The effective use of non-oxidizing biocides is critical for maintaining microbial control in water treatment applications. A variety of methods and approaches may be used to improve and optimize biocide efficacy in these systems. Results of efficacy testing have shown improved killing of bacteria by biocides with the addition of selected surfactants and adjuvants. Enhanced microbial control may also be achieved by combinations of certain biocides which display a synergistic effect on mixed populations of organisms. Studies have also shown the benefit of rotating or alternating biocides to control resistant organisms which may develop following use of low doses of a single biocide. Collectively, these approaches may enhance the degree of control achieved in a given water system. Other ways to provide additional knowledge on biocide dosing and microbial control may be achieved through the use of model biofilm systems. Many variations of biofouling models have been used to evaluate biocide dosing strategies under simulated dynamic conditions; however, these systems were complicated and time-consuming to operate. Newer methods are available using microtiter plates for evaluating biocide efficacy against biofilm populations on surfaces. Other factors which contribute to effective use of biocide include the ability to monitor the biocide concentration to achieve more accurate dosing and using rapid microbiological methods for assessing microbial contamination.
The proper use of industrial biocides is critical to a successful treatment program to reduce microbial populations on critical surfaces as well as reducing the total microorganism level introduced into the bulk water from external sources. Various biocide technologies have been used successfully in water treatment applications for many years.1 These include oxidizers, such as chlorine, bromine, ozone, chlorine dioxide, and peracetic acid, and non-oxidizing biocides, including isothiazolones, quats, DBNPA (dibromonitrilopropionamide), bromonitropropanediol (BNPD; bronopol), glutaraldehyde, and dithiocarbamates. Many common treatment programs employ non-oxidizers in conjunction with standard oxidant programs for a broad-based approach to microbial control. The overall efficacy of any given biocide is a function of its general spectrum of activity, mechanism of action, stability under environmental conditions, and compatibility with systems components and additives. Microbial fouling is a major concern in water treatment applications affecting a variety of operational problems, including microbially influenced corrosion, reduction in process efficiency (heat transfer and evaporative cooling), system cleanliness, and potential health concerns.2-5 Various groups of microorganisms are well recognized as major causes of these problems. These include aerobic and anaerobic bacteria, fungi, algae, and protozoa. The most diverse group of these organisms is bacteria, which include general slime forming organisms (Pseudomonas, Enterobacter, Klebsiella), health-related species (Legionella pneumophila), sulfate reducers (Desulfovibrio), acid-producers (Clostridium), and filamentous types (Sphaerotilus, Leptothrix),. The cyanobacteria (formerly known as blue-green algae) are also common to cooling tower waters and include Phormidium, Anabaena, Oscillatoria, and Anacystis. Green algae in cooling water systems include Chlorella, Scenedesmus, Chlorococcum, Ulothrix, and Spirogyra. Fungal contaminants (Aspergillus, Saccharomyces, Rhodoturula) occur less frequently, but are still considered proble
