Predominance of free-swimming ciliates and flagellates in activated sludge indicate?

The predominance of free-swimming ciliates and flagellates in activated sludge indicates a high food-to-microorganism (F/M) ratio along with a low solids retention time (SRT). In the context of wastewater treatment, a high F/M ratio means there is a sufficient amount of organic material available for microorganisms to consume, promoting rapid growth.

When there is an abundance of nutrients and organic matter, it creates an environment that favors the reproduction of free-swimming protozoa like ciliates and flagellates, as they thrive in these conditions. They play an essential role in the biological treatment process by consuming bacteria and organic material and can help maintain a balanced microbial community within the activated sludge.

Conversely, a low SRT implies that the activated sludge is not retained in the system for a long time, which can lead to a less stable microbial community composed mainly of protists that flourish quickly in nutrient-rich environments.

Therefore, the presence of these organisms indicates dynamic biological activity where the system is well-fed with substrate, leading to their proliferation. This scenario aligns with the characteristics associated with a high F/M ratio and low SRT, confirming why this answer is accurate.