What is the likely reason for bubbling in filter #5 when backwashing filter #6 with closed influent and effluent valves?

The occurrence of bubbling in filter #5 while backwashing filter #6, particularly with both influent and effluent valves closed, suggests a malfunction in the washwater system. A faulty washwater valve can fail to properly manage how water flows during backwashing, potentially allowing air to be introduced into the system. When the washwater valve is not functioning correctly, it can lead to air entrapment, which manifests as bubbling in nearby filters, in this case, filter #5.

The other options address issues related to the physical condition of the filters or operational procedures. A clogged filter media would typically restrict flow and impede backwashing processes, not cause bubbling. Excessive backpressure is a concern during backwashing but is more likely to result in operational difficulties rather than bubbling in another filter. An overfilled filter may lead to overflow or improper filtration but does not specifically correlate with bubbling in this context.

By understanding the role of the washwater valve and the dynamics of backwashing, it's clear that a malfunction in this component directly contributes to the bubbling phenomenon in the adjacent filter.