Nitrifying biofilms deprived of organic carbon show higher functional resilience to increases in carbon supply

In this study, we investigated the impact of acute and chronic changes in carbon supply on inter-guild competition in two moving bed biofilm reactors (MBBR), operated with (R1) and without (R0) external organic carbon supply. Two continuously nitrifying MBBRs were operated with three operating phases over 72 days. In phase 1 (day 1-52), reactors R0 and R1 were operated at influent C/N mole ratios of 0 and 1, respectively. In this phase, acute carbon inhibition tests were performed. Phase 2 (day 52-59) was designed to investigate the effect of an increase in the C/N ratio. On day 52, the C/N ratio in both reactors was increased to 3 by refilling both reactors with a new synthetic medium (C/N = 3) and resuming continuous operation with this new medium. In phase 3 (day 59-72), the medium flow rates to both reactors were doubled (on day 59), while maintaining the same influent C/N of 3. In this phase, we investigated the combined impact of a chronic increase in the C/N and the loading rate of ammonia and organic carbon. In phases 2 and 3, both reactors had similar operating conditions. Microbial communities were characterized by Illumina sequencing of v3+v4 amplicons.