Cooling water systems provide a suitable environment for the growth of bacteria, algae and occasionally fungi. The efficacy of industrial biocides is typically determined by monitoring reduction in viable cell counts. An alternative approach is to measure a parameter of microbial activity such as respiration. The effect of methylchloro / methylisothiazolone biocide
(MCMI) on bacterial respiration was determined using an enriched synthetic cooling water and actual cooling water samples. Addition of MCMI resulted in rapid inhibition of oxygen uptake ( <= 10 minutes) by the mixed population of bacteria, whereas reduction in viable counts (two to six-log decrease) was generally not observed until four to six hours. These studies demonstrated MCMI as a fast-acting biocide and
supported the current mode of action model for isothiazolone biocides.
Keywords: cooling water, biocide, microbial activity, bacterial respiration, isothiazolone biocide, oxygen uptake