• Municipal water• Ice cube Industry Urban & leisure market • Hotels and resorts • Hospitals • Swimming Pools • Domestic Sanitize Bio Industry: • Poultry Growers • Egg Production • Swine farms • Meat and milk farms • Fish and Shrimp Processing • Fruit and VegetableHere you can find few supporting documents to prove that Chlorine Dioxide is more effective than any Chemical like bleaching powder , Sodium Hypo , Ozone etc. For every application we are having the ready case study for your support.

Chlorine dioxide

Chlorine dioxide

IUPAC name

Chlorine dioxide

Other names

Chlorine (IV) oxide

Chloryl

Identifiers

CAS number 10049-04-4

PubChem 24870

ChemSpider 23251

UNII 8061YMS4RM

EC number 233-162-8

MeSH Chlorine+dioxide

ChEBI CHEBI:29415

RTECS number FO3000000

Gmelin Reference 1265

Jmol-3D images

Properties

Molecular formula ClO2

Molar mass 67.45 g mol−1

Appearance Yellow to reddish gas

Odor Acrid

Density 2.757 g dm

Melting point −59 °C (−74 °F; 214 K)

Boiling point 11 °C (52 °F; 284 K)

Solubility inwater 8 g dm−3 (at 20 °C)

Solubility soluble in alkaline

and sulfuric acid

solutions

kH 4.01 x 10−2 atm-cu

m/mole

Acidity (pKa) 3.0(5)

Thermochemistry

Std molar

entropySo298

257.22 J K−1 mol−1

Std enthalpy of

formationΔfHo298

104.60 kJ/mol

Hazards

MSDS ICSC 0127

EU Index 017-026-00-3

EU classification

O T+

C N

R-phrases R6, R8, R26, R34, R50

S-phrases (S1/2), S23, S26, S28,

S36/37/39,S38, S45, S6

1

LD50 292 mg/kg (oral, rat)

Chlorine dioxide is a chemical compound with the formula ClO2. This yellowish-green gas crystallizes as bright orange crystals at −59 °C. As one of several oxides of chlorine, it is a potent and useful oxidizing agent used in water treatment and in bleaching.

Structure and bonding

Comparison of three-electron bond to the conventional covalent bond.

The two resonance structures

The molecule ClO2 has an odd number of valence electrons and therefore, it is a paramagnetic radical. Its electronic structure has long baffled chemists because none of the possible Lewis structures is very satisfactory. In 1933 L.O. Brockway proposed a structure that involved a three-electron bond. Chemist Linus Pauling further developed this idea and arrived at two resonance structures involving a double bond on one side and a single bond plus three-electron bond on the other. In Pauling's view the latter combination should represent a bond that is slightly weaker than the double bond. In molecular orbital theory this idea is commonplace if the third electron is placed in an anti-bonding orbital. Later work has confirmed that the HOMO is indeed an incompletely-filled orbital.

PreparationChlorine dioxide is a compound that can decompose extremely violently when separated from diluting substances. As a result, preparation methods that involve producing solutions of it without going through a gas phase stage are often preferred. Arranging handling in a safe manner is essential.

In the laboratory, ClO2 is prepared by oxidation of sodium chlorite:

2 NaClO2 + Cl2 → 2 ClO2 + 2 NaCl

Over 95% of the chlorine dioxide produced in the world today is made from sodium chlorate and is used for pulp bleaching. It is produced with high efficiency by reducing sodium chlorate in a strong acid solution with a suitable reducing agent such as methanol, hydrogen peroxide, hydrochloric acid or sulfur dioxide. Modern technologies are based on methanol or hydrogen peroxide, as these chemistries allow the best economy and do not co-produce elemental chlorine. The overall reaction can be written;

Chlorate + Acid + reducing agent → Chlorine Dioxide + By-products

The reaction of sodium chlorate with hydrochloric acid in a single reactor is believed to proceed via the following pathway:

HClO3 + HCl → HClO2 + HOClHClO3 + HClO2 → 2 ClO2 + Cl2 + 2 H2OHOCl + HCl → Cl2 + H2O

The commercially more important production route uses methanol as the reducing agent and sulfuric acid for the acidity. Two advantages by not using the chloride-based processes are that there is no formation of elemental chlorine, and that sodium sulfate, a valuable chemical for the pulp mill, is a side-product. These methanol-based processes provide high efficiency and can be made very safe.

A much smaller, but important, market for chlorine dioxide is for use as a disinfectant. Since 1999 a growing proportion of the chlorine dioxide made globally for water treatment and other small-scale applications has been made using the chlorate, hydrogen peroxide and sulfuric acid method, which can produce a chlorine-free product at high efficiency. Traditionally, chlorine dioxide for disinfection applications has been made by one of three methods using sodium chlorite or the sodium chlorite - hypochlorite method:

2 NaClO2 + 2 HCl + NaOCl → 2 ClO2 + 3 NaCl + H2O

or the sodium chlorite - hydrochloric acid method:

5 NaClO2 + 4 HCl → 5 NaCl + 4 ClO2 + 2 H2O

All three sodium chlorite chemistries can produce chlorine dioxide with high chlorite conversion yield, but unlike the other processes the chlorite-HCl method produces completely chlorine-free chlorine dioxide but suffers from the requirement of 25% more chlorite to produce an equivalent amount of chlorine dioxide. Alternatively,hydrogen peroxide may efficiently be used also in small scale applications.

Very pure chlorine dioxide can also be produced by electrolysis of a chlorite solution:

2 NaClO2 + 2 H2O → 2 ClO2 + 2 NaOH + H2

High purity chlorine dioxide gas (7.7% in air or nitrogen) can be produced by the Gas:Solid method, which reacts dilute chlorine gas with solid sodium chlorite.

2 NaClO2 + Cl2 → 2 ClO2 + 2 NaCl

These processes and several slight variations have been reviewed

Handling properties

At gas phase concentrations greater than 30% volume in air at STP (more correctly: at partial pressures above 10 kPa ), ClO2 may explosively decompose intochlorine and oxygen. The decomposition can be initiated by, for example, light, hot spots, chemical reaction, or pressure shock. Thus, chlorine dioxide gas is never handled in concentrated form, but is almost always handled as a dissolved gas in water in a concentration range of 0.5 to 10 grams per liter. Its solubility increases at lower temperatures: it is thus common to use chilled water (5 °C or 41 °F) when storing at concentrations above 3 grams per liter. In many countries, such as the USA, chlorine dioxide gas may not be transported at any concentration and is almost always produced at the application site using a chlorine dioxide generator. In some countries, chlorine dioxide solution below 3 grams per liter in concentration may be transported by land, but are relatively unstable and deteriorate quickly.

UsesChlorine dioxide is used primarily (>95%) for bleaching of wood pulp, and for the disinfection (called chlorination) of municipal drinking water.

BleachingChlorine dioxide is sometimes used for bleaching of wood pulp in combination with chlorine, but it is used alone in ECF (elemental chlorine-free) bleaching sequences. It is used at moderately acidic pH (3.5 to 6). The use of chlorine dioxide minimizes the amount of organochlorine compounds produced. Chlorine dioxide (ECF technology) currently is the most important bleaching method world wide. About 95% of all bleached Kraft pulp is made using chlorine dioxide in ECF bleaching sequences.

Chlorine dioxide is also used for the bleaching of flour.

Water chlorinationThe Niagara Falls, New York, water treatment plant first used chlorine dioxide for drinking water treatment in 1944 for phenol destruction. Chlorine dioxide was introduced as a drinking water disinfectant on a large scale in 1956, when Brussels, Belgium, changed from chlorine to chlorine dioxide. Its most common use in water treatment is as a pre-oxidant prior to chlorination of drinking water to destroy natural water impurities that produce trihalomethanes on exposure to free chlorine. Trihalomethanes are suspect carcinogenic disinfection by-products associated with chlorination of naturally occurring organics in the raw water. Chlorine dioxide is also superior to chlorine when operating above pH 7, in the presence of ammonia and amines and/or for the control of biofilms in water distribution systems. Chlorine dioxide is used in many industrial water treatment applications as a biocide including cooling towers, process water, and food processing.

Chlorine dioxide is less corrosive than chlorine and superior for the control of legionella bacteria. Chlorine dioxide is superior to some other secondary water disinfection methods in that chlorine dioxide: 1) is an EPA registered biocide, 2) is not negatively impacted by pH 3) does not lose efficacy over time (the bacteria will not grow resistant to it) and 4) is not negatively impacted by silica and phosphate, which are commonly used potable water corrosion inhibitors.

It is more effective as a disinfectant than chlorine in most circumstances against water borne pathogenic microbes such as viruses, bacteria and protozoa – including the cysts of Giardia and the oocysts ofCryptosporidium.

The use of chlorine dioxide in water treatment leads to the formation of the by-product chlorite, which is currently limited to a maximum of 1 ppm in drinking water in the USA. This EPA standard limits the use of chlorine dioxide in the USA to relatively high quality water or water, which is to be treated with iron based coagulants (Iron can reduce chlorite to chloride).

Other disinfection uses

It can also be used for air disinfection and was the principal agent used in the decontamination of buildings in the United States after the 2001 anthrax attacks. After the disaster of Hurricane Katrina in New Orleans, Louisiana and the surrounding Gulf Coast, chlorine dioxide has been used to eradicate dangerous mold from houses inundated by the flood-water. Sometimes it is used as a fumigant treatment to 'sanitize' fruits such as blueberries, raspberries, and strawberries that develop molds and yeast.

Chlorine dioxide is used for the disinfection of endoscopes, such as, under the trade name Tristel. It is also available in a "trio" consisting of a preceding "pre-clean" with surfactant and a succeeding "rinse" with deionised water and low-level antioxidant.

Chlorine dioxide also is used for control of zebra and quagga mussels in water intakes.

Chlorine dioxide also was shown to be effective in bedbug eradication.

Other usesChlorine dioxide is used as an oxidant for phenol destruction in waste water streams and for odor control in the air scrubbers of animal byproduct (rendering) plants. It is also available for use as a deodorant for cars and boats, packaged as a chlorine dioxide generating package activated by water, and left in the boat/car overnight.

