Which microorganisms are commonly found in a well-settling mixed liquor?

The correct choice identifies rotifers, stalked ciliates, and free swimming ciliates as microorganisms commonly found in well-settling mixed liquor. This mix typically signifies a healthy biological treatment process in wastewater management where microorganisms play a crucial role in breaking down organic matter.

Rotifers are microscopic animals that contribute to the biological treatment process by grazing on bacteria, thereby helping to control bacterial populations and promoting the growth of beneficial microorganisms. Stalked ciliates are particularly important in this ecosystem as they help to aggregate floc and settle well due to their structure, enhancing the sedimentation characteristics of mixed liquor. Free swimming ciliates also play a significant role in this environment by feeding on bacterial populations and maintaining the balance of microorganisms necessary for effective treatment.

In contrast, the other options contain microorganisms that may not primarily contribute to well-settling mixed liquor in the same significant way. While yeasts and bacteria are relevant to the biological treatment process, they don't specifically highlight the settlement characteristics as well as the ciliates do. Nematodes and protozoa, although they exist in these environments, are generally less critical in promoting the settling of mixed liquor compared to ciliates. Algae and fungi are important in various aquatic ecosystems but are typically associated with different conditions and