مطهرات ومعقمات المياه

disinfectants

by
colonel.dr
bahaa badr
chemical consultant

Most water treatment processes, either separately or in combination, can be trusted to remove or destroy all bacteria in the water. Ascertaining the water is safe and as possible it will remain that way, it should be disinfected or sterilized before it leaves the treatment plant.

A different has to be established between sterilization and disinfection by carefully defining them as follows:

Water sterilization consists in the killing of all organisms living in the water.

Disinfection consists in the killing of organisms that may cause disease. Disinfection should be harmless, unquestionable by the consumer, and it should be measurable by simple tests.

Chemicals used for disinfection should preferably be those retaining their disinfection power long after water has been release from the plant to provide some protection against recontamination.

Chlorine is the disinfectant almost universally employed and the term chlorination is commonly used to indicate broadly that the water has been treated with an agent capable of killing all organisms capable of producing disease.

The equipment used to applying chlorine must be reliable and it must be capable of operating within narrow margins of accuracy. It must be kept in mind that too little chorine is ineffective and too much chlorine may cause odors and tastes. The amount of chlorine fed to the water should be proportioned to the volume of water flow and to the chlorine demand of the water.

Good mixing of the chlorine and the water has to be ascertained and the point of mixing should be so located as to ensure there will be a 20 to 30 minutes contact period between the two elements before the first consumer is served.

Chlorine and water contact period serves the double purpose of providing the time necessary to destroy the pathogenic organisms in the water and reduce the effect on consumer of possible overdoses of chlorine with the accompanying odors and tastes. The contact period is a function of the amount of residual, which is the chlorine remaining after initial contact with the water. If a low residual is required, a longer contact time is necessary.

Quality of the water can only be substantiated with good equipment and good installations accompanied by effective operation and maintenance.



Chorine is also used for other purposes such as:

 Algae control;
 Reservoirs growths control;
 Pipelines organic growths prevention;
 Coagulation with chlorinated copperas; and
 Control and neutralization of odors and tastes.

Chlorination Efficiency

The main factors affecting water chlorination efficiency include:

 Amount and type of present chlorine;
 Relationship between the forms of chlorine in the water after chlorination;
 Presence and extend of chlorine demand;
 Contact time between chlorine and water;
 Temperature; and
 Water acidity or alkalinity.

Water treatment theory assumes that organisms causing typhoid fever are not more resistant to chlorine than the coliform organisms. The amount of chlorine applied, the time and temperature factors, and the chlorine residuals used have given the basis to this assumption.

Chlorine Action

When chlorine is added to water, the resulting reaction is:

Cl2 + H2O HOCL + HCl

Chlorine + Water Hypochlorous acid + Hydrochloric acid

The products of this reaction do not persist since the hypochlorous acid dissociates into hydrogen ions, H+ and hypochlorite ions, OCl-.

Both reactions are reversible and both are dependent on the pH of the water. The first equation predominates at low pH and the second reaction predominates at high pH. The OCl- ions react immediately with ammonia ions present in the water to form various chloramines compounds. After the demand of the ammonia ions has been satisfied, free chlorine will be available.

The available free chlorine is much more effective than combined chlorine as a bactericidal agent under most conditions and acts much more rapidly.



At temperatures below 10o C free chlorine is approximately half as effective as it is at temperatures above 18o C. Chlorine is also most effective when the water pH is 7 of below and increasingly large concentrations of chlorine are required when the pH goes above 7.

Chlorine Usage Forms

The most commonly used forms of chlorine are liquid chlorine and hypochlorites. Chlorine is applied either alone or in combination with ammonia to form chloramines. Chlorine residuals may be of two types:

1. Combined residual, when the chlorine is combined with natural or added ammonia; and
2. Free residual, when enough chlorine is added to produce available free chlorine either directly or by destruction of the ammonia present.

A free chlorine residual may persist through a considerable portion of the distribution system, thus providing some assurance in localizing contamination that is not of a massive nature.