Safety issues in water and supplementsChlorine dioxide is toxic, hence limits on exposure to it are needed to ensure its safe use. The United States Environmental Protection Agency has set a maximum level of 0.8 mg/L for chlorine dioxide in drinking water. The Occupational Safety and Health Administration (OSHA), an agency of the United States Department of Labor, has set an 8 hour permissible exposure limit of 0.1 ppm in air (0.3 milligrams per cubic meter (mg/m3) for people working with chlorine dioxide.

On July 30, 2010 and again on October 1, 2010, the United States Food and Drug Administration (FDA) warned against the use of the product "Miracle Mineral Supplement" or "MMS", which when made up according to instructions produces chlorine dioxide. MMS has been marketed as a treatment for a variety of conditions, including HIV, cancer, autism, and acne. The FDA warnings informed consumers that MMS can cause serious harm to health, and stated that it has received numerous reports of nausea, severe vomiting, and life-threatening low blood pressure caused by dehydration, among other symptoms, such as diarrhea.

Chlorine dioxide

Chlorine dioxideThe quest for the disinfectant replacement of chlorine resulted in several possible candidates. Although no one disinfectant is perfect, Chlorine dioxide is a very good alternative due to its characteristics.

1. What is stabilised Chlorine dioxide?Like ozone and chlorine, chlorine dioxide is an oxidizing biocide and not a metabolic toxin. This means that chlorine dioxide kills microorganisms by disruption of the transport of nutrients across the cell wall, not by disruption of a metabolic process. Stabilised chlorine dioxide is ClO2 buffered in an aqueous solution. Adding an acid to the required concentration activates the disinfectant.

2. How does it work?

Of the oxidizing biocides, chlorine dioxide is the most selective oxidant. Both ozone and chlorine are much more reactive than chlorine dioxide, and they will be consumed by most organic compounds. Chlorine dioxide however, reacts only with reduced sulphur compounds, secondary and tertiary amines, and some other highly reduced and reactive organics. This allows much lower dosages of chlorine dioxide to achieve a more stable residual than either chlorine or ozone. Chlorine dioxide, generated properly (all chlorine dioxide is not created equal), can be effectively used in much higher organic loading than either ozone or chlorine because of its selectivity.

3. How effective is it?

The effectivety of chlorine dioxide is at least as high as chlorines, though at lower concentrations. But there are more and important advantages.

1. The bactericidal efficiency is relatively unaffected by pH values between 4 and 10;2. Chlorine dioxide is clearly superior to chlorine in the destruction of spores, bacteria's,

viruses and other pathogen organisms on an equal residual base;3. The required contact time for ClO2 is lower;4. Chlorine dioxide has better solubility;5. No corrosion associated with high chlorine concentrations. Reduces long term maintenance

costs;6. Chlorine dioxide does not react with NH3 or NH4+;7. It destroys THM precursors and increases coagulation;8. ClO2 destroys phenols and has no distinct smell;9. It is better at removing iron and magnesia compounds than chlorine, especially complex

bounds;4. How is it applied?

Chlorine dioxide can be used in two ways. The first is the on-site generation through a special process. The second is the possibility to order Chlorine dioxide in its stabilised form (SCD).

SCD is activated on-site whenever its usage is desirable. It can be dosed into an existing or new process where disinfection is required.This makes it an easy-to-use, safe and versatile disinfectant.

The dosing system is compact, safe, flexible and low on maintenance.

Where is it applied?

Prevention and control

In the prevention and control of legionnaires disease causing microbes, chlorine dioxide has taken an eminent roll. The specific characteristics of the disinfectant make sure ClO2 gets the job done where others fail.

Biofilm in the piping can protect legionella from most of the disinfectants.

Chlorine dioxide however removes the biofilm and kills the bacteria, spores and viruses.

Other advantages are:

1. The bactericidal efficiency is relatively unaffected by pH values between 4 and 10;2. The required contact time for ClO2 is lower;3. Chlorine dioxide has better solubility;4. Chlorine dioxide does not react with NH3 or NH4+;5. It destroys THM precursors and increases coagulation;6. ClO2 destroys phenols and has no distinct smell;

Biofilm removal and control

A biofilm is a layer of microorganisms contained in a matrix (slime layer), which forms on surfaces in contact with water. Incorporation of pathogens in biofilms can protect the pathogens from concentrations of biocides that would otherwise kill or inhibit those organisms freely suspended in water.

Biofilms provide a safe haven for organisms like Listeria, E. coli and legionella where they can reproduce to levels where contamination of products passing through that water becomes inevitable.

It has been proven beyond doubt that chlorine dioxide removes biofilm from water systems and prevents it from forming when dosed at a continuous low level. Hypochlorite on the other hand has been proven to have little effect on biofilms.

Cooling towers treatment

Cleaning and disinfecting cooling towers is essential for several reasons. Most of which are well known. Clean pipes mean higher heat exchange efficiency, pump lifetime improvement and lower maintenance costs.

Most people however, are unfamiliar with the fact that cooling towers pose a possible health risk. The high temperature condition is ideal for the growth of several pathogen organisms (like legionella).

The usage of chlorine dioxide comes with several advantages:

• It is a very powerful disinfectant and biocide;• It prevents and removes biofilm;• Unlike chlorine, Chlorine dioxide is effective at pH between 4 and 10. No dumping and filling with fresh water required;• The corrosive effects of chlorine dioxide are minimal compared to the corrosive effects of plain tap water;• The bactericidal efficiency is relatively unaffected by pH values between 4 and 10. Acidisation, therefore is not required;• Chlorine dioxide can be used as a spray. All parts therefore, can easily be reached;• And last but not least: less environmental impact.

Scrubbers

Scrubbers are similar in design to cooling towers. The primary difference between the two is that scrubbers are pressurized systems, while cooling towers are vacuum systems. Scrubber's re-circulate water and spray it across the top of the system, counter-currently to the airflow. The function of re-circulating water is to absorb odour-causing species from the air.

Chlorine dioxide added to the re-circulated water reacts rapidly with odour-causing species that have been absorbed in the water, as well as those species that remain in the air. Usually, a very low chlorine dioxide residual, around 0.2-ppm, is sufficient to ensure odour control.

Potable water disinfection

Chlorine dioxide has been used for years in potable water disinfection (US since 1944). The need arose when it was discovered that chlorine and similar products formed some dangerous DPD's (disinfection by-products) like THM (trihalomethanes).

Since then many UK and US based water companies have started using ClO2. There are however more reasons to use chlorine dioxide:

1. The bactericidal efficiency is relatively unaffected by pH values between 4 and 10;

2. Chlorine dioxide is clearly superior to chlorine in the destruction of spores, bacteria's, viruses and other pathogen organisms on an equal residual base;3. The required contact time for ClO2 is lower;4. Chlorine dioxide has better solubility;5. No corrosion associated with high chlorine concentrations. Reduces long term maintenance costs;6. Chlorine dioxide does not react with NH3 or NH4+;7. It destroys THM precursors and increases coagulation;8. ClO2 destroys phenols and has no distinct smell;9. It is better at removing iron and magnesia compounds than chlorine, especially complex bounds;

Vegetables washing

Chlorine dioxide is an excellent product for washing vegetables. The ability to kill spores, viruses and fungi at low concentrations is essential.

ClO2 is a proven product that can be used to solve several food-related problems. It does not affect taste, odour or appearance. It is safe to use and complies with food regulations. Below are some examples where chlorine dioxide has been applied.

• Apples: control of E.Coli and listeria bacteria's• Potatoes: protection against "late blight" and "silver scurf"• Lettuce, celeries and onions: compared to hypochlorite the vitamin-c content resulted higher and the potassium content lower• Citrus fruits: protection against "green mould" and "sour rot" proved to be successful at several pH values, low concentrations and limited contact time.

Hot and cold water systems

The advantages in using chlorine dioxide with hot and cold water systems have already been shown at the descriptions on biofilm and legionella. There are however more advantages:

1. The bactericidal efficiency is relatively unaffected by pH values between 4 and 10;2. Chlorine dioxide is clearly superior to chlorine in the destruction of spores, bacteria's,

viruses and other pathogen organisms on an equal residual base (even cryptosporidium and giardia);

3. The required contact time for ClO2 is lower;4. Chlorine dioxide has better solubility;5. No corrosion associated with high chlorine concentrations. Reduces long term maintenance

costs;6. Chlorine dioxide does not react with NH3 or NH4+;7. It destroys THM precursors and increases coagulation;8. ClO2 destroys phenols and has no distinct smell;9. It is better at removing iron and magnesia compounds than chlorine, especially complex

bounds;

Chlorine dioxideChlorine dioxide is mainly used as a bleach. As a disinfectant it is effective even at low concentrations, because of its unique qualities.When was chlorine dioxide discovered?Chlorine dioxide was discovered in 1814 by Sir Humphrey Davy. He produced the gas by pouring sulphuric acid (H2SO3) on potassium chlorate (KClO3). Than he replaced sulphuric acid by hypochlorous acid (HOCl). In the last few years this reaction has also been used to produce large quantities of chlorine dioxide. Sodium chlorate (NaClO3) was used instead of potassium chlorate.2NaClO3 + 4HCl ® 2ClO2 + Cl2 + 2NaCl + 2H2OWhat are the characteristics of chlorine dioxide ?Chlorine dioxide (ClO2) is a synthetic, green-yellowish gas with a chlorine-like, irritating odor. Chlorine dioxide is a neutral chlorine compound. Chlorine dioxide is very different from elementary chlorine, both in its chemical structure as in its behavior. Chlorine dioxide is a small, volatile and very strong molecule. In diluted, watery solutions chlorine dioxide is a free radical. At high

concentrations it reacts strongly with reducing agents. Chlorine dioxide is an unstable gas that dissociates into chlorine gas (Cl2), oxygen gas (O2) and heat. When chlorine dioxide is photo-oxidized by sunlight, it falls apart. The end-products of chlorine dioxide reactions are chloride (Cl-), chlorite (ClO-) and chlorate (ClO3

-).At –59°C, solid chlorine dioxide becomes a reddish liquid. At 11°C chlorine dioxide turns into gas. Chlorine dioxide is 2,4 times denser than air. As a liquid chlorine dioxide has a bigger density than water.Can chlorine dioxide be dissolved in water?One of the most important qualities of chlorine dioxide is its high water solubility, especially in cold water. Chlorine dioxide does not hydrolyze when it enters water; it remains a dissolved gas in solution. Chlorine dioxide is approximately 10 times more soluble in water than chlorine. Chlorine dioxide can be removed by aeration or carbon dioxide.

