If the settleometer shows slow settling, and there is shiny dark foam on the aerator surface, what could be the problem?

When the settleometer shows slow settling and there is shiny dark foam present on the aerator surface, it indicates potential issues with the biological treatment process. A high Mean Cell Residence Time (MCRT) suggests that the microorganisms within the system have been retained for an extended period. This can lead to excessive growth of microorganisms, resulting in a population that produces lighter, less dense flocs that do not settle well.

The shiny dark foam can be an indicator of rising biomass, particularly filamentous organisms that are commonly associated with a high MCRT. These filamentous bacteria can cause foam to form on the surface, affecting the efficiency of aeration and separation processes. Hence, a high MCRT can result in a situation where the solids do not settle well in the clarifier, leading to operational challenges.

In this context, the interpretation of slow settling paired with the observation of foam aligns with the consequences of having an excessively extended MCRT, making it the most plausible diagnosis among the choices presented.