CHAPTER 22 CONTROL OF MICROBIAL ACTIVITY

Transcription

1 CHAPTER 22 CONTROL OF MICROBIAL ACTIVITY All water treatment processes are affected by the presence of microbes. Many oxidation-reduction reactions are biologically mediated. In most cases, microbial effects are detrimental to the water-using process or system. However, certain industrial operations put microbes to work in a useful way: the activated sludge process uses microbes for digestion of organic wastes; microbes are used for the fermentation of beverages; microbial enzymes are useful for leather processing; and bacteria are used in the recovery of metal values especially copper from tailings, the residue of mineral beneficiation processes. Disinfection of municipal drinking water and sterilization of food processing and hospital equipment are examples of applications of biocides (chemicals toxic to microbes) where the goal is to kill all microbes. However, in the treatment of nonpotable water, a complete kill is often costly and not always necessary. Cooling water in utilities, steel mills, refineries, and other industrial plants is treated to control microbe populations at levels that experience has proven to be tolerable to the system without complete sterilization. Papermaking systems, unlike cooling water systems, are designed to operate with large amounts of suspended solids, so the tolerable levels of microbe populations are considerably higher than for cooling water. The tolerable microbe count in a paper mill varies with the type of paper being made and machine operating conditions, such as ph and temperature. Planning an effective microbial control program for a specific water treatment process requires an examination of: 1. The types of organisms present in the water system and the associated problems they can cause. 2. The population of each type of organism that may be tolerated before causing a significant problem. Typical microbes encountered in water treatment and the problems they cause are summarized by Table Bacteria, the largest group of troublesome organisms, cause the most varied problems. They are usually classified in water treatment by the types of problems they cause: slime-forming bacteria, iron-depositors, sulfate-reducers, and nitrifying bacteria. Each group has its preferred environment and thrives in specific areas of a water system. Aerobic bacteria, for example, require oxygen, so they are found in aerated waters such as in a cooling tower basin or white water in a

2 TABLE 22.1 Typical Microorganisms and Their Associated Problems Type of organism A. Bacteria 1. Slime-forming bacteria 2. Spore-forming bacteria 3. Iron-depositing bacteria 4. Nitrifying bacteria 5. Sulfate-reducing bacteria 6. Anaerobic corrosive bacteria B. Fungi Yeasts and molds C. Algae D. Protozoa E. Higher life forms Type of problem Form dense, sticky slime with subsequent fouling. Water flows can be impeded and promotion of other organism growth occurs. Become inert when their environment becomes hostile to them. However, growth recurs whenever the environment becomes suitable again. Difficult to control if complete kill is required. However, most processes are not affected by spore formers when the organism is in the spore form. Cause the oxidation and subsequent deposition of insoluble iron from soluble iron. Generate nitric acid from ammonia contamination. Can cause severe corrosion. Generate sulfides from sulfates and can cause serious localized corrosion. Create corrosive localized environments by secreting corrosive wastes. They are always found underneath other deposits in oxygen deficient locations. Cause the degradation of wood in contact with the water system. Cause spots on paper products. Grow in sunlit areas in dense fibrous mats. Can cause plugging of distribution holes on cooling tower decks or dense growths on reservoirs and evaporation ponds. Grow in almost any water which is contaminated with bacteria; indicate poor disinfection. Clams and other shell fish plug inlet screens. paper machine wire pit. Anaerobic bacteria, on the other hand, don't use oxygen and obtain their energy from reactions other than the oxidation of organic substances. The reduction of sulfur in sulfate to the sulfide ion is an example. Since anaerobes don't need oxygen, they are found in oxygen-deficient areas, such as under deposits, in crevices, and in sludges. Iron-depositors occur in water high in ferrous iron, which they convert to insoluble ferric hydroxide and which becomes part of the mucilaginous sheath around the cell. These deposit and accelerate corrosion rates, which produces additional soluble iron, further increasing the population of iron-depositors in the system. The cycle accelerates until the whole system is plugged with iron deposits (Figure 22.1). Nitrifying bacteria oxidize ammonia to nitrate. This nitrification reaction sometimes occurs in iron removal filters, accompanied by a reduction of oxygen and ph. These bacteria are often found in ammonia plants where leakage of

3 FIG Iron-depositing bacteria initiated the tuberculation attack on this steel distribution pipe. FIG The development of pits filled with voluminous products of corrosion from the attack of sulfate-reducing bacteria on steel pipe. ammonia into cooling water encourages their growth. A ph drop caused by the conversion of ammonia to nitrate is often the clue to their presence. Sulfate-reducing bacteria are found in many systems subject to deposit problems. The sulfides produced are corrosive to most metals used in water systems, including mild steel, stainless steel, and aluminum. Evidence of the sulfate-reducers is the unique pit etched on the metal surface, sometimes in the form of concentric rings (Figure 22.2). Many bacteria secrete a mucilaginous substance that encapsulates the cell (Figure 22.3), shielding it from direct contact with water, so that the cell is protected from simple toxic biocides. Control of encapsulated bacteria usually requires both oxidation and dispersion of the protective sheath so that the biocide can reach the cell. FIG A biofilm in development. Biofilm is the mucus-like coating produced by slimeforming bacteria. Organisms are Pseudomonas aeruginosa at 7000 X.

