On-line monitoring and quantitative analysis of biofouling in low-velocity cooling water system

Abstract

To monitor and analyze the biofouling phenomenon caused primarily by microbial suspended solids (MSS), a ‘biofouling tube’ apparatus consisting of a carbon steel tube, a differential pressure transmitter, a corrator, and a cooling water circulation device was designed and fabricated. By measuring continuously the pressure drop across the biofouling tube and using the correlation between the fluid’s friction factor and surface roughness, we quantitatively analyzed the influence of MSS concentration, temperature, and fluid velocity on the rate of biofilm growth on the metal surface. The result indicated that the fluid velocity had the most profound impact, e.g., lowering the linear velocity from 0.3 to 0.15 m/sec resulted in about a four times higher biofouling rate. Up to 50 ppm MSS, the biofouling rate was proportional to the MSS concentration. The biofouling rate at 35°C was about 1.75 times higher than that of 45°C., probably due to the diminishing effect of thermolabile microorganisms exposed at 45°C. It was also demonstrated that the biofouling could be significantly reduced by incorporating cooling water treatment programs such as protective pre-filming and adding corrosion inhibitors. This apparatus, if installed on-site at a sidestream of a cooling water system, could be used to monitor the biofouling tendency on-line and to suggest timely preventive maintenance actions.