The presence of light, stiff, billowy white foam in an aeration tank typically indicates what condition?

The correct answer highlights a condition in the aeration tank where the presence of light, stiff, billowy white foam indicates a high food to microorganism (F:M) ratio. A high F:M ratio occurs when there is an abundance of organic matter relative to the available microorganisms in the aeration tank. This means that there’s more food for the microbes than there are microbes to consume it, leading to a situation where the bacteria can become overfed.

In this scenario, the excess food can promote the growth of certain types of microorganisms, particularly those that produce excess extracellular polymeric substances (EPS), which contribute to the formation of foam. The specific characteristics of the foam described — light, stiff, and billowy — suggest that it is a healthy foam produced under conditions of excess food.

To manage a high F:M ratio effectively, increasing the rate of wasting (removing biomass from the system) is generally advised. This helps to reduce the concentration of microorganisms relative to the food available, thus helping to re-establish balance in the aeration tank and improve treatment efficiency. Therefore, recognizing this foam condition as an indication to decrease the wasting rate is unsuitable, as doing so would exacerbate the imbalance caused by the high F:M ratio.