Although biological wastewater treatment systems use microbial consortia containing a wide diversity of organisms across several trophic levels, information related to trophic information is currently limited. This study evaluated sludge characteristics in terms of mass measurement, microscopic observation, 16S and 18S rRNA gene sequencing approaches, and assessment of the natural abundance of stable isotope ratios. Lab-scale sponge units consisting of sponge media as a supporting material for sludge retention were fed with sewage at 25 °C for about 300 days. Microscopic observations confirmed that biotic community succession and a drastic increase in the number of organisms occurred. Moreover, 16S and 18S rRNA gene sequencing approaches showed a change in abundance of bacterial and eukaryotic organisms, including a wide-diversity of organisms at both lower and higher trophic levels. The isotopic analysis revealed that the stable nitrogen isotope ratio increased drastically from 0.9‰–2.7‰ on Day 83 to 5.8‰–8.6‰ on Day 298. The nitrogen stable isotope of sludge and diversity indices of bacterial and eukaryotic communities increased from high to low loading conditions. These results indicate that succession and trophic transfer took place within the bacterial and eukaryotic community under substrate competition and predation pressure in the sponge units. Further detailed analysis of microbial consortia with an emphasis on trophic status could provide a unique indication in the wastewater treatment process.
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