I
JOHN J. MAGUIRE
W. H. & L. D. Betz, Philadelphia, Pa.
Biological Fouling in Recirculating Cooling Water Systems Chlorine, bromine, chlorinated phenols, copper salts, and quaternary ammonium compounds prevent interference with heat rransfer caused by slime and algae in recirculating cooling water systems
B m m s E cooling represents the largest single use of water by industry, this use of water is usually the first problem attacked in a water conservation program. A recirculating system, in which a cooling tower, spray pond. evaporative condenser, and the like dissipate heat, permits great economy in make-up water requirements, usually decreasing them by an average of 95y0as compared with once-through operation. Aside from the reduction in make-up water used, it is possible to secure more effective corrosion control in a recirculating system than in a oncethrough system, because the quantities of corrosion inhibitor required are within the range of economic feasibility, which is often not the case with oncethrough systems. However, the use of recirculating systems can increase and complicate biological fouling. For example, exposure of the circulating water to sunlight in cooling towers and spray ponds favors the growth of algae. Increases in temperature and in concentration of dissolved and suspended solids, and the greater incidence of air-borne contamination usually increase the slime-forming potential due to fungi and bacteria. Difficulties Due to Algae and Slime Biological fouling in cooling water systems is the result of excessive growth and development of different members of the lower forms of plants-namely, algae and fungi. In general, the principal difference between algae and fungi is that algae are capable of manufacturing their own food, whereas fungi are not. Algae commonly are found in all surface water supplies. They range in size from unicellular microscopic kinds
2 162
to some salt water species more than 100 feet long. Although these plants are usually found in fresh and salt water, a few species are terrestrial. The species of primary importance in cooling water systems are the microscopic fresh water types, although, under unusual conditions, the larger marine types also may become an industrial menace. Because sunlight is necessary for the growth and development of algae, recirculating cooling water systems are plagued with the problem of algal growths to a greater extent than oncethrough systems, where the cooling water is not usually exposed to direct light. Abundant algal growths in spray ponds and in cooling towers interfere with proper water distribution and thus prevent the desired cooling. Large masses of algae plug screens and restrict flow in pipelines and pumps. Living algae in the presence of sunlight carry on processes of photosynthesis which manufacture food and liberate oxygen. Masses of living algae on metal surfaces accelerate corrosion in the form of pitting because of the depolarizing action of the oxygen thus released. Accumulations of dead algae lodged in heat exchange units also set up local corrosion cell action, which can cause severe pitting of the metal. The second type of biological fouling is commonly referred to as slime. Slime is an accumulation of microorganisms and whatever inorganic andl’or organic debris may become embedded in the mass. The microorganisms usually found in these deposits are members of the filamentous fungi, yeasts, bacteria, and occasionally some protozoa. Slime may contain dead algae entrapped in the mass. This type of biological fouling may occur either in illuminated or in dark areas. In general, dark areas
INDUSTRIAL AND ENGINEERING CHEMISTRY
or areas of low light intensity favor this type of biological fouling, because these microorganisms do not utilize light to manufacture their food-they are totally dependent upon the food available in the cooling water system. Most bacteria and fungi found in cooling systems are the common air-borne types. The slime-forming bacteria have a gelatinous capsule which not only entraps other materials that add to its bulk, but also protects the bacteria against the action of chemical and physical agents. Slime deposits on process equipment so retard heat transfer as to cause a serious loss of efficiency. Slime deposits on metal surfaces create local cell action and aggravate pitting tendencies. Figure 1 shows two steel test specimens freshly removed from a cooling water system, each coated with gelatinous slime deposits. Figure 2 shows the specimens after drying. The slime has dried and flaked off, exposing areas of tuberculation, resulting from pitting of the metal. Control of Biological Fouling
