A sand filter is a pressure vessel filled with graded sand that traps particles as small as 20 microns while pool water flows from top to bottom through the filter bed. The pump drives the flow, the multiport valve controls direction, and the pressure gauge tells you when to backwash. Below you will find exactly how each component works and which filter medium suits your pool best.
Last updated: 2026-03-22
How a sand filter works: the core technology
A sand filter has five main components: the pressure vessel (tank), the sand bed, the diffuser plate at the top, the laterals (collector arms) at the bottom, and the standpipe running down the centre. Together they form a simple but highly effective filtration system.
The pressure vessel is a fibreglass or polypropylene tank rated for pressures up to 2.5 bar (36 psi). The multiport valve sits on top. Inside, the standpipe hangs vertically from the valve down to the bottom, where the laterals collect filtered water.
Filter sand fills the tank to roughly two-thirds capacity. The diffuser plate at the top spreads incoming water evenly across the sand bed, preventing channelling.
The path water takes through the sand filter
During normal filtration the water follows this route:
- The filter pump sends water from the skimmer and main drain to the multiport valve.
- The valve is set to Filter, directing water into the top of the tank.
- Water spreads across the diffuser plate and sinks through the sand bed.
- Particles larger than 20 microns get trapped between the sand grains.
- Clean water reaches the laterals at the bottom of the tank.
- It travels up the standpipe back to the multiport valve.
- The valve sends the clean water to the return jets in the pool.
The recommended filter velocity is 40 to 50 m3 per m2 of filter area per hour. A 500 mm (20-inch) filter has about 0.20 m2 of surface area, giving a maximum flow rate of roughly 10 m3/hr (44 US gpm).
Tip
Write down the gauge reading immediately after a backwash. This is your reference value. When pressure climbs 0.3 to 0.5 bar (5 to 7 psi) above that baseline, it is time to backwash again.
How the sand bed captures particles
Filtration happens through three simultaneous mechanisms. Mechanical straining catches particles larger than the gaps between sand grains. Adsorption causes smaller particles to stick to grain surfaces through electrostatic forces. Biological filtration develops as beneficial bacteria colonise the sand and break down organic matter.
After a few hours of operation, a thin dirt layer forms on top of the sand bed. This layer acts as an additional fine filter, improving filtration from 20 microns down to about 10 microns. That is why a slightly dirty filter actually cleans better than a freshly backwashed one.
The downside: as debris accumulates, resistance increases. The pump delivers the same force, but water passes through more slowly. The pressure gauge climbs. Once it rises 0.3 to 0.5 bar above your clean baseline, backwashing is necessary.
All multiport valve positions explained
The multiport valve is the control centre of your sand filter. A standard six-way valve has the following positions:
| Position | Water path | When to use |
|---|---|---|
| Filter | Water flows top-to-bottom through sand, clean water returns to pool | Daily normal operation |
| Backwash | Water flows bottom-to-top through sand, dirty water goes to drain | When pressure rises 0.3–0.5 bar above baseline |
| Rinse | Water flows top-to-bottom through sand, then to drain | 30 seconds after every backwash |
| Waste | Water goes straight to drain, bypasses the sand | Vacuuming heavy debris from the pool floor |
| Recirculate | Water returns to pool without passing through the filter | After adding flocculant, testing the pump |
| Closed | All ports sealed | During pump maintenance |
Some valves include a seventh position: Winterize. This leaves all seals slightly open to prevent pressure damage during freezing months.
Tip
Never rotate the multiport valve while the pump is running. The sudden pressure change can damage the spider gasket inside the valve — a replacement gasket costs 30 to 80 euros.
How pressure builds inside the filter
The pressure gauge on top of the filter shows internal resistance in bar or psi. Right after a backwash, pressure is at its lowest — typically 0.5 to 1.0 bar (7 to 15 psi). This is your clean baseline.
As the sand bed traps more debris, resistance grows. The pump still delivers the same force, but water moves through the bed more slowly. The result: pressure rises. At 0.3 to 0.5 bar above your baseline, it is time to backwash.
A gauge reading above 1.5 bar (22 psi) is a warning sign. Possible causes include a clogged sand bed, a blocked lateral, or a closed return line. Shut the pump off immediately and investigate before restarting.
If pressure is unusually low (below 0.3 bar / 4 psi), the pump may be drawing air. Check the pump lid, the O-ring, and all suction-side plumbing for leaks.
Choosing the right filter media for your sand filter
You are not limited to quartz sand. Three popular filter media are available, each with distinct strengths:
Quartz sand: the classic choice
Quartz sand graded at 0.4 to 0.8 mm is the most widely used filter medium. It catches particles down to about 20 microns, costs roughly 10 to 15 euros per 25 kg bag, and is available everywhere. A 500 mm filter requires about 75 to 100 kg of sand.
The downside is relatively coarse filtration and a tendency to clump after 5 to 7 years. Calcium deposits and biofilm cause grains to stick together, creating channels through the bed that let dirty water pass unfiltered.