Table 1: the solubility of chlorine dioxide in watertemperature (°C) pressure (mm Hg) solubility (g/L)

25 3.0125 34.5 1.8225 22.1 1.1325 13.4 0.6940 8.4 2.6340 56.2 1.6040 18.8 0.8340 9.9 0.4760 106.9 2.6560 53.7 1.1860 21.3 0.5860 12.0 0.26

How can chlorine dioxide be stored?The best way to store chlorine dioxide is as a liquid at 4 ºC. At this state it is fairly stable. Chlorine dioxide cannot be stored for too long, because it slowly dissociates into chlorine and oxygen. It is rarely stored as a gas, because it is explosive under pressure. When concentrations are higher than 10% chlorine dioxide in air, there is an explosion hazard. In a watery solution, chlorine dioxide remain stable and soluble. Watery solutions containing approximately 1% ClO2 (10 g/L) can safely be stored, under the condition that they are protected from light and heat interference. Chlorine dioxide is rarely transported, because of its explosiveness and instability. It is usually manufactured on site.How is chlorine dioxide produced?Chlorine dioxide is explosive under pressure. It is difficult to transport and is usually manufactured on site. Chlorine dioxide is usually produced as a watery solution or gas. It is produced in acidic solutions of sodium chlorite (NaClO2), or sodium chlorate (NaClO3). For large installations sodium chlorite, chlorine gas (Cl2), sodium hydrogen chlorite (NaHClO2) and sulphuric or hydrogen acid are used for the production of chlorine dioxide on site.To produce chlorine dioxide gas, hydrochloric acid (HCl) or chlorine is brought together with sodium chlorite.The to main reactions are:

2NaClO2 + Cl2 ® 2ClO2 + 2NaCl(Acidified hypochlorite can also be used as an alternative source for chlorine.)

And:5 NaClO2 + 4HCl ® 4 ClO2 + 5NaCl + 2H2O(One disadvantage of this method is that it is rather hazardous.)

An alternative is:2 NaClO2 + Na2S2O8 ® 2ClO2 + 2Na2SO4

Chlorine dioxide can also be produced by the reaction of sodium hypochlorite with hydrochloric acid:HCl + NaOCl + 2NaClO2 ® 2ClO2 + 2NaCl + NaOH

The amount chlorine dioxide that is produced varies between 0 and 50 g/L.What are the applications of chlorine dioxide?Chlorine dioxide has many applications. It is used in the electronics industry to clean circuit boards, in the oil industry to treat sulfides and to bleach textile and candles. In World War II, chlorine became scarce and chlorine dioxide was used as a bleach.Nowadays chlorine dioxide is used most often to bleach paper. It produces a clearer and stronger fiber than chlorine does. Chlorine dioxide has the advantage that it produces less harmful byproducts than chlorine. Chlorine dioxide gas is used to sterilize medical and laboratory equipment, surfaces, rooms and tools. Chlorine dioxide can be used as oxidizer or disinfectant. It is a very strong oxidizer and it effectively kills pathogenic microorganisms such as fungi, bacteria and viruses. It also prevents and removes bio film. As a disinfectant and pesticide it is mainly used in liquid form. Chlorine dioxide can also be used against anthrax, because it is effective against spore-forming bacteria.Chlorine dioxide as an oxidizerAs an oxidizer chlorine dioxide is very selective. It has this ability due to unique one-electron exchange mechanisms. Chlorine dioxide attacks the electron-rich centers of organic molecules. One electron is transferred and chlorine dioxide is reduced to chlorite (ClO2

- ).

Figure 2: chlorine dioxide is more selective as an oxidizer than chlorine. While dosing the same concentrations, the residual concentration of chlorine dioxide is much higher with heavy pollution

than the residual concentration of chlorine.By comparing the oxidation strength and oxidation capacity of different disinfectants, one can conclude that chlorine dioxide is effective at low concentrations. Chlorine dioxide is not as reactive as ozone or chlorine and it only reacts with sulphuric substances, amines and some other reactive organic substances. In comparison to chlorine and ozone, less chlorine dioxide is required to obtain an active residual disinfectant. It can also be used when a large amount of organic matter is present.

The oxidation strength describes how strongly an oxidizer reacts with an oxidizable substance. Ozone has the highest oxidation strength and reacts with every substance that can be oxidized. Chlorine dioxide is weak, it has a lower potential than hypochlorous acid or hypobromous acid.The oxidation capacity shows how many electrons are transferred at an oxidation or reduction reaction. The chlorine atom in chlorine dioxide has an oxidation number of +4. For this reason chlorine dioxide accepts 5 electrons when it is reduced to chloride. When we look at the molecular weight, chlorine dioxide contains 263 % 'available chlorine'; this is more than 2,5 times the oxidation capacity of chlorine.Table 2: the oxidation potentials of various oxidants.oxidant oxidation strength oxidation capacityozone (O3) 2,07 2 e-hydrogen peroxide (H2O2) 1,78 2 e-hypochlorous acid (HOCl) 1,49 2 e-

hypobromous acid (HOBr) 1,33 2 e-chlorine dioxide (ClO2) 0,95 5 e-

The following comparisons show what happens when chlorine dioxide reacts. First, chlorine dioxide takes up an electron and reduces to chlorite:ClO2 + e- ® ClO2

-

The chlorite ion is oxidized and becomes a chloride ion:ClO2

- + 4H+ + 4e- ® Cl- + 2H2O

These comparisons suggest that chlorine dioxide is reduced to chloride, and that during this reaction it accepts 5 electrons. The chlorine atom remains, until stable chloride is formed. This explains why no chlorinated substances are formed. When chlorine reacts it does not only accept electrons; it also takes part in addition and substitution reactions. During these reactions, one or more chlorine atoms are added to the foreign substance.Table 3: the availability of chlorine per mol weightagent available chlorine (%)chlorine (Cl2) 100bleaching powder 35-37calcium hypochlorite (Ca(OCl)2) 99,2commercial calcium hypochlorite 70-74sodium hypochlorite (NaOCl) 95,2industrial bleach 12-15house hold bleach 3-5chlorine dioxide 263,0monochloramine 137,9dichloramine 165,0trichloramine 176,7

Does chlorine dioxide oxidize in the same way as chlorine?Contrary to chlorine, chlorine dioxide does not react with ammonia nitrogen (NH3) and hardly reacts with elementary amines. It does oxidize nitrite (N02) to nitrate (NO3). It does not react by breaking carbon connections. No mineralization of organic substances takes place. At neutral pH or at high pH values, sulphuric acid (H2SO3) reduces chlorine dioxide to chlorite ions (ClO2

-). Under alkalic circumstances chlorine dioxide is broken down to chlorite and chlorate (ClO3

-) :2ClO2 + 2OH- = H2O + ClO3

- + ClO2-

This reaction is catalyzed by hydrogen (H+) ions. The half life of watery solutions of chlorine dioxide decreases at increasing pH values. At low pH, chlorine dioxide is reduced to chloride ions (Cl- ).Does chlorine dioxide produce byproducts?Pure chlorine dioxide gas that is applied to water produces less disinfection byproducts than oxidators, such as chlorine. Contrary to ozone (O3), pure chlorine dioxide does not produce bromide (Br-) ions into bromate ions (BrO3

-), unless it undergoes photolysis. Additionally chlorine dioxide does not produce large amounts of aldehydes, ketons, keton acids or other disinfection byproducts that originate from the ozonisation of organic substances.What are the disinfection applications of chlorine dioxide?Drinking water treatment is the main application of disinfection by chlorine dioxide. Thanks to its adequate biocidal abilities, chlorine dioxide is also used in other branches of industry today. Example are sewage water disinfection, industrial process water treatment, cooling tower water disinfection, industrial air treatment, mussel control, foodstuffs production and treatment, industrial waste oxidation and gas sterilization of medical equipment.How does chlorine dioxide disinfect?Chlorine dioxide disinfects through oxidation. It is the only biocide that is a molecular free radical. It has 19 electrons and has a preference for substances that give off or take up an electron. Chlorine dioxide only reacts with substances that give off an electron. Chlorine, oppositely, adds a chlorine atom to or substitutes a chlorine atom from the substance it reacts with.How does disinfection by chlorine dioxide work?Substances of organic nature in bacterial cells react with chlorine dioxide, causing several cellular