Many different types of chemical agents are employed for control of microorganisms in recirculating cooling water systems. The primary purpose of the chemical agent is to kill and/or inhibit the growth and accretion of organisms. If a given toxicant is present in a sufficient quantity or is highly toxic, the organisms are killed; at levels below its killing concentration, the chemical only inhibits the growth of the organism. At extremely dilute concentrations the chemical agent may actually stimulate the groi\rth of microorganisms. Inhibiting substances are known as bacteriostatic agents, whereas chemical agents that kill bacteria are known as bactericides. A dilute solution of a bactericide may act as a bac-
RE-USE OF WATER B Y I N D U S T R Y teriostatic agent when its concentration is too low to kill bacteria. Chlorine. Chlorine is probably the agent most widely employed to control microbiological deposits in recirculating systems. I n the absence of substances that cause a high chlorine demand, chlorine treatment is usually the most economical. A recent survey ( 4 ) of 16 Gulf Coast refineries and chemical plants shows that continuous chlorination is employed in 24 towers and intermittent chlorination in 10 towers. Commercial algicides are used in 26 towers, either alone or in conjunction with chlorine. One tower is treated with sodium hypochlorite and in only one tower are no slime-control measures employed. Chlorine is highly toxic and acts quickly to kill bacteria. A chlorine residual of 0.5 to 1.0 p.p.m. will usually destroy most microorganisms. However, because chlorine acts on all oxidizable material, the chlorine demand of a water is greatly increased by the presence of organic matter, hydrogen sulfide, and ferrous iron. Thus, sufficient chlorine must be added to develop the residual necessary for toxicity and to satisfy the demand of other materials present in the water which are readily oxidized or absorb chlorine. Chlorine is stable in neutral, acid, and alkaline waters, although its germicidal efficiency is reduced somewhat under alkaline conditions. When large amounts of chlorine are required, it is generally obtained in liquid form and fed to the system by means of a chlorinator. Because of its corrosivity, the use of chlorine involves some hazards in handling, al-
though, satisfactory chlorination equipment is available. The amount of chlorine required for the control of biological fouling in any individual system is governed by numerous factors. There may be wide variations in chlorine requirements between different tower systems in the same plant. Chlorine requirements are affected by the quality of the make-up water to the system, water temperatures, air-water ratio, amount of contamination by reducing agents, and type and amount of microorganism contamination. The manner of chlorine feed, whether continuous or intermittent, and the residual chlorine for control of the problem are also individual to a specific system and vary between towers in the same plant. For control of slime and algae in any system, it is necessary that sufficient chlorine be fed to secure the necessary residual chlorine content of the treated water, and that this chlorine residual be maintained in the system long enough for microorganism control. Chlorine Residual. Although a chlorine residual of 0.5 to 1.0 p.p.m. normally kills most microorganisms contaminating cooling water systems, in many cases a free chlorine residual rather than a combined chlorine residual must be present. Chlorine reacts with nitrogenous matter in the water to form chloramines. I n the form of chloramines, the disinfecting properties and oxidizing power of chlorine are considerably reduced and increased contact time or increased chlorine residual must be provided for bacterial reduction. The chlorine-,
Figure 1. Slime coating on test specimens as removed from cooling water system
Figure 2.
