Protecting Cooling Water Systems – Part 2
Brad Buecker, Buecker & Associates, LLC
Posted 7/30/2024
Introduction
Cooling systems are vital and integral components at thousands of industrial plants. Properly designed and operated chemical treatment systems, preventive maintenance of such, and monitoring are absolutely critical to minimize failures that otherwise may cause partial or full plant outages. In the next three parts of this series, we will examine the water-side issues of microbiological fouling, scale formation, and corrosion that require attention during both the design and operation of open-recirculating cooling systems like those discussed in Part 1 of this series.
The Cooling System Triangle
A symbolic representation of the primary water-side issues and their interdependence is shown below.
Control of all three must be considered in a holistic manner, as each may be influenced by the others, although biofouling often causes the most severe and prompt difficulties. Biofouling is the subject of this installment. A fourth increasingly important factor, which would convert the triangle to a diamond, is the potential environmental impact of water treatment chemistry, especially regarding compounds such as phosphates that might be in cooling tower blowdown. Treatment programs that were once commonplace may no longer be allowed or may be severely restricted due to discharge regulations. (2) We will discuss some of these environmental influences later in the series.
Biofouling: The Common Elephant in the Room
Microorganisms are present everywhere in our environment. The microbes that most affect cooling water systems are bacteria, algae, and fungi, and they enter cooling towers from the air flowing through the tower and from makeup water. We will primarily consider bacteria in this section.
Free-floating bacteria are not a problem in cooling systems, but if the organisms settle and form sessile colonies, the situation changes radically. When bacteria attach to surfaces, some organisms, of which there are numerous species, immediately begin to generate a protective, polysaccharide (slime) layer. The slime collects other microorganisms and suspended solids, and deposits can rapidly accumulate as shown in Figures 2a and b.
Immediately obvious is that slime/silt deposits can greatly restrict flow. Also, the slime matrix is more insulating than many mineral deposits.
Fouling is problematic in other ways. Deposits in general can establish corrosion cells where the area beneath the deposit becomes anodic to bare metal and begins to corrode. The presence of small anodes in a large cathodic field can generate pitting and through-wall penetrations in a short time frame. Beyond this particular corrosion issue is that some bacteria, such as the sulfate-reducers, release metabolic byproducts, in this case hydrogen sulfide, that are very destructive to many metals including steels. This attack is known as microbiologically induced corrosion (MIC).
Furthermore, established colonies will generate higher life forms including protozoa and amoeba. These organisms can harbor Legionella pneumophila bacteria that multiply within the cells. Legionella was first discovered in 1976 when it infected American Legionnaires attending a convention in Philadelphia. Nearly three dozen people died, and many more became ill. The bacteria were traced to fine water droplets in the exhaust plume of a cooling tower on the roof of the convention hotel. The droplets entered the intake of an air handling unit, spread through the hotel, and were inhaled by the guests (including my parents, who suffered no long-term effects). These organisms can appear in many water systems (including hospitals, spas, produce watering systems in grocery stores, etc.) that are not properly treated to prevent microbiological fouling.
Proper design, operation, and water chemistry monitoring are key factors in minimizing colony formation.
Modern Microbiological Control
For the large majority of recirculating and once-through cooling waters, oxidizing biocides are the backbone of microbial control programs, as this chemistry usually represents the most cost-effective method for maintaining system cleanliness.
Numerous references suggest 1893 as the year in which chlorine was first applied as a drinking water biocide. (1)Chlorine gas became the standard for drinking water and then cooling water applications for many years. One-ton cylinders were a common method of storage and supply. When chlorine is added to water the following reaction occurs:
Cl2 + H2O ⇌ HOCl + HCl Eq. 1
HOCl, hypochlorous acid, is the killing agent, and it functions by penetrating cell walls and oxidizing internal cell components. Safety is obviously a major issue with handling chlorine cylinders and feeding chlorine gas, and years ago many industrial facilities switched to liquid sodium hypochlorite (NaOCl, aka bleach), with a common active chlorine concentration of 12.5%. The change to bleach allowed for use of metering pumps to feed the chemical, but a problem that many operators encountered was pump binding due to bleach’s tendency to vaporize in piping locations around the pump. Pump designs have evolved to prevent or relieve vapor lock.
An alternative to bleach is on-site hypochlorite generation.