processes to be interrupted. Chlorine dioxide reacts directly with amino acids and the RNA in the cell. It is not clear whether chlorine dioxide attacks the cell structure or the acids inside the cell. The production of proteins is prevented. Chlorine dioxide affects the cell membrane by changing membrane proteins and fats and by prevention of inhalation. When bacteria are eliminated, the cell wall is penetrated by chlorine dioxide. Viruses are eliminated in a different way; chlorine dioxide reacts with peptone, a water-soluble substance that originates from hydrolisis of proteins to amino acids. Chlorine dioxide kills viruses by prevention of protein formation. Chlorine dioxide is more effective against viruses than chlorine or ozone.Can chlorine dioxide be used against protozoan parasites?Chlorine dioxide is one of a number of disinfectants that are effective against Giardia Lambia and Cryptosporidium parasites, which are found in drinking water and induce diseases called 'giardiasis' and 'cryptosporidiosis'. The best protection against protozoan parasites such as these is disinfection by a combination of ozone and chlorine dioxide.Can microorganisms become resistant against chlorine dioxide? Chlorine dioxide as a disinfectant has the advantage that it directly reacts with the cell wall of microorganisms. This reaction is not dependent on reaction time or concentration. In contrast to non-oxidizing disinfectants, chlorine dioxide kills microorganisms even when they are inactive. Therefore the chlorine dioxide concentration needed to effectively kill microorganisms is lower than non-oxidizing disinfectant concentrations. Microorganisms cannot built up any resistance against chlorine dioxide.Can chlorine dioxide be used against bio film?Chlorine dioxide remains gaseous in solution. The chlorine dioxide molecule is powerful and has the ability to go through the entire system. Chlorine dioxide can penetrate the slime layers of bacteria, because chlorine dioxide easily dissolves, even in hydrocarbons and emulsions. Chlorine dioxide oxidizes the polysaccharide matrix that keeps the bio film together. During this reaction chlorine dioxide is reduced to chlorite ions. These are divided up into pieces of bio film that remain steady. When the bio film starts to grow again, an acid environment is formed and the chlorite ions are transformed into chlorine dioxide. This chlorine dioxide removes the remaining bio film.What are the disinfection byproducts of chlorine dioxide?The reaction process of chlorine dioxide with bacteria and other substances takes place in two steps. During this process disinfection byproducts are formed that remain in the water. In the first stage the chlorine dioxide molecule accepts an electron and chlorite is formed (ClO3). In the second stage chlorine dioxide accepts 4 electrons and forms chloride (Cl-). In the water some chlorate (ClO3), which is formed by the production of chlorine dioxide, can also be found. Both chlorate and chlorite are oxidizing agents. Chlorine dioxide, chlorate and chlorite dissociate into sodium chloride (NaCl).Can chlorine dioxide be used to disinfect drinking water?In the 1950's the biocidal capability of chlorine dioxide, especially at high pH values, was known. For drinking water treatment it was primary used to remove inorganic components, for example manganese and iron, to remove tastes and odors and to reduce chlorine related disinfection byproducts.For drinking water treatment chlorine dioxide can be used both as a disinfectant and as an oxidizing agent. It can be used for both pre-oxidation and post-oxidation steps. By adding chlorine dioxide in the pre- oxidation stage of surface water treatment, the growth of algae and bacteria can be prevented in the following stages. Chlorine dioxide oxidizes floating particles and aids the coagulation process and the removal of turbidity from water.Chlorine dioxide is a powerful disinfectant for bacteria and viruses. The byproduct, chlorite (ClO2

-), is a weak bactericidal agent. In water chlorine dioxide is active as a biocide for at least 48 hours, its activity probaly outranges that of chlorine.Chlorine dioxide prevents the growth of bacteria in the drinking water distribution network. It is also active against the formation of bio film in the distribution network. Bio film is usually hard to defeat. It forms a protective layer over pathogenic microorganisms. Most disinfectants cannot reach those protected pathogens. However, chlorine dioxide removes bio films and kills pathogenic microorganisms. Chlorine dioxide also prevent bio film formation, because it remains active in the system for a long time.How much chlorine dioxide should be dosed?For the pre- oxidation and reduction of organic substances between 0,5 and 2 mg/L of chlorine dioxide is required at a contact time between 15 and 30 minutes. Water quality determines the required contact time. For post- disinfection, concentrations between 0,2 and 0,4 mg/L are applied. The residual byproduct concentration of chlorite is very low and there are no risks for human health.

Can chlorine dioxide be used to disinfect swimming pools? For swimming pool disinfection the combination of chlorine (Cl2) and chlorine dioxide (ClO2) can be applied. Chlorine dioxide is added to the water. Chlorine is already present in the water as hypochlorous acid (HOCl) and hypochlorite ions (OCl-). Chlorine dioxide breaks down substances, such as phenols. The advantages of chlorine dioxide are that it can be used at low concentrations to disinfect water, that it hardly reacts with organic matter, and that little disinfection byproducts are formed.How much chlorine dioxide should be dosed?The amount of disinfectant required needs to be determined first. This amount can be determined by adding disinfectant to the water and measuring the amount that remains after a defined contact time. The amount of chlorine dioxide that is dosed depends upon the contact time, the pH, the temperature and the amount of pollution that is present in the water.Can chlorine dioxide be used to disinfect cooling towers?Chlorine dioxide is used to disinfect the water that flows through cooling towers. It also removes bio films and prevents bio film formation in cooling towers. The removal of bio film prevents damage to and corrosion of equipment and piping and causes the pumping efficiency to be improved. Chlorine dioxide is also effective in removing Legionella bacteria. The circumstances in cooling towers are ideal for the growth of Legionella bacteria. Chlorine dioxide has the advantage that it is effective at a pH between 5 and 10 and that no acids are required to adjust the pH.What are the advantages of the use of chlorine dioxide?AdvantagesThe interest in the use of chlorine dioxide as an alternative for or addition to chlorine for the disinfection of water has increased in the last few years. Chlorine dioxide is a very effective bacterial disinfectant and it is even more effective than chlorine for the disinfection of water that contains viruses. Chlorine dioxide has regained attention because it is effectively deactivates the chlorine-resistant pathogens Giardia and Cryptosporidium. Chlorine dioxide removes and prevents bio film. Disinfection with chlorine dioxide does not cause odor nuisance. It destroys phenols, which can cause odor and taste problems. Chlorine dioxide is more effective for the removal of iron and manganese than chlorine, especially when these are found in complex substances.Does chlorine dioxide form chlorinated disinfection byproducts?The use of chlorine dioxide instead of chlorine prevents the formation of harmful halogenated disinfection byproducts, for example trihalomethanes and halogenated acidic acids. Chlorine dioxide does not react with ammonia nitrogen, amines or other oxidizable organic matter. Chlorine dioxide removes substances that can form trihalomethanes and improves coagulation. It does not oxidize bromide into bromine. When bromide containing water is treated with chlorine or ozone, bromide is oxidized into bromine and hypobromous acid. After that these react with organic material to form brominated disinfection byproducts, for example bromoform.Is the chlorine dioxide concentration needed for sufficient disinfection high?The use of chlorine dioxide reduces the health risk of microbial pollutions in water and at the same time decreases the risk of chemical pollutions and byproducts. Chlorine dioxide is a more effective disinfectant than chlorine, causing the required concentration to kill microorganisms to be much lower. The required contact time is also very low.Does the pH value influence chlorine dioxide efficiency?Contrary to chlorine, chlorine dioxide is effective at a pH of between 5 and 10. The efficiency increases at high pH values, while the active forms of chlorine are greatly influenced by pH. Under normal circumstances chlorine dioxide does not hydrolyze. This is why the oxidation potential is high and the disinfection capacity is not influenced by pH. Both temperature and alkalinity of the water do not influence the efficiency. At the concentrations required for disinfection, chlorine dioxide is not corrosive. Chlorine dioxide is more water-soluble than chlorine. In the last few years better and safer methods for chlorine dioxide production have been developed.

Figure 3: the influence of pH on efficiency is larger for chlorine than for chlorine dioxideCan chlorine dioxide be used combined with other disinfectants?Chlorine dioxide can be used to reduce the amount of trihalomethanes and halogenated acidic acids, formed by the reaction of chlorine with organic matter in water. Before the water is chlorinated, chlorine dioxide is added. The amount of ammonium in the water decreases. The chlorine that is added afterwards, oxidizes chlorite into chlorine dioxide or chlorate. Ozone can also be used to oxidize chlorite ions into chlorate ions. By the use of chloramines, nitrification can take place in the distribution network. To regulate this, chlorine dioxide is added. Byproducts control by chlorine dioxide can take place in combination with adequate disinfection, especially the reduction of bromine containing trihalomethanes and halogenated acidic acids that originate from the reaction of bromine containing water with natural organic matter. Chlorine dioxide itself combined with bromine does not form hypobromous acid or bromate, while chlorine and ozone do. Chlorine dioxide has excellent anti-microbiological qualities without the non-specific oxidation of ozone.What are the disadvantages of the use of chlorine dioxide?Is chlorine dioxide explosive?When producing chlorine dioxide with sodium chlorite and chlorine gas, safety measures must be taken with regard to the transport and use of chlorine gas. Sufficient ventilation an gas masks are required. Chlorine dioxide gas is explosive. Chlorine dioxide is a very unstable substance; when it comes in contact with sunlight, it decomposes.During chlorine dioxide production processes, large amounts of chlorine are formed. This is a disadvantage. Free chlorine reacts with organic matter to form halogenated disinfection byproducts.

Does chlorine dioxide form byproducts?Chlorine dioxide and its disinfection byproducts chlorite and chlorate can create problems for dialysis patients.

Is chlorine dioxide effective?Chlorine dioxide is generally effective for the deactivation of pathogenic microorganisms. It is less effective for the deactivation of rotaviruses and E. coli bacteria.

What are the costs of chlorine dioxide use?Chlorine dioxide is about 5 to 10 times more expensive than chlorine. Chlorine dioxide is usually made on site. The costs of chlorine dioxide depend upon the price of the chemicals that are used to produce chlorine dioxide. Chlorine dioxide is less expensive than other disinfection methods, such as ozone.What are the health effects of chlorine dioxide?Chlorine dioxide gasWhile using chlorine dioxide as a disinfectant, one has to keep in mind that chlorine dioxide gas can escape from a watery solution containing chlorine dioxide. Especially when disinfection takes place in a sealed space, this can be dangerous. When chlorine dioxide concentrations reach 10% or more in air, chlorine dioxide becomes explosive. Acute exposure of the skin to chlorine that originates from the decomposition of chlorine dioxide,

causes irritations and burns. Eye exposure eyes to chlorine dioxide causes irritations, watering eyes and a blurry sight. Chlorine dioxide gas can be absorbed by the skin, where it damages tissue and blood cells. Inhalation of chlorine dioxide gas causes coughing, a sore throat, severe headaches, lung oedema and bronchio spasma. The symptoms can begin to show long after the exposure has taken place and can remain for a long time. Chronical exposure to chlorine dioxide causes bronchitis. The health standard for chlorine dioxide is 0,1 ppm.Development and reproductionChlorine dioxide is thought to have effects on reproduction and development. However, there is too little evidence to ground this thesis. Further research is required.MutagenityThe Ames test is used to determine the mutagenity of a substance. The Ames test uses Salmonella bacteria that are genetically modified. No bacterial colonies are formed, unless they come in contact with a mutagenic substance that alters genetic material. Tests show that the presence of 5-15 mg/L ClO2 increases the mutagenity of water. It is difficult to prove the mutagenity of chlorine dioxide and chlorine dioxide byproducts, because the substances are biocides. Biocides usually kill the indicator organisms that are used to determine mutagenity.