ammonia process has been used in distribution systems to reduce chlorine demand. There is, however, a significant difference in the germicidal effects of a chlorine residual, dependent on the form in which the chlorine is present in the treated water. The free available chlorine residual is defined as that portion of the total residual chlorine which will react chemically and biologically as hypochlorous acid or hypochlorite ion. I n this form, chlorine exerts the most potent bactericidal effect. The combined available chlorine residual is defined as that portion of the total residual chlorine which will react chemically and biologically as chloramine or organic chloramines. I n this form, chlorine is a relatively mild bactericide and oxidizing agent. It is possible to control slime and algae growths adequately with a combined chlorine residual in systems where the problem is not too severe or there is a relatively long contact time. However, in the presence of resistant microbiological complexes or where the contact time in the system is short, it is necessary to develop a free residual chlorine concentration in the circulating water. If an adequate free chlorine residual can be maintained in the circulating water for a sufficient time, any normal biological fouling can be brought under control. Many reported failures of chlorination to control the problem in recirculating cooling water systems can be ascribed to failure to maintain a free chlorine residual of sufficient concentration for a sufficient length of time, Contact Time. Because of the con-
Specimens o f Figure 1 after drying
VOL. 48, NO. 12
DECEMBER 1956
2163
tinuous contamination of a recirculating system with make-up water and airborne organisms, continuous chlorination should prove the most effective method of adding chlorine. With the continuous maintenance of an adequate chlorine residual, the maximum contact time permitted by system characteristics is secured. The contact time can be calculated by the simple formula: Contact time, days
=
T7
BD
where
V
= volume of water contained in
BD
=
system, gallons blowdown and windage losses, gallons per day
With most systems, contact times in excess of 1 day can be achieved with continuous chlorination. This time is far in excess of the 15-minute contact time usual in the disinfection of potable water. However, the importance of a free residual chlorine concentration is emphasized by investigations ( 7 ) that have shown that for a 100% bacterial kill with the same contact time, approximately 25 times as much combined residual chlorine is required as with a free o requires residual. T o obtain a 1 0 0 ~kill approximately 100 times as long acontact period for a combined chlorine residual as for the same amount of a free residual chlorine. While continuous chlorination is obviously effective, it also is the most expensive method. Continuous chlorination of a recirculating cooling water system is not always practical and may not be economical. Because of its volatility, chlorine is aerated from the water in passing over a tower or spray pond. The presence of organic agents in the cooling water increases the chlorine demand, as does the presence of reducing agents due to process leaks. If contaminants such as hydrocarbons, mercaptans, and sulfides are present in excessive quantities, it may be impossible to feed sufficient chlorine for control of slime. Complete sterility of the circulating water is not possible because of continuous contamination by air-borne organisms. However, to control organic growths, it is necessary only to reduce the bacterial population to the point where troublesome accumulations are avoided. Intermittent chlorination is common because of its economy. I t is usually possible to control biological fouling adequately by program of intermittent chlorination. The schedule of chlorine feed and the residual maintained must be tailored to the particular problem. However, it is generally enough to chlorinate daily until a free residual of 1 p.p.m. of chlorine has been maintained for 4 hours on the cooling water returned
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to the tower. The period of chlorination may exceed 4 hours because of the time required to raise the residual to 1 p.p.m.> at which level it should be maintained for 4 hours. This basic schedule can be adjusted in accordance with the severity of the problem and plant conditions. General Comparison of Chlorination Programs Program Remarks Continuous Most effective chlorination Most costly Free residual Not always technically or economically feasible because of high chlorine demand
Combined residual
Intermittent chlorination Free residual Combined residual
Less effective Less costly Inadequate for severe problems Usually effective Less costly than continuous chlorination Least effective Least costly
There are many variations in a program between continuous chlorination to a free residual as the most effective and most expensive and intermittent chlorination to a combined residual as the least effective and least expensive. While chlorination is usually an overall satisfactory method for control of slime and algae growths, it is not uncommon for bacteriological growths to build up gradually even when a free chlorine residual is maintained. Under such conditions, it is necessary temporarily to increase the chlorine residua1 and/or contact time, or to apply a slimecontrol agent of another type intermittently to assure continued adequate control of these growths. Liquid chlorine is the cheapest source of chlorine. Other chlorine-releasing agents such as calcium and sodium hypochlorite are rarely used in recirculating systems. Where the installation expense for feeding liquid chlorine is not justified, slime-control agents other than chlorine compounds are usually employed. Bromine. Because bromine is similar to chlorine in its chemical properties, the usefulness of bropine in microorganism control was investigated. Bromine has been applied to recirculating cooling water systems, particularly on the West Coast. The conclusions reported from several installations are conflicting. Some plants apparently have controlled slime satisfactorily by use of bromine alone. I n other applications, it has been concluded that bromine is most effective when used as an intermittent “shock” treatment in conjunction with continuous chlorination.
