What is the most likely cause of pitting observed in the impeller of the RAS pump?

Pitting observed in the impeller of the Return Activated Sludge (RAS) pump is most commonly attributed to cavitation. This phenomenon occurs when there are rapid changes in pressure within the pump, leading to the formation of vapor bubbles in the liquid. When these bubbles collapse or implode, they create shock waves that can erode the metal surfaces of the impeller, resulting in pitting.

Cavitation is particularly concerning in pumps because it not only affects the efficiency and performance of the pump but also can lead to significant physical damage over time. The high-velocity impact of collapsing bubbles can lead to localized wear and tear, which is visually evident as pitting.

In contrast, the other options do not typically cause pitting in the same manner. Corrosion from chemicals may lead to general degradation of surfaces but does not usually manifest as pitting caused by pressure changes. Overheating can damage materials but is more likely to warp or stress components rather than create pitting directly. Physical damage, while it can occur, is usually the result of external factors rather than operational phenomena like cavitation. Thus, understanding cavitation's role in pump operation is crucial for identifying and preventing the pitting of impellers.