Pool Filter Types Explained: Sand, Cartridge, and DE Filters
You can have perfect chemistry and still have dull, hazy water if the filter is not doing its job. Chlorine kills algae and bacteria; the filter physically removes the particles, dead algae and debris that make water look cloudy. This guide explains how the three main types of pool filter work, which suits which pool, and how to look after the one you have.
What does a pool filter actually do?
The pump pushes pool water through the filter, which traps suspended particles and sends cleaner water back to the pool. As the filter collects dirt, its resistance to flow rises and the pressure gauge climbs. Cleaning it — backwashing a sand or DE filter, or hosing a cartridge — brings the pressure back down. A filter that is never cleaned filters poorly and starves the pump; a filter that is cleaned too often never builds up the fine layer of trapped dirt that helps it catch smaller particles.
How does a sand filter work?
A sand filter is a tank filled with specially graded filter sand, or an alternative glass medium, sitting on a set of slotted pipes called laterals at the bottom. Water enters at the top, passes down through the sand, and dirt is caught between the grains. To clean it, you backwash: the multiport valve reverses the flow, lifts the sand bed and flushes the trapped dirt out through the waste line.
Strengths: simple, robust, forgiving and inexpensive to run. The multiport valve is easy to understand, and there is very little to go wrong.
Weaknesses: it catches coarser particles than the other two types, and backwashing uses water.
Best for: most residential pools in Gauteng, where cloudy water usually comes from the chemistry, the pump hours or tired sand rather than from the type of filter.
When does filter sand need changing?
Filter sand does not last for ever. Over a few years the grains wear smooth, and in hard-water areas calcium binds them into clumps. Water then channels through the gaps instead of through the whole bed, and dirt goes straight back into the pool. The signs:
- The water stays dull or slightly cloudy even when chlorine and pH are right.
- The pressure climbs straight back up soon after a backwash.
- Sand appears on the pool floor, usually from a cracked lateral.
- It has simply been several years since the last change.
A sand change and filter service replaces the media and lets us check the laterals, standpipe and valve while the tank is open. We price it by the number of bags your filter takes, which is printed on the filter label: R1,450 for a 3-bag filter, R1,550 for 4 bags, R1,650 for 5 bags and R1,750 for 6 bags.
How does a cartridge filter work?
A cartridge filter passes water through a pleated fabric element. The large surface area catches finer particles than sand, and there is no backwashing: you lift the cartridge out and hose it clean, and replace it when the fabric wears or will no longer come clean.
Strengths: finer filtration, no backwash water, and a compact footprint.
Weaknesses: more hands-on. Cartridges need regular cleaning, and in a pool with heavy leaf or dust loads they clog quickly. Replacement cartridges are an ongoing cost, and calcium and body oils can blind the fabric, so an occasional soak in a filter cleaner helps.
Best for: smaller pools, pools that are usually covered, and owners who want to avoid backwash water.
What is a DE filter?
A diatomaceous earth (DE) filter uses fabric grids coated with a fine powder made from the fossilised shells of tiny organisms called diatoms. The powder forms a layer that catches the finest particles of the three types. When the pressure rises, the filter is backwashed or opened, cleaned and recharged with fresh powder.
Strengths: the finest filtration and the clearest water.
Weaknesses: the most maintenance and the most parts to service, and the powder must be handled carefully — wear a dust mask and avoid breathing it in. DE filters are less common in South African homes, so spares and service can be harder to find.
Best for: owners who want the clearest possible water and are happy to maintain it.
Which filter is best for my pool?
For most family pools in Centurion, Pretoria and Midrand, a correctly sized sand filter is the practical choice. It copes well with storm debris and dust, it is easy to live with, and when the water is not clear the fix is usually elsewhere: pH, chlorine, pump hours or old sand. A cartridge filter makes sense if you are short of space or want to avoid backwash water and do not mind cleaning it. A DE filter is for clarity enthusiasts.
Whatever the type, the filter has to be matched to the pump. A pump that is too powerful for its filter pushes water through too fast and filters worse, not better. If your pump has been replaced with a bigger one, check that the filter was sized for it; our pool pump repairs page covers sizing a pump to its filter.
How often should I backwash a sand filter?
Backwash by the gauge, not the calendar. Note the pressure just after a backwash — that is your clean reading. Backwash when the gauge has risen roughly 50–70 kPa above it, and after heavy storms. Backwashing on a fixed schedule regardless of pressure wastes water and can make the filter work worse.
- Switch the pump off. Never move the multiport valve while the pump is running.
- Set the valve to Backwash and run the pump until the water in the sight glass runs clear.
- Switch off, set the valve to Rinse, and run briefly to resettle the sand.
- Switch off, return the valve to Filter, and switch the pump back on.
Check your municipality's rules on where backwash water may go, and top the pool up afterwards so the level stays at least halfway up the weir opening.
Why has my filter stopped working?
- Channelling through old or clumped sand.
- A cracked lateral, which sends sand back into the pool.
- A worn multiport gasket, which leaks water to waste even on Filter — a common "leak" that turns out to be the valve.
- A cracked tank or a split lid.
- A filter too small for the pool, which can never keep up however long the pump runs.
Where the tank itself is the problem, we sell sand and cartridge filters in our online pool shop and fit them. Quoting a replacement filter costs nothing.
Frequently asked questions
Why is there sand in my pool?
Usually a cracked lateral at the bottom of the filter. It can also be sand that was not rinsed after a backwash. The laterals are checked during a sand change.
Why is my pool cloudy after a sand change?
New sand carries fine dust, so the water can look slightly hazy for a filter cycle or two. Backwash and rinse, and it clears.
Can I switch from sand to glass media?
In many sand filters, yes. Follow the filter manufacturer's guidance on the grade and the quantity.
How do I know how many bags my filter takes?
It is on the filter label or in the manual. If the label has faded, send us a photo of the tank and we will identify it.
Talk to us
WhatsApp or call Crystal Reef on 081 577 5051, or email sales@crystalreef.co.za. We service filters across Midrand, Centurion and Pretoria. If your water keeps turning green despite a good filter, read why your pool turns green, and if the pump is the weak link, read pool pump problems.