Monitoring of biofilm in cooling water system by measuring lactic acid consumption rate

Biofilm formation in cooling water systems causes many problems such as increase of the frictional resistance in tubes and decrease in the heat exchange capacity of heat exchanger. Monitoring of biofilm formation in the system is necessary to avoid biofilm related problems and optimize biocide addition. However, detection of biofilm formation in the whole system is difficult. To solve this problem, a novel and simple method for monitoring biofilm was developed in this study. Biofilm consists of living microorganisms, and nonliving organic and inorganic substances. Microorganisms in the biofilm consume substrate in the circulating cooling water. Therefore substrate consumption rate reflects biofilm activity in the system. Experiments were scaled up stepwisely; i.e. beaker-scale, bench-scale, and operating plant experiments. The beaker-scale experiment was conducted to select suitable substrate. Among the eight substrates examined, lactic acid was the most bio-consumable one by the biofilm formed on carbon steel coupon. The lactic acid consumption rate (Rlac) was estimated at 273 mg/(m2 h). The Rlac value was compared before and after the hydrogen peroxide wash of the bench-scale plant. The Rlac before the wash was 36.3 mg/(m2 h). On the other hand, the Rlac after the wash was negligible. An experimental study was next conducted in the operating cooling water plant. A rapid decrease of lactic acid concentration in the circulating water of a relatively unclean system was shown. On the other hand, Rlac in a clean system was low. This simple method provided us information on biologically active biofilm formation in the system.