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The Dirt on Filter Sand

Sand filters have always been an industry mainstay. Here’s what techs need to know about taking care of them.

adapted from a PoolSpaNews article by Bob Dumas

There are several reasons for the sand filter’s staying power. First, it’s relatively easy to use and maintain. Second, it works well in pools with high bather loads, which makes it ideal for public and commercial installations.

Still, like any piece of pool equipment, sand filter problems occasionally arise. They usually manifest in one of three symptoms: shorter cycles between backwashing, water clarity issues and sand returning to the pool. 

Symptom 1: Short filter cycles

When the time between backwashing keeps growing shorter, the filter is telling you something is wrong. This could be happening for various reasons, ranging from water chemistry problems to equipment sizing issues.

Bad chemistry

If the pool water isn’t balanced, it can affect filtration. For example, if the pH is too high, calcium buildup can occur and overtax the filter. If chlorine levels are too low, algae blooms can appear and clog the sand bed.

Thus, before placing blame on the filter, veteran techs say you should always make sure the water is balanced.

You also need to balance the water according to the Saturation Index and change the sand if necessary. If this doesn’t fix the problem, turn your attention to the filter.

Dirty sand

This is probably the most common cause for short filter cycles. The sand has become clogged with oil and dirt, which has reduced the viable filtering area. Normally, this is something that only happens every three to five years.

“When the sand gets old, it starts to pack and harden up,” Jarden says. “This will decrease the amount of filtering area.

Another thing is that sand starts out with jagged edges and it’s what traps the particles. After a time, those edges start to round off and the sand won’t filter as well.”

Changing out the old medium and adding new sand remedies the problem.

Worn-out O-rings or gaskets

Regardless of how well a filter is maintained, gaskets and O-rings will wear out over time. When this happens, water can bypass the filter and head directly back to the pool, causing cloudiness. It also can cause leaks, resulting in water loss.

Check the spider gasket in the multiport valve or the O-rings in the push-pull type of backwash valve.  Then take note of any excessive wear and tear, and replace as needed. 

Water chemistry issues

High pH and/or total alkalinity, which can cause cloudy water, make the filter work harder. This may lead to other problems. Also, low chlorine levels can permit algae to thrive and add a green- or rust-colored tint to the water. Spores from algae blooms can accumulate in the medium and mitigate the filtration process.

One trick to reduce the risk of algae is to remove some of the organic material that has built up on top of the filter’s sand bed.

“The top 1 inch of sand has been covered with suntan lotions, baby oil and other things that are channeling down the walls and not being filtered at all,” Scallorn says. “Stick your hand in there and stir
it up. On a side-mounted tank, I put it in backwash mode, [insert] my hand and stir it while it’s backwashing out the top of the tank.”

Symptom 3: Sand in the pool

When a significant amount of sand is finding its way back into the pool, something may be wrong with the filter.  Consider these possibilities:

Laterals gone bad

Broken laterals are one of the primary causes of 
sand flowing back into the pool. This will require replacing the damaged parts. The sand will have to be removed and the laterals inspected. The laterals are the horizontal tubes that radiate from the center of the interior filter assembly where the air tube is located.

Dislocated air tube

When sand appears in the pool in the wake of a backwashing, a displaced air tube is likely the culprit. Open the filter and check to see that the tube is still properly secured. This job can usually be done without removing all the sand.

Pump is too large

If the pump is oversized, it can break up sand particles and allow them to pass through the laterals or the drain. It can also force the water right through the medium without proper filtration. In such cases, the pump needs to be replaced with a smaller one.

A look inside: The sand in this filter hardened so much because of improper water balance that it was as hard as cement and could not be removed without destroying the filter.
Improper backwashing

You need to allow enough time for sufficient backwashing. Experts say that two to three minutes should be adequate. “Most [sand filters] have a sight glass and that’s how I know when the backwash is complete — when it runs clear,” Scallorn says. “But you have to run it long enough.

“If you fail to do it long enough on the backwash and rinse cycles, you could end up blowing a cloud of dust back into the pool,” he adds. Some service pros recommend cycling the pump on and off two or three times during the process to get rid of any additional waste.

Insufficient flow​

This might not necessarily be the fault of the system, but it will result in poor filtration nonetheless. Industry standards call for 20 gpm per square foot of filter space to backwash a sand system correctly. If the flow is less than that for the filter and backwash cycles, a stronger pump and/or bigger pipes should be installed.

“If you push [the water] too slowly, it won’t pack the filter [medium] properly,” Jarden says.

“In turn, when you backwash and there is not enough force to lift the sand and loosen the dirt, the filter won’t clean as well.”

Symptom 2: Water clarity issues​

As previously mentioned, cloudy water can be a chemistry problem. But once that variable has been eliminated, you can focus on the filter as the source of the issue. It could be as simple as changing out the sand, but consider these other possible causes if the pool water still isn’t clear: