Advanced cooling water treatment concepts (Part 5)

Editor's note: This is the fifth 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.

Read Part 4 here.

The previous part to this series provided an overview of oxidizing biocides, which have for many years been utilized for cooling water microbiological control. However, conditions may exist in which supplemental chemical treatment is needed to control microbial growth or to attack sessile colonies that resist the oxidizers. In these situations, non-oxidizing biocides may be quite valuable. Non-oxidizers may also be needed to attack macro-fouling organisms such as zebra mussels. This installment provides fundamental details of this chemistry.

As was noted in Part 3, if bacteria form sessile colonies, the microorganisms may develop substantial immunity to oxidizers by producing protective biofilms that consume the chemical(s). The use of a non-oxidizing biocide on a periodic basis, for example once or twice per week for a relatively short duration, can help to control microbial growth. Whereas oxidizing biocides typically damage cell walls and cause death by leakage of organism internals (lysis), many of the non-oxidizers penetrate slime and then cell walls to react with cell compounds that are necessary for life. (1)

The compounds have varying degrees of efficacy and may target some organisms over others. Both efficacy and residual chemical decomposition are typically influenced by water conditions including pH and temperature. Let's examine several of the most common non-oxidizers.

2,2-dibromo-3-nitrilopropionamide (DBNPA)

DBNPA is a halogenated amide that is widely used in water treatment and pulp and paper applications, and in the oil-field serves to treat makeup water for fracturing fluids. The compound irreversibly reacts with sulfur-containing amino acids in cell internals and causes death.

DBNPA acts very quickly. Also, residual concentrations rapidly hydrolyze to less toxic by-products. The rapid decomposition is environmentally advantageous, for if the discharge passes through a retention pond, deactivation chemistry may not be needed. The optimum pH range for maximum DBNPA efficacy is 4-8. The hydrolysis rate increases with increasing pH and the compound rapidly loses potency above pH of 8. Hydrolysis also increases with increasing temperature. DBNPA is deactivated by sulfides and bisulfite or sulfite reducing agents. DBNPA also reacts with ammonia and is not stable to UV light.

DBNPA is effective for other applications. For example, a number of years ago the author, in consultation with an experienced chemical supplier, selected DBNPA to relieve microbiological fouling in reverse osmosis (RO) units for high-purity makeup water treatment at a power plant. Most RO membranes have a polyamide base material that contains nitrogen, which reacts irreversibly with chlorine. This makeup system had activated carbon filters to remove chlorine ahead of the RO unit.

Typically, however, some organisms survive chlorination and then blossom once the chemical is removed. (Also, an activated carbon bed removes oxidizers in the top few inches, leaving the remainder of the bed as a great spot for incubation of surviving microbes.) Serious membrane fouling may result, which occurred in this system. Feed of DBNPA for one hour twice per week solved the problem.

2-Bromo-2-Nitropropane-1,3-diol (Bronopol)

Bronopol is used extensively in water treatment applications and, like DBNPA, has seen some applications in the oilfield. Bronopol is particularly effective against Pseudomonas bacteria. The compound appears to function by differing mechanisms depending on whether conditions are aerobic or anaerobic. Bronopol is not a fast acting biocide. The compound may release formaldehyde upon decomposition, but the formaldehyde is not responsible for the biocidal properties.

Bronopol will hydrolyze in aqueous solutions, with the rate being much faster at alkaline pH. Increasing temperature also increases the hydrolysis rate. The optimum pH range forbronopol efficacy is 5–9. Bronopol will react with and become deactivated by sulfides and sulfite-based reducing agents.

Isothiazolones

The most common formulation for cooling water treatment has a 3:1 mixture of CMIT and MIT. The CMIT and MIT concentrations in registered products usually have either 1.5 percent or 4 percent active ingredient. Industrial formulations may contain a stabilizer(s), including cupric nitrate, magnesium nitrate, or potassium iodate. Also available is a 1.5 percent active product stabilized with bronopol. The compounds are broad spectrum but slow-acting bactericides that also show good reactivity towards fungi. Apart from cooling water applications, MIT, in very slight concentrations, serves as a common anti-microbial agent in some detergents.

