Chloromethyl-methylisothiazolone (CMIT/MIT) biocide is widely used for microbial control in a variety of industrial water treatment applications. It has been shown to be effective versus various types of bacteria, algae and fungi. Sulfate reducing bacteria (SRB) are of considerable interest for industrial water treatment due to their impact on biofouling and microbially influenced corrosion. This paper will provide results of planktonic and biofilm efficacy studies with CMIT/MIT biocide versus various strains of Desulfovibrio. Results showed that low levels of isothiazolone biocide (1-6 ppm active) provided control of SRB in liquid lab media and on surfaces in recirculating biofilm systems. Isothiazolone biocide is as highly effective versus SRB as it is against other slime forming bacteria, such as Pseudomonas species, as long as the biocide is stable in the system and sulfide levels are low. In sour systems (high sulfide), the biocide is degraded and its antimicrobial efficacy is reduced. The main pathway of degradation of CMIT/MIT by sulfide is presented. The importance of system cleanliness (biofilm accumulation) on biocide efficacy is discussed.
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. Various groups of microorganisms (aerobic and anaerobic bacteria, fungi, algae, and protozoa) are well recognized as the major causes of these problems.1-8 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). Various types of bacteria have been associated with accelerating the corrosion of metals, specifically defined as microbiologically-influenced corrosion (MIC). These most frequently involve anaerobic sulfate-reducing bacteria (SRB), such as Desulfovibrio and Desulfotomaculum and acid-producing Clostridium. Aerobic bacteria, including sulfur oxidizers (Thiobacillus), iron/manganese oxidizers (Gallionella, Leptothrix), and general slime forming organisms (Pseudomonas, Sphaerotilus), have also been linked to MIC. The factors involved in MIC and treatment schemes for control have been the subject of extensive research efforts in recent years. The state of the art in MIC detection and control has been reviewed recently.5-7 A variety of biocides, including aldehydes, quats, phenolics, and isothiazolones have been used successfully for SRB biofouling and MIC control in oilfield injection, cooling water, papermill, and metalworking fluid systems.1-8 Performance data on these biocides was generated from a combination of laboratory and field sources. Biocide efficacy is most easily evaluated using measurements of microbial growth and activity with planktonic or suspended cultures, including the API RP 38 test.9 Model system approaches are less frequently utilized, but provide additional control over key industrial process variables, environmental conditions, and dosing options. These systems are capable of monitoring biofouling on industrial surfaces under specific and highly controlled operating conditions.10-17 Previous studies with isothiazolone biocides have shown effective control over SRB's under anaerobic conditions when dosed alone or in combination with other biocides.10-15,17 Isothiazolones are fast-acting biocides, inhibiting growth, metabolism, and biofilm development.
