Commercial Sand Filtration

by brittney_cutler_2 | January 23, 2022 8:30 am

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Ensuring cleaner and clearer water for pools

By John Watt

Every aquatics facility manager values the importance of cleaner, clearer pool water. To help ensure the best water quality in a commercial pool, the equipment needs to be operating in peak condition. Properly functioning sand filters play a key part in keeping the water in top shape for patrons.

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

To prevent a sand filter from becoming clogged with caliche, professionals must know about a phenomenon called channelling, including how to identify it, how it occurs, and the steps required to prevent it.

What is channelling?

Understanding the basics of sand filter operation can help ensure against the problems that cause channelling.

When the system kicks into filtration mode, the diffuser at the top of the filter evenly sprays water over the surface of the sand bed. The water then settles down through the sand and goes back into the pool, presumably cleaner than when it entered.

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

Channelling occurs when water travels through smaller paths in the filter sand, which prevents it from flowing evenly through the sand bed. 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 laterals, without actually passing through the sand.

Essentially, the water flows right down the centre of the channel to the bottom of the filter and right back into the pool. When this happens, the water is no longer being filtered, resulting in poor clarity.

Operators can identify when channelling begins by performing a regular visual inspection of the sand. When looking down into the filter, the entire surface of the sand bed should be flat. If valleys and gorges are present, channelling has started.

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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?

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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.

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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.

Excessive water flow

Accelerated water flow can also cause channelling. If the water runs too fast and under too much pressure, it will come out of the diffuser, hit the side of the tank, and run all the way down—rather than spreading over the surface of the sand bed.

To check the flow rate, it is recommended to attach a vacuum and pressure gauge to the pump, then convert the vacuum and pressure reading to total dynamic head (TDH). Applying the TDH to the manufacturer’s pump curve will determine the flow rate at its maximum with a clean filter. If the maximum flow rate exceeds that of the filtration system, there will be too much water flow through the filter.

To take control of the water flow, facilities will benefit from having a variable-speed pump. This will allow pool operators to run the water at lower speeds, thereby maximizing the efficiency of the filtration system and increasing the speed allowance for backwashing.

Conclusion

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

John “MacGyver” Watt has spent the last 25 years working for Pentair in a variety of positions, including field service technician, regional service manager, national trainer, and product specialist. He now works for Pentair’s application engineering and new product development for the U.S., Canada, and Latin America. He can be reached via email at john.watt@pentair.com.

Source URL: https://www.poolspamarketing.com/trade/features/commercial-sand-filtration/

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