For most garden pools of about 10 m³ and up, sand filtration is the best choice — it’s versatile, cheap to run, and easy to maintain. Cartridge filtration makes sense for smaller above-ground pools and hot tubs, where it wins on simplicity and water savings. And if you want to get the most out of a sand filter, swap the silica sand for glass media: it catches finer particles and lasts roughly twice as long. In this guide we’ll go through all three options in detail, including figures, a comparison table, and correct sizing.
What does filtration actually do — and why can’t chemistry replace it?
Pool water needs two things: physical removal of dirt and chemical disinfection. Filtration handles the first — capturing dust, pollen, leaf debris, cosmetics, skin cells, and dead algae that would otherwise cloud the water. Chemistry (chlorine, oxygen, or salt with electrolysis) handles the second: it kills the bacteria and microorganisms that pass through the filter.
The key is understanding that these are two systems working together, not instead of each other. Even the best chlorination won’t remove mechanical debris from the water — the water will be sterile but cloudy. And conversely, a perfect filter without disinfection won’t stop bacteria and algae from multiplying. When filtration can’t keep up, chemical consumption rises sharply, because disinfectant gets “used up” on organic debris. A typical result of neglected filtration is green pool water, which you then have to rescue with shock chlorination.
The heart of the system is the pump, which drives water from the skimmer through the filter vessel and back into the pool through the jets. What determines how clean your water is above all is the filter medium — and that’s exactly where sand, cartridge, and glass filtration differ.
How does sand filtration work?
Sand filtration is the long-standing standard for garden pools. The pressure vessel holds a layer of special silica sand (usually a 0.4–0.8 mm grain size) through which water flows from top to bottom. Debris gets trapped between the grains, and the cleaned water drains back into the pool through a drainage system at the bottom. The commonly quoted filtration fineness is around 20–40 microns — for comparison, a human hair is about 70 microns.
A home filter typically holds 25 to 100 kg of sand, depending on the vessel’s diameter. A complete sand filtration setup for a family pool costs roughly €160–600, with the sand itself costing in the tens of euros per 25 kg bag.
What is backwashing and why is it necessary?
As the sand clogs up, resistance rises and flow drops. The solution is backwashing: you switch the six-way valve to the “backwash” position, water flows through the filter in reverse, lifts the sand, and flushes the trapped debris out to waste. A brief “rinse” cycle follows to settle the bed. The whole process takes 2–3 minutes and uses roughly 150–400 litres of water, depending on the filter’s size.
Backwash when the filter’s pressure gauge reads about 0.2–0.3 bar above the reading after a clean media fill — in season, typically once every 1–2 weeks. The water used for backwashing is the main running cost of sand filtration.
How does cartridge filtration work?
Cartridge filtration replaces sand with a removable insert made of pleated polyester material. Water flows through the cartridge’s pleats, which, thanks to their large surface area, typically capture particles from 10–20 microns — somewhat finer than classic sand. Cartridge units are smaller, lighter, and cheaper: basic models for above-ground pools start at around €60, with more powerful sets costing up to €240.
The key difference lies in maintenance: cartridges aren’t backwashed. You remove the cartridge, rinse it with a hose (ideally you have two and rotate them), occasionally soak it in a cleaning solution, and replace it after one to two seasons. This eliminates the hundreds of litres of water used for backwashing — which is why cartridges are also standard in hot tubs, where only a small volume of water is involved.
The downside is regular hands-on work (cleaning roughly every 2–4 weeks in season) and ongoing costs for replacement cartridges. In large, heavily used pools, cartridges clog quickly, so this technology is best suited to smaller above-ground pools up to about 20 m³, hot tubs, and swim spas.
Glass media: upgrading a sand filter
The third option isn’t a separate filter type but a media swap in a classic sand vessel: instead of sand, you pour in filter glass — granulate made from recycled glass, or premium activated glass (e.g. AFM). Manufacturers and industry retailers consistently cite three main benefits.
- Finer filtration. While sand captures particles from around 20–40 microns, glass media is commonly quoted at around 5–10 microns, and even less for activated glass. The glass surface also carries a slight negative charge that attracts even very fine particles.
- Less water for backwashing. Glass is lighter, and debris tends to sit on the surface rather than deep in the bed, so backwashing is shorter — industry tests and manufacturers report backwash water savings of around 20–25% and longer intervals between backwashes.
- Longer lifespan. Glass media typically retains its effectiveness for 7–10 years, while sand is recommended to be replaced every 3–5 years. Glass is also less prone to biofilm formation and “channelling,” where water flows through without being filtered.
The trade-off is a higher purchase price — glass media typically costs roughly two to three times as much as sand. But thanks to its lower density you need about 15–20% fewer kilograms, and with its longer lifespan the total cost works out comparable, often better. For most sand filter owners, this is the simplest upgrade with immediately visible results.
