Florida Wastewater Practice Test

What is the major cause of repeated sludge bulking in most activated sludge treatment plants?

1. High F/M loading rate and low MCRT

2. Low F/M loading rate and high MCRT

3. High MLSS concentration

4. Low SVI values

The correct answer is: High F/M loading rate and low MCRT

The major cause of repeated sludge bulking in activated sludge treatment plants is primarily associated with a high food-to-microorganism (F/M) loading rate combined with a low mean cell residence time (MCRT). When the F/M ratio is high, it indicates that there is an abundant amount of organic matter available for the microorganisms in the system. In such conditions, microbial populations can grow rapidly, but if the MCRT is low, there isn’t enough time for the microorganisms to settle effectively. This imbalance can lead to buoyant, filamentous bacteria dominating the biomass, which inhibits proper settling and results in sludge bulking. Proper management of both F/M ratio and MCRT is essential to maintain a healthy activated sludge system and prevent this issue. On the other hand, factors such as low F/M loading rates or high MCRT often promote stabilization of the sludge and can prevent bulking conditions. High mixed liquor suspended solids (MLSS) concentration can also be beneficial for settling if managed correctly, and low solids volume index (SVI) values typically indicate good settling characteristics, which are contrary to the conditions that cause bulking. Thus, understanding and controlling the dynamics of F/M loading and MCRT is crucial for preventing repeated sludge bulking