INDUSTRIAL AND ENGINEERING CHEMISTRY
The use of bromine is not n4despread. Probably the most important single factor retarding interest in bromine is the hazard involved in its handling. Bromine is a very active oxidizing agent, both as liquid and as vapor. Contact with the liquid or inhalation of the vapor presents a serious hazard. The maximum allowable concentration in air is less than half that of chlorine (5). The liquid rapidly attacks the skin and other tissues to produce irritation and burns that heal very slowly. The vapor concentration considered safe for 8-hour exposure is less than 1 p.p.m., and a concentration of 10 p.p.m. can hardly be tolerated (2). Because of its hazardous nature and relatively high cost, bromine is unlikely to find wide use for microorganism control in cooling water systems. Chlorinated Phenols. Phenolic materials have been used extensively for germicidal purposes in medicine. Many phenolic materials, and particularly the chlorinated phenols, have found use in industrial microbiological control. The toxicity of these materials varies widely between different species of bacteria and fungi. Table I (6, 7) illustrates the inhibiting concentration of phenolic and chlorinated phenolic compounds to standard test organisms. The bacteria used in these tests were Aerobacter aerogenes, a capsulated nonspore.-forming organism, and Bacillus mycoides, a sporeforming bacterium. The fungi were Aspergillus niger and Penicillium expansum. A material may be inhibitory to the growth of bacteria at relatively low concentrations, as in the case of 4,G-dichlorophenol, but require high concentrations of 2000 p.p.m. to exhibit inhibitory power against Aspergillus niger. The sodium salts of tetrachlorophenol and pentachlorophenol are inhibitory at low concentrations against the fungi and Bacillus mycoides, but concentrations of 400 and 200 p.p.m. are necessary to inhibit the growth of Aerobacter aerogenes. The results obtained by a given laboratory procedure do not necessarily indicate the dosage required to control microorganisms in the field nor the relative effectiveness of each material under different field conditions. Rather, such data indicate that there are wide variations in the effectiveness of a toxicant against different organisms and that a microbiological complex sometimes can be effectively controlled only through a combination bf several treatments. The chlorinated phenols most widely used in recirculating cooling water systems are the sodium salts of trichlorophenol and pentachlorophenol. Sodium pentachlorophenate is probably in most widespread use, either alone or combined in commercial formulations with sodium trichlorophenate. Sodium pentachlorophenate is a soluble and stable material. I t does not react with most inor-
RE-USE OF W A T E R B Y I N D U S T R Y Table 1.
Compound
Relative Toxicity of Phenolic Compounds Inhibiting Concent rat ion, P .P.M. Aerobacter Bacillus Aspergillus Penicillium aerogenes mycoides niger expansum
Chloro-o-phenylphenol
40 35 55 30
2-tert-Butyl-4-chloro-5-methylphenol o-Benzyl-@-chlorophenol 4, 6-Dichlorophenol Sodium salt of o-Phenylphenol 2,4,5-Trichlorophenol Chloro-2-phenylphenol 2-Chloro-4-phenylphenol 2-Bromo-4-phenylphenol 2,3,4,6-Tetrachlorophenol Pentachlorophenol
ganic or organic chemicals which may contaminate a cooling water system. The author’s experience has shown that a broader spectrum of biological activity can be achieved by stimultaneous use of more than one chlorinated phenolic compound, such as, for example, a trichlorophenate and a pentachlorophenate. Additional benefits may be gained by the combined use of metallic ions such as copper and zinc. Where a concentration of 200 p.p.m. of a single phenolic compound may be required for satisfactory control of organic growths, combinations of phenolic compounds have been successful in the range of 60 to 100 p.p.m. Because of the toxicity of the chlorinated phenols to fish and animals, it may be necessary to control the concentrations in the blowdown water from the cooliiig system. These materials must be handled with care, as the dust is irritating to the mucous membranes of the eyes, nose, and throat. If the concentrated material is permitted to remain in contact with the skin, a rash may develop. With ordinary precautions in handling, however, no difficulties are to be expected. Continuous 9s. Shock Treatment. With the chlorinated phenols, as with chlorination, many different feeding programs can be employed, ranging from the continuous maintenance of a high concentration in the circulating water to the other extreme of intermittent addition of the material in low concentration at infrequent intervals. As with chlorination, the slime-control program must be fitted to the conditions of the individual system. I n any program for the continuous maintenance of a given toxicant concentration in the circulating water, the concentration must be maintained a t an adequate level. One program originally suggested for sodium pentachlorophenate involved continuous feeding of the material, so as to maintain a concentration of approximately 20 p.p.m. in the system. This method has been unsuccessful because 20 p.p.m. is inadequate for inhibition of the microbiologi-