4 Yeasts and molds can live on dead or inert organic matter. Fungi are often found on wooden structures, such as cooling tower fill and supporting members, and sometimes under bacterial or algal masses. Fungal attack of wood usually means permanent loss of strength of the wood structure, so protection of the wood requires control of fungi from the time the structure is put into service. Periodic testing of the wood to determine its resistance to fungal attack is an important maintenance step. Very thin sections of wood specimens taken from susceptible locations are examined to determine the extent of attack if there has been any (Figure 22.4). With few exceptions, algae need sunlight to grow, so they are found on open, exposed areas, such as cooling tower decks or on the surface of reservoirs, ponds, FIG Microtome of wood section from a cooling tower, showing fungal attack. and lakes. Most algae grow in dense, fibrous mats that not only plug distribution piping and flumes, but also provide areas for subsequent growth of anaerobic bacteria under the algae deposits. A century ago, a Danish biologist, Christian Gram, developed a method of staining bacterial cultures as a means of separating them into two broad categories as an aid to identification: those that retain a blue color produced by an iodine treatment are called Gram-positive; those not retaining the blue color and accepting a red dye following iodine treatment are Gram-negative. Most aquatic microbes are Gram-negative. They all are negatively charged colloids (have a negative zeta potential), a property not related to the Gram staining technique. Because they are negatively charged colloids, they are affected by cationic polymers and biocides.

5 PHYSICAL FACTORS AFFECTING MICROBE GROWTH Many species of microbes indigenous to soil, water, and vertebrate organisms thrive in a rather broad temperature range of 10 to 45 0 C. Nature has produced select organisms that can live at temperatures as low as O 0 C and as high as 10O 0 C. Higher temperatures kill all common microbes, but scientists report finding life in hot springs and adjacent to ocean vents on the sea floor at temperatures of over 20O 0 C. Denaturation of proteins, which causes coagulation within the cell, occurs at temperatures below 7O 0 C. Commercial pasteurization is a denaturation process. Milk is usually pasteurized at 63 0 C by holding that temperature for 30 min; if the temperature is raised to 72 0 C, pasteurization is completed in only 15 s. This process kills all disease-producing (pathogenic) organisms but does not produce sterile milk; some microbes remain to cause the milk to spoil in time. Most actively growing microbes of interest in water treatment technology are killed at 7O 0 C in less than 5 min. Although pasteurization has a long history of success in food processing, it has never been reported in use for water disinfection, except in occasional emergencies where a community water supply may be contaminated and the public is warned to boil all drinking water until the crisis has passed. Maintaining low temperatures is not an effective means of killing microbes. At O to 5 0 C, organisms become dormant. Freezing kills many cells, but those that survive are capable of complete recovery from the shock. One procedure used to preserve microbes involves freezing cells rapidly at -7O 0 C and then removing the ice crystals as vapor (sublimation). This process is lyophilization. Dry heat results in dehydration of all cellular matter and oxidation of intracellular constituents. Sterilization of laboratory media is usually carried out in an autoclave at C. Sterilization of glassware is done with dry heat at 16O 0 C for two hours. Moisture is required for microorganisms to grow actively. Many species of pathogenic organisms are killed quickly by drying. However, organisms in the spore or cyst state can survive low moisture environments; and, if transported by wind or animals to a location where moisture levels suit them, they revive and form new colonies. To prevent attack by microbes, lumber and other vulnerable materials are dried to less than 20% moisture content. Organisms containing chlorophyll are able to use the radiant energy of the sun or artificial lighting to convert CO 2 to carbohydrates, which they need for cell synthesis. However, not all radiant energy is useful to the cell and certain frequencies of radiation are harmful. Radiation is therefore one method of microbe control. Short-wavelength forms of energy, such as gamma rays (0.01 to 1 A) and x- rays (1 to 100 A) are particularly useful. These create free hydrogen and hydroxyl radicals and some peroxides when they pass through the cell, causing cell damage or death. These forms of radiation are hazardous. Energy in the ultraviolet region ( A) is also useful for killing microbes. In this case, the energy is absorbed by the nucleic acids, creating chemical reactions that are lethal to the cell. This form of energy, however, has poor penetrating ability, so the use of ultraviolet light for disinfection requires a treatment unit of special design so that the energy does not have to penetrate deeply into the water. Ultraviolet sterilization uses about 0.2 kwh of electric energy per thousand gallons of water treated (0.05 kwh/m 3 ), so it is economically attractive in situations where the microor-

