Conclusion

By using different ordination methods it was possible to identify general differences in the microbial communities of 32 Danish WWTPs based on 16S rRNA amplicon sequencing. Principal Components Analysis (PCA) and Principal Coordinates Analysis (PCoA) of Bray-Curtis dissimilarities both revealed largely overlapping clusters of samples from the individual WWTPs, indicating considerably similar microbial communities between large groups of samples from different WWTPs. General differences between clusters of WWTPs were evident in most of the ordination methods used to analyse the samples, where groups of similar WWTPs were different from other groups of WWTPs. The differences were considered to be the result of a combination of both quantitative and qualitative differences, where both variation in the read abundances of common OTUs and the presence of unique OTUs were important to consider. The latter was evident in Canonical Correspondence Analysis (CCA), where the Ribe and Esbjerg E+W WWTPs seemed to have several unique OTUs, which were absent in the other WWTPs. The 5 most abundant genera (65 OTUs) in all samples were found to be Tetrasphaera, Candidatus Microthrix, Trichococcus, Rhodobacter, Rhodoferax which together made up roughly 20% of the total number of reads, and specifically Tetrasphaera was generally abundant in all 32 WWTPs.

The correlation between variation in the microbial communities and four different design characteristics of the WWTPs were investigated and were found to be significant (P<0.001). According to CCA, WWTPs with a high amount of industrial wastewater in the influent were also found to have a slightly different microbial community composition than those with a lower content. Furthermore, variation in the microbial communities were observed over the years 2006-2015, where the year 2014 were significantly different from the other years, presumably due to different laboratory handling of the samples. Lastly, general differences in the microbial communities of all 32 WWTPs as explained by which time of the year the samples were taken, were minor with no particular bacteria corresponding to each seasonal period.