25 6 5 0.7
200 20 60 45 60 400 200
i
200 15 30 20 15 7 4
35 95 80 2000
35 75 80 40
150 15 55 65 150 20 25
150 7 30 50 80
30 30
cal complexes existing in most recirculating systems. O n the other hand, continuous feeding is necessary in particularly difficult cases which cannot be controlled by shock treatment. I n one plant using a make-up water with a high degree of sewage contamination, all shock treatment programs were ineffective and the problem was solved finally by the continuous maintenance in the system of a 200 p.p.m. concentration of the toxicant. Whether the feeding program is continuous or intermittent, the concentration of slime-control agent developed in the system must exceed the minimum inhibitory concentration. Otherwise, continuous maintenance of a noninhibitory concentration, or shock treatment to such a level, will not solve the problem. Because it is necessary to develop an effective concentration of the slime-control agent in the system, continuously or intermittently, intermittent feeding is desirable from an economic standpoint. Unless the system has an unusually low retention time, there will be no marked difference in the inhibitory concentration required with continuous us. intermittent feed. I n setting a schedule for intermittent feeding of the slime-control agent, it is necessary to determine the concentration to be developed in the system and the frequency with which the shot will be repeated. The theoretical depletion of the slime-control agent from the system can be determined by the formula: log c,
=
log
c, -
BD X T 2.303V
where
C, Ci
= final concentration, p.p.m. = initial concentration, p.p.m. = blowdown and windage loss,
V T
= system capacity, gallons
BD
gallons per minute =
time, minutes
A program that has been found practical in shock treatment is to repeat the slug of treatment when the concentration has been depleted to 25% of the original.
O n this basis, the formula above can be simplified as follows: T
V 1.385 -
BD
where
T V
BD
= retention time, days = =
system capacity, gallons blowdown and windage loss, gallons per day
Solving this equation for T will indicate the frequency with which the slug of treatment should be repeated. The formula is independent of the initial concentration developed by the slug of treatment. This formula is useful in setting a feeding schedule and is valid for slime-control agents which do not volatilize in passage over the tower and do not react with other substances in the system. I t is desirable to check the schedule in any particular case by analyses of the circulating water for the slime-control agent. However, once the schedule has been checked, a regular program of shot feeding can be established, feeding the slime control agent every 3, 5 , etc., days as necessary. Copper Salts. Algae are sensitive to very small amounts of copper ion, and copper sulfate was one of the earliest materials used as a n algicide. The copper ion in distilled water solutions is also toxic to bacteria, but this effect is nullified by the presence of other ions and organic matter. Although algae in general are sensitive to low concentrations of copper, there is considerable variation in the killing dose for different species. Table I1 shows the lethal dosage of copper sulfate for several common types of algae ( 3 ) . Although copper salts are toxic to fish, various fish exhibit different degrees of sensitivity toward the copper ion (Table 111) ( 3 ) . Investigations ( 8 ) have shown
Table II. Copper Sulfate Requirements for Treatment of Various Organisms Organism Copper Sulfate, P.P.M. Diatomaceae Asterionella Nitzchia Synedra Chlorophyceae Closterium Microspora Ulothrix Cyanophj ceae Anabaena Clathrocystes Microcystis Protozoa Euglena Synura Uroglena
VOL. 48, NO. 12
0.12-0.20 0.50 0.36-0.50 0.17 0.40
0.20 0.12-0.48 0.12-0.25 0.20 0.50 0.12-0.25 0.05-0.20
DECEMBER 1956
21 65
Table 111. Limiting Safe Dosage of Copper Sulfate for Fish Fish Copper Sulfate, P.P.M. Trout Carp Suckers Catfish Pickerel Goldfish Perch Sunfish Black bass
0.14 0.33
0.33 0.40 0.40 0.50 0.67 1.35 2.00
