Florida Wastewater Practice Test

A low F/M ratio, high MCRT, low SVI, and high MLSS concentrations are all characteristic of?

1. New sludge

2. Old sludge

3. Primary sludge

4. Secondary sludge

The correct answer is: Old sludge

The characteristics mentioned—low Food to Microorganism (F/M) ratio, high Mean Cell Residence Time (MCRT), low Sludge Volume Index (SVI), and high Mixed Liquor Suspended Solids (MLSS) concentrations—are indeed indicative of old sludge. A low F/M ratio suggests that there is a minimal amount of food available for the microorganisms, which typically occurs in systems that have been operating for a longer period, allowing the microorganisms to grow and thus reach higher concentrations. This relationship highlights why you would expect older sludge to have a lower F/M ratio, as the microorganisms have adapted to lower food availability over time. The high MCRT indicates that the microorganisms are retained in the system for an extended amount of time before being removed, which is characteristic of older sludge. This retention allows for the population of microorganisms to mature and stabilize, which is critical for effective treatment. Low SVI is a sign of good settling characteristics, which is often found in mature activated sludge. Older sludge typically has well-flocculated particles, leading to a lower SVI value, indicating that the sludge will settle more effectively in the clarifier. High MLSS concentrations indicate a dense population of microorganisms in the mixed liquor, which aligns with the characteristics