6 ganisms are not shielded by large agglomerated masses or by suspended solids. This method of disinfection is widely used in ultrapure water systems. Osmosis is the diffusion of water through a semipermeable membrane separating two solutions of different solute concentrations. The water flows in a direction to equalize the concentrations. When microbes are placed in 10 to 15% salt solutions or 50 to 70% sugar solutions, the water inside the cells is extracted by the surrounding medium. This dehydrates the cells so they are unable to grow or are killed. This technique is used commercially to preseve food. Bees use this principle to preserve honey, concentrating it by fanning the comb with their wings. The interfaces between a liquid and a gas (such as surrounding a bubble of air in water), between two liquids (oil droplets in water), and between a solid and a liquid (sand grains in water) are characterized by unbalanced forces of attraction between the molecules of water at the surface and those in the fluid body. These forces are closely associated with the metabolic processes of the microbe. The cell must be able to accumulate nutrients at its surface for assimilation, and waste products must be eliminated from the cell and carried away. Therefore, the growth and well-being of a cell are influenced by surface forces in the surrounding aquatic environment. Substances having surface tension depressing effects (surfactants) tend to have a detrimental effect on microbes if the concentration is high enough. These materials can alter cell division, growth, and survival. Surfactants are often used to increase the effectiveness of biocides by dispersing cell colonies and protective sheaths to allow the toxicant to contact the cells. Some toxicants are themselves surface active (such as phenols and quaternaries) and tend to accumulate on cell surfaces by adsorption. This prevents entrance and utilization of food substances by the cell. Quaternaries will sometimes cause leakage of cellular material out of the cell wall by changes in surface tension at the membrane surface. CHEMICAL FACTORS AFFECTING MICROBIAL GROWTH Microbes have been found to exist in the broad ph range of 1 to 13. However, the most common microbes associated with water algae and bacteria usually maintain their internal ph at 7, so they prefer a neutral aquatic environment. Generally, yeasts and molds favor depressed ph, in the range of 3 to 4. Dilute alum solution is sometimes contaminated by fungi, causing the plugging of alum feed lines and rotameters while at the same time the solution is free of bacterial growth. Bacteria and fungi can both contribute to industrial problems over a ph range of 5 to 10. Other chemical factors, discussed in earlier chapters, include the presence of organic matter to serve as food for the microbes, and a supply of the common nutrients such as nitrogen and phosphorus required for cell metabolism. One of the surprising facts of microbe life is that there is such a profusion and variety of forms that some can almost always be found that will resist damage by, or even thrive on, chemicals that are toxicfto animal and plant life. For example, phenol was one of the early chemical biocides used for sterilization in medical practice, yet at low concentrations up to 100 mg/l, which is sometimes found in coke plant wastes it is readily digested in activated sludge waste treatment

7 plants. Similarly, some bacteria thrive in wastewaters that contain herbicides, pesticides, cyanide, arsenic compounds, and a variety of other chemicals normally considered toxic. METHODS FOR CONTROLLING MICROBIAL ACTIVITY For practical reasons, in most industrial water systems, only limited use can be made of the physical conditions that inhibit or destroy microbial life. For example, heating water may control microbial activity, but if the water is used for cooling purposes, this is not useful. Radiation is sometimes used, but its adoption on a widespread basis would require the development of more efficient energy sources and better designs of equipment to expose the water to the radiant energy. Among chemical conditions that might be used for microbe control, ph is the only likely candidate for practical results. Even this is limited unless the system water can be kept at a ph over 10. However, ph does have important effects on the performance of biocides, as shown later when chlorine reactions are discussed. Available chlorine as HOCI,mg/L Contact time, min FIG Relationship of time versus concentration of HOCl required for 99% destruction or inactivation of a common bacteria (Escherichia coli) and a virus, at 0 to 6 C. (From Water Pollution Control Federation Publication Deeds & Data, January 1980.)

