The difference in removal efficiency observed between the two units immediately after the second spike and during the third spike may also be related to the different microbial communities present within the two schmutzdeckes. The removal of total coliforms progressively decreased within the schmutzdecke of unit B1 simultaneous to the decrease of Bacteroidetes and the increase of Gammaproteobacteria. On the other hand, the drop in bacterial removal observed in unit B2 was less severe. During this NH125 period, Proteobacteria and Bacteroidetes were the most abundant phyla. At the end of the study, low bacterial removal was observed in both schmutzdeckes, and was associated with the extremely low amount of Bacteroidetes (< 2%).
Methanol was used to prepare the stock solutions of carbamazepine, E1, E2, gemfibrozil and phenazone, while water was used to prepare the stock solution of caffeine. The presence of methanol during the first two spikes represented an additional carbon source in the feed water. During the second spike in which the six PhACs were simultaneously added, methanol represented the primary carbon source. A limited impact of methanol on the microbial community was expected during the first spike in which only caffeine and E2 were added as well as during the third spike in which only caffeine was added.