# Protozoan Communities and Their Contribution to Predation on E. coli in Aerobic Granular Sludge

## TL;DR

This study explores how protozoa in wastewater treatment systems help remove harmful bacteria like E. coli, with larger granules showing higher predation efficiency.

## Contribution

The study identifies protozoan community distribution and their predation role in E. coli removal across different granule sizes in AGS systems.

## Key findings

- Sessile ciliates like Epistylis and Vorticella dominate protozoan populations in AGS size fractions.
- Larger AGS fractions (>1 mm) show higher predation capacity due to increased Epistylis abundance.
- Predation contributes an additional 0.5 to 2.5 log10 CFU mL–1 E. coli removal beyond non-predatory processes.

## Abstract

Protozoa contribute
to water purification through predation in
wastewater treatment systems. Full-scale aerobic granular sludge (AGS)
reactors treating municipal wastewater contain AGS of varying sizes,
with those larger than 2 mm dominating. These size fractions exhibit
different sludge morphologies and microbial communities. To date,
little is known about protozoan communities and their role in the
removal of human-associated bacteria (like pathogens) in AGS plants,
particularly across different size fractions. This study conducted
uptake experiments with fluorescent Escherichia coli, as a model for human-associated bacteria, followed by microscopic
observation to investigate protozoan communities and their predatory
behavior in six AGS size fractions and activated sludge collected
from full-scale municipal wastewater treatment plants. Sessile ciliates,
particularly Epistylis and Vorticella, dominated protozoan populations across
six AGS size fractions, with Epistylis being more abundant in larger AGS fractions (>1 mm) and Vorticella in smaller fractions (<1 mm). Additionally,
microcosm experiments under aerobic (including predation) and anoxic
conditions (excluding predation) revealed that predation was likely
to be the main E. coli removal pathway,
contributing an additional 0.5 to 2.5 log10 CFU mL–1 reduction over a combination of non-predatory biological
and abiotic processes. Larger AGS fractions showed greater predation
capacity, linked to higher Epistylis abundance, while activated sludge, dominated by Vorticella, resembled smaller AGS fractions with lower predation capacity.
These findings advance the understanding of the distribution of protozoan
communities and their contribution to E. coli removal by predation in AGS wastewater treatment.