OSHA, CTI, AWT, CDC - all of which agree that a microbiological control program should combine various biocides, but is this justified?
Biofouling is a frequent and persistent problem in industrial water systems. The control ranges of temperature and pH, aeration, and abundance of nutrients in these systems provide an excellent environment for the growth of several biological species. Problems associated with biological growth include decreased heat transfer efficiency, microbiologically influenced corrosion (MIC), increased downtime, blockage of filtration/screens, and potential health concerns. As water enters the system, organic material begins to attach to the clean internal surfaces by an adsorption process (Figure 1). Once there they create a protective self-secreted extracellular polymeric substance as a defense strategy. This biofilm formation helps cells adapt and react to the changing environment and can more readily communicate exchange genetic materials and resources. What’s worse, these areas enhance the ability of biological species to reduce their sensitivity to antimicrobials, or biocides.
For decades, biocides have been utilized in the water treatment industry to actively destroy microbiological species and minimize the effects of microbiologically-related activity. Biocides fall into two broad categories, oxidizing and non-oxidizing biocides and treatment programs often combine biocides from both categories to effectively use the attributes from each type.
Oxidizing biocides tend to be effective against most microbiological species by means of chemical oxidation of the cellular structure and subsequent cell lysis. They act by directly attacking the cell walls of organisms and any other organic material they contact. These biocides are less effective when they encounter species that have a protective cover such as the oocysts of Cryptosporidium and Giardia. Additional limitations can include their corrosiveness, incompatibility with corrosion and scale inhibitors, and cooling water pH. Chlorine, bromine, ozone, chlorine dioxide, and peracetic acid are all included under the oxidizing umbrella and action mechanisms are similar. Oxidizing chemicals are used due to their effectiveness, rapid biodegradation, and low cost. A tremendous amount of attention should be given when deciding which oxidizing biocide will perform the most effective in any given application. Outside of cost and environmental impacts, pH and reaction time should be addressed (Figure 2).
Figure 2: Biological Effectiveness of Bromine vs. Chlorine
Chlorine and Bromine are two of the popular choices in today’s treatment programs. Chlorine is most effective in the pH range of 6.5 – 7.5 and begins to lose its effectiveness above this. Cooling Towers tend to operate above 8.0 pH and will drastically reduce the available hypochlorous acid (HClO) present. Bromine is effective at a higher pH and the active Hypobromous acid (HOBr) will present a better residual.
Typically, a low residual oxidant is used to significantly reduce the planktonic population in a system. Maintaining a surplus of 0.3 ppm to 0.5 ppm is often sufficient at minimizing organic growth and can be fed based upon water usage or an ORP setpoint.
Non-Oxidizing biocides are typically more adequate at controlling microorganisms due to their greater persistence. They are poisons and kill microbiological species by interfering with normal cell functions. Although non-oxidizers tend to be more expensive, they are less likely to cause internal corrosion and can be more compatible with inhibitors. A drawback is the rigorous testing program these biocides undergo to determine their effectiveness and may require a pesticide applicator license depending upon the state. (Figure 3) provides advantages/disadvantages to common non-oxidizing biocides in the industry. Reaction times, bacterial presence, safety, and price are often considered when determining which biocide(s) will be most effective. Non-oxidizing biocides bring additional value to a treatment program as they can work synergistically when in combination with other oxidizing and non-oxidizing biocides.
Figure 3: Advantages/Disadvantages of Common Non-Oxidizing Biocides
Often non-oxidizers are intermittently slug-fed into a system based upon the biocide’s retention time or depletion rate. The retention time of a biocide in a cooling tower (time elapsed until target re-dose point) can be expressed as T = 1.40 (V) / BD where T = elapsed time in days, V = system capacity in gallons, and BD = bleed-off and windage loss in gallons per day. There is a growing interest in the use of solid biocides in small cooling systems where dosing, handling, and transportation can prove to be problematic. The solid non-oxidizing family consists of DBNPA, bronopol, CMIT, and MIT.
Over time, microbiological species learn to adapt to the biocides and evolve into new strains that are resistant to them. Once this occurs, biocides cease to be effective. A robust water treatment program typically uses an oxidizing biocide supplemented with a non-oxidizer effectively to use the attributes of each type of chemical. There may be a synergism by which the two biocides work together due to their working mechanisms that enhance the efficacy and improve biological control. This dual biocide method provides both quick kill and extending control against microorganisms.
I must add that there are some caveats to this “alternating biocide” rule of thumb. Where the concentration of organic material in a cooling system remains low, the use of one non-oxidizer alone can prove to be efficacious and cost conservative. A slow-releasing DBNPA product can be just as effective as a continuous chlorine feeding program. It has been shown that feeding an oxidant in conjunction with a dispersant product can also provide adequate microbiological protection.
Although using a combination of oxidizing and non-oxidizing biocides for industrial water treatment is widely accepted, there are exceptions that should be considered. Factors such as cooling tower location, system volume, concentration cycles, makeup water quality, and cost help determine what biocide program is most suitable. As always, consult a professional water treater prior to making these decisions.
Figure 1: Stages in the Biofouling Process