Microbiological problems are a common occurrence in cooling water systems. These living organisms are present in the air, soil and water. Under the proper conditions, these microscopic plants and animals grow into large colonies that block water flow, impede heat transfer, destroy wood, induce corrosion and cause offensive odors. If left unchecked, microbiological growths will cause a rapid degradation of the cooling system.
Types of Microorganisms
Microbes are grouped into three broad categories: algae, bacteria and fungi.
Algae are single or multicelled organisms that contain chlorophyll, the green pigment of plants. These organisms use chlorophyll in the respiration process called photosynthesis to convert carbon dioxide and water into energy-rich carbohydrates, releasing oxygen in the process. The energy for the photosynthesis process comes from sunlight. The most common species of green algae found in cooling water systems are Chlorella, Scenedesmus, Pediastrum and Oocystis.
Bacteria are single cell organisms ranging in size from less than 0.5 microns to 3.0 microns. Bacteria cells are spherical (cocci), cylindrical (bacilli), or helical (spirilla).
Heterotrophic bacteria obtain their food from organic sources. Most bacteria are heterotrophic aerobes that use oxygen to break down simple sugars into carbon dioxide and water. Some types of heterotrophic bacteria are classified as anaerobes. Anaerobic bacteria do not use oxygen for cellular respiration. They obtain their energy by the fermentation process. Fermentation is the process of converting glucose into alcohol or lactic acid. Some strains of bacteria are facultative anaerobes. They can live in the presence or absence of oxygen.
Bacteria often produce a gelatinous slime as a by-product of their metabolism. These slimes are thought to assist in trapping and storing nutrients for cellular respiration. It is the bacterial slimes and associated odors that are normally the first physical evidence of the presence of bacteria in the system.
Autotrophic bacteria use inorganic matter as a source of nutrients. Chemosynthetic bacteria oxidize inorganic compounds such as ammonia, nitrite, sulfur, ferrous iron or hydrogen gas, releasing usable energy in the process. Iron bacteria are typical of this type of organism. Photosynthetic bacteria use chlorophyll to trap light energy for the respiration process. These organisms grow anaerobically in the light, using hydrogen sulfide to produce sulfide or sulfate as a by-product of the respiration process.
Fungi are multicelled plants that lack chlorophyll. Fungi growths can be troublesome in the operation of cooling towers in that some species cause fungal deterioration of the wooden support structures by feeding on the cellulose or lignin components of the wood. Fungi from the Ascomycetes group cause soft rot in cooling tower lumber. Basidiomycetes are the causative agent for white rot and brown rot.
Problems Microbes Can Cause in Cooling Water Systems
Restrict water flow: Algae mats and bacterial slimes often restrict water flow. Algae grow in large colonies or mats on the distribution decks of cooling towers. These colonies block the distribution ports or plug the water sprays. Slime-forming bacteria secrete a gelatinous slime that can block flow through a heat exchanger. The slime also traps dirt and debris that accumulates in the exchanger, further blocking water flow.
Reduce heat transfer: Normally, mineral scales are the primary cause of reduced heat transfer in cooling systems. However, slime growths and other microbiological debris can also insulate the heat transfer surface, causing a dramatic reduction in heat transfer rates.
Odors: One unpleasant by-product of microbiological growths is odor. These are the damp, musty, septic odors given off by many bacteria. Other organisms, like sulfate-reducing bacteria, emit a rotten-egg smell caused by the hydrogen sulfide liberated as a by-product of their metabolism.
Wood attack: Fungi attack the wooden structures of cooling towers causing wood rot. Three types of attack are common; brown rot, white rot, and soft rot. Brown-rot and white-rot are caused by Badisiomycetes that consume the lignin and carbohydrates in the interior of the wood. The outside surfaces of the wood remain fairly sound. Soft-rot is caused by Ascomycetes that attack the wet surfaces of the tower lumber. Here the fungi consume the cellulose components of the wood, leaving the lignin relatively in tact.
