Monitoring and control of biofouling in power utility open recirculating cooling water systems

Abstract

Surveys of open recirculating cooling water systems at 12 fossil fired power stations and their corresponding raw water supplies were carried out. It was established that all the raw water supplies and recirculating cooling waters contained aerobic and anaerobic bacteria, anaerobic acid producing bacteria, Thiobacillus, Nitrobacter, sulphate reducing bacteria and algae, with the exception of two potable water supplies. Analysis of the numbers of microorganisms as well as system inspections revealed that each system was unique and no generalisations in terms of presence or activity of microorganisms could be made. Biodispersant/biocide cooling water treatment programmes were monitored at four fossil fired power stations by means of microbiological analysis of Robbins Device biofouling monitors and bulk water. The use of combinations of biodispersants and biocides effectively controlled microbiological growth in all the cooling water systems, but the treatment products produced different effects in different systems. A Robbins Device, modified Robbins Device, a Pedersen Device and the Barry's Device biofouling monitors were evaluated in an open recirculating cooling water system during three different biodispersant dosing regimes. Statistically significant differences were found in the numbers of bacteria recovered from the devices with the exception of the aerobic bacteria when no biodispersant was added and the H2S producing bacteria when biodispersant was slug dosed. Consistently higher numbers of bacteria were recovered from the modified Robbins Device. A corrosion monitoring device, using the linear polarisation technique was evaluated to determine its suitability as a technique for the monitoring of sessile microorganisms. Bacterial attachment occurred uniformly on the electrodes of the device and on corrosion coupons. The addition of a biocide to the bulk water of two pilot rigs resulted in a statistically significant decrease in c01Tosion rate in these two rigs when compared to the untreated controls.