Advanced cooling water treatment concepts (Part 4)

Editor's note: This is the fourth installment of a multi-part series by Brad Buecker, President of Buecker & Associates, LLC.

Read Part 1 here.

Read Part 2 here.

Read Part 3 here.

For decades, oxidizing biocides have served as the core treatment for microbiological control in cooling systems. Chlorine is the most well-known biocide, but the evolution of scale/corrosion control programs, and the related shift from slightly acidic to moderately basic pH, have influenced that choice in many cases. (1) Alternative or modified oxidizers may be more effective; and for difficult conditions, supplemental non-oxidizing biocides may also be beneficial. The next two parts of this series examine many of the most important developments for micro- and macro-biological control.

Numerous references suggest 1893 as the year in which chlorine was first applied as a drinking water biocide, with rapid development of the technology by the early 1900s. Chlorine gas, typically supplied in one-ton cylinders, became the method of storage at many facilities, drinking water and otherwise. 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 then oxidizing internal cell components. Due to safety issues with gaseous chlorine, many industrial facilities switched to liquid sodium hypochlorite (NaOCl, aka bleach), with a common active chlorine concentration of 12.5%. Another popular alternative, of which the MIOX® name is most well-known, is on-site sodium hypochlorite generation by salt water electrolysis. This process eliminates the need for bleach storage.

A common control range for cooling water chlorine concentration is 0.2-0.5 ppm, subject to the chlorine demand, which we will examine shortly. The efficacy and killing power of chlorine are significantly affected by pH due to the equilibrium nature of HOCl in water, as shown below.

HOCl ⇌ H+ + OCl– Eq. 2

OCl– is a weaker biocide than HOCl, possibly because the charge on the OCl– ion does not allow it to effectively penetrate cell walls. The dissociation of hypochlorous acid dramatically increases in relation to pH.

Because many cooling tower scale/corrosion treatment programs now operate at an alkaline pH near or slightly above 8.0, modified oxidizing chemistry may be a better choice than basic chlorine, as will be outlined. Also, hypochlorous acid can react with other compounds that are often present in recirculating cooling and process waters. The most prominent are ammonia and organics. The sum of these non-antimicrobial reactions is referred to as "chlorine demand." The reactions consume chlorine and lower the concentration available to attack microbes. Some reactions can produce halogenated organics, whose discharge concentration may be regulated.

A rather popular answer to these issues has been bromine chemistry, where a chlorine oxidizer (bleach is again the common choice) and sodium bromide (NaBr) are blended in a makeup water stream and injected into the cooling water. The reaction produces hypobromous acid (HOBr), which has similar killing powers to HOCl, but functions more effectively at alkaline pH.

HOCl + NaBr ⇌ HOBr + NaCl Eq. 3

Figure 2 compares the dissociation of HOCl and HOBr as a function of pH.

As is clearly evident, at pH of 8.0 80% of the HOBr remains un-dissociated.

Like hypochlorous acid, hypobromous acid is a strong oxidizer that also has a halogen demand. However, unlike chlorine which reacts irreversibly with ammonia, the bromine-ammonia reaction is reversible, which leaves bromine free for activity towards microbes. Bromine can also form halogenated organics.

Another issue that can be problematic is the permitted oxidizer feed duration (hours per day), which, based on this author's experience, has been influenced by regulations for once-through cooling systems. Common for the National Pollutant Discharge Elimination System (NPDES) permits of once-through systems is oxidizer feed for a maximum of two hours per day per unit. This restriction was implemented to minimize exposure of aquatic organisms to residual biocide at the cooling water outlet. Beyond that issue, in some NPDES permits the effluent total residual oxidant (TRO) concentration limit was reduced from a once-standard 0.2 ppm value to much lower levels. Compliance requires feed of a reducing agent such as sodium bisulfite (NaHSO3) or perhaps even gaseous sulfur dioxide (SO2) to reduce the effluent TRO concentration. A similar two-hour time limit has appeared in the permits for systems with cooling towers, even though the blowdown is much smaller and easier to treat than once-through discharge. The upshot is that a two-hour time limit for biocide feed allows microorganisms the remaining 22 hours each day to become established. This makes it even more imperative to properly maintain and operate chemical feed systems. When "bugs" gain a foothold, problems can quickly escalate.

The author, who tracked steam condenser performance for years at two power plants, (2) was once involved in a project to shock chlorinate a condenser that had become fouled due to malfunction of the biocide feed system. Condenser cleanliness factors had dropped to very low levels. Even though the shock chlorination killed the microbes, only part of the tenacious slime layer peeled away, and the condenser cleanliness only recovered by approximately 50%. A mechanical scraping at the next unit outage was required to remove the remaining material.

Conjunctional feed of biodispersants/surfactants can often assist biocide efficacy by opening pathways for the biocide to penetrate the slime layer. Several types of surfactants are available, including anionic alkylbenzene sulfonates, alkyl polyglycosides, and nonionic ethylene oxide/propylene oxide (EO/PO) type polymers. (3)

Most common are anionic compounds that prevent agglomeration of smaller particles by absorbing onto surfaces, thus increasing the negative surface charge to induce particle repulsion. Because these biodispersants have a negative charge, they may interact with cationic species present in the water (especially calcium in highly-cycled cooling towers), and lose effectiveness. Biodetergents are typically nonionic molecules that will not react with other compounds in the water treatment program. They exhibit good efficiency in hard waters. Their stability is an important factor in removing biofilms.