Corrosion: Microbiological growths are the causative agent for corrosive attack in cooling water systems. The term for this is “microbiological induced corrosion”, or MIC. Certain organisms, such as sulfate-reducers and slime-formers, secrete an acidic by-product of their metabolism, such as hydrogen sulfide or hydrochloric acid. This locally reduces the pH and causes accelerated attack on the underlying metal. Slime deposits and algae also form localized oxygen differential cells that lead to underdeposit corrosion of the metal.
Health issues: The bacteria present in cooling water can pose a health risk. The well-publicized 1976 outbreak of Legionnaires disease in Philadelphia is an example. The causative agent for this episode, which killed 34 conventioneers, was eventually traced to the hotel’s cooling tower. Since then, other occurrences of Legionnaires disease have been linked to other cooling towers, showers, and the misting devices used in grocery store vegetable displays.
Microbiological Control Methods
Microbiological problems are controlled by the implementation of a rigorous chemical treatment program. These programs use biologically toxic chemicals to control the growth of algae, bacteria, mold and fungi in the system. Two types of biocides are commonly used for this purpose: oxidizing biocides and non-oxidizing biocides.
Chlorine dissolves in water to form hypochlorous acid and hypochlorite ion according to the following chemical reactions:
Cl2 + H2O ——— HOCl + H+ + Cl–
HOCl ——- ——– OCl– + H+
Hypochlorous acid Hypochlorite
Hypochlorous acid exhibits a faster disinfection rate than hypochlorite ion. The extent to which hypochlorous acid dissociates to form hypochlorite ion is dependent on the pH of the water. At lower pH’s, the ratio between hypochlorous acid and hypochlorite ion is increased. This increases the disinfection rate.
The disinfection rate is also dependent on its dosage and contact time in the system. A 1 ppm dosage of chlorine produces a 99% kill rate in less than 30 seconds at a pH of 6.5. At a pH of 8.5, a 99% kill is achieved in about 5 minutes.
Chlorine combines with organics and ammonia to form chlorinated organics and chloramines. The combined chlorine is not as available to react with bacteria and, therefore, exhibits a less toxic effect on these organisms than does hypochlorous acid and hypochlorite ion. When test results are reported for chlorine, a distinction is made between the amount of free chlorine and the amount of combined chlorine in the system. The sum of the free and combined chlorine is reported as total chlorine.
Various forms of chlorine are used in water treatment. This includes gaseous, liquid and solid products. All forms of chlorine react in the same way; they dissolve in water to form hypochlorous acid and hypochlorite ion. The product label indicates the active chlorine percentage and the available chlorine content (ACC). The active chlorine percentage is the percent by weight of the actual chemical product and does not include inert components. The available chlorine content, or ACC, is the relative oxidizing power of the product as compared to chlorine gas which is assigned by convention an ACC of 100%.
Chlorine gas, a greenish-yellow chemical with a burning odor, contains 100% active chlorine. Although gaseous chlorine is the least expensive of the chlorine products to use, the handling and storage of the compressed gas cylinders normally restricts its use to larger systems. In addition, SARA Title III legislation mandates extensive emergency planning and reporting procedures be implemented when gaseous chlorine is stored on site.
Sodium hypochlorite or liquid chlorine contains 10 to 13% active chlorine and has an available chlorine content of about 10%. Unlike gaseous chlorine, which tends to decrease the pH and alkalinity of the cooling water, liquid chlorine contains caustic soda as a stabilizer, which tends to increase the pH.
Calcium hypochlorite is available as a granular material or in tablet form. It has an available chlorine content of 65%. Although it disinfects just like gaseous or liquid chlorine, one side effect is that it also increases both the calcium hardness and alkalinity of the cooling water. This increases the scaling tendency of the water.
Stabilized chlorine is made by combining chlorine with cyanuric acid. The cyanuric acid inhibits the depletion of active chlorine by ultraviolet light. This can be advantageous in towers with large open distribution decks that are exposed to direct sunlight. As the product is applied, however, the cyanuric acid levels increase. Cyanuric acid concentrations above 100 ppm tend to slow down or “lockup” the activity of the free chlorine, rendering it far less effective as a bactericide and algaecide.
