What Does a Pool Filter Do? - The Actual Job of Your Filter Explained
A pool filter removes suspended particles from the water that the pump cycles through it - dirt, dead algae, body oils, debris, and microorganisms too small to see. Without it, your pool water would be cloudy within days even with correct chemistry. This guide explains exactly what the filter does and what it cannot do.
The Basic Job: Remove Particles
A pool filter has one core function: remove suspended particles from the water as the pump forces water through it.
Water from the pool enters the filter housing under pump pressure, passes through a filter media (sand, cartridge pleats, or DE-coated grids), and exits back to the pool through the return jets. The media physically traps particles that are larger than a certain size and lets the cleaned water pass through.
The filter is not a chemical treatment. It does not kill algae, does not sanitise water, and does not balance pH or alkalinity. It removes the physical debris load from the water.
What Exactly Gets Filtered Out
A pool filter removes:
- Visible debris - leaves, grass, insects, and large particles that the skimmer basket misses
- Fine particulate - dust, sand, dirt tracked in by swimmers
- Dead algae - algae killed by chlorine shock treatment breaks into fine particles; the filter captures them (this is why you run the filter for 8-24 hours after shocking)
- Body oils and sunscreen - partially, though these can coat the filter media and require chemical cleaning to fully remove
- Pollen - fine pollen particles are a significant clouding agent in spring; a good filter captures them
- Microorganism debris - dead bacteria and other organic matter after sanitiser treatment
What the filter does not remove:
- Dissolved chemicals (chloramines, metals, phosphates)
- Algae spores smaller than the filter’s micron rating
- Bacteria and viruses while alive - chlorine kills these, then the filter captures the dead cells
- Total dissolved solids - TDS requires a partial drain and refill to reduce
The Three Filter Types and How They Work
Sand Filters
Water enters the tank at the top, passes downward through a bed of pool-grade silica sand (typically 50 lb to 300 lb depending on tank size), and exits through lateral tubes at the bottom.
Sand traps particles through a combination of physical interception and a phenomenon called filter cake - as the sand bed loads with debris, the trapped particles themselves become part of the filtration surface and help capture finer particles. This is why you should not backwash a sand filter too frequently: the filter cake improves performance up to a point.
Particle capture size: 20-40 microns
Sand filters need backwashing when pressure rises 8-10 psi above the clean baseline. Full sand replacement is needed every 5-7 years.
Cartridge Filters
Water passes through pleated polyester filter elements (cartridges). The pleated design creates a large surface area in a compact housing - a standard residential cartridge has 100-500 square feet of filter surface despite the filter being roughly the size of a trash can.
Cartridge filters do not backwash. When pressure rises, you remove the cartridge, rinse it with a garden hose, and reinstall. Chemical cleaning (acid soak or filter cleaner solution) is needed 1-2 times per season to remove oils and scale that physical rinsing cannot dissolve.
Particle capture size: 10-15 microns
DE Filters (Diatomaceous Earth)
Water passes through fabric grids coated with diatomaceous earth - a powder made from fossilised diatom skeletons. Under a microscope, DE has a complex lattice structure with openings in the 2-5 micron range.
DE filters provide the finest filtration of the three types and are the reason some pools with DE filtration look exceptionally clear. The grids need to be recharged with fresh DE powder after each backwash.
Particle capture size: 2-5 microns
The Filter and the Sanitiser Work Together
The filter and the chlorine (or other sanitiser) are complementary systems, not substitutes for each other.
| Task | Filter | Sanitiser |
|---|---|---|
| Remove debris particles | Yes | No |
| Kill bacteria | No | Yes |
| Kill algae | No - captures dead cells | Yes |
| Clarify cloudy water | Yes - over time | No |
| Remove chloramines | No | Partial (break-point chlorination) |
Cloudy water with correct chemistry almost always means the filter is not running enough hours, has reached capacity and needs cleaning, or has a media problem. Cloudy water with incorrect chemistry means the sanitiser is not doing its job.
Both problems produce the same symptom - cloudy water - which is why you always check both chemistry and filter condition when troubleshooting.
How Long the Filter Needs to Run
The filter can only clean the water that actually passes through it. A filter that runs 4 hours per day on a 20,000 gallon pool is only turning the water over about once - some water may not pass through the filter at all if circulation patterns allow dead zones.
The minimum recommended run time is one complete turnover per day - meaning the total pool volume passes through the filter at least once. For most residential pools this is 6-10 hours per day during swim season.
- How Long Should a Pool Filter Run? - run time formula and table by pool size
Filter Media Comparison
| Feature | Sand | Cartridge | DE |
|---|---|---|---|
| Filtration level | 20-40 microns | 10-15 microns | 2-5 microns |
| Cleaning method | Backwash | Rinse + chemical soak | Backwash + recharge |
| Media replacement | Every 5-7 years | Every 1-3 years | Add after each backwash |
| Water needed to clean | High (backwash uses 200-400 gallons) | None (rinse only) | High (backwash) |
| Best for | Large pools, easy maintenance | Water conservation, fine filtration | Clearest water, fine filtration |
Keeping Your Filter Working Well
A filter that is never cleaned will eventually bypass - water finds the path of least resistance through gaps in clogged media and the filter stops removing particles effectively. Pressure gauge readings are the most reliable maintenance signal: check it weekly and clean the filter when it rises 8-10 psi above the post-cleaning baseline.
Pool filter cleaner on Amazon - a dedicated filter cleaning product dissolves oils and scale that physical rinsing leaves behind. Use it on cartridge and DE filters 1-2 times per season.
Related Guides
- How Long Should a Pool Filter Run? - daily run time by pool size
- How Often to Clean Pool Filter - cleaning frequency by filter type
- Pool Filter Pressure Too High - diagnosing elevated pressure
- DE Filter vs Sand Filter vs Cartridge - choosing the right filter type
- What Size Pool Filter Do I Need? - sizing by pool volume
Frequently Asked Questions
What does a pool filter do?
Does a pool filter clean the water or sanitise it?
What does a pool filter not remove?
How do I know if my pool filter is working?
What is the difference between a pool pump and a pool filter?
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