Control of the three major microbiological classes, bacteria, algae and fungi, and macroorganisms including zebra mussels, is essential for healthy cooling tower operation. For many years, chlorine was the treatment chemical of choice. However, chlorine use is declining due to environmental, safety and performance-related issues. This article examines the positive and negative aspects of chlorine and of replacement oxidizing biocides.
Microbiocides generally belong to one of two groups: oxidizers and non-oxidizers. The former attack cells by oxidizing (an electron transfer reaction) microorganism cell components. Non-oxidizers react with cell components via different chemical processes. Oxidizing biocides are still the most common biological control agents, and even though chlorine use is declining, it continues to be an important player in the cooling water treatment industry.
When chlorine is injected into a cooling water stream, it disproportionates into hypochlorous and hydrochloric acid as follows:
Cl2 + H2O Æ HOCl + HCl
HOCl is the oxidant that attacks cell structures. An increase in pH increases the dissociation of HOCl into the hypochlorite ion (OCl-):
HOCl H+ + OCl-
Although both HOCl and OCl- are oxidants, OCl- is a much weaker disinfectant, possibly because the charged OCl- ion has a more difficult time penetrating the cell wall. Chlorine`s biocidal efficiency greatly decreases as the pH rises above neutral. (See sidebar.)
For years, the most popular cooling water treatment program at many facilities was low-level sulfuric acid feed to control calcium carbonate formation, with supplemental feed of chromate and zinc for corrosion inhibition. This suited chlorine as a microbiocide because the mildly acidic pH maintained the chlorine residual predominantly as HOCl. However, chromate discharges to the environment have been banned due to the potential release of toxic hexavalent chromium. Modern cooling tower treatment programs operate in the less corrosive alkaline range of pH 8.0 to 9.0, in which advanced calcium carbonate scale inhibitors have replaced sulfuric acid. Such programs do not favor chlorine as a microbiocide. This problem has been exacerbated by the development of more efficient cooling tower fill (Figure 1), whose close spacing makes the material susceptible to pluggage.
Safety issues are another factor in chlorine`s reduced popularity. Chlorine gas is quite hazardous, and regulations governing its storage and leak detection are becoming increasingly stringent. Rather than deal with the safety requirements for gaseous chlorine, many plant managers are opting for alternatives.
The potential for formation of chlorinated organics has also become an important issue. Many halogenated organics are known or suspected carcinogens, and tighter restrictions are being placed on the amount of allowable halogenated organics. In 1979 the Environmental Protection Agency (EPA) set an interim maximum contaminant level (MCL) of 0.100 ppm for total trihalomethanes (TTHM`s). The agency has proposed to reduce the MCL to 0.080 ppm, and may lower the standards even further in the future.
In 1982, the power industry was required to meet optimized technology-based standards for chlorine use. The requirements limited the maximum chlorine discharge from cooling towers to 0.5 ppm, with an average discharge of 0.2 ppm for no more than two hours per day. In 1985 the EPA announced more stringent ambient water quality criteria, which applied to all industries. These guidelines limited fresh-water chlorine concentrations at the boundary of a calculated effluent mixing zone to 0.011 ppm over a four-day average, or 0.019 ppm average for one hour. Restrictions for salt water are even more stringent at 0.0075 ppm and 0.013 ppm, respectively.
The regulations have made it particularly difficult to use chlorine to control cooling water biological fouling. The situation has been further complicated by the spread of macrofouling species, the most notable of which are zebra mussels and Asiatic clams. Continuous or semi-continuous chlorination is necessary to control the growth of macrofoulers, especially adults, but continuous chlorination is expensive, particularly when bleach is the biocide. Continuous chlorination can also harm non-target organisms in once-through cooling systems. Many facilities must dechlorinate their cooling water prior to discharge in order to comply with water quality standards. This process typically requires the feed of a reducing agent, such as sodium bisulfite, into the effluent.
