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Basics: Filtration

Basics: Filtration

The Basic Principles of Pool Maintenance

Swimming pools come in many shapes, sizes, can be made from a variety of materials. They also utilise various different types, brands and styles of equipment. How to look after your pool will vary according to your pool but there are a two principles that all pools abide by: filtration and sanitation. To look after your pool properly it is important to understand these.


Pool filtration in the simplest terms is just a way of removing impurities such as dirt, organic matter and other debris from your pool. There are multiple levels of filtration starting with the skimmer and pump baskets. These allow water to flow through while catching leaves and other large debris. Even though these are technically filters very few people will refer to them as such. When referring to a pool filter most people are referencing the main pool filter which catches smaller debris and particles. This will be located near your pump. It is probably the largest piece of equipment on your pool, and will look something like the filters in the image below.

An assortment of pool filters

Pool filters come in three main types: Media, Cartridge, or Diatomaceous Earth (DE). By far the most popular is the media filter (also known as a sand filter). A media filter works by passing water through a large quantity of fine media such as sand or glass beads. Small particles get trapped in the tiny spaces between the media while the water passes through and returns to the pool. Cartridge filters work by passing water through a cartridge - a fine paper-like element that sits inside the filter tank. The paper-like substance is actually a fabric material full of tiny holes large enough for water molecules to pass through but not large enough for dirt, dust and other impurities to pass through.

Left: As water passes through media impurities are filtered out. Right: Water passes through fabric of a cartridge element

Diatomaceous Earth (DE) filters work through a combination of the two of these filtration types. It works by means of multiple elements similar to a cartridge filter which is coated in DE powder. DE Powder is a super fine silica powder made up of the fossilised remains of tiny microbial organisms called Diatoms. This creates a very fine filter capable of removing impurities as small as 3 microbes.

Most impurities that get caught in your filter are very fine and you won't notice them in your pool until there are a lot of them. As a result many people don't even realise that the filtration process is happening. Over time all those tiny particles build up in the filter blocking the gaps or holes where water would normally pass through. Without cleaning the filter will get completely blocked and water will not be able to pass through or be filtered. As this happens you will start to lose suction, the pump will have to work harder and the water will start to go cloudy as the tiny particles are not filtered out.

The best way to determine whether a filter need cleaning is to monitor the filter pressure. All filters should have a pressure gauge (whether it works or not is another matter). Once the pressure rises by 20kPa or more you should consider giving the filter a clean according to the manufacturer's instructions.

Maintaining water flow

A filter by itself is useless as it has no way to move the water. It is always accompanied by a pump which sucks water out of the pool and either pushes or pulls it through the filter where impurities get caught. Clean water is returned to the pool. It is very important to ensure that this water flow is maintained, as filtration won't happen without it. Blockages can occur anywhere along the suction or return line but the most common places are, of course at the filtration points - the skimmer basket, pump basket and the pool filter. Make sure that these are cleaned regularly. Some pools will also have a suction powered pool cleaner such as a Kreepy Krauly or Baracuda. The head of these pool cleaners can sometimes get jammed by large twigs or leaves. These also need to be cleaned regularly to ensure water flow is maintained.

Pool pumps work by spinning an impeller round at very high speeds. An impeller is kind of like a fan that sucks water in from the middle and using centrifugal force pushes it out through vanes in the sides. 

These vanes are very small and can easily get blocked by small debris if it is allowed past the skimmer and pump baskets. If either basket breaks be sure to replace it immediately or you may end up with a pump repair bill. You should also be careful not to allow any debris to pass through when emptying the baskets. Below you can see two examples of debris caught in pump impellers. This stops or severely restricts water flow leading to poor filtration.

Filtration is vitally important to water quality as it removes dirt, debris and small particles. Filters require regular cleaning. This means regularly emptying skimmer baskets, pump baskets, checking pool cleaner heads, and cleaning pool filters. When emptying the baskets it is important to be careful that no debris gets through to the pump impeller as that will cause blockages in the impeller vanes.

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