Thermophilic and mesophilic bacteria in biofilms associated with corrosion in a heat exchanger

Biofilm development on AISI-1020 carbon steel coupons installed at the outlet of a heat exchanger was evaluated at the thirtieth and the sixtieth days of exposure. Water temperature varied between 41 and 60 degrees C. The most probable number technique (MPN) was applied to quantify mesophilic and thermophilic species of aerobic, anaerobic, and sulphate-reducing bacteria (SRB) in planktonic and sessile phases. The results showed predominance of thermophilic aerobic bacteria in both phases, corresponding to 9.5 +/- 0.8 x 10(6) cells/ml in the planktonic phase. In biofilms, maximal aerobic cell concentration, 7.8 +/- 0.6 x 10(8) cells/cm(2), was registered at the sixtieth day. An increase in the number of thermophilic anaerobic and SRB with elapsed time was also observed. The results obtained after 60 days were 5.8 +/- 0.4 x 10(7) and 8.9 +/- 0.9 x 10(4) cells/cm(2) for anaerobic and sulphate-reducing bacteria, respectively. Scanning electron microscopy showed a varied composition of species in the biofilms and corrosion on the carbon steel surfaces after biofilm removal.