Chlorine has long been used as a disinfectant for domestic water supplies and for the removal of tastes and odors from water. A number of factors determine the amount of chlorine required in an open industrial water system. These include chlorine demand, contact time, pH and temperature of the water, the volume of water to be treated and the amount of chlorine lost by aeration as the treated water passes over the industrial tower. In addition, many areas of the country have chlorine discharge limits which also may determine the maximum amount of chlorine that can be used in a given plant.

When chlorine gas encounters water, it hydrolyzes to form two acids, hypochlorous and hydrochloric acid, respectively.

Hypochlorous acid will ionize according to this reversible reaction:

HOC1 <=> H⁺ + OC1^-

The amount of hypochlorous acid, as opposed to hypochlorite ion, determines the biocidal efficacy.

Hypochlorous acid is an extremely powerful oxidizing agent. It easily diffuses through the cell walls of microorganisms and reacts with the cytoplasm to produce chemically stable nitrogen-chlorine bonds with the cell proteins. Chlorine oxidizes the active sites on certain coenzyme sulfhydryl groups which constitute intermediate steps in the production of adenosine triphosphate, which is essential to respiration. Hypochlorous acid is estimated to be twenty times more reactive (effective) as a microbiocide than the hypochlorite ion.

The pH of the industrial water is directly responsible for the extent of ionization of hypochlorous acid. The acid state is favored by low pH. As shown in Figure 5-19, at pH 5.0 there will be very little ionization. At pH 7.5 there will be approximately equal amounts of acid and hypochlorite ion. Above a pH of 8.0, the cost-effectiveness of other oxidizing microbiocides should be strongly considered as alternatives to chlorine. It is in these more alkaline environments that oxidizing toxicants like chlorine dioxide and bromine donors have gained widespread popularity. Chlorine becomes ineffective as a microbiocide at pH 9.5 or greater, as a result of total ionization. A pH range of 6.5-8.0 is considered practical for chlorine-based microbial control programs, since lower pH values would increase system corrosion and higher pH values favor alternate oxidizing technologies.

Many industrial water treatment programs operate in an alkaline pH range. This is possible because the newer, more sophisticated antifoulants available for scale control and inhibitor formulations are effective over a wide pH range. As effective as alkaline programs may be in controlling scale and corrosion, they can weaken a chlorine-based microbiological control program.

There are a number of basic types of chlorination programs, among them prechlorination and postchlorination, terms which simply refer to the point at which the system is disinfected. In industrial water systems, the most prevalent program is called "breakpoint" chlorination. Chlorine is dosed to the system initially to satisfy the demand of the system and then to attain the desired free residual chlorine level.

"Chlorine demand" of the system refers to the amount of chlorine which will react with contaminants before any chlorine is left unreacted. Organic matter, including algae and slime, tower lumber and the presence of hydrogen sulfide, sulfur dioxide and certain nitrogen compounds, all exert a chlorine demand which must first be satisfied if a suitable concentration of residual chlorine is subsequently to exist in the water.

Figure 5-20 shows a breakpoint chlorination curve for the chlorination of a water containing only ammonia-nitrogen.

If the ratio of chlorine to ammonia-nitrogen is less than 5:1, monochloramines will form. This is represented on the curve by the area labeled NH₂C1. If the ratio is greater than 5:1, dichloramines and nitrogen trichloride start to form and will continue until the ratio reaches 10:1; this is shown by the areas labeled NHC1₂ and NCl3,. During this reaction, there is a decrease in the residual chlorine within the system. After this "breakpoint," any further addition of chlorine results in a "free available residual concentration," a technical term which merely refers to that unreacted chlorine in the system.

The formation of chloramines contributes to the combined chlorine residual. There is widespread belief that chloramines are more harmful environmentally than chlorine itself. These compounds do have toxicity, but will only be effective at a pH of 9.5 or greater. Only hypochlorous acid and hypochlorite ion can produce free residual chlorine. Most programs aim to have some free chlorine as a ready disinfectant in the event of increased microbial activity.

Because chlorine produces acids when added to water, there is a corresponding reduction in the alkalinity of the water. For every ppm of chlorine gas added, 1.2 ppm of alkalinity is neutralized. When chlorine reacts with iron or hydrogen sulfide, there is also reduction of alkalinity.

In the first reaction, 0.9 ppm of alkalinity is neutralized per ppm of iron oxidized. The reaction involving hydrogen sulfide results in a much more extensive alkalinity neutralization. Ten ppm of alkalinity are neutralized for each ppm of hydrogen sulfide in the system as a result of the reaction of the hydrochloric and sulfuric acids produced.

Chlorine is an excellent algaecide and sporicide. Free residual chlorine at intermittent levels of 0.5 ppm and slightly above are usually enough to control most microbial growths. A longer chlorine contact time is usually required for the eradication of blue-green algae because of their extremely dense slime capsules.

Chlorine is also an excellent bactericide in most circumstances. However, some strains of Aerobacter, Pseudomonas and Desulfovibrio can develop strong resistance. Iron bacteria can be controlled at low chlorine residuals, but once a deposit problem develops, high chlorine levels are required.

Chlorine in solution will eventually form chloride ions, which are relatively harmless. However, residual chlorine discharged with the tower bleedoff can continue to react with such contaminants in the receiving stream as

ammonia, nitrogen and phenols to produce chloramines and chlorinated phenolics which are toxic to fish.

Gas chlorination equipment is costly and generally requires a relatively large capital investment for many smaller plants. Chemical usage costs also contribute to the cost of chlorine programs. High chlorine usage results from recirculating waters contaminated by ammonia, phenol and organic matter.

Many plants have found it advantageous to chlorinate their recirculating systems at night in an effort to prolong their chlorine residuals since sunlight (ultraviolet light) breaks down chlorine.

Because chlorine is destroyed by sunlight and lost by aeration as it passes over the industrial tower, feed is generally downstream from the tower, just before the heat exchange areas or directly into the tower basin through a PVC distribution header. Feed can be continuous or intermittent, although the latter is usually preferred, since it gives good control at more reasonable cost.

Chlorine levels should be monitored at the end of the system; existence of a residual at that point assures the presence of a residual throughout the system.

Because the amount of chlorine added to the system is directly proportional to the alkalinity reduction, many plants find it necessary to suspend acid feed during chlorination periods in order to avoid low pH excursions.

There are two basic chlorine feed systems. Dry gas chlorinators apply chlorine gas to the system through suitable diffusers. Solution-feed chlorinators mix chlorine with water and then discharge into the system. Either design is available with manual or automatic operation