Both CMIT and MIT are incompatible with hydrogen sulfide and other sulfide-containing compounds. Therefore, if sulfur reducing bacteria (SRB) are present, isothiazolones may not be very effective. Elevated pH (> 9.5) will shorten the half-life of CMIT, but MIT is stable even at a pH above 10. Isothiazolones are deactivated by sodium bisulfite.

Glutaraldehyde

Glutaraldehyde is often used in industrial water treatment applications, including oil and gas operations, the paper industry and for medical instrument sterilization. Within cells, the compound deactivates two essential amino acids, lysine and arginine, which are essential for cell metabolism. Efficacy is greatest within an alkaline pH range of 7-10, but the compound is more stable at an acidic pH. Glutaraldehyde will react irreversibly with amines or ammonium ions to reduce biocidal efficacy.

Quaternary Amines

Quaternary amines, or "quats" as they are commonly termed, have been utilized extensively in a wide variety of cooling and process water applications. The molecules are positively-charged, with four alkyl groups attached to a central nitrogen atom. One or more of the alkyl groups consists of a methyl, benzyl, decyl (C10), coco (C14), or soya (C18) group. Quats are commonly fed in combination with other biocides. Quats are also used as filming-amine corrosion inhibitors.

Quats have surfactant properties and therefore solubilize cell membranes, leading to cell damage and death. (2) The compounds are especially effective when used in combination with other biocides that also attack cell walls.

Foaming is a concern with quaternary amines, but low-foaming compounds are now available. The surfactant properties of quats can inhibit the separation of oil/water emulsions in oilfield production systems, and hard water may decrease the biocidal activity of the compounds. (3) Quats can react with negatively-charged scale and corrosion inhibitors, which reduces efficacy.

Oxidizing biocides are lethal to clams, mussels, and so forth when these creatures are in the larval stage, but if organisms become established or adult organisms have a pathway into cooling systems, the situation can be entirely different. A classic case is zebra mussels, where, as was noted in Part 3 of this series, the mussels will attach to surfaces, including each other, with thin filaments known as byssal threads. They then reside comfortably by filtering the flowing cooling water. The mussels can sense oxidizing biocides, and when feed is initiated for the two hours per day (or whatever time period is allowed by the plant's NPDES permit), they will "clam up" (pardon the pun) until the toxic conditions disappear, upon which they will merrily resume filtering the cooling water for food.

Oxidizers are lethal to adult organisms if plant personnel can obtain a variance for continuous chemical feed for perhaps two or three weeks. The long feed duration eventually forces the organisms to re-open or re-activate, upon which the oxidizer causes damage. However, regulatory agencies are often reluctant to grant such variances.

Non-oxidizers can be of benefit in these cases, as many macro-organisms do not detect the chemical presence and continue to filter water. The most effective include the quaternary amines mentioned above.

Non-oxidizing compounds pose environmental risks and potential toxicity to other aquatic organisms. Accordingly, they cannot be utilized without permission from the plant's environmental regulators, with the application specifics incorporated into the facility's NPDES discharge permit. The permit may require feed of a material such as clay or bentonite to the discharge stream to adsorb and deactivate residual concentrations, although as was noted above, some compounds, if given sufficient retention time in a holding pond, will decompose naturally.

As with any chemical, following proper safety procedures when handling non-oxidizers is very important. Plant personnel must be wearing all protective personnel equipment required for any particular chemical, and should follow all handling procedures to the letter. Safety data sheets (SDS) must be available at the feed site, with a second copy located in a central location such as the plant control room.

Of the various mechanisms that can cause difficulties in cooling systems, micro- and sometimes macro-fouling can be by far the most serious. If organisms become established, growth can be very rapid and damaging. The first line of defense is a well-designed, maintained, and operated oxidizing biocide feed system, but this may not be sufficient for challenging conditions. Non-oxidizing biocide feed effectively supplements oxidizers, but storage, handling, and feed of these chemicals must be based on a firm foundation of safety and adherence to regulatory guidelines.

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].