The generator produces a mixed oxidant from three common consumables: water, salt, and electricity. Saltwater electrolysis generates chlorine and some peroxide. The process requires no storage of hazardous chemicals.
Chlorine/Bleach Alternatives
Prior to the 1980s, the common scale/corrosion inhibitor treatment for open recirculating systems consisted of sodium dichromate/acid feed, with pH typically maintained within a range of 6.5-7.0, or thereabouts. (We will examine scale/corrosion inhibition technology in greater detail in parts 3 and 4 of this series.) Concerns over the toxicity of acid/chromate programs (especially in regard to the formation of hexavalent chromium, Cr6+), led to inorganic/organic phosphate programs that operate near or slightly above a pH of 8.0. Unfortunately, the efficacy and killing power of chlorine significantly decline with rising pH, per the equilibrium nature of HOCl in water, as shown below.
HOCl ⇌ H+ + OCl– Eq. 2
OCl– is a much weaker biocide than HOCl, probably because the charge on the OCl– ion does not allow it to effectively penetrate cell walls.
As is evident, the dissociation of HOCl is much greater at a pH of 8 than in the 6.5-7.0 range of the former acid/chromate programs. Chlorine/bleach may not be the best oxidizer choice when combined with modern scale/corrosion control chemistry.
Bromine Chemistry
A popular answer to this difficulty has been bromine chemistry, where a chlorine oxidizer (bleach is the common choice) and sodium bromide (NaBr) are blended in a slipstream and injected into the cooling water. The reaction produces hypobromous acid (HOBr), which has similar killing powers to HOCl.
HOCl(aq) + NaBr(aq) ⇌ HOBr(aq) + NaCl(aq) Eq. 3
HOBr functions more effectively than HOCl at alkaline pH. Figure 7 shows the schematic of an HOBr generating system, with which the author directly monitored.
A difficulty with the halogens, and especially chlorine, is their reaction with organics and nitrogen compounds, particularly ammonia. These reactions represent the halogen “demand,” and reduce the effective biocide concentration. The reactions can also produce halogenated organics that may be regulated in the plant’s discharge permit.
Halogen Stabilizers
Several chemical compounds are available that can stabilize chlorine and bromine and then release the oxidizers gradually, and where they are most needed. Stabilized halogens typically exhibit a lower oxidizing power than the parent halogen, but this reduced oxidizing power offers benefits with respect to microbial control in that it reduces undesirable reactions with the protective slime. Three classes of stabilizers dominate the market: sulfamate, dimethylhydantoin, and isocyanurates.
A supplemental treatment technique is feed of a bio-surfactant prior to the oxidizer. These molecules typically consist of a non-polar, hydrophobic carbon chain with a charged molecule at one end. The hydrophobic portion penetrates the organic slime matrix, with the active group being attracted to water. Surfactants/penetrants can loosen the slime layer allowing better penetration by the chemical agent. Most effective is to feed the surfactant perhaps a half-hour or so before the biocide to allow better penetration of the killing agent.
Chlorine Dioxide
Chlorine dioxide (ClO2) is a gas at room temperature that is stable and soluble in water to a maximum concentration of approximately 3,000 ppm. It must be prepared on site via the reaction of either sodium chlorite (NaClO2) or sodium chlorate (NaClO3) with an additional oxidizing agent under acidic conditions. Chlorine dioxide is more expensive than the halogens, but modern production techniques have lowered the cost.
Because chlorine dioxide exists as a gas in solution, it is easily stripped by aeration in cooling towers. ClO2 should be introduced below the surface of receiving waters to minimize losses. Handling of the chemical precursors for chlorine dioxide, which may include sulfuric acid, require attention to safety, although modern ClO2 generators are typically designed with safety in mind. Strict adherence to operational guidelines is important.
Monochloramine
Chloramines have served for microbial control in water systems for over a century. In water containing ammonia, continued chlorine feed will produce a series of chloramines, starting from monochloramine (NH2Cl), then dichloramine (NHCl2), and finally nitrogen trichloride (NCl3). The solution reaches “breakpoint” once all ammonia has been consumed, upon which free chlorine appears. Monochloramine is the compound of interest for modern biofouling control, and technologies are now available to produce a pristine stream of NH2Cl for this purpose. When compared with sodium hypochlorite, monochloramine is less reactive but almost equivalently toxic. The reduced reactivity allows it to penetrate biofilms and attack underlying organisms. However, monochloramine generally needs a longer contact time than hypochlorite to achieve the desired microbial destruction.