A popular substitute for chlorine is bromine (Br2). Like chlorine, bromine reacts with water to produce a hypohalous acid, in this case HOBr. Bromine has nearly the same oxidizing power as chlorine, but it offers several advantages over chlorine in certain conditions. First, the dissociation of HOBr occurs at a higher pH than HOCl (Figure 2), which makes it more effective in alkaline environments. Second, bromine does not react irreversibly with ammonia as does chlorine. Chloramines are much less effective disinfectants than free chlorine, which makes chlorination of ammoniated waters problematic. Third, bromine is less corrosive than chlorine to copper alloys.
Bromine may be introduced into a cooling water system by several different methods. Most common is to react liquid sodium bromide (NaBr) with chlorine or sodium hypochlorite in a sidestream loop of the cooling water makeup. Chlorine activates the bromide salt to hypobromous acid as follows:
NaBr + HOCl Æ HOBr + NaCl
Sodium bromide, being the bromine analog of common table salt (NaCl), is stable and may be stored in a simple bulk tank. NaBr is usually supplied as an aqueous solution of approximately 40 percent concentration. The sodium bromide and chlorine or sodium hypochlorite should be fed separately into the slipstream to obtain at least a 100:1 dilution. This prevents the formation of undesirable bromate byproducts.
Like chlorine, bromine is toxic to non-target organisms and it can form halogenated organics. For these reasons plant cooling water discharges containing bromine are regulated similarly to chlorine, although some states or EPA regions have established more restrictive standards for bromine residuals.
Even though the cost of sodium bromide adds to the total delivered cost of the oxidants, users often find it possible to reduce the overall quantity of oxidant required to achieve the equivalent performance. Frequently, the reduction in chlorine consumption more than offsets the cost of the sodium bromide, especially where liquid sodium hypochorite is the chlorine source.
For smaller cooling water systems, solid bromine donors may be a cost-effective alternative to the arrangement mentioned above. One of the most common solid biocides is bromo- chloro-dimethyl-hydantoin, or BCDMH, which releases bromine as it dissolves in water. Several of the major water treatment vendors supply solid bromine or chlorine donors in granules, pellets, or tablets along with a feed system. As water passes through the dissolving vessel, the BCDMH dissolves at a controlled rate to release HOBr and HOCl:
C5H6O2N2BrCl + 2H2O Æ C5H8O2N2 + HOBr + HOCl
Solid donor systems are also available for strictly chlorine-releasing products. Some of the most common chlorine-based solid donors include dichloro-dimethyl-hydantoin, calcium hypochlorite [Ca(OCl2)] and chlorinated isocyanurates. The latter two compounds are widely used as swimming pool chemicals, but will also work well in some cooling water applications. Specific feed systems are available for each halogen donor. All of these products are strong oxidizers and must be handled and stored properly; it can be hazardous to use one compound in a feeder designed for another.
Prominent advantages of the solid halogen donors are:
Handling--no potential for liquid spills;
Stability--stable compared to bleach;
Effectiveness--strong oxidizers that work well at alkaline pH and in the presence of ammonia;
Water Chemistry--less corrosive to system materials; do not significantly alter cooling water pH; and
Environmental and Safety Factors--no chance of toxic gas or liquid release.
The solid halogen donors are best for systems with a low or moderate chlorine demand, and where simple operation is desirable. Solid donors are generally more expensive than chlorine and even bleach, and can be slow to dissolve at water temperatures below 60 F.
Sodium hypochlorite (NaOCl) may be a suitable non-gaseous alternative. It comes in bulk solution, at a concentration ranging from 10 percent to 12.5 percent as NaOCl. The hypochlorite can be metered directly into the cooling system. An important point to remember about NaOCl is its effect on cooling water pH. Gaseous chlorine lowers the pH due to the production of both HOCl and HCl. Sodium hypochlorite tends to raise the pH.
Sodium hypochlorite will degrade over time to form sodium chloride, oxygen, and sodium chlorate (NaClO3). Temperature and impurities greatly affect decomposition. (See sidebar for specifications and storage guidelines for sodium hypochlorite.)
