The response of a bacterial biofilm community in a simulated industrial cooling water system to treatment with an anionic dispersant

The effect of a dispersant on the microbial community in a simulated open r ecirculating cooling water system was determined by continuous operation of the system over two consecutive periods of 196 and 252 d, respectively. An open recirculating cooling water system feeding a modified Robbin's Device with synthetic cooling water to simulate the environment of an industrial cooling water system was set up. Planktonic and biofilm (mild steel and Nyl on(R)) samples were taken weekly in 1997 (196-d period) and fortnightly in 1998 (252-d period). Each biofilm was scraped off and diluted in 10-ml 1 x phosphate-buffered saline (PBS). Serial dilutions were performed and plated onto R2A agar (pH 8.0) to obtain the predominant culturable bacteria. The diversity was determined by allocating groups according to colony morpholog y, diameter and colour. Diversity was calculated according to the Shannon-W eaver Index. During the first run (1997), dispersant was added on day 57 to a final concentration of 15 mg l(-1) for 49 d, stopped for 49 d and dosed at 30 mg l(-1) for 41 d. The second run entailed adding dispersant to a fin al concentration of 30 mg l(-1) on day 98 for 70 d, stopping dosing for 56 d and resuming dosing at 30 mg l(-1) for another 28 d. The 2-year evaluatio n period demonstrated that the biofilm-removing action of the dispersant de creased to a point where it was not effective at all. Our results showed th at the synthetic dispersant evaluated was only effective initially, but was ineffective in controlling biofouling on Nylon(R), and to a lesser degree on mild steel at the recommended (15 mg l(-1)) as well as at double the rec ommended concentration in the long term. The release of cells from biofilms observed when dispersant dosing was terminated, supports the notion that a community attaching in the presence of the surface active agent was select ed for. The decreased efficacy may therefore be due to a selection of strai ns able to remain attached and/or attach in the presence of the dispersant as demonstrated by shifts in the biofilm communities on both Nylon(R) and m ild steel.