What is the primary indication of poor settling sludge in an aeration tank?

The primary indication of poor settling sludge in an aeration tank is the presence of billowing white foam. This foam typically forms when there are high levels of suspended particles and microorganisms, which can prevent proper settling. When the sludge does not settle effectively, it can lead to an increased formation of foam due to trapped air bubbles within the mixture.

This billowing white foam is often associated with various operational issues, such as excess biomass or filamentous organisms, which can arise from nutrient imbalances or other conditions in the aeration tank. The presence of foam not only indicates poor settling but can also interfere with the treatment process by reducing the overall efficiency of the system.

While clear water above the tank suggests that some settling may be happening, it does not provide a complete picture of the sludge quality or settling characteristics. High volumes of sludge could indicate aggressive settling, but it doesn't directly point to the settling characteristics themselves. Low dissolved oxygen levels are crucial for the biological processes in the aeration tank, but they are not a direct indicator of sludge settling quality. Thus, billowing white foam serves as a clear and direct sign of poor settling conditions in the sludge.