What is the term for a depression created by turning the RAS up too much?

The term "ratholing" refers to a condition in wastewater treatment where excessive return activated sludge (RAS) flow causes a depression or a hollow area in the sludge blanket within sedimentation tanks. This typically occurs when the RAS is set too high, leading to an imbalance that disrupts the settled sludge layer. When ratholing happens, it can negatively affect the separation of solids and liquids, thus reducing the efficiency of the treatment process.

In contrast, backflow pertains to liquids flowing backward through a system, which is not directly related to the RAS setting. Foaming involves the formation of foam on the surface of wastewater treatment facilities, which is a separate phenomenon typically associated with increased surfactants or biological activity. Siphoning refers to the movement of liquids through a pipe due to pressure differences, not stemming from RAS adjustments. Understanding these distinctions helps clarify why ratholing is specifically associated with problems resulting from improper RAS flow management.