To help ensure the best water quality in a commercial swimming pool, aquatics facility managers need to ensure that the filters are operating in their peak condition.

Many facilities utilize sand filters because they are highly effective when they are backwashed properly and the flow rate is kept on target. Unfortunately, improper backwashing and flow can lead to poor water quality or worse, a sand filter filled with what is known as caliche, a rock-like material that must be chiseled out in order to get the filter back into working order.

To prevent a sand filter from becoming clogged with caliche, a professional must know about a phenomenon called “channeling” – how to identify it, understand how it occurs and what steps are required to prevent it.

Channeling 101

Understanding the basics of the sand filter operation can help safeguard against the problems that cause channeling.

When the system kicks into filtration mode, the diffuser at the top of the sand filter sprays water over the surface of the sand bed, allowing for even dispersion of the water. Then the water settles down through that sand and goes back into the pool, presumably cleaner than when it entered.

For these filters to operate correctly, the pool water must flow evenly throughout the sand bed. This enables the sand to properly filter out the debris in the water.

The term channeling is used by industry professionals to describe when pool water travels through channels that have formed in the filter sand, which prevents it from flowing evenly through the sand bed. These channels are smaller paths that look like valleys and gorges. When the sand becomes dense and compacted, it hardens, and the water is forced through a very small channel from the top of the filter down to the filter laterals without actually passing through the sand. Essentially, the water flows right down the center of the channel then down to the bottom of the filter and right back into the pool. When this occurs, the water is no longer being filtered, resulting in poor water clarity.

Often, operators can quickly identify when channeling begins to occur by performing a regular visual inspection of the sand. When looking down into the filter, you should see that the entire surface of the sand bed is flat. If you see valleys and gorges, that indicates that channeling has started.

In addition, if you see pea gravel at the top of the sand, this is another sign of channeling. This occurs because, during the backwash cycle, water can’t get from the bottom to the top. Water is forced through the same channel and the pea gravel is pushed up from the filter floor all the way up to the top surface of the filter.

Besides the visual inspection, don’t be afraid to stick your hand into the sand bed and take a sample – something like a core sample. Make sure the top layer of the sand bed is catching the debris. Below that, the sand should be fairly clean. If you have mud 6 inches down into the sand bed, you likely have a problem.

Why does channeling happen?

Most often, channeling occurs from improper backwashing. Either there isn’t enough pump pressure to move the required volume of water, or the pool operator is running the backwash filter cycle too short.

Short-cycling: Most often, the sand in the filter becomes dense and compacted when operators “short-cycle” the backwash process. This also makes up the most common cause of improper backwashing.

Remember: The objective of backwashing is to lift the sand bed and rinse all the dirt and debris out. This keeps the sand loose and flat, allowing water to flow through and be filtered by clean sand. Unfortunately, pool operators often use the sight glass to monitor the backwash. This is not an effective method: If the water looks clear, operators often end the cycle. But at the rate that water moves through a backwash system, golf balls could be flowing through and the operator would never see them in the sight glass! As a result, operators often stop the backwash cycle before the prescribed three minutes, leaving debris behind in the sand.

To make matters worse, because the sand bed has been ‘fluffed’ in the shorter-cycle process, that same debris can burrow its way deeper and deeper into the filter. This debris acts like a glue and causes the sand to clump together. The debris combined with the weight of the sand compresses into a mud that eventually turns into caliche. Occasionally you will even see a chunk of caliche at the top of the filter when a piece breaks off and the water forces the chunk to the surface of the sand bed during the backwash cycle. Unfortunately, once the caliche forms, it requires pool operators to enter the filter and remove it. The larger pieces of caliche may need to be broken up with a hammer to get them out of the filter.

Operators indeed say that three minutes of backwashing feels like a long time, especially as you watch water draining out of the pool. But three minutes is the time required to do the job correctly. To ensure proper backwashing, use a stop watch and make sure the cycle runs the full three minutes.

Excessive water flow: In general, a pool pump that pushes water through the filter too fast impedes the filters’ ability to perform its job correctly. As a rule of thumb, the faster the water moves through a filtration media, the worse job it does.

This also is another cause of channeling. If the water runs too fast and under too much pressure, it will come out of the diffuser at such a high rate that it will hit the side of the tank and run all the way down the side of the tank rather than spreading over the surface of the sand bed. If you were to look inside the filter when this is happening you would see a hump around the outside edge of the filter that can also lead to channeling.

To check the flow rate, be sure to attach a vacuum and pressure gauge to the pump then convert the vacuum and pressure reading to total dynamic head (TDH). Apply the TDH to the manufacturer’s curve for the pump, to give you the flow rate at its maximum with a clean filter. Make sure the maximum flow rate does not exceed the maximum flow rate for the filtration system, or you will end up with too much water flow through the filter.

To take control of the water flow, facilities benefit from having a variable-speed pool pump. This allows you to run the water at lower speeds, thereby maximizing the efficiency of the filtration system but then also allowing you to increase the speeds for backwashing.


To keep filters working in their peak condition, operators must prevent channeling in the sand media by ensuring proper backwashing and water flow through the filter. By doing frequent visual inspections inside the filter, running complete 3-minute backwashing cycles, and making sure the water flow from the pump is appropriate, commercial pool operators should be able to maintain cleaner and clearer water and avoid expensive filter repairs.

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