Pea gravel at the top of the sand is another sign of channelling. This occurs during the backwash cycle if water cannot get from the top to the bottom. It is forced through the same channel and the gravel is pushed up from the filter floor to the top surface.
A core sample can also be taken following a visual inspection. It is important to ensure the top layer of the sand bed is catching the debris. The sand below this layer should be fairly clean. Any sign of mud in the lower level likely signifies a problem.
Why does channelling happen?
Channelling often results from improper backwashing. There may not be enough pump pressure to move the required volume of water, or the pool operator may be running the backwash filter cycle too short.
Short-cycling
The sand in the filter becomes dense and compacted when operators ‘short-cycle’ the backwash process. This is the most common cause of improper backwashing.
The objective of backwashing is to lift the sand bed and rinse out all dirt and debris. This keeps the sand loose and flat, allowing water to flow through and be filtered by clean sand.
Unfortunately, pool operators frequently use sight glass to monitor the backwash. This is not an effective method, as they will usually end the cycle if the water looks clear. As a result, the backwash cycle will be stopped before the prescribed three minutes, leaving debris behind in the sand.
To make matters worse, the same debris is able burrow deeper into the filter through the fluffed sand. From there, it acts like a glue and causes the sand to clump together. The debris combined with the weight of the sand compresses into a mud, which eventually turns into caliche.
Occasionally, a chunk of caliche may be seen at the top of the filter when a piece breaks off and the water forces it to the surface of the sand bed during the backwash cycle. Unfortunately, once the caliche forms, pool operators must enter the filter and remove it. The larger pieces of may need to be broken up with a hammer.
While three minutes of backwashing feels like a long time, it is required to prevent channelling and any of its related issues. To ensure proper backwashing, it is recommended to use a stopwatch and make sure the cycle runs its full course.