Two forms of stabilized chlorine are available for cooling water systems: Trichlor (trichloroisocyanuric acid), a 90% ACC product, and Dichlor (dichloroisocyanuric acid), a 56 to 62% ACC product. Trichlor is available in a slow-dissolving tablet while Dichlor is applied as a faster dissolving granular material.
Chlorine dioxide is a yellow-green gas with a disagreeable odor similar to chlorine. Unlike chlorine, however, chlorine dioxide is an unstable material and must be generated on-site. The chlorine generator uses a 2-pump or 3-pump system to mix the reactants in the reaction chamber of the generator to produce a gaseous effluent containing from 500 to 2000 mg/L of chlorine dioxide (ClO2).
Chlorine dioxide exists in solution as ClO2. Its disinfection rate is not affected by pH nor does it react with ammonia to form chloramines. This is an advantage in systems that have high organic loadings or ammonia contamination. The major disadvantage of chlorine dioxide lies with the operation and maintenance of the on-site generator. This, combined with the higher cost of chlorine dioxide, has limited its use to systems with high organic loadings or process contaminants that render chlorine ineffective.
Bromine chemistry is very similar to that of chlorine.
Br2 + H2O —– HOBr + H+ + Br–
HOBr —— OBr– + H+
Bromine dissolves in water to form hypobromous acid and hypobromine. As with chlorine, the concentration of hypobromous acid strongly influences the disinfection rate of bromine. At pH 8.5, 60% of the bromine exists in the acid form, whereas, less than 10% of chlorine is present as the acid. This explains why bromine exhibits a faster disinfection rate than does chlorine at pH’s at or above 8.5.
Typical bromine dosages are 0.1 to 2.0 ppm applied either continuously or intermittently. Because of the activity of combined bromine compounds, bromine residuals are reported as total bromine instead of the free, combined and total residuals used in chlorine chemistry.
Bromo-chloro-hydantoins: Because of its hazardous nature, bromine is attached to a chemical carrier to produce a slow-release bromine product. Two forms of this material are commonly used 1-bromo-3-chloro-5-methyl-5-ethylhydantoin (BCMEH) and 1-bromo-3-chloro-5,5-dimethylhydantoin (BCDMH). The combined bromine and chlorine content in each product is about 88%.
Because of the limited solubility of BCDMH and BCMEH, they must be fed using a by-pass chemical feeder called a brominator. The bromine tablets or granules are placed in a cylindrical feeder that employs a high flow of water to dissolve the chemical at a controlled rate.
Ozone has received new recognition as an effective microbiological control agent in cooling water systems. It is the strongest oxidizing biocide and is extremely toxic to all organisms. In addition, it does not form undesirable oxidation by-products as does chlorine.
Ozone is unstable and must be generated on-site with an ozone generator. Dry air is passed through an electric arc to product the ozone gas. The ozone is dissolved into a sidestream flow of water from the cooling system. Control of the ozone dosage is maintained by an ORP indicator system.
Isothiazolin exhibits a broad spectrum of activity and is particularly effective against algae, bacteria and sulfate-reducers. Its activity is not affected by pH. It is also a non-foaming material and produces no chemical odors in the system. Isothiazolin is readily degraded in water and soil, and it does not concentrate in fish tissue or persist in the environment.
Isothiazolin is effective at low dosages. The product is sold as a 1.5% active solution of 5-chloro-2-methyl-4-isothiazolin-3-one. The recommended dosage of the product is approximately 50 ppm for algae, 150 ppm for bacteria and 200 ppm for sulfate-reducing bacteria. It’s effectiveness is diminished by high concentrations of amines, sulfides and strong reducing agents.
Extreme caution should be used when working with Isothiazolin. Contact with the eyes causes immediate and irreversible damage. Skin contact produces a chemical burn that is slow to heal. For these reasons, the product should always be applied with proper safety precautions in place.
Quaternary ammonium compounds are a class of amine-based, cationic biocides that include N-alkyl dimethyl benzyl ammonium chloride, N-alkyl 1,3-propanediamine, and poly(oxyethylene dimethyliminio) ethylene (dimethyliminio)-ethylene dichloride), also known as WSCP or Busan 77 from Buckman Labs. These biocides are effective over a broad spectrum and are especially effective against bacteria and algae. They are also effective over a broad pH range of 6.5 to 9.5.