When gases occur in freshwater

Effluents are often complex mixtures of poisons. If two or more poisons are present together in an effluent they may exert a combined effect on an organism wich is additive. Some gases that can harm aquatic freshwater life are gases such as chlorine, ammonia and methane. Chlorine is very additive in combination with copper. It does not normally occur in the environment except as a yellow gas on rare occasions. It's a manufactured substance and the byproducts of chlorine (organochlorines and dioxins) are persistent in the environment. One of the largest uses of chlorine is in the paper industry. Chlorine is first used to break down the lignan that holds the wood fibers together. Then chlorine is used to bleach the paper to make it white. The effluent or wastewater containing dioxins and other organochlorines are then dumped into streams and waterways. These ingredients are highly toxic and carcinogenic. Once in the waste stream, they come into contact with other organic materials and surfactants and combine to form a host of extremely toxic organic chemicals.The water becomes polluted; the fish become contaminated; animals eat the fish and people eat the contaminated animals and fish. It is so widespread that it would be difficult to find any human being who does not have detectable levels of dioxin in his/her blood.Some environmentalists call for a ban on the use of chlorine as bleach in the pulp and the paper industry around the Great Lakes.Introduction water disinfectionWater disinfectionWater purification has largely developed in the past century. Drinking water disinfection has decreased the number of outbreaks of waterborne diseases, such as cholera and typhoid (figure 1).

Figure 1: decrease in the number of deaths as a result of typhoid in the USA (1900-1920), compared to the number of people that use treated water

In developing countries there is usually not enough clean drinking water or sewerage. In these countries waterborne diseases cause many people to be ill or to die, mainly fragile groups such as young children, elderly people and people with a weakened immune system (AIDS patients and organ transplant receivers).

The larger part of pathogenic microorganisms 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 treatement step.

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, disinfection 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, which originates largely from swimmers. The contamination contains microorganisms, among other things.To prevent contagion of swimmers 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 mayor problem in cooling towers, because this promotes corrosion and blocks the system.Another problem in cooling towers, as well as ventilation systems, is the development of legionella bacteria. These bacteria spread through aerosols and can cause legionnaires disease. This is 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 the disinfection of cooling water.

In the early seventies it was discovered that disinfection byproducts can form during water disinfection by means of chemical disinfectants. When this was discovered research started on the development and health effects of these byproducts. Today there are legal standards indicating maximum levels of disinfection byproducts in drinking water. Methods to lower the concentration of disinfection byproducts in drinking water have also been researched.

Necessity of water treatmentComposition of water

When we open the tap, clean tasty water flows out. Water undergoes several purification steps before it flows from taps.

Water that is used for drinking water production contains water molecules and a large variety of other substances. One of the properties of water is that it easily dissolves other susbtances. Water that falls to earth during rain showers dissolves substances, particles and gasses such as oxygen, which can be found in air. Contaminants that are present in air also dissolve in rain water. When

surface water flows on earth it also dissolves several different substances, such as sand particles, organic matter, microorganismss and minerals. Water that settles into the ground and

becomes groundwater often contains large amounts of dissolved minerals, as a result of contact with soils and rocks. Human activities, such as agriculture and industrial waste and sewer water

discharge cause a number of pollutants to enter the water.Self-cleansing capacity of water

Water has the capacity to cleanse itself. Contaminants are removed from water during biological processes. When water settles onto the ground, groundlayers will cause filtration to occur. Contaminants are broken down, or will stay behind in the ground layer. The self-cleansing capacity of water is not strong enough to produce clean drinking water. This is a consequence of the quantity and variety of industrial and agricultural contaminants that have entered surface and groundwater for many decades.In the 1970’s it was discovered that industrial discharges and waste water discharges were the cause of water contamination. Immediately after this discovery measures were taken to prevent water pollution. Waste water must meet legal standard before it can be discharged. To meet the standards water is purified before it is discharged.

Despite of these measures water often still needs treatment before it is suitable for use as drinking water.During water purification waste water is treated to become drinking water which meets legal standards in the physical, bacteriological and chemical area. The water may not contain an odour or flavour, and it should be bright and chemically stable (non-corrosive).

The kind of treatment water needs, strongly depends upon the composition and quality of the water. Water treatment contains two process steps: physical removal of solid particles, mainly minerals and organic matter and chemical disinfection; killing or deactivating microorganismss in water.

History of drinking water treatmentHistory of drinking water treatment

Humans have been storing and distributing water for centuries. Before, when people lived as hunters/ collectors, river water was applied for drinking water purposes. When people permanently stayed in one place for a long period of time, this was usually near a river or lake. When there were no rivers or lakes in an area, people used groundwater for drinking water purposes. This was pumped up through wells.When the human population started growing extensively, the water supply was no longer sufficient. Drinking water needed to be extracted from a different source.

About 7000 years ago, Jericho (Israël, figure 1) stored water in wells that were used as sources. People also started to develop drinking water transport systems. The transport took place through simple channels, dug in the sand or in rocks. Later on one also started using hollow tubes. Egypt used hollow palm trees and China and Japan used bamboo strunks. Eventually one started using clay, wood and even metal.In Perzia people searched for underground rivers and lakes. The water went through holes in rocks into the wells on the plains.

Around 3000 B.C., the city of Mohenjo-Daro (Pakistan) used a very extensive water supply. In this city there were public bathing facilities with water boiler installations and bathrooms.

In ancient Greece spring water, well water, and rainwater were used very early on. Because of a fast increase in urban population, Greece was forced to store water in wells and transport it to the people through a distribution network. The water that was used was carried away through sewers,

along with the rainwater. When valleys were reached, the water was lead through hills under pressure. The Greek where among the first to gain an interest in water quality. They used aeration basins for water purification.The Romans were the greatest architects and constuctors of water distribution networks in history. They used river, spring or groundwater for provisioning. The Romans built dams in rivers, causing lakes to form. The lake water was aerated and than supplied. Mountain water was the most popular type of water, because of its quality.For water transport the aquaducts where built. Through these aquaducts water was transported for tens of miles. Plumming in the city was made of concrete, rock, bronze, silver, wood or lead. Water winnings were protected from foreign pollutants.After the fall of the Roman empire, the aquaducts were no longer used. From 500 to 1500 A.D. there was little development in the water treatment area. In the Middle Ages countless cities were manifested. In these cities wooden plumming was used. The water was extracted from rivers or wells, or from outside the city. Soon, circumstances became highly unhygenic, because waste and excrements were discharged into the water. People that drank this water fell ill and often died. To solve the problem people started drinking water from outside the city, where rivers where unpolluted. This water was carried to the city by so-called water-bearers.

The first drinking water supply that supplied an entire city was built in Paisley, Scotland in 1804 by John Gibb, in order to supply his bleachery and the entire city with water. Within three years, filtered water was transported to Glasgow.In 1806 Paris operated a large water treatment plant. The water settled for 12 hours, before it was filtered. Filters consisted of sand and charcoal and where replaced every six hours.In 1827, the Englishman James Simpson built a sand filter for drinking water purification. Today, we still call this the number one tribute to public health.

What is water disinfection?Water disinfection means the removal, deactivation or killing of pathogenic microorganisms. Microorganisms are destroyed or deactivated, resulting in termination of growth and reproduction. When microorganisms are not removed from drinking water, drinking water usage will cause people to fall ill.Sterilization is a process related to disinfection. However, during the sterilization process all present microorganisms are killed, both harmful and harmless microorganisms.

MediaDisinfection can be attained by means of physical or chemical disinfectants. The agents also remove organic contaminants from water, which serve as nutrients or shelters for microorganisms. Disinfectants should not only kill microorganisms. Disinfectants 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 recontaminated.

For chemical disinfection of water the following disinfectants can be used:- Chlorine (Cl2)- Chlorine dioxide (ClO2)- Hypo chlorite (OCl-)- Ozone (O3)- Halogens: bromine (Br2), iodene (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

How does disinfection work?

Chemical inactivation of microbiological contamination in natural or untreated water is usually one of the final steps to reduce pathogenic microorganisms in drinking water. Combinations of water purification steps (oxidation, coagulation, settling, disinfection, filtration) cause (drinking) water to be safe after production. As an extra measure many countries apply a second disinfection step at the end of the water purification process, in order to protect the water from microbiological contamination in the water distribution system. Usually one uses a different kind of disinfectant from the one earlier in the process, during this disinfection process. The secundairy disinfection makes sure that bacteria will not multiply in the water during distribution. Bacteria can remain in the water after the first disinfection step or can end up in the water during backflushing of contaminated water (which can contain groundwater bacteria as a result of cracks in the plumbing).

Disinfection mechanism

Disinfection commonly takes place because of cell wall corrosion in the cells of microorganisms, or changes in cell permeability, protoplasm or enzyme activity (because of a structural change in enzymes). These disturbances in cell activity cause microorganisms to no longer be able to multiply. This will cause the microorganisms to die out. Oxidizing disinfectants also demolish organic matter in the water, causing a lack of nutrients.

Necessity of drinking water disinfectionThe larger part of pathogenic microorganisms is removed from water during the primairy water purification steps. However, water disinfection is still necessary in order to prevent drinking water from being harmful to our health.

MicroorganismsMicroorganisms can be found commonly in nature. Invisible to bare eyes, microorganisms are present in soils, air, food and water. Before humans are born, we are free from microorganisms. Through consumption of food and air we are exposed to microorganisms soon after we are born. The microorganisms will remain present on and in our bodies. Most microorganisms are harmless and will contribute to a number of vital processes in the human body, such as the metabolism. But there are also microorganisms which can cause disease or which are harmful to people with low resistance to disease.Pathogenic microorganisms in the water have a number of specific properties which distinguish them from chemical contaminants. They are living organisms. They are not dissolved in water, but they will coagulte or attach to colloids and solids in water.

Types of pathogenic microorganismsPathogenic microorganisms in drinking water can be divided up into three types: bacteria, viruses and parasitic protozoa. Bacteria and viruses can excist in both surface water and groundwater, whereass parasitic protozoa can be found mainly in surface water.