Oxidizer Monitoring
Regular analysis of oxidizing biocide residual is extremely important to ensure consistent and proper dosages.
Prior to development of on-line monitoring methods, the standard procedure for either free or total halogen measurement was the DPD method. DPD is the acronym for N,N-diethyl-p-phenylenediamine. It reacts with oxidants in the water to form a colored complex, whose intensity is directly proportional to the amount of oxidant present.
Mature, on-line oxidizing biocide measurement is available. A standard industry instrument is shown below.
The analyzer extracts a sample, and per the DPD chemistry mentioned above, injects the chemical and a buffer solution. Following the proper reaction time, the instrument displays the oxidizer concentration, flushes the spent sample, and extracts a new sample. Thus, the analyzer can provide round-the-clock analyses, separated only by the time needed for reagent reaction. On-line monitoring can be of great benefit in promptly detecting biocide feed system malfunctions or water chemistry changes that increase biocide demand. Similar to grab-sample DPD analyses, either a free- or total-residual reagent is available for the analyzer.
Cooling waters typically contain numerous oxidizing or reducing compounds. Oxidation-reduction potential (ORP) measurement is an analytical technique that, as its name implies, provides data regarding the oxidizing or reducing power of the solution. The oxidizing potential of the oxidizing biocides usually dwarfs other species. ORP can serve as a good supplement to chlorine monitoring.
Non-Oxidizing Biocides
While oxidizing chemicals normally serve as the foundation of cooling water biocide programs, microorganisms can develop partial immunity. And, as been noted, the slime produced by bacterial colonies also protects organisms. Feeding a non-oxidizing biocide on a periodic basis, e.g., once or twice per week for an hour or so, can help control microbial growth. Non-oxidizers typically penetrate cell walls to then react with compounds within the cell that are necessary for life. Programs can be tailored to address the most troublesome organisms in a system, whether they be bacteria, algae, or fungi. Additional details regarding non-oxidizers may be found in Reference 1.
Note: Plant personnel must receive approval from the proper regulatory authorities before making a change, or even testing, chemicals that are not currently authorized in the existing plant discharge permit(s).
Conclusion
The key takeaway from this installment is that proper design, control and proactive maintenance of biocide feed systems is essential for cooling system reliability. Large plants may have dozens of cooling systems, with cooling towers scattered around the facility. Two of the illustrations in this article hint at the extreme problems that can arise if microbiological colonies become established. Critical is having the parts in the storeroom and repair procedures readily available to promptly correct any equipment malfunctions. And, of course, reliable instrumentation is needed to quickly detect upset conditions. Microbes will very quickly adapt to their environment if the treatment program is not consistent.
References
- B. Buecker (Tech. Ed.), “Water Essentials Handbook”; 2023. ChemTreat, Inc., Glen Allen, VA. Currently being released in digital format at www.chemtreat.com.
- Buecker, B. and R. Aull, “Cooling Water Treatment: Fundamental Information for The New Kids in Town”; paper for the 42nd Annual Electric Utility Chemistry Workshop, June 4-6, 2024, Champaign, Ill.
Brad Buecker
Brad Buecker is president of Buecker & Associates, LLC, consulting and technical writing/marketing. Most recently he served as a senior technical publicist with ChemTreat, Inc. He has many years of experience in or supporting the power industry, much of it in steam generation chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power (Springfield, IL, USA) and Kansas City Power & Light Company's (now Evergy) La Cygne, KS, USA, station. His work has also included eleven years with two engineering firms, Burns & McDonnell and Kiewit, and he spent two years as acting water/wastewater supervisor at a chemical plant.
Buecker has a B.S. in chemistry from Iowa State University with additional course work in fluid mechanics, energy and materials balances, and advanced inorganic chemistry. He has authored or co-authored over 250 articles for various technical trade magazines, and has written three books on power plant chemistry and air pollution control. He is a member of the ACS, AIChE, AIST, ASME, AWT, the Electric Utility Chemistry Workshop planning committee, and he is active with the International Water Conference and Power-Gen International.