Guide written by:
Patrick, Handyman, Bath
Sand filters, diatomite filters, cartridge filters and sock filters... Your filtration system should be scaled according to your pool. To avoid getting confused among glass beads, zeolite, diatomite and particle size, follow our buying guide to help you choose your pool filtration system!
- Pool basin volume
- Sand filter
- Cartridge filter
- Diatomite filter
- Sock filter
How to choose my pool filtration system?
The purpose of a pool filtration system is to purify your pool water to make it clear, hygienic and free of impurities.
The water is drawn in via skimmers, passes through filters and is then reintroduced to the pool basin via outlet nozzles.
Depending on your installation, water can pass through an electrolyser, a saltsterilizer or a booster on its route around the filtration cycle. The principal filter, positioned downstream of a number of prefilters that exclude larger debris, is capable of removing even the smallest particles suspended in the water. The most sophisticated filters achieve a maximum grain size of 5 microns. Your filter should be carefully scaled to ensure water clarity. The other key element is the pool pump, which enables water circulation in the pool pipe network. These two elements are intricately connected: the sizing of the filter dictates the pump power necessary for efficient filtration.
What are the different types of pool filters?
Swimming pool filter performance is measured by the water clarity it produces, or in other words, the size of particles it's capable of removing.
There are four main types of filters:
- Sand filters are the most common type, offering impressive value for money;
- Cartridge filters, which have an elongated shape, are made of paper or synthetic material;
- Diatomite filters are cylindrical in shape;
- Sock filters are sometimes used on smaller swimming pools.
Each type has its advantages and disadvantages.
Paper or synthetic material
How does a sand filter work?
A sand filter consists of a cylinder two-thirds filled with a granular material of an appropriate grain size to produce good filtration.
The size of particles the filter can exclude depends on the grain size of the filter material as well as the rate at which the water passes through it.
Despite the generic name "sand filter", these devices can also contain zeolite or glass beads as additional filter materials.
What's best: sand, zeolite or glass beads?
Here's the deal...
- Sand: favoured by most pool owners for its low price, ease of use and foolproof maintenance. It's a good idea to include two differentgrain sizesfor optimal filtering - a layer of grain size 2-4mm at the bottom and a layer of 0.4mm on top. It's also possible to get an intermediate grain size of 0.6mm, likewise giving good results. This type of filter provides filtration to a grain size of about 35 microns. Using flocculants can further improve performance.
- Zeolite: a mineral of volcanic origin. Its filtration capacity is far superior to sand - but it's substantially more expensive too. The cost is somewhat offset by the reduced requirement for water treatment products such as clarifiers. The fineness of filtration provided by zeolite makes it a competitor to the diatomite filter. Use and maintenance are identical to a pure sand filter, and the two substances can also be combined. The sand can be replaced with zeolite without modifying your installation.
- Glass beads: a material based on polished recycled glass. Glass beads have only recently come on the market and can be used to improve filtering performance over sand alone and have the advantage of not producing a surface film like sand and zeolite. Washing therefore takes less time, thus saving water.
If you're stumped by the prospect of having to choose between these three, stick with sand - it will give excellent results in most private pools, at low cost and with minimal maintenance.
How does a diatomite filter work?
A diatomite filter is in the form of a cylinder containing a membrane. This membrane contains diatomaceous earth - a white powder derived from microscopic algae fossils.
During filtration, the diatomite attaches to the membrane and acts as a filter. This process produces very fine filtration to a grain size of 3 microns and gives crystal-clear water.
However, maintenance is more laborious than with a sand filter. The filter membrane must be disassembled and descaled every year and the diatomite replaced. In addition, it's advisable to oversize the filter in order to avoid clogging the membrane.
Why choose a cartridge filter?
With this type of device, filtration is provided by a cartridge contained within a casing. The size of filter needed depends on the volume of the pool basin. The cartridge filter generally gives a finer grade of filtration than the sand filter. However, product quality can vary; filtration grain size is indicated on the packaging. The filter may be made of paper or synthetic material.
Specifically suited to smaller pools, cartridge filters require regular manual cleaning (including dismantling the filter). Cartridges have a limited service life and should be replaced regularly. One of the advantages of the cartridge filter is that it doesn't need to be washed like a sand filter. This can be decisive if it's impossible to include a wastewater pipe in your installation. One way to cut down cleaning frequency is to increase the filter capacity, providing a larger area of exchange and hence delaying clogging.
Top tip: purchase two cartridges; then, when cleaning becomes necessary, swap them over and you'll have more breathing space to clean the clogged one!
Pocket filter, sock filter... What's the deal?
Reserved for above-ground pools receiving occasional use, the pocket filter consists of a cloth pocket or "sock" that is cleaned regularly.
Since it's subject to wear, the sock must also be changed regularly.
The requirement for frequentcleaning can quickly become tedious.
Its fineness of filtration places it at the bottom of the pile compared to the other types of filter.
However, the pocket filter is commonly used in one-piece filtration blocks.
What filtration capacity to choose for your swimming pool filter?
One more thing! Choosing the type of filter isn't enough - you've also got to scale its capacity to the size of your pool. Whatever type it is, filter capacity is measured in terms of water flow rate.
To obtain good quality water, the full volume of your pool should be filtered within six hours maximum, and ideally four.
Calculate the volumeof your pool, divide by 4 and add a safety margin of 1.3 to get the minimum flow of your filter (F).
Example: Inground pool of 40m3: 10m3/h will recycle all the water in 4 hours, so choose a filter that will tolerate a flow of 13m3/h.
For sand filters, an additional factor must be considered: filtration speed, which must not exceed 50m/h. This dictates the minimum filter area (A) of your filter, where A = F / 50. This in turn dictates the minimum diameter of your filter.
In our case, with a flow of F = 13m3/h, the area must be at least (13m3/h / 50m/h) = 0.26m2, giving a minimum diameter of 575mm.
For those of you who aren't into maths, don't fret, most of the time the filtering area is given by the manufacturer!
All you need do now is choose a pump capable of sustaining an appropriate flow rate for your filter!
Learn more about swimming pool equipment...
To find out more about swimming pool equipment, follow our editors' advice and discover their other guides:
Guide written by:
Patrick, Handyman, Bath, 12 guides
Professional maintenance and electronics training. I don’t ask to use my tools on the weekend. The car, the boat, the house with its electric appliances, the garden and its swimming pool, they all remind me that they need my attention. In the absence of an unlimited budget, I try to expand my areas of expertise. There’s nothing I like quite as much as being in the weekend workshop with my buddies. Everyone benefits from his experience which ends up being more-or-less successful, it must be said. But that’s part of the charm of DIY, helping each other and realizing a project in a pleasant way. So, if I can help you, it would be a pleasure.