BacteriaBacteria are single-cell organisms, shaped like a sphere, spiral or rod. They can excist as individual bacteria or in bacterial chains, bundles or pairs. Bacteria are the most abundant lifeform on earth. They are between 0,4 and 14 μm in length and about 0,2 to 12 μm in width. Consequentially they can only be viewed under a microscope. Bacteria feed on fluid nutrients. They can reproduce by means of DNA replication, causing a bacteria to split into two independent cells. In ideal circumstances this process taken about 15 to 30 minutes.

Some types of bacteria can form spores. These spores contain a protective layer which is heat resistant and can protect bacteria from a lack of moist and food.

Bacteria play a role in various processes. Some bacteria breack down organic matter and play an important ecological role, other assist in the human metabolism.

VirusesViruses are organisms which can cause infections and which only reproduce in living host cells. When viruses excist outside host cells, they are inactive. Viruses contain a protective shell. They are shaped like a spear, sphere or wire and they are so small (between 0,02 and 0,09 μm) that they can slip through filters which capture bacteria.Contrary to bacteria and parasitic protozoa, viruses contain only one type of nucleic acid (RNA or DNA). They cannot reproduce, but instead take over the metabolism of the host cell and make sure the DNA is copied in the host cell, causing new viruses to develop.Contrary to bacteria, viruses are not naturally present in the human body. When people are infected with a virus it usually leaves the body through secretion. When secreation takes place water can be contaminated with viruses. When the water is not thoroughly disinfected, other people can be infected with viruses.

Figure 1: three different types of viruses

Parasitic protozoaParasitic protozoa are single-cell organisms. They have a very complex metabolism and feed on solid nutrients, algae and bacteria which are present in multiple-cell organisms, such as humans and animals. Multiplication take place through splitting of the cells. Various types of parasitic protozoa are spread in sleeping, protected form as a cyste or oocyste. Oocysts of Cryptosporidium and cysts of Giardia can be found in waters throughout the world as a consequence of fecal pollution. As cysts the pathogens are resistant to chlorine disinfection. Parasitic protozoa can be removed by means of filtration or chlorine dioxide application.

The odds of infectionThe odds of infection depend upon the type of pathogen, the way in which it is transferred, the infective dose and persistence of the microorganism, and the resistance of the person that is infected.The infective dose means the number of microorganisms that need to enter te body before the disease occurs. This dose is very low for viruses and parasitic protozoa. The persistence of a microorganism depends upon the viable time of the microorganism, when it is not present in a human host. Bacteria are commonly the least persistent microorganisms, and protozoic cysts are the most persistent ones.Young children, elderly people and sick people have a lower resistance to disease and are therefor more fragile. When a person is infected the pathogens multiply within the host, causing the risk of illness to rise. Not every person that is infected with a pathogen falls ill. People that do become ill will spread a disease easily, mainly through secretion.

Secretion and sewer waterWhen water flows through a certain area, it collects all kinds of substances and gives these off in other areas. Microorganisms also enter the water. The larger part of microorganisms which cause waterborne diseases originate from human or animal feaces.

Figure 2: E. Coli bacteriaOne drop of feaces contains millions of microorganisms. In the faeces of cattle there can be millions of E. Coli bacteria (figure 2), Giardia cysts (figure 3) and Cryptosprodium spores (figure 4). In chicken faeces pathogenic bacteria such as Salmonella and Campylobacter can be present. When fertilizers are applied to land, rain can cause bacteria to rinse out te surface water or groundwater, causing the microorganisms to contaminate water.

Figure 3: Cryptosporidium spores Figure 4: Giardia cysts

Sewer or waste water cannot be discharged into the environment untreated. The larger part of purified waste water ends up in rivers, lakes and oceans. Sometimes heavy rainfall can cause sewer systems to flood, causing untreated water to end up in surface water or groundwater. Not every country purifies water before it enters surface or groundwater. Mainly developing countries lack sanitary facilities. The water can contaminate water that is used for drinking water purposes, causing the risk of infection with diseases carried by waterbone microorganisms to become very high. this is a particular risk when drinking water is not treated at all. When septic tanks are used for waste water treatment, pathogenic microorganisms can contaminate surface water and groundwater sources.

Not all pathogenic microorganisms in water originate from faeces. Legionella (figure 5) can be found commonly in water and easily multiplies in the water distribution system. There are also other pathogenic microorganisms that can be found commonly in surface water.

Figure 5: Legionella bacteria

History of water disinfectionHistory of drinking water disinfectionThe link between water quality and health has been known since the early ages. Clear water was considered clean water. Swamp areas were associated with fever.Disinfection has been applied for centuries. Two basic rules dating back to 2000 B.C. state that water must be exposed to sunlight and filtered with charcoal and that impure water must be purified by boiling the water and than dipping a piece of copper in the water seven times, before filtering the water. Descriptions of ancient civilisations were found about boiling water and water storage in silver jugs. To realize water purification copper, silver and electrolysis were applied.

Disinfection has been applied for several decades. However, the mechanism has been known for only one hundred years.In 1680 Anthony van Leeuwenhoek developed the microscope. His discovery of microorganisms was considered a curiosity. It took scientists another two hundred years before they started using the microscope to distinguish microorganisms and other pathogens.The first multiple filter was developed in 1685 by the Italian physician Lu Antonio Porzo. The filter consisted of a settling unit and a sandfiltration unit. In 1746 the French scientist Joseph Amy received the first patent for a filter design, which was applied in households by 1750. The filters consisted of wool, sponges and charcoal.

Figure 1: John Snow Figure 2: contaminated water pump spreads choleraFor the past centuries humans have suffered from diseases such as cholera and the plague. The origin of these diseases was misinterpreted. It was said that the diseases were a devine punishment or were caused by impure air or the alignment of the planets.In 1854 a cholera epidemic caused many deaths in the city of Londen. John Snow, an English doctor (figure 1), discovered that the cholera epidemic was caused by a contaminated water pump (figure 2). He prevented a spread of the epidemic by closing down the contaminated water pump. After that scientists have performed bacteriological studies to research the development, existence and identification of microorganisms and the removal of microorganisms from drinking water.

In the nineteenth century the effect of disinfectants, such as chlorine, was discovered. Since 1900 disinfectants are largely applied by drinking water companies to prevent the distribution of diseases and to improve water quality.

Waterborne diseasesContagion by pathogenic microorganismsInfectious diseases caused by pathogenic bacteria, viruses and protozoan parasites are among the most common and widespread health risk of drinking water. People are introduced to these microorganisms through contaminated drinking water, water drops, aerosols and washing or bathing.Some waterborne pathogenic microorganisms spread by water can cause severe, life-threatening diseases. Examples aretyphoid fever, cholera and Hepatitis A or E. Other microorganisms induce less dangerous diseases. Often, diarrhoea is the main symptom (figure 1). People with low resistance, mainly elderly people and young children, are vulnerable to these diseases as well.No acces to clean drinking waterWorlwide, 1,2 billion people do not have access to clean and safe drinking water, and 2,4 billion people lack sanitation. Every year, 5 million people die of waterborne diseases.Developed countriesMost waterborne diseases occur worldwide. In developed (western) countries, contagion is prevented by drinking water purification and by hygienic measurements. But even in developed countries, people can fall ill from waterborne diseases. This is caused by using insufficiently disinfected water, by implementing non-hygienic food preparation and by insufficient personal hygiene.Developing countriesIn developing countries, waterborne diseases are a major problem which contributes to the vicious circle that people are in. In many developing countries, there is a lack of medicine to treat ill people. Vaccination is usually very scarce as well. Many people weaken because of waterborne disease and, as a result, are more susceptive to other infections. Their physical capacity decreases and they cannot work and provide their families with money and food. A lack of sufficient nutritional food weakens people, especially children, even further. They become even more susceptible to diseases. Children run behind at school, because they cannot be educated when they are ill. Waterborne diseases frustrate the economic development of many people. The appearance of HIV in developing countries makes more people susceptive to infectious diseases. During wars

and natural disasters (floods) many people are infected with waterborne diseases. Diseases are easily spread because water treatment and sewage no longer function or are lacking completely.To improve the economical progress of developing countries, water contamination and spread of infectious diseases must be handled. This is achieved through (drinking) water treatment, sewage, waste and sewage water treatment and education on personal and food hygiene.Occurence of waterborne diseasesIt is im possible to represent the number of waterborne microbiological infections (figure 2). This has several causes. Diseases are misdiagnosed or not reported. Sometimes it is difficult to demonstrate the source of a water related disease. Both swimming in contaminated water and the microbiological or chemical quality of drinking water can cause illness.

Figure 2: the number of waterborne diseases in the United States from 1971 to 1992Disinfection remains importantGroundwater usually has a good microbiological quality, because it is prefiltered through various ground layers. Those ground layers function as a natural filter; microorganisms and other particles are removed when the water seeps down. Afterwards, the water still needs treatment, because not all pollutions can be removed biologically. Groundwater can be contaminated by sewage water or waste water pollutions.Even when water treatment is applied, one has to watch out for outbreaks of waterborne diseases. Water that is used for drinking water purposes can be prepared from surface water, groundwater or recycled water. This water can be contaminated by pathogenic microorganisms and other pollutants. Sufficient disinfection is needed to prevent diseases.New waterborne diseasesInfection routes change throughout the years. In the last twenty years a number of pathogenic diseases have appeared, even in developed countries, that cannot be prevented by traditional water treatment.For example: in 1993 in Milwaukee, USA, 400.000 people fell ill from using drinking water that was contaminated by Cryptosporidium cysts. In the year 2000, 2.300 people fell ill in Walkerton, Canada, because of E. coli O157:H7. Other pathogenic microorganisms that can be found in drinking water are caliciviruses, Heliobacter bacteria, Mycobacteria and Giardia Lambia. In the future more pathogenic microorganisms will emerge and spread through water, because of agricultural magnification, increased population growth, increased migration and climate change. Pathogenic microorganisms can also emerge because they built up resistance to disinfectants.