8 Since neither physical nor aquatic chemical conditions can be changed in a practical way to control microbial growth, toxic chemicals must be applied as biocides. The two commonly used types are oxidizing and nonoxidizing. Regardless of which type is used, there is a relationship for all chemical biocides that expresses effectiveness, measured as percent kill or inactivation, concentration of biocide applied to the water, and time of contact of the biocide with the organism or virus. This was first discovered by H. Chick in 1908 and further developed by H. E. Watson in the same year, and is now designated as the Chick-Watson law: N/N 0 = exp(-^rt) where TV 0 represents the bacterial population at the time zero TV is the reduced population at time t, after biocide application exp = base of natural logarithm system, k is a rate constant c is the concentration of the biocide, mg/l n is an empirical value / is the time of contact, min As an illustration of this law, Figure 22.5 shows the concentration versus time relationship for application of chlorine to water for destruction of Escherichia coli, the coliform bacteria used as an indicator organism in evaluating disinfection of municipal water supplies, and for inactivation of polio virus 1. This graph is typical of all biocidal charts, demonstrating the Chick-Watson law. In each case, the graph is specific for both the organism and the water supply, as there may be side reactions of the biocide with other constituents of the water. OXIDIZING BIOCIDES (SEE ALSO CHAPTER 19) Chlorine gas, the chemical biocide most commonly used in the United States, hydrolyzes rapidly when dissolved in water according to the following equation: Cl 2 + H 2 O -H + H- Cl- + HOCl (1) Hydrolysis occurs in less than a second at 65 0 F (18 0 C). Hypochlorous acid (HOCl) is the active ingredient formed by this reaction. This weak acid tends to undergo partial dissociation as follows: HOCl - H + + OCl- (2) This reaction produces a hypochlorite ion and a hydrogen ion. Depending on ph and concentration, chlorine in water exists as free chlorine gas, hypochlorous acid, or hypochlorite ion. Figure 22.6 illustrates the distribution of these components at varying ph values. Above ph 7.5, hypochlorite ions predominate, and they are the exclusive form when the ph exceeds 9. The sum of the hypochlorous acid and hypochlorite ions is defined as free available chlorine. Hypochlorite salts such as calcium hypochlorite ionize in water to yield these two species, depending on ph. Ca(OCl) 2 - Ca OCr (3) 2OCr H- H 2 O HOCl H- OCT H- OH~ (4)

9 Thus, the same equilibria are established whether elemental chlorine or hypochlorite is used for chlorination. Liquefied chlorine is available in bulk or in cylinders. A typical installation for feeding chlorine is shown in Figure 22.1a and b. Percent un-ionized form (HOCI or HOBr) Percent ionized form (OCI or OBr ) FIG Distribution of hypochlorite and hypobromite in water as affected by ph. Chemicals other than chlorine gas that liberate hypochlorite ions are compared to one another in oxidizing power on the basis of "available chlorine." The available chlorine values of a number of disinfectants are shown in Table Chlorine is a strong oxidizing agent capable of reacting with many impurities in water including ammonia, amino acids, proteins, carbonaceous material, Fe 2+, Mn 2+, S 2 -, and CIST. The amount of chlorine needed to react with these substances is called the chlorine demand. Chlorine reacts with ammonia to form three different chloramines: HOCl + NH 3 NH 2 Cl (monochloramine) + H 2 O (5) NH 2 Cl + HOCl NHCl 2 (dichloramine) + H 2 O (6) NHCl 2 + HOCl NCl 3 (trichloramine) + H 2 O (7) These chloramine compounds also have biocidal properties; they are referred to as the combined residual chlorine. In general, the chloramines are slower acting than free residual chlorine, but have the advantage of being more effective at ph values above 10. Chloramines may also be more persistent in a water system. Breakpoint chlorination is the addition of sufficient chlorine to satisfy the chlorine demand and produce free residual chlorine. When breakpoint chlorination is used, the ammonia nitrogen content is destroyed and the residual chlorine remaining will be almost entirely free available chlorine (Figure 22.8).

10 FIG (a) Typical chlorination system. In this plant, eight chlorinators each meter up to 8000 Ib Cl 2 /day to municipal sewage plant effluent. Each is supplied by a chlorine evaporator (in background, right). (Courtesy of Wallace & Tiernan.) (b) Ton cylinders of chlorine are mounted on a scale to monitor consumption by weight loss, and the discharge is converted to Cl 2 gas by the evaporator on the right. The chlorine supply room is isolated from operators in the control room. (Courtesy of Wallace & Tiernan.)

