In wastewater treatment, what happens when excess clogging occurs?

In wastewater treatment systems, excess clogging can significantly impact the flow dynamics within the system. When a clog occurs, it creates a restriction in the flow path. This leads to a decrease in the available cross-sectional area for the wastewater to pass through. As a result, the pressure within the system (or the head) must increase to overcome this restriction and maintain flow.

However, once the clogging becomes excessive, the flow can become unstable. In many cases, the head required to push the wastewater through the restrictive segment can exceed the pump and system capacity, resulting in a decrease in effective flow movement. This can lead to conditions where the pressure decreases at the pump inlet due to increased friction losses and turbulence caused by the clog.

Ultimately, as head decreases—due to the inability of the pumps to maintain the pressure needed to push through the clog—velocity can effectively increase at the point of restriction as water is forced to accelerate through a narrower opening. But this increase in velocity occurs under conditions of reduced head, which is contrary to the assumptions in the other options where head and velocity behave differently or remain constant despite the clogging.

Thus, the situation created by excessive clogging aligns with the answer indicating a decrease in head coupled with an increase in