The improved safety and convenience of sodium hypochlorite versus gaseous chlorine has a tradeoff in cost. Chlorine gas in ton cylinders costs $0.15 to $0.20 per pound delivered. By contrast, 12.5 percent sodium hypochlorite costs around $0.60 per gallon ($0.059 per pound) delivered in bulk. This corresponds to $0.50 per pound on an equivalent Cl2 basis, or approximately three times the cost of gaseous chlorine.
Chlorine dioxide (ClO2) is a powerful oxidizer with excellent biocidal properties. It offers several potential advantages compared to chlorine. Chlorine dioxide:
does not form halogenated organics,
is less corrosive to copper alloys than chlorine,
does not react with ammonia and primary amines,
efficiently destroys phenols and sulfides,
is not affected by pH like chlorine and works well in alkaline waters, and
is more effective against mollusks.
These advantages come at a price, however. Chlorine dioxide is a very reactive compound that is hazardous to transport at practical concentrations. It must be generated on-site from other reactive chemicals that also present some handling risks. Chlorine dioxide does not react with water or ionize in solution, thus it remains as a dissolved gas that is easily stripped across a cooling tower. The principal reaction product, chlorite, affects some forms of aquatic life at low levels. Chlorite can be difficult to neutralize with reducing agents.
Chemical costs for sodium chlorite are typically in the $0.50 to $1.00 per pound range for a 25 percent aqueous solution. This corresponds to $2.00 to $4.00 per pound of active ClO2. The costs of bleach or acid for reaction, along with the 80 percent to 95 percent efficiency of the ClO2 generation reactions, can push the price above $3.50 per pound, or over seven times the cost of chlorine. Still, there are some situations where the effectiveness of chlorine dioxide may offset the added cost.
Ozone (O3) is a powerful, short-lived oxidant that is generated by passing air through an electric discharge. The air stream is then bubbled into the cooling water through a diffuser.
Ozone is too reactive to transport or store on-site, so it must be generated at the point of use. This requirement has so far limited ozone`s application to relatively small cooling systems. Ozone is the most powerful oxidant of all, but its oxidizing strength can be a mixed blessing. Ozone rapidly destroys biofilms and produces sparkling clear water, but it also breaks down most scale and corrosion inhibitors, and it slowly degrades plastic, rubber and gasket materials. Similar to chlorine dioxide, ozone does not ionize in water but remains a dissolved gas, which can be stripped during passage through the cooling tower. Ozone offers environmental advantages in that its residuals are short-lived, it does not produce halogenated reaction products, and it breaks down into oxygen, which can be beneficial. However, ozone can produce secondary oxidants if bromide is present, and it can also produce aldehydes and ketones via reaction with organics.
One of the major advantages of ozone is that no hazardous chemicals are transported, stored or handled on-site. For small cooling systems in sensitive locations, ozone can be an attractive alternative to chlorine. Researchers continue to investigate methods of ozone treatment for larger systems. p
Figure 1. Cooling tower film fill.
At a pH of 6.5 tests have shown that a 1.0 ppm chlorine solution will kill 99 percent of all microorganisms within 30 seconds. At this pH, a large percentage of the dissolved chlorine exists as HOCl. The percentage rapidly decreases as pH rises.
Thus, in an alkaline cooling water treatment program with a pH between 8 and 9, the available hypochlorous acid is well below 50 percent (Figure 2).
Sodium hypochlorite will decompose into oxygen, sodium chloride, and sodium chlorate. The decomposition rate is affected by temperature and by the catalytic action of some metals, most notably iron and copper. A specification for bulk quantities of sodium hypochlorite should contain the following:
Iron concentration less than 0.5 ppm;
Copper concentration less than 1.0 ppm; and
pH range from 11.0 to 11.2.
Temperature can have a dramatic impact on hypochlorite solutions. For example, the half-life of a hypochlorite solution is reported to be 800 days at a temperature of 59 F. At 77 F the half-life drops to 220 days, and at 140 F, the half-life is only 3 days. Bulk storage tanks of sodium hypochlorite should be kept as cool as possible by sun-shading, painting them white, or both.
Fiberglass-reinforced-plastic is the recommended construction material for bulk storage tanks. Poly ethylene was once the material of choice, but cracking problem shave been reported after exposure to hypochlorite.
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