Factors that influence water disinfectionCT:This stands for the contact time between disinfectant and microorganism and the concentration of disinfectant. CT is used to calculate how much disinfectant 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 minutesWhen a particular disinfectant is added to water, it does not only react with pathogenic microorganisms, but also with other impurities, such as soluble metals, particles of organic matter and other microorganisms. The utilization of a disinfectant for reactions with these substances make up the disinfection demand of the water. The disinfection demand must first be satisfied, before 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 to result in 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.The CT is commonly used to determine the affectivity of a particular disinfectant against a certain microorganism under specified conditions. There is a difference between the relative affectivity of chemical disinfectants against different microorganisms. Often a certain level is added to the CT, for example 99%. This means that 99% of the microorganisms are deactivated by the disinfectant. CT can be used to compare the affectivity 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 en 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 a lot higher than that of chlorine used against E. Coli bacteria and Polio virus.Table 1. Comparison of CT values for the 99% inactivaton of microorganisms at 5 °C

Organism Free chlorine(pH 6-7)

Chloramines(pH 8-9)

Chlorine dioxide(pH 6-7)

Ozone(pH 6-7)

E. coli bacteria 0,034 - 0,05 95 - 180 0,4 - 0,75 0,02Polio virus 1,1 - 2,5 770 - 3740 0,2 - 6,7 0,1 - 0,2Giardia lambia cyst 47 - 150 - - 0,5 - 0,6

The type of microorganismDisinfectants can effectively kill pathogenic microorganisms (bacteria, viruses and parasites). Some microorganisms can be resistant. E. coli bacteria, for example, are more resistant to disinfectants than other bacteria and are therefore used as indicator organisms. Several viruses are even more resistant than E. coli. The absence of E. coli bacteria does not mean that the water is safe. Protozoan parasites like Cryptosporidium and Giardia are very resistant to chlorine.The age of the microorganismThe affectivity of a particular disinfectant also depends upon the age of the microorganism. Young bacteria are easier to kill than older bacteria. When bacteria grow older, they develop a polysaccharide shell over their cell wall, which 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, contact time is sufficient. Bacterial spores can be very resistant. Most disinfectants are not effective against bacterial spores.Water that requires treatmentThe 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.TemperatureThe temperature also influences the affectivity of disinfection. Increasing temperatures usually increases the speed of reactions and of disinfection. Increasing temperatures can also decrease disinfection, because the disinfectant falls apart or is volatized.

Conditions for water disinfectionConditions for water disinfection

Drinking water disinfection is linked to other water purification steps. A proper disinfection can only take place when water is already purified to a certain extent. The circumstances will than be suitable for disinfection, because the larger part of pathogenic microorganisms present in the water will be removed during primairy water purification steps.Dissolved and floating particles must be removed from water, because these may react with disinfectants to disinfection byproducts and because they are a substrate for microorganisms. Moreover, microorganisms are harder to remove from water when adsorption to floating particles in water takes place. The concentrations of floating particles in water must be low when disinfection is apllied, preferably below 1 mg/L. Chemical substances that are present in water through human or natural causes may also influence disinfection. The substances react with disinfectants to disinfection byproducts. This causes the concentration of disinfectants needed to properly remove microorganisms to be much higher. It is also harder to maintain a residual concentration.

Adequate waste water treatment can make the disinfection of drinking water more efficient. This is often neglected. When waste water treatment is insufficient, water that is polluted with all kinds of pathogenic microorganisms and chemical pollutants will end up in the environment. This negatively influences the environment, mainly surface water quality. Surface water is used for the production of drinking water. To enhance waste water quality, waste water is purified. The purification process includes a disinfection step.

Swimming pool water treatmentTypes of swimming pools

There are various types of swimming pools. Swimming pools differ in function (tropical swimming pool, sauna), size and conditions such as water temperature, cleaning system and water disinfection mechanism. A division can be made in indoor and outdoor swimming pools.There is a difference between swimming pools with a water recirculation system and swimming pools that undergo constant water refreshment. In circulation pools, the water is recicled from a water purification system. Part of the water is separated and carried away after treatment. The pool will be filled up with fresh water.When a swimming pool has a continual flow, the water is continually refreshed. The water that is retreated from the pool is discharged into the sewer, or it will be transported to a water purification plant.Most public swimming pools apply water recirculation.

Swimming pool water treatment

Swimming pool water must undergo treatment, in order to remain clear and clean, free from harmful substances, bacteria, viruses, algae and other pathogens and suitable for use by swimmers.

Purification steps

Swimming pool water is treated by means of various purification steps (figure 1). The water is first transported from swimming pools to a water purification plant (1). In the water purification plant, it will flow through a hair removal filter (2), which removes raw pollutions, such as hairs, plasters and leaves, from water. After that, a flocculant (3) is added, which causes smaller colloids to bind together. Colloids are visible floating particles of organic matter, such as skin tissue and textile fibers. This group of pollutants also concerns colloidal pollutants, such as saliva, soap remains, cosmetic products and skin fats. When these pollutants are abundant, they cause turbidity.Parameters that indicate the presence of undissolved particles are water turbidity and potassium permanganate (KMnO4) demand of the water. Potassium permanganate is used as an indicator of organic matter oxidation.Floating particles are removed from water in a sandfilter. The sandfilter is backflushed periodically. Finally, pollutants are discharged into the sewer.

Swimming pool pollutionPollutants in swimming pools

The water in public swimming pools contains microorganisms and unwanted substances, which derive from the skin and excretion products of swimmers. Swimmers cause many pollutants to enter the water, such as bacteria from saliva, excretion products, pollution from swimwear, skin tissue, sebum, sweat, nose and throat saliva, hairs, cosmetics and ammonia (NH3). Vouching for clean swimming pool water through constant refreshment is often too expensive. Furthermore, this does not solve the problem of pollutants that remain on swimming pool walls. The water is recicled, causing pollutant and pathogen concentrations to increase. Microorganisms will multiply and this causes the risks of contagion to increase.

Dissolved pollutants

Swimming pool water contains dissolved pollutants, such as urine and sweat and other excretion products of swimmers. Sweat and urine largely consist of water, ammonia and ureum. These substances also contain kreatine, kreatinine and amino acids. The components of sweat and urine are not harmful for human health. However, when these products react with disinfectants in the water, such aschlorine, unwanted reaction products can be formed which consist mainly of chloramines.The water can contain dissolved pollutants that derive from disinfectants and cleansing agents that are used to clean swimming pools. Dissolved pollutants are largely removed by oxidation. This means that pollutants are decomposed by chemicals. Substances that are not or partly decomposed during the oxidation process and oxidation products are removed from the circulation system by gradual refreshment of swimming pool water.

Health effects of swimming pool pollutants

Swimmers are susceptive to pathogenic microorganisms in swimming pool water. As a result of cooling and water uptake, the resistance of the mucous membranes of swimmers can weaken, causing them to become more susceptive to pathogens in swimming pool water and air, and even to pathogens that are present in their own bodies. Microorganisms that enter the water through excretion by swimmers can cause a large variety of conditions. Most pathogenic microorganisms cause diarrhoea or skin rashes. Certain microorganisms can cause serious symptoms, such as paralysis, brain inflammation, heart inflammation, jaundice, fevers, vomiting, diarrhoea and respirational or eye infections.Pathogenic mciroorganisms that can be found in swimming pool water are bacteria, viruses and parasitic protozoa. Young children, elderly people and people with damaged immune systems are more susceptive to infections caused by these species and will fall ill more easily. People that have an untreated form of cancer may also suffer from lower resistance to waterborne diseases. Not every person that is infected will fall ill, but hen or she can still cause contagion of other people.In developing countries, the number of people with damaged immune systems is increasing, as a result of an increase in the number of AIDS patients in the last 20 years.

Conditions for water disinfectionDisinfection of swimming pool waterMicro-organisms polluted swimming pools. Every swimmer adds 1.000.000 to 1.000.000.000 microorganisms to the water. The water itself contains microorganisms, as well. After oxidation a disinfectant must be added to the water to kill pathogenic microorganisms.Demands on disinfectantsDisinfectants used for swimming pool water disinfection must meet certain demands. They should be harmless and non-irritating to swimmers and attendants. They must be active in small concentrations and remain their activity for a long time.Contrary to drinking water disinfectants, disinfectants for swimming pool treatment must be active in the pool itself, because pollutions and pathogenic micro-organisms are constantly added to the water. Therefore the water has to maintain a residual disinfectant concentration. The disinfectant must be easily traced and measured and should be safe to use.Disinfection methods for swimming pool waterIn some countries, sodium hypochlorite is used for both oxidation and disinfection of swimming pools. When it is added to water, sodium hypochlorite increases the pH value. It is better to use chlorine as a disinfectant and an oxidizer at a pH value of 6,5. Often, acid is added to lower the

pH value.Demands on swimming pool conditionsChlorine-based disinfectants are among the most frequently applied disinfectants and oxidizers for swimming pool treatment. Chlorine is usually added as hypochlorous acid (HOCl) or hypochlorite (OCl-). Chlorine kills pathogenic microorganisms that are present in the water. When too much chlorine is present, it can cause eye and mucous membrane irritation, as a result of chloramine formation.Threshold and maximum levels are set for chlorine concentration. For available chlorine the minimum concentration in swimming pools is set to 0,5 milligram per litre. The maximum level is set to 1,5 mg/l. When using cyanic acid (stabilizer) minimum and maximum values are set to respectively 2,0 and 5,0 mg/L. For outdoor swimming pools and indoor pools smaller than 20 m2, the maximum level is set to 5,0 mg/l.Lowering the chlorine concentration is undesirable, because this increases the risk of waterborne diseases.Alternative disinfectants can be used as well, these decrease the required amount of chlorine or cause chlorine addition to be irrelevant.The pH value is measured daily. It should be between 6,8 and 7,8. At a pH of 7,0, the amount of free chlorine present is 70%, while this concentration decreases to 20% at pH of 8,0.The water and air temperature in swimming pools is usually high. Furthermore the humidity is high. This influences the activity of disinfectants and the behaviour of substances that are formed in the swimming pool during disinfection. When sodium hypochlorite is used, chlorine gas is formed due to reactions with the acid that is added to lower the pH of the water. Chlorine gas must be removed, because it can be harmful to human health and corrosive on materials. Chloramines, formed through reactions of ureum and chlorinated disinfectants, are corrosive as well.