Organo-tin compounds like bis (tri-n-butyl tin) oxide (TBTO) are often blended with “quats” to enhance the biological activity of the product. It is particularly effective for the control of wood-rotting fungi. TBTO absorbs into the cellulose to give longer term protection.
Organo-sulfur compounds such as dimethyldithiocarbamate, disodium ethylene-bis-dithiocarbamate, and methylene bis(thiocyanate) are enzyme poisons that are effective against molds, yeast and bacteria. MBT is excellent against sulfate reducing bacteria. It is also used in metal working fluids to control bacteria growths and odor. These products are best applied at neutral pH, since the degradation rate increases with pH.
DBNPA or 2,2-dibromo-3-nitrilopropionamide is a broad spectrum biocide that is effective at low concentrations. It is non-foaming and is not affected by anionic dispersants or organic contaminants in the water. DBNPA hydrolyzes at high pH, so it is best applied at or near neutral pH. Upon discharge it does not accumulate in the environment.
Glutaraldehyde is a broad spectrum biocide that is insensitive to sulfides. It is non-ionic and tends to tolerate salts and hardness well. It is deactivated by ammonia, however, along with primary amines and reducing agents.
Decylthioethaneamine (DTEA) is effective within the pH range of 7.5 to 9.0. It has a slight amine odor and tends to foam, but is effective when dosed at recommended levels.
Tetrahydro-3,5-dimethyl-2H-1,3,5-thiazine-2-thione (DMTT) exhibits a slight sulfurous odor at use concentration. It is effective within the pH range of 6.5 to 9.0 and does not foam.
Selecting the Right Biocide for the Job
Biocide Application Methods
Biocides are added to cooling water systems either intermittently or continuously. Intermittent application involves the addition of the biocide in a slug dose. The concentration increases rapidly after addition and then dissipates over time. The depletion rate is determined by the following equation.
Retention time = ln (c/co) = -b(t-to)
c = concentration after time t
co = initial concentration at to
b = blowdown rate
V = total system volume
The depletion rate increases with increased blowdown. Systems with minimal volumes and high bleedoff rates have a very short retention time. This means the biocide may not have sufficient time to work in the system.
The frequency of biocide addition is often determined by operating experience with the cooling tower. As a general rule of thumb, the biocide should be added when the final concentration, as determined by the depletion rate, is 10% of the initial concentration.
The other option is to feed the biocide continuously. This is frequently the case in systems that are chlorinated. The chlorine feed system is calibrated to maintain a continuous free chlorine residual of 0.5 to 3.0 ppm. Other biocides can be fed continuously as well. Continuous feed methods avoid the high and low concentrations created by intermittent biocide feed.
Monitoring the Microbiological Activity
The effectiveness of the microbiological control program is best determined by periodic checks on the microbiological activity in the system. This includes physical inspections for algae growth as well as laboratory tests for sessile and planktonic bacteria populations.
Dip slide test method is an easy-to-run test for monitoring bacteria populations in cooling tower systems. The dip slide or paddle slide contains a growth media on each side of a two-sided plastic test paddle. The slide is immersed in a water sample, placed in a clear plastic incubation vial and incubated for 24 hours at 30 oC. At the end of the incubation period the number of colonies on the slide is compared to a standard growth density chart. Optimum control is achieved when the total bacteria population is maintained below 106 organisms per ml.
Heterotrophic plate count (HPC) method is used to determine the number of live heterotrophic bacteria. Three methods for HPC are recognized by the EPA; the pour plate, spread plate and membrane filter techniques. In the pour plate and spread plate methods a sample of water is used to inoculate a Petri dish containing a suitable agar growth media. In the membrane filter technique, a water sample is filtered through a 0.45 disk. The disk is then placed directly on the agar media. The Petri dish is incubated for 48 hours and the number of colony-forming units (CFU) is counted. The results are reported as the number of CFU’s per ml of sample.