11 Chlorine also reacts with organic nitrogen in water. This is found in components of living cells, protein, polysaccharides, and amino acids. The toxicity of chlorine is thought to be derived not from the chlorine itself or its release of nascent oxygen, but rather from the reaction of the HOCl with the enzyme system of TABLE 22.2 Available Chlorine of Chlorination Chemicals Material Chlorine gas (Cl 2 ) Chlorine dioxide (ClO 2 ) Hypochlorites (OCl) Calcium, HTH, Ca(OCl) 2 Sodium, NaOCl Industrial grade Domestic grade Lithium, LiOCl, laundry grade Chlorinated isocyanuric acid (CONCl) 3 Percent available Cl the cell. The superiority of HOCl over OCl may be due to the small molecular size and electrical neutrality of HOCl, which allow it to pass through the cell membrane. On-site production of hypochlorite from seawater or brine is becoming popular as it limits the exposure of operating personnel to chlorine gas or hypochlorite compounds. An installation of such a system is shown in Figure Chlorine reacts with a variety of organic materials, and attention is being directed to the presence of chlorinated compounds in water thought to be pro- Chlorine residual,mg/1 (after fixed period,15mm minimum) This curve is typical of contaminated water supply Theoretical This curve is typical of clean water supply Breakpoint: Left of breakpoint Cl 2 is combined; to the right, it is free Chlorine applied, mq/l (breakpoint can be estimated at 10 X NH 3, mg/1) FIG Breakpoint chlorination curves showing reaction of Cl 2 with N- compounds.

12 The Cl: Br ratio can be varied to produce a Br 2 residual, a Cl 2 residual, or a mixture, as conditions may vary and require a ratio change. With high ammonia levels, the bromamines that form degrade more rapidly than do chloramines, so they are less persistent in the environment. The combination reduces the total residual chlorine and aids in compliance. Like chlorine, bromine residuals exist either as unionized or ionized species in water, as seen in Figure At ph 7.5, 50% of the available chlorine is present as HOCl, with the balance as OCl". With hypobromous acid, at ph 7.5, over 90% of the oxidant is present as HOBr, the more active form, just as HOCl is more active than OCl", as mentioned earlier. These curves demonstrate why bromine residuals provide better biocidal performance than chlorine in systems operating at higher ph values. In an 8-month field study, a utility system treated with the activated bromide-chlorine blend required only about 10% of the oxidant used by a similar unit treated with chlorine alone, and in this period, the 0.2 mg/l chloduced by chlorination. Chloroform is one of these materials. Because of concern for the potentially adverse physiologic effects of these chlorinated compunds, regulatory agencies are severely restricting chlorine applications to large effluent flows. For example, treatment of utility station condenser water with chlorine may be limited to a total period of only 2 h/day at residuals averaging not over 0.2 mg/l Cl 2 (EPA limit mandated July 1, 1984). FIG An electrolytic cell designed to produce chlorine from brine. This unit is generating Cl 2 from seawater for chlorination of sewage plant effluent. (Courtesy of Electrolytic Systems Division, Diamond Shamrock.) For this reason, the electric utility industry has studied chlorine minimization techniques and alternatives to chlorination to meet the strict discharge limits without jeopardizing condenser performance. One such alternative is an activated bromide program: a chlorine-bromide mixture produces oxidant species that penetrate the biofilm; chlorine activates a bromide compound, according to the reaction HOCl H- Br" -* HOBr H- Cl" (8)

13 rine residual specified by the EPA was never exceeded. (Note: The conventional total chlorine test reacts with both chlorine and bromine.) Chlorine dioxide, ClO 2, is used to a limited extent in water treatment for the control of taste and odor problems and for the degradation of phenol. It is used extensively in the pulp and paper industry for bleaching. This compound must be generated from the reaction of chlorine with sodium chlorite as shown: 2NaClO 2 + Cl 2-2ClO 2 + 2NaCl (9) Generally an excess of chlorine is used to drive the reaction to completion. For safety and to preserve its stability, the material is generated on-site. Since chlorine dioxide does not react with ammonia, it is useful in systems containing ammonia. The next most common oxidizing biocide is ozone, O 3. This is in common use commercially throughout Europe in preference to chlorine and is finding growing acceptance in certain municipalities in the United States for disinfection of potable water. It is also used in certain waste treatment applications to avoid the residual chloramines that result from the usual chlorination of wastewater effluent. Ozone is produced on-site by an electric corona discharge through air or oxygen. A typical ozone generator is shown in Figure Oxidizing biocides such as chlorine, hypochlorites, and organochlorine materials will kill all organisms in the system quickly, if the free chlorine comes into direct contact with the organisms long enough and at a strong enough dosage FIG A battery of three 110 Ib/day (49 kg/day) ozonators installed in a municipal plant. These produce O 3 from air. (Courtesy oflnfilco Degremont, Inc.)