Comparison: sand vs. cartridge vs. glass in numbers
The table below summarizes indicative figures for family pools in 2026. We give filtration fineness as ranges commonly seen from manufacturers and in industry comparisons — the exact value always depends on the media’s grain size, condition, and flow rate.
| Criterion | Sand filtration | Cartridge filtration | Sand vessel with glass |
|---|---|---|---|
| Purchase price of the setup | €160–600 | €60–240 | same as sand + €40–160 for the media |
| Filtration fineness (commonly quoted) | approx. 20–40 µm | approx. 10–20 µm | approx. 5–10 µm |
| Operating effort | low — valve backwash | medium — manual cartridge cleaning | low — fewer backwashes than sand |
| Backwash water use | 150–400 l per backwash, every 1–2 weeks | no backwash (hose rinse only) | roughly 20–25% less than sand |
| Media lifespan | sand 3–5 years | cartridge 1–2 seasons | glass 7–10 years |
| Best for | pools from about 10 m³ | smaller pools up to about 20 m³, hot tubs | anyone who wants the most from a sand filter |
In short: cartridges win on purchase price and water savings, sand on low maintenance and media cost, glass on water quality and lifespan. You can’t really go wrong with any of the three — as long as you get the sizing right.
How to size filtration correctly
The basic rule: filtration should turn over the pool’s entire volume in roughly 4 hours. The rule-of-thumb formula is simple — flow rate (m³/h) = pool volume (m³) / 4. For a 4×8 m pool with a 1.4 m depth (about 45 m³), you need filtration with a real flow rate of around 11 m³/h; for a 3.6 m above-ground pool with 10 m³, 2.5–3 m³/h is enough.
Watch out for two details. First, manufacturers quote the pump’s maximum flow rate “at zero head” — once you factor in the resistance of the pipework and filter, actual flow is noticeably lower, so it pays to build in a small margin. Second, for a sand filter the flow rate must also match the vessel’s diameter: too powerful a pump on a small vessel will push debris straight through the sand, so a bigger pump doesn’t automatically mean cleaner water.
During the season, filtration should turn over the pool’s entire volume 2–3 times a day, which, with correct sizing, corresponds to 8–12 hours of running time per day. A practical rule of thumb: filtration time in hours ≈ half the water temperature in °C (so at 26°C, roughly 13 hours).
When to replace the media, and how to spot a clogged filter?
The most reliable indicator is the pressure gauge on the filter vessel. After replacing the media or backwashing, note the “clean” operating pressure — once it rises by 0.2–0.3 bar, it’s time to backwash the sand or clean the cartridge. A second signal is a weaker jet stream from the nozzles and worsening water clarity.
- Sand: backwash every 1–2 weeks in season, full replacement after 3–5 years. Rounded-off grains stop holding onto debris even though the filter keeps “running.”
- Glass media: backwash less often than sand, replace after 7–10 years.
- Cartridge: rinse every 2–4 weeks, deep-clean with a solution 1–2 times per season, replace after 1–2 seasons or when the pleats are frayed or pressure returns to high readings right after cleaning.
If pressure instead drops noticeably below the usual value, the problem is usually upstream of the filter — a clogged pump pre-filter, a blocked skimmer, or air being drawn in.
The most common mistakes in choosing and running filtration
- Undersized filtration. The cheapest set “for pools up to 30 m³” often only handles half that in practice. Calculate using the volume/4 formula and treat manufacturer figures as a maximum, not an operating value.
- Too short a filtration time. Two hours a day isn’t enough even for a small pool. Without 2–3 volume turnovers a day, debris builds up and chemistry has to “catch up” at a higher cost.
- Delayed backwashing or cleaning. A clogged filter has lower flow, so you’re effectively filtering less even though the pump runs for the same length of time.
- Old sand. Sand doesn’t last forever — after some years the grains round off and the filter just “spins the water.” If the water is cloudy despite correct chemistry and sufficient run time, the media is the first suspect.
- Filtering only while swimming. Filtration should run on a daily schedule, ideally during the day, when the pool is under load and sunlight is encouraging algae growth.
Frequently Asked Questions (FAQ)
Is sand or cartridge filtration better?
It depends on the pool. For in-ground and larger above-ground pools from about 10 m³, sand (or glass) filtration is more practical — it handles heavy use well, and maintenance is a matter of turning a valve. For small above-ground pools, hot tubs, and swim spas, cartridge filtration is more suitable: it’s cheaper, more compact, and doesn’t use water for backwashing.
Is it worth switching from sand to glass media?
In most cases, yes — ideally at the point when the sand needs replacing anyway. For roughly two to three times the price of sand, you get finer filtration, fewer backwashes, and a 7–10 year lifespan, so the price difference typically pays for itself over the media’s life.
How long should filtration run each day?
In season, 8–12 hours a day, so the pool’s entire volume is turned over 2–3 times. As a rule of thumb, “half the water temperature” helps: at 28°C, roughly 14 hours. Outside the season and with the pool covered, you can shorten this.
How do I know it’s time to backwash the filter?
Use the pressure gauge: backwash when pressure rises 0.2–0.3 bar above the reading after the last cleaning. A weaker jet stream from the nozzles is a helpful secondary signal. But don’t backwash unnecessarily often — slightly “broken-in” sand filters a bit finer than freshly backwashed sand.
Does good filtration replace pool chemistry?
No. Filtration removes mechanical debris but doesn’t kill bacteria or microscopic algae. You always need both: correctly sized filtration and basic disinfection with pH control. Good filtration does, however, significantly cut chemical consumption.
Whether you choose sand, cartridge, or glass media, what matters most is correct sizing and regular operation — clean water is then 80% done, and chemistry just fine-tunes the rest. You’ll find filter units, pumps, replacement cartridges, and filter media in our pool technology category, and if you’re curious what happens when filtration can’t keep up, read our guide on rescuing green pool water.
