Dark, greasy-looking foam in the aeration basin is commonly associated with which condition?

• A. High MLSS, sludge age, and low F/M
• B. Low MLSS and high F/M
• C. High DO and short sludge age
• D. Presence of nitrifying bacteria only

Answer: (A)

Foaming in the aeration basin tends to occur when there is a high solids concentration in the mixed liquor along with a long sludge age, which lowers the food-to-microorganism ratio. With a high MLSS and extended sludge retention, the microbial community shifts toward organisms that produce more extracellular polymers and filamentous forms. These substances stabilize foam and can trap fats and greases from the wastewater, giving the foam a dark, greasy appearance. In this situation, the low F/M ratio encourages these foam-forming organisms to dominate.

If the solids are low and the food supply per microorganism is high, there’s less tendency for stable, greasy foam to develop. A high dissolved oxygen level with a short sludge age also reduces the opportunity for filamentous organisms to proliferate and stabilize foam. Simply having nitrifying bacteria present doesn’t by itself explain dark greasy foam; it’s the combination of high MLSS, long sludge age, and low F/M that best fits this condition.