14 level. They also retain their effectiveness because organisms cannot adapt to or become resistant to chlorine. However, oxidizing biocides also react with contaminants like H 2 S, NH 3, pulp lignins, wood sugars, and other organics. This increases the amount of chlorine required for biocidal effects. They are not persistent, and they decay quickly after the chemical feed stops. They do not penetrate slime masses, so they do not reach subdeposit microbes. Thus oxidizing biocides require complementary treatments to improve their effectiveness. These include biodispersants to remove existing slime masses and to prevent organisms from settling on heat transfer surfaces; penetrants to permeate organic masses and to expose and kill subsurface organisms; and biocides for control of organisms in systems contaminated with H 2 S, NH 3, and other reducing agents. NONOXIDIZING BIOCIDES Nonoxidizing biocides offer a way to control microbial activity in systems incompatible with chlorine, such as water systems high in organic matter or ammonia. With few exceptions (e.g. copper sulfate), they are organic chemicals. They provide the following features: 1. Activity independent of ph 2. Persistency 3. Control of organisms such as fungi, bacteria, and algae Since all of these benefits are usually not available from a single penetrating biocide, individual ingredients are formulated into proprietary products designed to increase overall performance in very specific applications, e.g., paper machine systems, open cooling water systems, and process water in food plants. Slug feed is the preferred method of application. Just as ammonia, sulfides, and reducing agents can interfere with the performance of oxidizing biocides, certain substances can reduce the effectiveness of nonoxidizing biocides; e.g., cationic biocides may react with anionic dispersants to cancel the effectiveness of both. Organic Compounds Methylene-bis-thiocyanate (MBT), (SCN)-CH 2 -(SCN), is a well-known organosulfur biocide. It is usually recommended for applications in paper mills and cooling systems where effluent limitations are strict, and where control of slime-forming bacteria is the main problem. Holding time and ph affect the half-life of MBT, which hydrolyzes in water to form less toxic substances. Figure illustrates the ph dependency of the half-life of MBT. At a ph of 11, it is destroyed in seconds. A continuous feed of 1 mg/l to the makeup of a cooling system will control organisms entering with the water. If an alkaline cooling water treatment program is being used, the relatively higher ph will cause the MBT to hydrolyze faster. In most cooling systems, the molecule will eventually be destroyed if holding time is sufficiently long. Most of the degradation products are volatile and are stripped in the tower.

15 A study to assess the effect of MBT on biological sewage treatment programs indicated that 0.5 to 2.0 mg/l had no measurable effect on BOD and suspended solids removal. So it is feasible to treat a system with methylene-bis-thiocyanate to control microbes and still have an effluent amenable to biological treatment. Percent MT remaining Time, hours FIG Degradation of methylene-bis-thiocyanate at ph 6, 7, 8, and 9. Dibromonitrilopropionamide (DBNPA) is also excellent where antibacterial activity and environmental acceptability are needed. DBPNA kills quickly, then decomposes to nontoxic compounds. Chlorinated phenols are highly effective against most common organisms, especially fungi and algae. Tower fill is sometimes sprayed with chlorinated phenols to increase the lumber's resistance to fungal attack. These compounds are effective biocides when fed directly to cooling water. However, because of toxicity and danger to the environment, chlorinated phenols are banned in the United States and many other countries. Organotriazine compounds, such as isothiazolones, are broad spectrum nonoxidizing biocides, particularly effective against bacteria, and active over a wide ph range. Isothiazolones are not inhibited by most organic and inorganic contaminants found in cooling waters, and are compatible with ionic and nonionic dispersants. They can be used with any of the oxidizing biocides. A typical example of the organometallic group of biocides is the compound bis-tributyl tin oxide (TBTO), (H 9 C 4 )3=Sn-O-Sn=5(C 4 H 9 )3. This compound is used to control fungi and algae. It also has a tendency to adsorb on surfaces in the system, especially on wood, and therefore analyses usually do not show tin levels as high as would be expected from dosage calculations. This adsorption provides for residual algae and fungi control after treatment is discontinued. Although its toxicity is greatest on fungi and algae, it also provides good control of anaerobic corrosive bacteria. CA TIONIC BIOCIDES If penetration of slime and algae masses is desired along with persistency, then amines and quaternaries (quats) are usually applied. The use of amines or quats with chlorination usually permits the dosage of chlorine to be reduced.

