California Wastewater Treatment Operator Practice Exam 2025 - Free Wastewater Treatment Operator Practice Questions and Study Guide.

There are several possible factors that could lead to thick billows of white sudsy foam in the aeration tank. However, option D, “MLSS is too low,” is the most suitable choice because it explains the underlying cause of the problem. Low MLSS (mixed liquor suspended solids) levels means that there are not enough microorganisms present in the aeration tank to consume the organic matter and other dissolved solids in the wastewater. As a result, these substances are not fully broken down and can create a buildup of foam on the surface of the tank. Option A, "High MLVSS concentration," describes a different situation where there are too many microorganisms present, which would typically result in a thick sludge rather than foam. Option B, "Inadequate aeration," could also potentially result in foam, but this would likely be caused by an excess of organic matter in the tank, not a lack of microorganisms. Option C, "Low pH levels," may cause some foaming, but it is not the primary factor in the formation of thick, billowing foam in the aeration tank.