During backwashing, if the surface wash is barely rotating with both valves closed, what is a probable issue?

When the surface wash is barely rotating while both valves are closed during backwashing, it's indicative of a problem with the washwater valve. A properly functioning washwater valve should allow for an adequate flow of water to ensure that the surface wash can rotate effectively and dislodge any accumulated debris from the filter media. If the valve is faulty, it may not be opening fully or could be obstructed, resulting in insufficient water being sent to the surface wash system.

The other options point to issues that would not specifically lead to the surface wash barely rotating with both valves closed. An insufficient water supply would typically result in low flow throughout the system but wouldn't specifically indicate that the wash water was barely rotating in the presence of closed valves. Similarly, filter media failure or clogged piping would manifest in different ways, such as a reduced flow or decreased effectiveness of the filter overall, rather than directly impacting the operation of the surface wash mechanism when both valves are closed.