ABSTRACT

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 I 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 ( 5 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.

INTRODUCTION

Recirculating cooling water systems provide an excellent environment ( pH, temperature, nutrients) for the growth of a variety of bacteria, algae and occasionally fungi. Uncontrolled growth of these organisms results in high levels of planktonic organisms and subsequent development of biofilms (l). The problems associated with excessive microbial activity and biofouling include microbiologically influenced corrosion, blockage of screens and distribution boxes, reduced heat exchanger performance, reduced cooling efficiency in the tower and potential health risks (2).

The use of effective biocide programs is an essential component of cooling water management. A common method of evaluating biocide performance in cooling water systems is to monitor viable cell counts before and after biocide addition. A limitation of viable cell counts is that many microorganisms have specific nutritional requirements and will not grow on standard plating media to give an accurate assessment of biocide performance.

An alternate and more rapid approach for determining the efficacy of biocides is to monitor a parameter of microbial activity ( metabolism) such as respiration (dissolved oxygen uptake) (3). Aerobic and facultative anaerobic microorganisms found in industrial systems utilize oxygen during growth. Therefore, monitoring oxygen consumption by these organisms provides a simple, rapid method for assessing biocide performance.

This paper describes studies to explore oxygen uptake as a method for determining biocide performance using a laboratory synthetic cooling water medium and actual cooling tower water. Initial methods development studies investigated the effect of nutrients ( type and concentration) and pH conditions on oxygen uptake in synthetic cooling water. Oxygen uptake monitoring was then used to investigate short and long term effects of methylchloro / methylisothiazolone (MCMI) biocide on aerobic respiration of typical cooling water bacteria.

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