16 Many of these biocides are surface-active and therefore can disperse slime masses. This allows chlorine and other toxicants to contact organisms which, under normal circumstances, they would not harm. The wettability of organic slime masses is increased so that the toxicants can reach under the mass to get at anaerobic corrosive bacteria. In a typical amine program in a cooling tower system, a once-per-month slug of an amine biocide at 75 mg/l for 48 h is used to supplement continuous chlorination at 0.5 to 1.0 mg/l free residual for 6 to 8 h/day. METALLIC COMPOUNDS The detection of mercury in aquatic organisms in the late 1960s and especially in tuna and other edible fish as a result of mercury discharges from chloralkali plants alerted the public to the potential threat of toxic metals in the environment. As a result of subsequent studies of the toxicity of heavy metals, some that had been used as biocides (mercuric salts) and others that were not used as poisons but had simply found their way into the environment indirectly (such as lead aklyls in gasoline), these compounds came under scrutiny and were withdrawn from the market or had their use greatly curtailed. One of the most common of the metallics used as a biocide is copper in ionic form. It has been applied as copper sulfate to ponds and reservoirs for many years for control of algae at a concentration usually below 1 mg/l. Its solubility falls off rapidly as ph increases, so a chelating agent, such as citric acid, is often applied to improve the effectiveness of treatment. Since algae have their individual seasons for blooming, the copper sulfate is usually applied only seasonally, and its effective dosage is below the concentration permitted in potable water. The mechanism of action against algae is not clear, but copper ions form complexes with amines; the effectiveness of copper ions is probably a result of their reaction with the essential amino acids. Toxic metals can be transported across the cell membrane more readily in nonionic form, so a variety of neutral metallo-organic compounds have been developed to increase the toxicity of certain heavy metals as biocides, as discussed earlier (TBTO). MICROBIAL MONITORING PROCEDURES In systems where microbe populations may be maintained within a certain acceptable control range and complete sterilization is not required, frequent microbe analyses should be run for monitoring the program. Sometimes only total counts are run to indicate overall microbe population levels. However, in systems such as industrial recirculating cooling water loops, analyses of more specific organisms are required. Changes in total count do not always indicate changes in fungi, anaerobic bacteria, sulfate-reducing bacteria, or algae. Since these organisms can be troublesome, they should be monitored specifically. In industrial waters, a microbial analysis usually expresses counts per 1-mL sample. Since a billion bacteria weigh on the order of 1 mg, a high count of 10,000,000/mL of sample represents only about 10 mg/l of suspended solids. Since there are interfering substances that can reduce the effectiveness of certain biocides, a change in microbial population may be due to causes other than the inherent toxicity of the biocide. So, a review of plant conditions (temperature,

17 chemical environment, sources of inoculation) should always accompany any sampling for microbe analysis. COOLING WATER SYSTEMS A typical report of those organisms most commonly found in industrial water systems is shown in Figure This analysis is typical of the weekly analyses From: Analysis No. B Date Sampled 12/ 1/83 Date Received 12/14/83 Sample Marked: Date Printed 12/21/83 Cooling Water - Ammonia Plant >» MICROBIOLOGICAL EVALUATION <«PHYSICAL APPEARANCE : Yellow Liquid TOTAL AEROBIC BACTERIA 500,000 Aerobacter 20,000 Pigmented 50,000 Mucoids <1,000 Pseudomonas 70,000 Others 360,000 TOTAL ANAEROBIC BACTERIA <10 Sulfate Reducers <10 IRON-DEPOSITING Gallionella Sphaerotilus TOTAL FUNGI <10 Molds <10 Yeast <10 ALGAE Filamentous Non-Filamentous OTHER ORGANISMS : Lab Comments: (All Counts Express Colony Forming Organisms per ML of Sample.) FIG Analysis a cooling system under control.

18 From: Analysis No. B Date Sampled 12/ 1/83 Date Received 12/14/83 Sample Marked: Date Printed 12/21/83 Ammonia Plant >» MICROBIOLOGICAL EVALUATION <«PHYSICAL APPEARANCE : Yellow Liquid with Floe TOTAL AEROBIC BACTERIA 14,000,000 Aerobacter 70,000 Pigmented 200,000 Mucoids <1,000 Pseudomonas 400,000 Others 13,330,000 TOTAL ANAEROBIC BACTERIA 20 Sulfate Reducers 20 IRON-DEPOSITING Gallionella Sphaerotilus TOTAL FUNGI <10 Molds <10 Yeast <10 ALGAE Filamentous Non-Filamentous OTHER ORGANISMS : Few Protozoa Lab Comments: (All Counts Express Colony Forming Organisms per ML of Sample.) FIG Analysis the system in Fig out of control because of an exchanger leak. for microbe counts taken on a cooling system water in an ammonia plant, sampled when the system is under control. The notation "NEG IN 1/1000" means that no organisms were observed in a 1/1000 dilution of the sampled water. The total count is usually higher than the sum of individual aerobic slime-forming bacteria shown above it. This is because there are many more types of aerobic bacteria in cooling water than those specifically reported as troublesome. Figure demonstrates changes in microbe populations produced by an

19 From: Analysis No. B Date Sampled 12/ 6/83 Date Received 12/12/83 Sample Marked: Date Printed 12/21/83 Well Water >» MICROBIOLOGICAL EVALUATION <«PHYSICAL APPEARANCE : Liquid with Floe TOTAL AEROBIC BACTERIA Aerobacter <10 Pigmented Mucoids Pseudomonas Others TOTAL ANAEROBIC BACTERIA 15 Sulfate Reducers 10 Clostridia 5 IRON-DEPOSITING Gallionella Sphaerotilus TOTAL FUNGI Molds Yeast ALGAE Filamentous Non-Filamentous OTHER ORGANISMS : Few Lab Comments: (All Counts Express Colony Forming Organisms per ML of Sample.) FIG A well water with low counts can still produce troublesome iron growths. ammonia leak into the cooling water system described above. This sudden influx of nutrient caused aerobic slime-formers to multiply. The resulting deposition of these aerobes increased the shelter for anaerobic corrosive bacteria underneath the deposits. Using analyses such as these, an industrial plant can optimize biocide usages to minimize water treatment costs while preventing unnecessary shutdowns caused by rampant microbial contamination.

