During MP operation, the abundance of Proteobacteria also decreased,stabilizing after 19 months at 11.4–14.9% of total reads. In addition to b-proteobacteria, decreases in the relative numbers of a-, c- and e-proteobacteria were noticed, for each class from approximately 7% of total reads at the beginning to less than 2.5%, 1.6% and 0.4%, respectively, after 37 months of MP operation.