Managing Cooling Water: Preventing Biological Fouling
This article, which explores why you should control microbiological growth in your industrial cooling water, is the fifth in an occasional series on water management basics and technologies.
Uncontrolled microbiological growth within a cooling water system can result in the formation of biological fouling layers (biofilm) on all surfaces in contact with the cooling water. The biofilm acts as a thermal insulator to decrease heat transfer efficiency in the production equipment and usually results in a substantial corrosion rate increase due to the formation of anaerobic areas under the fouling layer. The anaerobic areas create galvanic couple corrosion and form metabolic byproducts such as hydrogen sulfide, which attack the base metals. Severe cases of biological fouling have resulted in complete cooling system failures due to the biomass physically plugging cooling water passages in production equipment and cooling towers.
Waterborne diseases also are a major health and safety concern with the operation of many cooling water systems. For example, Legionnaire's Disease is a bacterial infection spread by inhaling particles containing legionella bacteria. One of the recognized primary sources of such particles is the windage, or drift, produced by the normal operation of a cooling tower, where cooling water is entrained in the airstream and subsequently discharged into the atmosphere. If the cooling water contains legionella, the resulting particles can be a source of infection.
Legionella is generally controlled by maintaining a biologically clean system, which is defined by the Centers for Disease Control (CDC) as having no visible biological debris in the system, and having dipsticks below 10E4 and adenosine triphosphate (ATP) below 2,000 relative light units (RLU). The Occupational Safety and Health Administration (OSHA) has stated that oxidizing biocides “such as chlorine and bromine have been proven effective in controlling legionella in cooling towers” while “little information exists on the demonstrated effectiveness of many commercial biocides.”1 OSHA has set recommended action levels for legionella bacteria in cooling water: below 100 cfu/ml, no action; from 100 cfu/ml to 1,000 cfu/ml, promptly clean and/or treat the system with biocides; and above 1,000 cfu/ml, immediately clean and/or treat with biocides, and take prompt steps to prevent employee exposure. Two tests per year are recommended to confirm that the legionella level in any cooling system is below the OSHA recommended action levels.
Chemistry of Oxidizing and Nonoxidizing Biocides for Industrial Waters
The current practice for controlling biological fouling is to periodically dose the cooling system with a biocide to kill as many of the organisms as possible. The dose makes the poison: A biocide does not work unless a critical dosage is reached and maintained for a set time period. The critical dosage point and time required for effective microbiological control varies substantially with the specific biocide used and the overall condition of the cooling water system. The most commonly used biocides can be separated into two major classes: oxidizing and nonoxidizing.
Oxidizing biocides function by chemically oxidizing -- and thereby destroying -- the cellular structure of the organism. Due to the destructive form of attack, it is impossible for any organism to develop a significant immunity to an oxidizing biocide. Oxidizing biocides usually are cost effective due to their low unit cost, rapid effect on the target organism and low effective dosage. Unfortunately, oxidizing biocides have some drawbacks. For example, some can decrease cooling water pH in an uncontrolled manner (gas chlorine and chlorine dioxide generators). Most oxidizing biocides increase the corrosive nature of the cooling water, and some, such as chlorine, produce undesirable byproducts from an environmental standpoint. Corrosion and scale control chemicals can be inactivated by contact with specific oxidizers, and none of the oxidizing biocides has any dispersant effect for removing dead microbiological growth and/or penetration of organic “slime” layers. Additionally, process contamination can neutralize many oxidizers, and the effectiveness of some oxidizers varies with the water pH. Table 1 lists some common oxidizing biocides along with their advantages and disadvantages.
Nonoxidizing biocides function by interfering with the metabolism of the organism. Due to the large variety of organisms, those that are immune to a particular nonoxidizing biocide will rapidly replace those that are killed by a single dosage. Following doses will become progressively less effective as the organism population shifts to those varieties that are immune to the biocide used. Due to this natural effect, at least two different nonoxidizing biocides, or an oxidizing and nonoxidizing biocide, should be used in biological control programs on an alternating basis.
Nonoxidizing biocides can be costly because of the high effective dosage required, long contact times and often high unit cost. However, most nonoxidizing biocides function in the presence of process contamination, and they do not appear to affect system corrosivity or corrosion and scale chemicals. Often these biocides can be targeted at a specific class of problem organism, and several have a definite dispersant effect for removing dead microbiological growth. Table 2 lists some of the nonoxidizing biocides commonly used along with their advantages and disadvantages.
A standard practice by many cooling water treatment suppliers is to use both an oxidizing and nonoxidizing biocide, alternated weekly or biweekly. For example, a typical combination would be to dose hydantoin twice a week, with carbamate as an alternative every other week.
The cost for oxidizing and nonoxidizing biocides varies substantially on a product basis, with the actual use cost complicated by the widely varying strengths and dosages of the various products, making cost comparisons difficult. The generic product use costs shown in table 3 were calculated as the dollars required to treat 1,000 gallons of cooling water based on typical product strengths, recommended average dosages, and average list prices for various biocides.
Several “all-in-one” products package a nonoxidizing biocide with an inhibitor chemistry for controlling corrosion, scale and deposition in one drum. However, these chemicals may not effectively control biological fouling due to the inability to alternate biocides or adjust the biocide dosage independent of the other inhibitors, and the particular chemistry of the biocide mandated by the other components in the product.
Controlling Legionella in Industrial Waters
With the noted OSHA recommendation to use an oxidizing biocide for controlling legionella, the overall selection and control of a biocide program has become much simpler than in the past. Any basic program should be based on a routine feed of an oxidizing biocide with the supplemental use of nonoxidizing biocides or biodispersants as required.
Routine dosing with bromine biocides two to three times per week appears to provide excellent control of legionella. A continuous feed of an oxidizing biocide is not required and actually has a number of negative effects on other portions of the water management program, such as increasing water corrosivity and destroying the effectiveness of some corrosion inhibitors, as well as adversely affecting many scale and deposition inhibitors. When using an oxidizer, a general rule for establishing proper dosage is have 0.5 to 1.0 mg/l total oxidizer in the treated water one hour after completing the biocide dose.
Dipstick culture plates and ATP test kits can be used to evaluate the effectiveness of a biocide program. As mentioned, test results should be no greater than 10E4 for dipsticks and 1000 to 2000 RLU for ATP. Higher values require increased biocide dosage levels or more frequent applications of biocides. Continued high readings indicate that a switch in biocide chemistry is required.
When calculating the initial dosage for any biocide, a good starting point is to determine the actual volume of the cooling system by the lithium salt dilution technique and use the supplier's suggested dosage. The dosage then can be adjusted based on the observed results, dipstick or ATP testing, and total oxidizer testing. If determining the actual system volume is not possible, the following “rules of thumb” provide good starting point volumes. For standard cooling tower systems, dose at 15 gal/ton, and for evaporative condenser systems, dose at 2 gal/ton.
1. OSHA Technical Manual, Section III, Chapter 7, online at http://www.osha.gov/dts/osta/otm/otm_toc.html.
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