20 A common problem in once-through cooling water systems is iron fouling. In many instances the iron fouling is actually a result of contamination with irondepositing bacteria such as Sphaerotilus or Gallionella. By routinely monitoring the makeup water with microbiological analyses, potential problems with iron depositors can be anticipated and treatment adjusted before the problem gets out of hand. Figure shows a typical makeup water analysis with iron-depositors present. A total count analysis would have indicated this system to be under good control and would have missed the potential for iron fouling from the iron-depositing bacteria. PAPER MILL WATER SYSTEMS In the manufacture of unbleached grades of paper or board, total counts of 30 to 40 million bacteria per milliliter can often be tolerated in the system without causing slime problems, because the solids are highly dispersed by residual lignin materials remaining in the system. By contrast, in the production of bleached grades, slimes and other microbial problems would be uncontrollable at such high total counts because of the absence of the dispersive lignins. The high temperatures of linerboard production systems often allow the machines to run virtually slime-free even though total counts may approach the 30 million to 40 million range. Cylinder machines can tolerate extremely high levels of bacteria because the machines run slowly and are relatively immune to production interruptions from slime. They usually produce heavyweight, multiply board which can incorporate slime spots without damage to its properties. Molds and yeasts are more responsible for slime problems in papermaking systems than are bacteria. Molds create more problems than yeasts because their threadlike branched form can trap fiber, filler material, and debris, and bind them into a tenacious deposit. Yeasts cause more problems than bacteria because they are considerably larger and have a tacky coating around the cell membrane. Figure shows an analysis of a deposit taken from a paper machine frame. The important features of this deposit are the high levels of yeasts, the presence of anaerobic corrosive bacteria, and the presence of coliform bacteria (E. coli). Deposits caused by aerobic bacteria require total counts many times higher than these, so this is not primarily a bacterial problem. The major culprit is yeast. Corrosion may be a problem in this system because of the high level of anaerobic corrosives associated with the deposits. Protozoa in the deposit are indicators that there are problems in treating the fresh water brought into the mill. Improper chlorination or filtration of the fresh water may be the cause. STORING, HANDLING, AND FEEDING PRECAUTIONS Because the purpose of biocides is to kill living organisms, the need for care in planning an effective but safe program to handle these toxic chemicals is obvious. Proper protective clothing, gloves, masks, and respirators must be worn by oper-

21 From: Analysis No. B Date Sampled 12/ 6/83 Date Received 12/12/83 Sample Marked: Date Printed 12/21/83 Tray Deposit (No. 1 Machine) >» MICROBIOLOGICAL EVALUATION <«PHYSICAL APPEARANCE : Green Deposit TOTAL AEROBIC BACTERIA 56,000,000 Aerobacter 50,000 Pigmented 1,000,000 Mucoids <10,000 Pseudomonas 5,000,000 Sporeformers Others <100 49,950,000 TOTAL ANAEROBIC BACTERIA 10,200 Sulfate Reducers 10,000 Clostridia 200 IRON-DEPOSITING Gallionella Sphaerotilus TOTAL FUNGI 20,100 Molds Yeast ,000 ALGAE Filamentous Non-Filamentous OTHER ORGANISMS : Many Protozoa Lab Comments: (All Counts Express Colony Forming Organisms per GRAM of Sample.) Microscopic Examination - Many Bacteria, Moderate Fibers and Fines. FIG Bioanalysis paper machine frame. ators responsible for charging tanks, adjusting feeders, and controlling the program by testing. Showers and eye baths must be readily accessible. Bulk supply has advantages in minimizing exposure of personnel to biocides and eliminating the problem of package disposal. If products are obtained in drums, provision must be made for proper drum handling, piping hookup, and drum disposal. (Figure 22.15). Not only is the safety of plant personnel at risk, but also the safety of the environment. It is important to know the fate of the

22 biocide selected when it reaches the waste treatment plant and eventually a receiving stream. Bench testing or pilot plant studies may be needed to assure the plant manager that the biocide program most effective in the plant will be compatible not only with discharge permit requirements, but also with the health of the receiving stream.