How Does a Pool Sand Filter Work? - The Complete Explanation
A sand filter traps debris as pool water flows downward through a bed of #20 silica sand. When the sand loads with debris and pressure rises, backwashing reverses the flow to flush it clean. This guide explains exactly what happens inside the tank at each stage.
Inside the Tank: The Key Components
A sand filter has five main components:
The tank: A sealed pressure vessel, typically fiberglass or ABS plastic. Rated to 50 PSI operating pressure. The tank itself is passive - it just holds everything else.
The multiport valve: Mounted on top of the tank. Six positions control which direction water flows and where it goes. FILTER is normal operation. BACKWASH is cleaning mode.
The standpipe: A central pipe running from the multiport valve down to the lateral assembly at the bottom. Filtered water travels up through this pipe to exit the filter.
The laterals: A ring of slotted plastic arms at the base of the standpipe. The slots (approximately 0.3mm wide) are narrower than a sand grain but wide enough to pass filtered water. They prevent sand from escaping into the plumbing.
The sand: #20 grade silica sand, 0.45-0.55mm grain size, filling approximately two-thirds of the tank. This is the actual filtering medium.
The Filtration Cycle - What Happens on FILTER
- The pool pump draws water from the skimmer and main drain
- Pressurized water enters the top of the filter tank through the inlet port on the multiport valve
- Water distributes evenly across the top of the sand bed
- Water flows downward through the sand - gravity and pump pressure push it through
- As water passes between the sand grains, particles larger than the spaces between grains (20-40 microns) get physically trapped
- Filtered water reaches the lateral assembly at the bottom
- Water enters the laterals through the narrow slots - the slots are too small for sand grains, so sand stays in the tank
- Filtered water travels up the standpipe to the multiport valve
- The valve routes the filtered water out the return port back to the pool
The entire cycle happens continuously as long as the pump is running.
Why Pressure Rises
Fresh sand has relatively large spaces between the grains - water flows through easily and pressure is at its lowest (your clean baseline).
As the filter operates, debris fills the spaces between grains. Each trapped particle slightly narrows the path that water must follow. Water must push harder to flow through the increasingly restricted bed. The pressure gauge measures this resistance.
At 8-10 PSI above baseline, the restriction is significant enough to noticeably reduce flow and filtration efficiency. This is the correct time to backwash.
The Backwash Cycle - Cleaning the Sand
On the BACKWASH setting, the multiport valve reroutes water in reverse:
- Water now enters from the bottom through the laterals (reverse direction)
- Upward flow lifts and agitates the sand bed - the grains tumble and release trapped debris
- Debris-laden water carries the particles upward and out the waste/backwash port
- The discharge water is dark and murky at first, gradually clearing as debris is flushed out
- The cycle runs until the sight glass shows clear water - typically 2-3 minutes
After backwashing, the RINSE cycle (30 seconds on the RINSE setting) re-settles the disturbed sand into a flat, even bed before returning to FILTER mode.
Why Sand Filters Need #20 Grade Sand
The grain size is precisely specified because it determines filtration quality and sand retention:
- Too fine (play sand, under 0.45mm): Grains pass through the lateral slots into the pool. Water flow resistance is too high. Filter clogs very quickly.
- Too coarse (over 0.55mm): Spaces between grains are too large. Debris passes straight through unfiltered. Water stays cloudy.
- #20 grade (0.45-0.55mm): Correct balance. Traps 20-40 micron particles. Held back by 0.3mm lateral slots. Flows at proper resistance.
What Type of Sand for Pool Filter - full explanation of sand grades and alternatives.
Sand Filter vs Other Filter Types
| Feature | Sand | Cartridge | DE |
|---|---|---|---|
| Filtration level | 20-40 microns | 10-15 microns | 2-5 microns |
| Cleaning method | Backwash | Remove and rinse | Backwash + recharge |
| Water usage | High (backwash) | None | Moderate (backwash) |
| Maintenance complexity | Low | Medium | Highest |
| Media lifespan | 5-7 years | 2-3 years | 5-10 years (grids) |
For a full comparison: DE Filter vs Sand Filter vs Cartridge
Related Guides
- How to Backwash a Pool Filter - the cleaning cycle in detail
- What Type of Sand for Pool Filter? - why grain size matters
- Pool Filter Pressure Too High - diagnosing pressure problems
- Sand Filters Complete Guide - all sand filter topics
Frequently Asked Questions
How does a pool sand filter work?
How fine does a sand filter filter?
Why does filter pressure rise over time?
How does backwashing clean the sand?
Why does a dirty filter actually filter better?
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