Tennessee Wastewater Operator Practice Exam

At the start-up of an activated sludge process, which microorganisms are likely to be dominant in the mixed liquor?

1. Filamentous organisms

2. Amoeboids and flagellates

3. Bacteria and protozoa

4. Nematodes and rotifers

The correct answer is: Amoeboids and flagellates

In the start-up phase of an activated sludge process, the predominant microorganisms in the mixed liquor tend to be amoeboids and flagellates. This is primarily because these organisms are early colonizers of new environments and are capable of thriving in varying conditions, including low nutrient scenarios. Amoeboids contribute to the floc formation process, which is critical in the activated sludge system as it helps in settling and separating the solids from the treated effluent. Additionally, flagellates serve a vital role in the microbial community by consuming bacteria, thereby helping in controlling bacterial populations and enhancing overall microbial diversity. While filamentous organisms may develop as the system matures, they are typically not dominant at the start-up. Similarly, while bacteria and protozoa are present, the specific dynamics at the beginning favor the growth of amoeboids and flagellates. Nematodes and rotifers are more indicative of a stable, mature system and generally appear later as the process becomes more established.