How Does a Pool Filter Work?

Posted by
John Woodard on September 01, 2023

Pool filters provide year-round safety and clarity to your swimming pool water. Without a filter, a pool poses risks of waterborne disease each time you swim. Not only would the pool be dangerous to swim in, but the water would also be murky, slimy, and unpleasant to look at. Because of the dangers of unclean pool water, a pool owner is responsible for the correct installation and upkeep of an effective pool filtration system. In this article, you can learn about the types of pool filters, how they work, and the pros and cons of each.

How does a pool filter work?

A pool filter removes dirt and debris from pool water by running it through a filter media of synthetic fabric, sand, or diatomaceous earth. Pool filters work in conjunction with pool chemicals to keep water clear and safe. While chemicals prevent the growth of bacteria and the spread of viruses in pool water, pool filters remove cloudiness, dirt, and debris that can also contribute to bacterial growth. Pool filters and chemicals are codependent. If a pool filtration system does not operate correctly, excess dirt can cause imbalances in the pool’s chemistry, leading to bacteria and algae growth. Likewise, imbalanced pool chemistry can damage pool filters, pumps, and the pool walls and floor.

What are the types of pool filters?

The three types of home pool filters are cartridge, sand, and diatomaceous earth (DE). The most common type of pool filter used in homes is sand because of its inexpensiveness. However, cartridge filters are common in areas where backwashing, the process of cleaning out a sand or DE filter, is prohibited. For many pool owners, a sand filter will provide adequate filtration to keep their water clean. For those who want more advanced filtration, cartridge and DE filtration are both excellent options.

Learn more: What is the best type of pool filter?

Pool filters: cartridge vs sand vs diatomaceous earth

Pool cartridge filters, sand filters, and diatomaceous earth filters are all effective forms of home pool filtration, but they each possess advantages over the others.

How cartridge filters work

A cartridge pool filter is a layer of synthetic fabric wrapped around a cylindrical core. The fabric on a cartridge filter is folded to allow for maximum filter surface area. As water passes through the filter, dirt and debris cannot pass through the fabric, resulting in cleaner output water. Most cartridge filters possess a micron rating between 5 and 20, meaning they can filter out debris as small as 5 to 20 micrometers, respectively. A pool filter cartridge must be cleaned at least once every 3 to 4 months, but they do not require backwashing like sand and DE filters. Rather, they can simply be hosed off to clear the fabric of trapped debris. The frequency of cartridge filter maintenance depends on the pool size and how much sunscreen and debris the water is subjected to. A cartridge pool filter can last anywhere from 1 to 3 years. To elongate the life of a pool filter cartridge, you must take care to allow as little debris into the pool as possible.

Learn more: Maintaining a pool filter cartridge

Advantages of cartridge pool filtration

  • Removes smaller particles than sand filtration
  • Does not affect pool water chemistry
  • Large filter surface area
  • Does not require backwashing

Disadvantages of cartridge pool filtration

  • Cartridges must be replaced twice as often as sand
  • Not possible with very large pools
  • Cartridges must be washed

How sand filters work

Pool sand filters use pressure to move water through a tank filled with sand. The jagged edges of sand grains filter out dirt and debris as the water flows through the tank. Sand filters typically contain a micron rating between 20 and 100, the largest among all pool filter types. When enough dirt and debris is trapped in the sand, it must be cleaned through a process called backwashing. The sand in a sand filter must be replaced when the grains become smooth and rounded, about once every 3 to 5 years. The lifespan of sand in a sand filter depends on how often the filter runs and how clean the pool is kept. Sand filtration is the least expensive of the three types, but it is also the least effective.

How to backwash a pool sand filter

Backwashing a sand filter is an essential maintenance step in keeping your pool water clear and your pump running smoothly. It involves running water backward through your sand filter to dislodge dirt and debris that can clog the system. You should backwash your sand filter at least once per month during the pool season or any time your filter pressure is 10 PSI or greater above its normal operating pressure. Follow these steps to backwash your pool’s sand filter:

  1. Switch off your pump at the breaker.
  2. Connect your backwash hose to the filter waste port.
  3. Roll out the backwash hose to an appropriate drainage area.
  4. Set the multiport valve to backwash.
  5. Switch the pump back on.
  6. Allow the pump to run until the water is clear.
  7. Switch the pump off and set the multiport valve to rinse.
  8. Switch the pump on and let it run for about 30 seconds.
  9. Switch the pump off and set the multiport valve to filter.
  10. Top off the pool’s water.
  11. Rebalance your pool’s chemicals.

