EFFECT OF CHLORINE AND CHLORINE BROMINE BIOCIDE TREATMENTS ON THE NUMBER AND ACTIVITY OF BIOFILM BACTERIA AND ON CARBON-STEEL CORROSION

The efficacy of NaOCl and a mixture of NaOCl and NaBr as biocides at reducing the numbers and metabolic activity of sessile bacteria on carbon steel in a moderately alkaline test medium were evaluated. The polarization resistances, measured by electrochemical impedance spectroscopy (EIS) of the carbon steel coupons, were evaluated prior to and during biocide treatments. Corrosion tubercles were observed on the carbon steel samples four days after inoculation with a consortia of five different physiological types of bacteria. NaOCl or NaOCl/NaBr was then added to the vessels containing the steel samples. Exposure to 2 ppm of residual chlorine or residual chlorine/bromine for two hours had little effect on the numbers of sessile bacteria or the bacterial metabolic activity, as measured by C-14-acetate incorporation into total lipids. Treatment for two hours with 16 ppm of residual chlorine or residual chlorine/bromine, followed by a 24-h treatment with 2 ppm of residual biocide, decreased sessile aerobic bacteria by two orders of magnitude and decreased sessile sulfate-reducing bacteria (SRB) by three orders of magnitude. Bacterial metabolic activity was decreased to the levels of the sterile controls following the 16 ppm treatment. Bacteria in the bulk solution were reduced from approximately 5 x 10(7) cells/mL to less than 1 cell/mL following the 16 ppm treatments. The carbon steel admittance increased when samples were treated with the high concentrations of biocides, compared to untreated controls. These results indicated that low levels of halogen biocide treatments may be ineffective at reducing sessile bacterial populations in moderately alkaline systems. Treatments with high concentrations of halogen biocide, though effective at reducing bacterial numbers and activities, increased the corrosion rate of the carbon steel.