Isothiazolone biocides have been used for microbial control in a variety of industrial water treatment applications. The most widely used product is a mixed isothiazolone biocide (methylchloromethylisothiazolone; MCMI). MCMI has broad spectrum efficacy versus bacteria, algae, and fungi. The other isothiazolone biocide is a new micro-emulsion technology based on 4,5-dichloro-2-n-octyl-4- isothiazolin-3-one (DCOI). DCOI is primarily an algicide for cooling water treatment and a fungicide for papermill applications. MCMI and DCOI inhibit certain dehydrogenase enzymes which disrupt metabolic activity and stop cell growth within minutes. The rate and extent of killing may be enhanced by addition of surfactants. MCMI controls microbial fouling on surfaces and prevents loss of heat transfer efficiency. MCMI efficacy versus problem-causing bacteria, including SRB?s and Legionella, has been demonstrated in controlled lab studies. Lab and field studies with DCOI showed excellent fouling control against algae in cooling towers. Isothiazolones are stable in process waters containing ammonia, high hardness, and oxidizing biocides. They are compatible with additive packages, materials of construction, ion exchange media, and ultrafiltration membranes. They are degraded by reducing agents and high pH (>9). MCMI and DCOI rapidly biodegrade in aquatic environments to non-toxic byproducts which do not affect waste treatment systems.
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 1-5. 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 sulfate reducers (Desulfovibrio), acid-producers (Clostridium), filamentous types (Sphaerotilus, Leptothrix), general slime forming organisms (Pseudomonas, Enterobacter, Klebsiella),and health-related species (Legionella pneumophila). 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 occur less frequently, but are still considered problematic in air washers applications, papermaking, and in deterioration of wood cooling towers. 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. These include oxidizers, such as chlorine and bromine products, and non-oxidizing biocides, including isothiazolones, quats, DBNPA, and glutaraldehyde. Many common treatment program employ non-oxidizers in conjunction with a standard oxidant programs for a broad-based approach to microbial control. The overall efficacy of any given biocide is a function of it?s general spectrum of activity, mechanism of action, stability under environmental conditions, and compatibility with systems components and additives.
This paper provides an overview of two isothiazolone biocides used in industrial water treatment applications. The most widely used isothiazolone biocide contains a blend of methylchloroisothiazolone and methylisothiazolone (MCMI). The second isothiazol