that fish mortality is not due entirely to the toxic effect of the copper ion in the treated water, but partially to clogging of the gills with dead organisms that have previously existed in the water. The effectiveness of copper salts in the control of algae is nullified at the higher , p H values, because the copper ion is precipitated as insoluble copper hydroxide. However, if copper salts are used in conjunction with surface active agents, toxicity is greatly increased and precipitation of the copper ion is prevented. Further improvements include the use of wetting agents which permit the copper ion to penetrate the slime and algae growths. Because copper sulfate solutions are corrosive to iron and steel, corrosionresistant feeding equipment is required. I n the control of slime and algae in cooling water systems, copper sulfate or a specially prepared copper-base algicide is generally slug-fed. The dosage as well as the frequency of feeding is governed by the results obtained. However, because recirculating cooling water systems are rarely afflicted with algae only, and bacteriological slime is also part of the problem, copper sulfate alone is not ordinarily used to control biological fouling. While copper can be considered practically specific for algae, other agents are required for bacterial
control and copper sulfate is usually fed in addition to or combined with other toxic agents. Quaternary Ammonium Compounds. A great number of quaternary ammonium compounds available to industry are effective germicides in many applications. Table IV (6, 7) shows that there is considerable difference in the effectiveness of various quaternaries against the test organisms. All those listed were relatively ineffective against the fungi, except for those containing mercury in the molecule. The bacterial properties of the quaternaries are reduced in the presence of soap, protein, and high ionic concentrations. Certain quaternaries react with or are absorbed by organic matter found in industrial recirculating cooling water systems and a loss of effectiveness results. Some volatilization also occurs over a cooling tower, increasing losses from the system. Despite these disadvantages from the standpoint of practical application to recirculating cooling water systems, some success has been achieved, particularly when quaternaries are combined with metallic ions, such as copper. The quaternaries are effective wetting agents and aid penetration of organic growth because of this property. I n general, like the chlorinated phenols, they have been most efficiently used in shot fashion. The periodic addition of high concentrations to the circulating water, repeated a t regular intervals, has been more effective in controlling organic growths than continuous maintaining of a lower concentration in the circulating water. Miscellaneous Toxicants. The inorganic mercurials, such as mercuric chloride, are considered too toxic to humans for use in recirculating cooling water systems. Organic mercurials have been developed in an effort to retain
Table IV.
Relative Toxicity of Quaternary Ammonium Compounds Inhibiting Concentration, P.P.M. Aerobacter Bacillus Aspergillus Penicillium nerogenes mycoides niger expansum Compound
Dilauryl dimethyl ammonium chloride Dilauryl dimethyl ammonium oleate Dodecyl trimethyl ammonium chloride Trimethyl ammonium chloride Octadecyl trimethyl ammonium chloride N-Alkylb enzyl-N, N,N-trimethyl ammonium chloride Mixture of alkyl-9-methyl benzyl ammonium chlorides Lactoxymercuriphenyl ammonium lactate Alkyl dimethyl benzyl ammonium chlorides 3,4-Dichlorobenzyl ammonium chlorides Phenylmercuric trihydroxyethyl ammonium lactate Phenylmercuric triethanolammonium lactate Mixture of alkyl dimethyl benzylammonium chlorides
2 166
> 3000 > 3000 > 3000 > 9000 > 3000 > 3000
>3000 >3000 >3000 > 3000
> 3000 >3000 >3000
1500
35 40 65 35
> 3000
55
500
4
150
15
40
15
65
6
700
3
> 3000 > 3000 > 3000 > 3000
15
1
35
45
20
4
65
70
200
7
> 3000
> 3000
500
> 3000 80
INDUSTRIAL AND ENGINEERING CHEMISTRY
> 3000 2500
800
the bactericidal properties of mercury, while reducing toxicity. Some of these compounds may be aerated readily from a cooling tower system, thus allowing only a short time of contact with the slime-forming organisms. Such ma terials should not be used in any cooling tower system where the spray may come in contact with humans or where, because of volatility, the compound will be present in the air breathed. In general, both the inorganic and organic mercurials are considered too toxic for use in cooling water systems. The mercury ion, even in low concentrations, adds appreciably to the corrosive qualities of a water. Potassium permanganate, like chlorine, is a powerful oxidizing agent and effective in the control of slime-forming organisms. However, it acts on all oxidizable material present, and manganese dioxide precipitates as a result of the