Learn more: How to accurately test pool water

Advantages of sand pool filtration

  • Least expensive
  • Low maintenance
  • Sand only needs changing every 5-8 years

Disadvantages of sand pool filtration

  • Lower micron rating than cartridge filters
  • Backwashing impacts water levels and chemistry
  • Small surface area

Pool sand filter

How diatomaceous earth filters work

A diatomaceous earth pool filter uses grids coated in diatomaceous earth powder to filter fine particulate matter. Like a sand filter, a DE filter removes impurities from water with a tank filled with a filter medium. However, rather than using sand, DE filters utilize diatomaceous earth powder on a cotton grid. While sand filters possess a micron rating of between 20 and 100, DE filters possess a much lower micron rating of between 1 and 6. This is the smallest micron rating among all pool filter types, allowing for the highest filtration performance among commercial pool systems.

DE filters must be backwashed at least once every month. When backwashed, about 60% to 80% of the diatomaceous earth is removed from the grids. Consequently, you must replenish the DE after each backwash. About one pound of DE powder should be used for every square foot of your pool. Diatomaceous powder costs around $1.50 to $2.00 per pound, so the expense of regular maintenance can add up quickly. As a result, DE filters are the most expensive of all pool filtration types.

How to backwash a diatomaceous earth pool filter

Backwashing a diatomaceous earth pool filter is an almost identical process to backwashing a sand pool filter. The only difference is DE must be added back to the grids after the backwash is completed. Follow these steps to backwash your DE filter:

  1. Switch off your pump at the breaker.
  2. Connect your backwash hose to the filter waste port.
  3. Roll out the backwash hose to an appropriate drainage area.
  4. Set the multiport valve to backwash.
  5. Switch the pump back on.
  6. Allow the pump to run until the water is clear.
  7. Switch the pump off and set the multiport valve to rinse.
  8. Switch the pump on and let it run for about 30 seconds.
  9. Switch the pump off and set the multiport valve to filter.
  10. Add new DE powder to the skimmer closest to the pump and filter.

Advantages of diatomaceous earth pool filtration

  • Removes smaller contaminants than cartridge or sand filtration
  • Highest quality filtration commercially available
  • Long filter lifespan

Disadvantages of diatomaceous earth pool filtration

  • Expensive
  • More difficult to maintain
  • Powder must be replenished after each backwash
  • Backwashing could be prohibited in your area

How often should a pool filter be replaced?

Certain pool filters require replacement more frequently than others. The amount of debris and sunscreen your water receives is another big factor in replacement frequency. Based on the amount of debris your pool receives, you can expect the following lifespans for each filter type:

  • Sand filter – 3 to 5 years
  • Cartridge filter – 1 to 3 years
  • Diatomaceous earth filter – 5 to 10 years

How much do pool filters cost?

Most pool filtration systems fall in the $300-$2000 price range with all materials, labor, and installation costs included. Sand pool filtration is the least expensive type, followed by cartridge filtration at the medium price point and diatomaceous earth at the high end. Here are the average pool filtration system initial costs in the United States:

  • Sand filtration - $300 to $1200
  • Cartridge filtration - $400 to $1600
  • Diatomaceous earth filtration - $500 to $2000

Maintenance costs vary per system. While DE filters can last for a long time, the DE powder must be replenished at each backwash, adding significant upkeep cost. Sand in a sand filter must only be replaced once every few years, and cartridge filters should be replaced every 1 to 3 years depending on use. As a result, both sand filters and cartridge filters are significantly less expensive to maintain than DE filters.

Learn more: How to open a pool for the summer in 10 steps

Explore additional cartridge filter options


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