Closed loop cooling systems provide a unique set of environmental and microbiological conditions for biocides to control microbial growth and fouling. Key factors for biocide selection include in-use stability, solvent content, and materials compatibility. A new biocide product based on the methylisothiazolone active ingredient was recently developed for use in closed loop cooling systems. The new biocide is water-based, effective versus a range of bacteria, stable in high pH and high temperature conditions, and contains no salts, metals, or organic solvents. Results of comparative efficacy and stability studies versus commercial biocides will be presented.
The proper use of industrial biocides is critical to a successful water 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, organobromines, and glutaraldehyde. 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.
Closed loop water treatment systems present unique challenges for industrial biocides relative to the open recirculating systems.1,2,3,4,5,6,7 Closed systems may see greater temperature extremes, are more difficult to sample and dose biocides, and often suffer from dead legs in the piping system compared to open recirculating systems. In addition, oxidizers are typically not used due to corrosion issues and the persistence of non-oxidizing biocides in these systems can significantly affect the microbial control program, especially in high pH or high temperature systems.1,3
Isothiazolone biocides are widely used in a variety of industrial water treatment applications for control of microbial growth and biofouling.8,9 The most frequently used product is a 3:1 ratio of 5-chloro-2- methyl-4-isothiazolin-3-one (CMIT) and 2-methyl-4-isothiazolin-3-one (MIT). CMIT/MIT has broad spectrum efficacy versus bacteria, algae, and fungi. DCOIT (4,5-dichloro-n-octyl-isothiazolin-3-one) was introduced several years ago for use as an algicide treatment for open recirculating cooling water systems. The most recent isothiazolone biocide developed for industrial water treatment is based on the MIT active ingredient alone and is targeted at long-term preservative applications with higher pH and temperature ranges, such as closed loop systems.10
This report describes a series of chemical stability and antimicrobial efficacy studies to demonstrate the potential of MIT alone as a new biocide product for use in closed loop systems. The studies include chemical stability (persistence) in high pH and high temperature waters (relative to other commercial biocides) and microbiological studies to characterize the mechanism of action and efficacy of MIT as a biocide.
The chemical structures of the isothiazolone biocides described in this paper are shown in Figure 1. CMIT/MIT is composed of a 3:1 ratio of CMIT:MIT at a final concentration of 1.5% total active ingredient.(1) MIT biocide is a 9.5% active solution in water.(2) 1,2-Benzisothiazolin-3-one (BIT) was included in certain studies for comparison.