reduction of the permanganate. The resulting sludge is the chief factor limiting the use of permanganate. Iodine and silver have been used to a limited extent in sterilization of drinking water and in swimming pool control, but have not been satisfactory in cooling water systems. Biological and medical literature refers to hundreds of chemical agents that are useful in controlling microbiological activity, but only a small fraction have been applied to control biological fouling in recirculating cooling water systems. Many of these agents are ruled from consideration immediately because of cost. Many are effective only in concentrations of 57,, lo%, or more. Others are ineffective under the conditions of use in a cooling water system, because of volatility, reaction with organic matter, reaction with inorganic ions naturally present, or chemical treatments for control of scale or corrosion. Few bactericidal and bacteriostatic agents can qualify for use in recirculating cooling systems, principally because of the factors of cost, effective concentrations, and compatibility in this environment. General Considerations in Selecting Slime Control Agents
Chlorine is probably the most widely used agent for micoorganism control in cooling water systems. Whether to use chlorine or another slime-control agent is usually decided by balancing continuous chemical operating cost against costs of equipment installation and maintenance. Where liquid chlorine can be used, chemical cost will usually be lower than with other toxicants, if chlorine is purchased in l-ton containers or tank cars. However, the cost of installing chlorinators and facilities for handling bulk chlorine shipments is appreciable. Maintenance of this equipment is an added cost.
RE-USE OF WATER BY INDUSTRY
\
O n the other hand, other slime-control agents are usually furnished in readily handled containers in liquid, powder, or briquet form. No feeding facilities are required where the toxicant is shot-fed and the material is added only once every several days. No continuous attention to feeding equipment is necessary and there is no installation cost to be written off. Where the system is sufficiently large and the problem sufficiently acute, the expense of installing and maintaining chlorination facilities is justified. Where the cooling water system is relatively small or there is only infrequent necessity for slime-control agents, it is more economical to employ agents other than chlorine. The simplicity of feeding and handling briquetted materials is particularly attractive in the smaller installations and where there is only periodic need for slime-control measures. Even where biological growths are normally controlled by the use of chlorine, additional slime-control agents are frequently needed. For example, if the cooling water is contaminated by reducing agents, chlorination facilities may be inadequate for the quantity of chlorine required. Addition of the chlorinated phenols or commercial combinations may be necessary to control organic growths during such periods. Even where the increased chlorine dosage can be handled by the feeding equipment installed, it may be more economical to use another slime-control agent than to employ greatly increased amounts of ohlorine. At present no single toxic agent is completely effective for the control of biological fouling in all types of industrial cooling water systems. There are faults inherent in all the toxicants in common use. Chlorine is highly effective and rapid in bactericidal action, but chlorine consumption is greatly increased by the presence of reducing agents. The chlorinated phenols and other materials may be toxic to fish in the blowdown discharge. Copper salts are more specific to algae than to bacteria and are rendered ineffective by precipitation at higher p H values. Quaternaries are affected by ions and organic matter. All these chemical agents are not equally economical and efficient. Selection of toxicants must be based on the microbiological associations encountered. Frequently, more than one toxicant is desirable to control, for example, bacteriological slime and/or algae. Methods and frequency of chemical feeding must be varied to suit the individual problem. Slime-control agents vary in the rapidity of their action and must be selected with a knowledge of the retention period in the system. Compati-
bility of the toxicant must be studied in relation to other treatments used for scale and corrosion control. Local conditions governing discharge of blowdown water from the cooling system must be considered. Selection of the proper method of control of microorganisms in recirculating cooling water systems, therefore, requires evaluation of numerous factors.