It is important to note the role of the dispersant is not to kill microbes but to assist the oxidizer. Dispersant feed ahead of biocide injection is often the most effective procedure, as the chemical "conditions" the biofilm, increasing the efficacy of the killing agent(s).

Several chemical compounds are available that can stabilize chlorine and bromine and then release the oxidizers gradually, and where they are most needed. A stabilized halogen typically exhibits a lower oxidizing power when compared with the parent halogen, but this reduced oxidizing strength actually offers several benefits with respect to microbial control in that it minimizes undesirable reactions such as those with the protective slime.

Three classes of stabilizers dominate the market: sulfamate, dimethylhydantoin, and isocyanurates. These compounds are available in solid form as tablets, granules, and powders. Each product has individual dissolution characteristics, which requires careful evaluation when designing the feed system. A common design for solid products feed is a small vessel in which the tablets/pucks can be loaded, and which then gradually dissolve in a cooling water slipstream that returns to the main system. Also possible are liquid feed systems, an example of which is stabilized hydantoin that can be fed to a slipstream simultaneously with sodium hypochlorite to generate the stabilized product.

Alternative Oxidizers

An issue that can be problematic with the halogens is, if sessile colonies have formed, the chemical is mostly consumed by the protective slime generated by the microbes. Little biocide may remain to attack the organisms underneath. The next sections examine two alternative oxidizers.

Chlorine Dioxide

Chlorine dioxide (ClO2) is a gas at room temperature that is stable and soluble in water. The compound cannot be stored and must be prepared on site. Years ago, when the author worked with this biocide, the generation technique consisted of blending chlorine gas and sodium chlorite (NaClO2, which can be stored on site) separately into a water slipstream that was reintroduced to the main cooling water. More modern and reliable systems are now available that react NaClO2 or sodium chlorate (NaClO3) with an oxidizing agent under acidic conditions. As with all chemicals, adherence to proper safety procedures is a must when loading the reactants into, and operating, the feed system.

Chlorine dioxide is more expensive than the halogens, but the compound exhibits a high degree of reaction selectivity, and it can penetrate biofilms to attack microbes. The selectivity is advantageous for other non-cooling applications, including phenol destruction and wastewater odor control. Because chlorine dioxide exists as a gas in solution, it is easily stripped by aeration, including when water passes through a cooling tower. Injection points should be carefully evaluated during project design to minimize vaporous escape of the biocide.

Chloramines

Chloramines have served for microbial control in potable water systems for over a century. The benefits are now being recognized for cooling water treatment. During conventional chlorine feed to a water supply, as chlorine concentration increases, a series of chloramines will appear, starting from monochloramine (NH2Cl) to dichloramine (NHCl2) and then nitrogen trichloride (NCl3). 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. Monochloramine is less reactive than the halogens but this can be a benefit against sessile colonies. The reduced reactivity allows the compound to penetrate biofilms and attack underlying organisms. However, monochloramine generally needs a longer contact time than hypochlorite to achieve the desired microbial destruction.

Other oxidizing compounds are available including hydrogen peroxide, peracetic acid, and ozone (generated on-site). However, these chemicals react very quickly with a wide variety of compounds, and thus are not typically utilized for large cooling water applications. They can be very effective for off-line cleaning of cooling tower fill. (4)

Ultraviolet light (UV) has proven effective in killing microorganisms in many applications, but for large flows where the water has significant turbidity, the light may not penetrate sufficiently to be effective. And, of course, UV light does not offer any residual effects. Perhaps we will investigate this technology in a future Power Engineering article.

This installment examined many of the primary properties of oxidizing biocides for microbiological control. The key takeaway is that feed systems should be in proper working order at all times. If sessile colonies develop, it may be extremely difficult to remove the slime layer that is generated. Subsequent outcomes include substantial loss of heat transfer in condensers and other heat exchangers, the potential for under-deposit corrosion generated either directly or indirectly by colonies, and cooling tower fill fouling. In the next part of this series, we will examine non-oxidizing biocides as supplemental treatment for both micro- and macro-biological control.

This discussion represents good engineering practice developed over time. However, it is the responsibility of plant owners, operators, and the technical staff to implement reliable programs based on consultation with industry experts. Many additional details go into the design and subsequent use of these technologies than can be outlined in a single article.

References

About the Author: Brad Buecker is president of Buecker & Associates, LLC, consulting and technical writing/marketing. Most recently he served as Senior Technical Publicist with ChemTreat, Inc. He has over four decades of experience in or supporting the power and industrial water treatment industries, much of it in steam generation chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power (Springfield, Illinois) and Kansas City Power & Light Company's (now Evergy) La Cygne, Kansas station. 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 may be reached at [email protected].