Stability of the bacterial community in a pulp mill effluent treatment system during normal operation and a system shutdown.

Currently, very little is known about the normal dynamics of microbial populations in wastewater treatment systems and the relationship between population dynamics and functional stability of treatment systems. We monitored the bacterial community in an oxygen activated sludge system at a pulp and paper mill during a 55-day period that included normal operation as well as an 11-day shutdown of the system and the subsequent start-up. Ribosomal intergenic spacer (RIS) length polymorphism fingerprints were very similar (57-88% similar) throughout the study period. Analysis of clone libraries of RIS-rRNA gene amplicons indicated that Proteobacteria affiliated with the genera Paracraurococcus and Acidovorax as well as a Green Nonsulfur Bacterium affiliated with the genus Roseiflexus were consistently among predominant members of the community. By comparison, wastewater treatment systems from different pulp mills yielded dissimilar fingerprints (9-17% similar), and their clone libraries had distinct predominant phylotypes. Our analysis strongly suggests that the composition of the bacterial community in the former treatment system was stable during normal operation as well as the shutdown and start-up. This stability coincided with functional stability, including consistent and nearly complete removal of biological oxygen demand.