Abstract
MIC often contributes to corrosion in paper mills despite the seemingly inhospitable conditions for microbial growth. Molecular microbiological methods most notably quantitative polymerase chain reaction (qPCR) were employed to examine MIC at three paper mills with unique operations and construction materials. Despite raw water treatment, qPCR quantification of total bacteria and specific MIC associated microbial groups revealed growth of substantial and diverse microbial populations which had not been identified with cultivation based methods. Moreover, qPCR quantification of several microbial groups highlighted their roles in MIC. At most facilities including one experiencing corrosion of UNS S31254, iron oxidizing bacteria (IOB) were detected at high concentrations (1.00 × 108 cells/g). The actions of IOB were confirmed by x-ray diffraction analysis demonstrating production of iron oxyhydroxides (e.g. hematite). Fermenting bacteria were also routinely detected. Along with direct impacts, volatile fatty acids and hydrogen produced during fermentation support growth of other anaerobic microorganisms linked to MIC. Consistent with tubercle formation and biofilm maturation, sulfate reducing bacteria (SRB) and methanogens were detected in some solid phase samples. Overall, the qPCR results suggest biomass growth within the system, IOB activity and tubercle formation followed by proliferation of fermenters and eventually SRB and methanogens under the deposits.
Introduction
Corrosion in the modern paper mill accounts for as much as 30% of maintenance expenses and has been estimated to cost the pulp, paper, and paperboard industry an average of nearly $6 billion annually.1 With the emphasis on reducing water usage and the shift toward closed systems, the severity and frequency of corrosion related issues in the industry have only increased. Moreover, equipment taken out of service has frequently been upgraded from carbon steel to 300 series stainless steel with expectations of service lives of 20 years and lower life cycle costs. In many cases however, equipment service lives have actually decreased significantly.
