How do protozoans help remove bacteria and particles during wastewater treatment?

In wastewater treatment processes, protozoans—particularly large ciliated communities—are essential for producing high-quality, clean effluent. They contribute to the removal of bacteria and particles through two primary mechanisms:1

• Grazing: Ciliated protozoans graze voraciously on dispersed bacteria that remain in the liquid after initial solid waste removal. This process is critical because treatment plants lacking these protozoans often produce turbid effluents with high levels of suspended solids and bacteria.1
• Flocculation: Protozoans possess the ability to flocculate, or aggregate, suspended particulate matter and bacteria. By forming these loose aggregates, the materials become easier to separate from the liquid during the final stages of the treatment process, where solids settle out in a settlement tank.1

In activated sludge systems, these protozoans specifically consume aerobic bacteria that have already flocculated, while in percolating filter plants, they serve as oxidizing agents that feed on the microorganisms—such as fungi, algae, and bacteria—that inhabit the substrates.1