# Similarity of activated sludge and treated wastewater with special reference to nitrifiers and their seasonal variability

## TL;DR

This study compares the microbial communities in activated sludge and treated wastewater, focusing on nitrifying bacteria and how they change with seasons.

## Contribution

The study introduces a method using 24-h composite samples and highlights seasonal variability in nitrifier populations as potential indicators of wastewater treatment changes.

## Key findings

- Using 24-h composite samples provides more representative microbial profiles in treated wastewater.
- Biodiversity decreases in colder months in both activated sludge and treated wastewater.
- Nitrifying bacteria show seasonal differences, suggesting they may indicate changes in treatment processes.

## Abstract

While the research into the activated sludge (AS) microbiome using next-generation sequencing (NGS) are increasingly being published, they still apply to a few wastewater treatment plants (WWTPs). Little information can be found in the scientific literature on the microbial composition of treated wastewater (TW) and bacterial activity in real WWTPs facilities. In this study, AS and TW are compared in terms of their physico-chemical parameters and bacterial community composition, with particular emphasis on nitrifying bacteria, based on the results from 16S rRNA amplicon sequencing using DNA and complementary DNA (cDNA) data and fluorescence in situ hybridization (FISH). Using a 24-h composite sample of TW rather than a grab sample yields highly representative microbial community profiles. The research revealed a decrease in biodiversity in months with lower sludge temperatures both in AS and TW. Differences among groups of nitrifying bacteria were observed during the monitoring of WWTP, suggesting that their presence in TW may not be a random occurrence but could serve as an indicator of physicochemical and operational changes within biological reactors.

The online version contains supplementary material available at 10.1038/s41598-025-34503-4.