Long-term dynamics of the microbial community in a full-scale wastewater treatment plant

• Structure of bacterial communities in full-scale WWTP was analysed.
• β-Proteobacteria, Actinobacteria & Chloroflexi formed 50% population regardless season.
• ANOVA showed that abundance of only few bacterial groups was differentiated by season.
• Total N was more decisive factor than BOD5 or COD in determining bacterial abundance.
• Substrate affinity of each ecophysiological bacterial group should be considered.

Fluorescence in situ hybridization was applied to investigate the microbial population dynamics in activated sludge of a full-scale wastewater treatment plant over a two-year-period. Statistical analysis, using Pearson's and Spearman's correlation coefficients, was performed to find relationships between quantified bacteria and environmental data. Differentiating factors were tested using analysis of variance (ANOVA). Betaproteobacteria, Actinobacteria and Chloroflexi were the most abundant bacteria in the activated sludge (on average 23%, 15% and 12% of all bacteria, respectively), but the relative percentages of these bacteria varied widely between samples. Alphaproteobacteria, Gammaproteobacteria, Nitrospira and Saprospiraceae accounted for 6%, 5%, 3% and 3%, respectively. Average percentages of Firmicutes and Deltaproteobacteria did not exceed 1% nor correlated with operational data. ANOVA showed that the abundance of only Actinobacteria, Saprospiraceae and Alphaproteobacteria were differentiated by season. The concentration of total nitrogen in the influent was a more decisive factor in determining the relative proportions of the predominant bacteria than BOD5, COD or sludge loading with organics. The percentages of Proteobacteria and Actinobacteria increased with total nitrogen concentration in influent, while the abundance of Chloroflexi decreased. Instead of using one parameter such as COD or BOD5, the concentration of different substrates in the inflowing wastewater should be considered.