Table 2 - Non-Oxidizing Biocides
to HOCl, but functions more effectively at alkaline pH. Figure 10 compares the dissociation of HOCl and HOBr as a function of pH. Figure 10. Dissociation of HOCl and HOBr vs. pH
Other alternatives include chlorine dioxide (ClO2 ), monochloramine (NH2Cl), and monobromamine (NH4Br). The latter two are weaker oxidizers than the others, but appear to be more effective at penetrating the protective slime layer that bacteria produce, to then directly attack the organisms. Also to be considered are onsite hypochlorite generating systems, as exemplified by the MIOX® technology, where no oxidizer needs to be stored. A factor that influences the efficacy of many oxidizers, and most notably the strong oxidizers such as chlorine and bromine is the ability, or lack thereof, to penetrate the polysaccharide (slime) layer, that bacteria form for protection. Sessile microbiological colonies may be almost impossible to destroy with standard treatment. Recently developed is a new halogen stabilizer/biodetergent, which is applicable for bleach-only oxidizing treatment. The product itself has no biocidal properties, and thus does not fall under regulatory guidelines, but it helps to stabilize chlorine and reduce losses from irreversible reactions. Also, the biodetergent portion of the formulation, as its name implies, disperses the biofilm formed by the organisms, and allows the biocide to contact the organisms directly. In many cases, oxidizer feed is limited to two hours per day, which gives microbes time to settle and form colonies during off times. A supplemental feed of a non-oxidizing biocide on perhaps a once-per-week basis can be quite effective in controlling biological growth. These non-oxidizers in conjunction with biodetergents reduce overall chlorine usage and do not produce halogenated byproducts such as THMs. Table 2 below lists properties of some of the most common non-oxidizers.
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Careful evaluation of the microbial species in the cooling water is necessary to determine the most effective biocides. Antimicrobial compounds should not be used or even tested without approval from the appropriate regulating agency. They must be incorporated into the plant’s National Pollutant Discharge Elimination System (NPDES) permit. Also, as with all chemicals, safety is an absolutely critical issue when handling biocides.
Conclusion Alternatives to fresh water are becoming increasingly common for industrial plant makeup. Rigorous pretreatment of these waters, and in particular POTW effluent is necessary to protect the plant, and in particular the cooling water system. And even with good pretreatment, cooling water process chemistry must not be neglected. The cooling system offers the ideal environment, warm and wet, for microbes, and if they are allowed to settle and establish colonies, severe fouling is typically the result. Also, new methods of scale/corrosion control offer great promise in improving cooling system efficiency and reliability.
References B. Buecker, Trip report from visit to the De Soto, Kansas wastewater treatment plant, March 2013, courtesy of Smith & Loveless. R.M. Post, P.E. and R.P. Kalokodimi, Ph.D., “The Development and Application of Non-Phosphorus Corrosion Inhibitors for Cooling Water Systems”; presented at the World Energy and Environmental Congress, Atlanta, Georgia, October 26, 2017.
CTI Journal, Vol. 40, No. 2