Chlorine Requirements

Most waters have a chorine demand which may be satisfied before chlorine is effective as a disinfecting agent. Chlorine demand is the difference between the amount of chlorine added to the water and the amount remaining at the end of a specified contact period.

The chlorine demand arises from waters with:

 Organic contents;
 Iron;
 Manganese;
 Nitrates; and/or
 Hydrogen Sulfides.

Most of these substances react rather quickly with the chlorine therefore, the chlorine residual measured after a brief period is taken as the index of the required quantity of chlorine to be applied.

When chlorine is applied to water in sufficient amounts, the first reactions cause the destruction of the chlorine-reducing compounds and there is no disinfecting action. As more chlorine is added, the next steps are the formation of chloro-organic compounds and ammonia-chlorine compounds, which have a slow disinfecting action. As still more chlorine is added, these compounds are destroyed and finally free chlorine is available in some ratio to the excess of chlorine added. This free chlorine has a rapid disinfecting action.

An effective disinfection requires free chlorine residuals of 0.2 mg/l under the most favorable conditions and residuals up to 0.4 to 0.8 mg/l otherwise. For waters that have a pH value not greater than 7, there should be a free residual of 0.2 mg/l after 10 minutes. For waters with a pH of 8, there should be a free chlorine residual of 0.4 mg/l or a combined residual of 1.8 mg/l. For a combined residual to be effective, the pH should be reduced below 9.

Measuring Chlorine Concentration

Concentration of chlorine in water is measured b the ortholodidine-arsenite (OTA) test. This test shows the available free chlorine residual and the combined chlorine residual. It also distinguishes by the colors produced by the nature of interfering substances such as ferric, nitric, and manganic compounds.

Chlorine Destruction of Virus

Hepatitis virus in clear water has been inactivated by 1.1 mg/l of combined residual or by 0.4 mg/l of free residual. Coxsackie virus appears to require a very heavy chlorine residual, from 7 to 46 times as much free chlorine as E. coli).

Poliomyelitis appears to be destroyed by a very small residual, 0.1 mg/l of free chlorine residual after 30 minutes at about pH 7. The cysts of amoebic dysentery are resistant to chlorine, about 3 mg/l of free residual will be required at pH 6. Tuberculosis organisms have been killed with residual of about 3 mg/l with 30 minutes contact.

Liquid Chlorine Application

Chlorine is a greenish-yellow gas, but under pressure it is converted into a liquid form and it is then shipped in steel containers, which are furnished in various capacities from 100 lb to 1 tone. At 18 oC the pressure required to maintain the liquid form is about 85 psi, but the container pressure increases with the temperature. When the liquid is drawn from the container, it changes to gas and a temperature drop occurs in the cylinder.

In most water works, liquid chlorine is applied by means of solution-feed chlorinators. Such a device takes liquid chlorine from the container, meters it, and mixes it with a small amount of water to form a strong chlorine solution. This solution is fed into the main usually by means of a water injector.
Direct feed chlorinators are sued on some small plants. A chlorinator of this type feeds the chlorine gas to the water through a diffuser by utilizing the pressure of the chlorine in the cylinder.

Chlorinator Installation


Chlorinators should be placed in a room heated to maintain a temperature of at least 12 degrees centigrades, the room should have a door opening outward to the outside air, ventilation should be provided, and a tight inside window should be implemented to permit supervision of the operations without entering the room.

Precautions should be taken to keep the chlorinator at a higher than that of the chlorine containers in order to prevent condensation as the gas enters the chlorinator. It must be remember that a concentration of 1 part chlorine in 100,000 parts of air is noticeable, 1 part chlorine in 50,000 parts air causes inconvenience, but 1 part chlorine in 1,000 parts air may cause death after exposure for 5 minutes.

Calcium and Sodium hypochlorites are used for chlorinating water in small installations and for sterilizing water mains. The formula of calcium hypochlorite is Ca (OCl)2, and that for sodium hypochlorite is Na OCl.

The term available chlorine is used to represent the total oxidizing power of a hypochlorite. It is equal to the OCl- value of the compound or twice the weight of the chlorine present as OCl- in the compound.

