Under which condition should sludge wasting normally be increased?

The presence of a dark brown scummy foam on the surface of the aeration tank typically indicates that there may be excessive growth of certain types of microorganisms, such as filamentous bacteria. This condition can lead to poor settling characteristics of the sludge and can create operational problems such as an increased risk of sludge bulking. To mitigate these issues and maintain optimal conditions for treatment, it is necessary to increase the rate of sludge wasting. By doing so, the excess microorganisms contributing to the problematic foam can be removed from the system, helping to restore better performance in the aeration tank.

The other conditions presented would not directly necessitate an increase in sludge wasting. For example, if the aeration tank is operating below optimal temperature, this may affect the biochemical processes but does not directly indicate a requirement for more sludge to be wasted. A suspended solids concentration below acceptable levels suggests that sludge wasting might actually need to be decreased to allow for more solids retention. Lastly, consistently low pH levels might indicate a need for pH adjustment rather than an increase in sludge wasting, as this may not correlate with excess microbial growth.