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Filtration Operation: Back to Basics

By John Watt

Cleaning Pool Filters

There are several ways to clean filters. The most common cleaning procedure is by backwashing. Below are detailed best practices and tips for each filter type.

Sand filter backwashing: First you must determine proper backwash flow rate (15 to 20 gpm per square foot). For example, a 3.1-square-foot sand filter needs no less than 46.5 gpm and no more than 62 gpm. Too much water flow and you could potentially lift the sand bed and pass it to waste. In a worse scenario, you could damage the laterals or filter tank from the sandblasting effect as the water is introduced at the bottom of the tank at a high velocity. Not enough water flow, and you will not remove the debris from the sand bed completely. As a result, the debris that is still in the filter works its way deeper into the filter. When this happens, the debris in the bottom of the filter can turn to "caliche" and the filter then must be replaced because the "caliche" that forms often cannot be removed because of its size and difficulty to break up.

Another problem with backwashing sand filters occurs when the filter is backwashed too frequently in desert or dusty environments. In some cases silt can pass completely through the filter when it is clean. This results in calls from pool owners explaining they have sand in their pool and that additional sand comes out of the pool returns when they vacuum. In order to diagnose this situation, begin by asking if when you brush the pool does the "sand" makes a pile or a cloud in the pool. If it makes a pile there could be an issue with the filter. If it makes a cloud it is not sand, but rather silt. If it is silt, the problem is that the sand filter is actually too clean and therefore should not be backwashed. When a sand filter is very clean, it allows the larger particulate to pass through the filter. This is a sign to stop backwashing the filter, and allow it to load up and then begin to trap the silt into the filter to help with this a clarifier or flocking agent can be added.

Determining how long to backwash a sand filter should not be determined by looking at the water flowing out of the backwash line. Sand filter backwashing should be done in 3 minutes. Set a stop-watch for a 3 minute backwash cycle; don't guess.

DE filter backwashing: Unlike sand filters, backwashing DE filters frequently does not affect the filtration rate. However, it does create extra work and expense for service techs. DE filters should be backwashed for the same 3 minutes, but the cycle should be broken down into a 1-minute backwash, then a 1-minute filtration for three separate cycles (make sure you shut the pump off each time you switch between backwash and filtration). This procedure helps to separate the DE coating from the grids so it can be removed from the filter. A DE filter should be backwashed at the same 10 psi increase over normal operating pressure as any other filter media. A DE filter should be torn down, cleaned and degreased at least twice a year. Body lotions and sunscreen collect on the grid material and cannot not be backwashed away. A degreasing agent should be used to complete the filter maintenance process. It's important to note that muriatic acid will permanently lock the oils and lotions into the grid material and is therefore not recommended.

DE filter recharging: When recharging the DE filter, always follow the manufacturer's recommendation as to the amount of DE used to charge the filter. The proper way to charge the filter is to use a 5 gallon bucket then add water and DE together to create a slurry. Slowly pour the slurry into the skimmer while the pump is running. This will ensure that the grids or elements are properly coated from top to bottom.

Cartridge filter backwashing: If you have a backwash valve on your cartridge filter, something has gone awry. Cartridge filters are not designed to withstand water flow in the reversed direction. Therefore it must be taken apart to clean. A cartridge filter is cleaned in the same manner as a DE filter teardown, hosing the surface debris off the cartridge is only the first step (if it is your only step you might as well not bother). Body oils and lotions are the number one contributor to a plugged cartridge filter. Getting the oils and lotions off the cartridge surface is the difference between frequent cartridge replacement and cartridges that last and run a longer time between cleanings. The oily buildup on the surface plugs up the pores on the cartridge that restricts water flow and it also it creates a very sticky surface. Cartridge filters trap dirt on the surface and when the filter is turned off the debris falls to the bottom of the tank to free up more surface area for the next cycle. If the surface of the cartridge is sticky the debris does not fall away causing the filter to plug up in a very short period of time.

Many pool service professionals now offer cartridge filter cleaning services. Not only do they hose off the filter, they soak the filter with muriatic acid and a filter cleaning agent, then re-soak the filter with a liquid chlorine bath to brighten and lighten the filter (making them look nicer when returned to the client). Finally, the filters need to air-dry before using them again. Allowing them to dry completely gives the cartridge time for the fibers to fluff back up, which is important because the fibers need to expand to be effective. If they can be pushed down easily, it reduces the filter cycle. Many service companies suggest the pool owner own two sets of cartridges so the client can continue to use their pool while the other set is being cleaned. The most effective system is for the service tech to take the filter media when they close the pool for the season, clean it at their shop, then return the filters ready-to-use in the spring when they re-open the pool. Since pools are typically at their dirtiest during spring opening season, those cartridges are filtering more debris than usual, some pool service professionals change out the filters after the pool startup. Once the water is clear, they then install new filters to provide a more productive filtration process over the course of the summer. This also extends the lifespan of the new filter cartridges.


On large commercial swimming pools, perform manual weekly checks. An automatic controller can be used to set a backwash schedule based on psi reading. Therefore an automatic controller can be an invaluable addition to a pool's filtration system. With an automatic backwash controller, the need for a service technician to be present to perform this maintenance procedure—although recommended—is not necessary because the parameters set in the controller will automatic the backwashing process. Most backwash cycles occur when a 10 to 15 psi increase occurs -starting with a clean filter psi reading. Controllers can also be set to backwash on specific days of the week or times of the day.


Understanding the relationship between the pool pump and the pool filter gets to the heart of obtaining clean and clear pool water. Using filter pressure psi readings and understanding the way in which variable speed pumps affect filter pressure readings are also key to truly understanding how the filter is performing and when it's time to clean the filter. Once you have your filters and pumps sized correctly for one another, regular filter cleaning will help ensure the cleanest, clearest water for both commercial and residential swimming pools. Be sure you have your filtration operation in full swing this summer.

John "MacGyver" Watt has spent the past 25 years working for Pentair in a variety of positions, including field service technician, regional service manager, national trainer and product specialist. John currently works for Pentair's application engineering and new product development for the United States, Latin America and Canada. The nickname "MacGyver" was given to him early in his career at Pentair because of his ability to overcome challenges with whatever materials he had on hand at the moment.