Florida Wastewater Practice Test

If the settleometer shows slow settling and there is shiny dark foam on the aerator, what might be the issue?

1. MCRT too low

2. MCRT too high

3. Insufficient aeration

4. Excessive food-to-microorganism ratio

The correct answer is: MCRT too high

A settleometer showing slow settling in conjunction with shiny dark foam on the aerator typically indicates issues related to the microbial activity and health within the wastewater treatment system. When the MCRT (Mean Cell Residence Time) is too high, this suggests that the microorganisms are remaining in the system for an extended period, leading to an accumulation of older cells that may not be effective in treating the wastewater. As microorganisms age, they can become less vigorous and their settling characteristics can degrade, leading to slower settling processes. The presence of shiny dark foam often points toward the overgrowth of certain types of microorganisms—often filamentous bacteria—which can thrive in environments where the MCRT is prolonged. These filaments can trap air and contribute to the foamy appearance, indicative of an imbalance in the microbial community. In this context, maintaining the proper MCRT is essential for ensuring that a balanced and effective microbial population is present to process the incoming waste and aid in successful settling. Thus, a high MCRT contributes to inefficiencies in the overall treatment process.