Fungus Attack of Cooling Tower Wood While fungus attack of cooling tower wood poses a problem different from the control of organic growths, it is also of biological origin. Various species of fungi preferentially attack different constituents of wood. Biological attack is not confined to any particular section of the cooling tower or any particular type of tower. The growth of wood-destroying fungi seems to be most noticeable in the mist sections of the tower, a t or beyond the eliminators, although fungus attack in the fill sections is not uncommon. Fungus attack on cooling tower wood was recognized as such only a few years ago. I t is likely that, in many cases, chemical attack of the wood has preceded biological attack and has predisposed the wood to biological attack. Several methods are under study a t the present time for the prevention of fungus attack. For many years, wood has been impregnated with preservatives for use as railroad ties, fence posts, and telephone and power line poles, Experiments are under way with cooling tower wood impregnated with preservatives, prior to the erection of the tower. Soluble preservatives can be expected to be leached from the wood by the circulating water. The production of insoluble preservatives within the wood by the double-diffusion method of the Forest Products Laboratory may provide the most lasting protection. A recent development has been the system of removing a tower cell from service, spraying the wood with a relatively strong solution of copper sulfate, continuing spraying, and recirculating the soiution for approximately 48 hours to permit diffusion of the solution into the wood. Then the process is repeated with a strong sodium chromate solution. This procedure makes use of the doublediffusion principle to precipitate copper chromate within the wood for control of fungus attack. Other agents considered for this use are sodium pentachlorophenate, arsenic acid, and zinc sulfate. T h e tower cell being treated must be removed from service during the operation and precautions must be taken to avoid contaminating the circulating water with the treating solutions. Introduc-
tion of large amounts of copper into the cooling water system will markedly increase the corrosion potential. Plating out of copper on steel surfaces will lead to intensive pitting of the metal. Where the cooling system has been contaminated with the treating solutions, corrosion rates of several times normal have resulted. Precautions must also be taken %gainst promiscuous discharge of these toxic agents to streams. Fungus attack may perhaps be controlled by toxicants employed in the regular program of slime control. Chlorination does not protect against fungus attack, because of aeration of chlorine from the water and dissipation of chlorine by reaction with the organics in the wood. However, nonvolatile and nonreactive slime-control agents are not aerated from the water and each droplet of water in the mist sections carries its proportionate share of the toxicant. I n some cases, the normal charge of slimecontrol agent has been added by spraying a concentrated solution directly into one cell with the fan reversed. When the next toxicant is added, it is sprayed into another cell. I n this way, the quantity of toxicant is not increased over the usual amount for slime control, but is added in high concentration to the area most susceptible to fungus attack. I t is too early as yet to determine whether fungus attack can be controlled in this manner. I t would be highly desirable to control the attack on cooling tower wood by the addition of chemical agents to the circulating water in a manner similar to and perhaps simultaneous with the control of slime and algae. T o this end, many fungicides are under study and it is likely that additional developments will appear in the near future.
literature Cited (1) Butterfield, C. T., J.Am. Water Works ASSOC. 40, 1305-12 (1948). (2) Dow Chemical Co.. Midland. Mich.. “Methods and Materials for Han: dling Liquid Bromine,” p. 1, 1950. (3) Hale, F. E., “Use of Copper Sulfate in Control of Microscopcc Organisms,” Phelps Dodge Refining Corp., New York, 1950. ( 4 ) Natl. Assoc. Corrosion Engrs., Corrosion 11, 61-3 (November 1955). ( 5 ) Sax, N. I., “Handbook of Dangerous Materials,” Reinhold, New York, 1951. (6) Shema, B. F., Conkey, J. H., “Relative Toxicity of Disinfectants Available for Use in the Pulp and Paper Industry, 1954 Supplement,” Biological Control Committee, American Paper and Pulp Assoc., New York, 1954. (7) Shema, B. F., Conkey, J. H., Tappz 36, 20A-30A (November 1953). (8) Smith, M. C., J . A m . Water Works ASSOC.43, 763-9 (1951). \
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RECEIVED for review April 10, 1956 ACCEPTED September 22, 1956 VOL. 48, NO. 12
DECEMBER 1956
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