Health effects of swimming pool disinfectionHealth effects of swimming pool disinfectionDisinfectants that are used for swimming pool disinfection water can affect human health. Too much chlorine can cause eczema and rashes. Water that has a high pH value increases susceptivity to these kind of ailments.When water is mobile, it comes in contact with a sufficient amount of of air. Carbon dioxide is released into the water, causing the pH value to decrease. When one applies chlorine, chlorine gas will evaporate.Free active chlorineHypochlorous acid (HOCl) and hypochlorite (OCl-) are the main components of free active chlorine. Swimming pool water has a high pH value and the amount of dissolved chlorine gas as free active chlorine is negligible. Free active chlorine hardly ever causes eye irritations. These only occur above concentrations of 20 mg/L. Dissolved chlorine and chlorine substances dehydrate hair and skin. The air above the pool contains chlorine gas concentrations between 0,01 and 0,1 mg/m3. These concentrations are far below the level that irritates respirational tracts. Through the formation of combined active chlorine, free active chlorine can cause irritation.Combined active chlorineCombined active chlorine is the generic term for reaction products produced by free active chlorine with organic and inorganic nitrogen pollutions. These pollutions are made of swimmers excretion products. Combined active chlorine is a complex mixture of partly unknown substances, such as chlorine ureum combinations, chloramines and chlorine kreatines.The irritating effects of combined active chlorine are often ascribed to chloramines (NH2Cl, NHCl2, NCl3). Chloramines are volatile substances that partly escape from water as gas. Like chloroform, chloramines cause the well known 'chlorine smell' in swimming pools. The formation of di- and trichloramines increases when the free active chlorine concentration is increased and the pH value is lowered. The typical 'chlorine smell' in swimming pools arises at ureum levels of 0,5 mg/L and free active chlorine concentrations of 1,0 mg/L. There is no relation between chloramine formation and ureum concentration.Monochloramines cause eye irritations. At normal pH levels in swimming pools, monochloramine is produced predominantly. Both di- and trichloramines irritate eyes. These substances reach the eyes through water and through the air above the pool. Trichloramine also irritates air tracts. Other chlorinated organic subtances are suspected to irritate as well, particularly chlorinated ammonia, kreatinine and urine acid.The combined active chlorine concentration in swimming pools should be below 1 mg/l.Disinfection byproducts: chloroform

Some disinfection byproducts, such as trihalomethane chloroform, are suspected carcinogenic.Trihalomethane concentration depends upon total organic carbon, the number of swimmers and the water temperature.Chloroform is the most important reaction product. Additionally other trihalomethanes, dichloromethane, tetrachloromethane, trichloroethene, bromodichloromethane and other chlorobromo hydrogen carbons can be found.Chloroform and similar substances are volatile. Some part escapes from water and swimmers are exposed through inhalation. Chloroform concentrations in swimming pools vary a lot. Chloroform concentrations are highest just above the water. The suggested health standard for chloroform 100 mg/m3, this concentration is found in indoor swimming pools.Swimming is one of the main sources of non-professional exposure to chloroform (over 70% percent after one hour of swimming). In outdoor swimming pools exposure is lower, because the wind ventilates the air above the water.Exposure to chloroform can be measured in blood plasma of swimmers. Swimmers that swim for a long time with great physical effort (competitive swimmers) take up most chloroform. Chloroform concentrations in air is the main factor that determines the amount of chloroform that is absorbed. The time interval, the number of swimmers and the chloroform concentration in water are less important factors.Effects of chloroformExposure to low concentrations of chloroform causes renal and liver defects. These can be demonstrated by enzymes in the blood which are indicators for renal- and liver functions. Epidemiological research shows there might be a relation between skin exposure to chlorinated organic substances and hypochlorite, and skin cancer. This relation has not been proved by laboratory animal testst. Long-term oral exposure of laboratory animals to chloroform through food caused liver cancer.Health complaints of swimming pool attendantsEspecially swimming pool attendants are exposed to swimming pool disinfection byproducts for a long period of time. Dutch research, carried out in 2001 by Abvakabo Nederland, concerned working conditions in swimming pools. Swimming pool attendants were interviewed. The research shows that a lot of employees suffer from forgetfulness, fatigue, chronic colds, voice problems, eye irritations, headache, sore throat, eczema and frontal sanus inflammation. Fertility problems are also mentioned. All problems are probably caused by working conditions. People work long hours in a warm, humid environment and are exposed to chemical substances. Swimming pool ventilation is often insufficient and volatile disinfection byproducts remain. If swimming pool attendants do not work, health problems vanish and a sense of health returns. Alternative disinfectants that produce less disinfection byproducts and improved ventilation can prevent or at least minimize problems.Epidemiologich researchEffects of exposure to swimming pool disinfectants and byproducts have been researched a couple of times. Competitive swimmers weekly spent many hours in swimming pools. During exercise their physical effort is large. Their inhalation is more deeply and more powerful than that of recreational swimmers. They inhale more air and absorb more chlorine products. Lung functions of swimmers decrease when they are swimming in pools that are disinfected by chlorine. Many competitive swimmers suffer from asthma. Complaints disappear when they swim in outdoor pools, because the wind removes gasses from the air above the pool. Children inhale more air per unit body than adults do. They relatively absorb more gaseous substances and the health risk for children is bigger. Swimming pools that are disinfected by chlorine gas can produce hydrochloric acid with sunlight. This causes the pH value to drop. when pH values drop below 3,6, swimmers can suffer from dental abrasion. Tooth enamel dissolves and the teeth become brittle and sensitive.

Cooling tower water What is a cooling tower?

A cooling tower is an installation that retreats heat from water by evaporation or conduction.The industries use cooling water in various processes. As a result, there are also various types of cooling towers. There are cooling towers that create process water that can only be used once, before it is discharged. There are also cooling towers that create water that can be reintoduced in the production process.When water is reused, it is pumped through the installation into the cooling tower. After the water is cooled, it is reintroduced into the production process. Water that needs to be cooled usually has a temperature of between 40 and 60°C.The water is pumped to the top of the cooling tower and will than flow down through plastic or

wooden shells. This causes drop formation. While flowing down, the water emits heat which mixes with the above air flow, causing it to cool down 10 to 20 ˚C.Part of the water evaporates, causing it to emit more heat. Water vapor can sometimes be observed over the cooling tower.To create an upward airflow, some cooling towers contain blades in the top, which are similar to ventilator blades. These blades cause an upward air flow inside the cooling tower. The water falls down into a basin and will be brought back into the production process from there.There are open and closed cooling tower systems. When a system is closed, the water does not come in contact with outside air. This causes pollution of cooling tower water by air pollutants and microorganisms to be insignificant. Also, microorganisms that are present in cooling tower water cannot end up in outside air. When a cooling tower is an open system, this phenomenon may occur.

Cooling tower water pollutionPollutants in cooling tower water

Water that is applied in cooling towers, even when this concerns tap water, often contains salts (such as chlorine, sulphates and carbonates), dissolved gases (such as oxygen and carbon dioxide) and metal ions (such as iron and manganese ions). The presence of these pollutants can cause a series of problems. The main problems that are caused, are fouling, limestone formation, corrosion and biological growth. The pollutants that are present depend on cooling tower building material, as well. Cooling towers are build of concrete, wood, plastic or metal.

Microorganisms

Bacteria and other pathogenic mircroorganisms are present everywhere throughout the environment. They can often be found in cooling tower water. When cooling towers contain an open recirculation system, microorganisms can spread from air to water. Microorganisms can rapidly multiply, when a substrate is present and a number of conditions are ideal for microbial growth. Examples are pH, temperature, oxygen concentration and nutrients. The nutrient content in water increases, because of water evaporation. Process leaks and water use can also cause the nutrient content in the water to increase. This can cause problems.

Bio film

When a significant microbial growth takes place, a slime layer is formed. This contains both organic and inorganic matter. Some microorganisms excrete polymers, which can form a gel-like network around cells after hydrolysis takes place. This is called a bio film. As a result of bio film formation, microorganisms can attach themselves to surface layers. This causes microorganisms to no longer be flushed away by cooling tower water. Bio films protect microorganisms from other microorganisms and from toxic disinfectants. This causes water disinfection to be much more difficult when a bio film is present.Bio film partly consists of microbiological cells and components. Bio film, which is very sticky, also contains organic and inorganic matter that is present in the water and is absorbed by the film. This concerns chemical precipitation, organic flakes and dead cell mass. Bio film consists of 90% water.

Bio film causes a number of problems, knowing: Within the protected slime layer microorganisms can cause a speedy corrosion, causing the walls of cooling towers and heat exchange systems to be corroded. The bio film prevents materials that cause corrosion protection from reaching the surface. Furthermore, microbiological reactions can accelerate corrosion reactions and microbial products can corrode materials.Bio film creates an isolation layer on heat-exchange systems, causing them to no longer function properly. Microorganisms present in the bio film accelerate oxygen uptake. This can cause an oxygen deficiency in the system. Some microorganisms switch to fermentative metabolisms and produce a number of organic acids, which causes a decrease in pH. Anaerobic bacteria form sulphide byproducts, which are corrosive.

Cooling tower water disinfectionCooling tower water disinfection

Cooling tower water disinfection must meet demands that differ from those of drinking water disinfection and swimming pool water disinfection. Disinfectants may not affect the system and must remove microorganisms that can affect the system, as well. Cooling tower water is not used as drinking water. As a result, it does not need to meet drinking water quality demands.Work-related exposure to cooling tower water or water vapor may occur. As a result, pathogenic microorganisms in cooling tower water, such as Legionella bacteria, must be deactivated.

Legislation for cooling tower water disinfectionDischarge demands

When cooling tower water is tapped from a river or lake, and must be discharged into the same water body after it has been used, it must meet certain discharge demands. Aditionally, the water temperature may not be too high, because warm water has a low oxygen content, which promotes algal growth. This can cause fish mortality and a decrease in water biodiversity.