Adaptation of biofilm communities in a feast-famine regime: implications for degradation of organic micropollutants

Feast-famine moving bed biofilm reactors (MBBRs) were found to be removing a number of organic micropollutants effectively from wastewater in previous studies. It was hypothesized that micropollutant-degrading organisms in the biofilm communities were possibly enriched by feast-famine selective pressure. We established a MBBR operated in feast-famine regimes (alternating influent/effluent wastewater) to test the hypothesis. The development of degradation kinetics of 36 micropollutants and the microbial communities in the biofilm were assessed simultaneously for 19 time points during the 70-day adaptation.

During this adaptation, 16S rRNA gene amplicon sequencing showed that the microbial communities shifted greatly from the initial biofilm composition in the first 8 days toward a more steady development afterwards. Ammonia oxidizing bacteria (Nitrosomonas) and nitrite oxidizing bacteria (Nitrospira) were strongly enriched (both > 18 % relative abundance at day 43), which led to high nitrification capability. Notably, the biofilm absorbed and nitrified ammonia during the feast regime, while releasing stored nitrate during the famine regime. Twenty-four out of studied 36 micropollutants showed enhanced reaction rate constants k (especially for propranolol up to 6600 %) during the adaptation. Maximum k values were observed between day 22 and 67 during the adaptation. DNA concentration in the biofilm was used as a proxy for biomass, and normalized reaction rate constants relative to the DNA concentration as k_DNA were used for understanding the degradation reaction rates of MPs per DNA concentration unit. During the adaptation, the DNA concentration continuously increased suggesting growth and accumulation of microorganisms. However, k_DNA of 21 micropollutants showed a decreased removal after day 11, which suggests the relative abundance of the respective degraders decreased while their absolute abundance increased. It suggests that the colonization rates of the MP degraders were slower than the non-degraders under the selective pressure of the feast-famine regime. By mining correlations between the microbial community and k_DNA of micropollutants, 88 operational taxonomic units (OTUs) belonging to different taxonomic groups were found to correlate significantly with removal rates of micropollutants (Pearson correlation coefficients, r > 0.5, p < 0.05). Thus, these identified OTUs are potential candidates as the degraders of the respective micropollutants. In summary, the feast-famine strategy was successful for enhancing the degradation of some compounds, but the feast-famine regime in this study was not successful in selecting microorganisms in biofilm with high removal capability for many micropollutants. Nevertheless, this study contributed to a better understanding of what occurred during the adaptation process of biofilms with potential for micropollutant degradation.