Glass media: finer filtration and longer life
Glass filter media (glass pearls or recycled glass granules) filter down to 5 microns, weigh about 20% less than quartz sand, and last 8 to 10 years. You need approximately 20% less weight than the sand equivalent.
The smooth surface resists biofilm formation. Glass media clumps less readily and requires less backwash water. The price is about 25 to 35 euros per 25 kg — two to three times more expensive than quartz sand. Over the full lifespan the cost difference narrows thanks to the longer replacement interval.
Zeolite: filtration plus ammonia removal
Zeolite is a volcanic mineral that filters to 5 microns and also binds ammonia (ammonium) from the water. This makes it especially useful for pools with heavy bather loads or indoor pools where ventilation is a concern.
The catch: you need to regenerate zeolite every 3 to 6 months by soaking it in a salt solution (100 grams of table salt per litre of water for 24 hours). Without regeneration it becomes saturated and loses its ammonia-binding capacity. It is also the most expensive option at 40 to 60 euros per 25 kg.
| Feature | Quartz sand | Glass media | Zeolite |
|---|---|---|---|
| Filtration fineness | 20 microns | 5 microns | 5 microns |
| Lifespan | 5–7 years | 8–10 years | 5–7 years (with regeneration) |
| Weight per 500 mm filter | 75–100 kg | 60–80 kg | 60–80 kg |
| Price per 25 kg | €10–15 | €25–35 | €40–60 |
| Binds ammonia | No | No | Yes |
How to size a sand filter for your pool
Your sand filter must match your pump’s flow rate and your pool’s volume. A filter that is too small forces water through at excessive speed, pushing debris straight through the sand bed instead of trapping it.
The rule of thumb: filter velocity should not exceed 50 m3 per m2 of filter area per hour. Calculate filter area with the formula: area = pi × (diameter / 2)².
| Filter diameter | Filter area | Max flow (50 m3/m2/hr) | Suits pools up to |
|---|---|---|---|
| 400 mm (16 in) | 0.13 m2 | 6.3 m3/hr (28 gpm) | 20,000 litres (5,300 gal) |
| 500 mm (20 in) | 0.20 m2 | 9.8 m3/hr (43 gpm) | 35,000 litres (9,200 gal) |
| 600 mm (24 in) | 0.28 m2 | 14.1 m3/hr (62 gpm) | 50,000 litres (13,200 gal) |
| 750 mm (30 in) | 0.44 m2 | 22.1 m3/hr (97 gpm) | 80,000 litres (21,100 gal) |
When in doubt, go one size up. A larger filter means lower velocity, better filtration, and longer intervals between backwashes.
What goes wrong with a neglected sand filter?
Without regular backwashing and periodic sand replacement, three problems develop.
Channelling: water finds the path of least resistance and carves channels through the sand bed. Dirty water passes through unfiltered. You will notice cloudy pool water despite a functioning pump.
Calcification: in hard water (above 300 mg/l calcium), sand grains cement together into a solid mass that barely allows water through. The pressure gauge spikes. The solution is replacing the sand or treating the filter with a specialized descaler.
Biofilm growth: bacteria and algae colonise the sand bed. If the pump sits idle for more than 48 hours, this can produce foul odours and a slimy mass inside the filter. Running the pump regularly and maintaining a free chlorine level of 1.0 to 1.5 mg/l prevents this problem.
Want to see how sand filters compare to cartridge filters? Read the sand vs cartridge filter comparison . For step-by-step backwash instructions, check the backwash guide . And for an overview of how the filter fits into the complete pool system, see how a pool works .
Frequently asked questions about sand filters
How does sand filter pool water?
Water flows top-to-bottom through a bed of quartz sand graded at 0.4 to 0.8 mm. Particles larger than about 20 microns get trapped between the sand grains. Over time a dirt layer forms on top of the sand bed, improving filtration down to roughly 10 microns.
What are the positions on a multiport valve?
A standard multiport valve has six or seven positions: Filter (normal operation), Backwash (flush dirt out of the sand), Rinse (settle the sand bed after backwash), Waste (send water straight to drain), Recirculate (bypass the filter), Closed (all ports sealed), and sometimes Winterize (relieve pressure on seals).
When should I replace the sand in my filter?
Replace quartz sand every 5 to 7 years, glass media every 8 to 10 years. Signs it is time: pressure stays high after backwashing, water clarity does not improve, or you find sand grains in the pool.
What is the difference between quartz sand, glass media, and zeolite?
Quartz sand filters down to 20 microns and is the cheapest option. Glass media filters to 5 microns, weighs 20% less, and lasts longer. Zeolite filters to 5 microns and binds ammonia, but costs the most and needs periodic chemical regeneration.
What should the pressure gauge read?
Note the gauge reading right after a backwash — that is your clean baseline, typically 0.5 to 1.0 bar (7 to 15 psi). When pressure rises 0.3 to 0.5 bar (5 to 7 psi) above that baseline, it is time to backwash. Readings above 1.5 bar (22 psi) indicate a clogged filter bed.