Hypochlorites Application

Hypochlorites are usually applied by dosing instruments specifically design for that purpose and featuring all kind of regulations. A simple way of applying it to the water in the absence of the dosing device is utilizing two barrels and connecting these to a box with a level controlled by a bloat and valve.

One container is filled, and the chemical in it is allowed to stand still long enough for undissolved material, which is mainly lime sludge, to settle out. The valve connecting it to the feed box is then opened. While one container is in use, the other can be filled and the sludge is allowed to settle out. The discharge line form each container should extend several inches above the bottom to prevent clogging by the settled material.
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Chlorine and Ammonia

Ammonia gas is packaged in containers the same as chlorine is and it is applied to water by ammoniators, which work on the same principles as the chlorinators, however, the equipment is not interchangeable because the two chemicals react differently with the materials in the feeders.

When phenols tastes are a problem, ammonia should be applied far enough in advance of chlorine to allow a thorough mixing with the water. Since the disinfecting reactions ae much slower with chlorine and ammonia that with chlorine alone, a contact period of 2 hours should be provided before the waters is used.

Special Methods of Disinfection

In special cases, it may be desirable to apply very large amount of chlorine to water. Some of those cases are:

 When tastes and odors can be destroyed by increasing the amount of chlorine applied;
 Where the water is turbid or contains organic matter; and
 When there is reason to believe that it also contains cysts of E. histolytica, which is the organism causing amoebic dysentery.

In these cases 5 to 15 mg/l of chlorine may be applied and when the desired contact period is achieved, the excess chlorine can be removed by adding a dechlorinating agent such as sodium thiosulfate or sulfur dioxide. These agents may be applied either as a solution or, in the case of sulfur dioxide, as a gas.

Activated carbon also removes chlorine from water. The amount of chlorine absorbed will theoretically be 12 times the weight of the pure carbon, but in practice it will absorb only 1.5 to 6 times its weight.

When chlorinated water has been dechlorinated, no residual remains and recontamination can occur. Therefore, after superchlorination and dechlorination, it is desirable to again chlorinate the water to provide a perceptible residual unless the water is to be consumed at once.

Prechlorination

Chlorine may be added to the water in advance of any treatment or between any two steps of treatment. Such practice is called prechlorination, as distinct from chlorination performed as the final step in treatment. Prechlorination may reduce the bacterial load on the plant or prevent undesirable algae growth, and in some plants it is effective in odor control.


Other Chemicals Used in Disinfection

CL2 (Chlorine dioxide) is an unstable compound that is normally generated at the point of use, by adding sodium-Chlorite solution to the chlorinator discharge line. Chlorine dioxide has and oxidizing power about 2.5 times as great as the chlorine alone, it is specially effective in the removal of tastes and odors when it is applied in amounts ranging from 0.5 to 1.5 mg/l.

CL2 is also relatively unaffected by changes in pH between 6 and 10, and it is therefore especially valuable in highly alkaline waters.

Due to the fact that CL2 is quickly taken up by organic matter, its use is restricted to nonpolluted waters in which its bactericidal values can be fully utilized.

Ultaviolet rays are used for disinfection, by passing the water through a thin sheet where a special ultraviolet lamp is placed. Any considerable organic content or turbidity reduces the efficiency of the process.

Iodine is a disinfectant, but in water supply has been used only for soldiers canteen sterilization.

Bromine has been used for treating swimming pool water.

Ozone is an unstable form of oxygen in which each molecule contains three atoms. Nascent oxygen, released when ozone is changed to a two-atom molecule, becomes available as a disinfecting agent. A perceptible residual, which can be measured by the orthotolidine test, remains after treatment. The recent application of ozone in water treatment has been limited to a few installation for reducing odors and tastes.

Silver has some sterilizing power, but it not adaptable to water purification.

Excess lime, when added in sufficient amounts to raise the pH of the water above 9.5, does bacterial reduction, but a complete kill cannot be relied upon and there is no residual effect after pH reduction.