Filtration Overview

The Basic Principles of Pool Maintenance

Swimming pools come in many shapes & sizes and 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 two principles that all pools abide by: filtration and sanitation. To maintain your pool it is important to have a basic understanding of these. This article is about the first of these:

Filtration

Pool filtration is a process of removing impurities such as dirt, organic matter and other debris from your pool by use of a medium that only allows water to pass through.

Types of Filter

The first step in pool filtration is the removal of large debris and leaves. This is achieved at the skimmer and pump baskets. The holes in these allow a lot of smaller debris to pass through but by catching the larger debris the baskets reduce the load on the main pool filter. These baskets can get clogged up very quickly if you get lots of debris in your pool and it is important to clear them out regularly.

Even though these baskets are technically a type of filter I tend not to refer to them as filters because it gets too confusing. 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 be the largest piece of equipment on your pool, and will look something like the filters in the image below.

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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) because it is the easiest to maintain.

Media Filters

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

Cartridge filters work by passing water through a cartridge - a fine, fabric element that sits inside the filter tank. The fabric is 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.

DE Filters

Diatomaceous Earth (DE) filters work through a combination of the two of these filtration types. It utilises multiple fabric elements (often called septums), similar to a cartridge. These are coated in a super fine media - DE powder, which acts in a similar way to the sand or glass in a media filter. 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.

Filter Operation and Maintenance

Cleaning

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.

One way to determine whether a filter needs 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 circulate 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 the impurities get caught. Clean water is then 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. 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 Pump Operation

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. 

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Blocked Impellers

These vanes are very small and can easily get blocked by small debris if it is allowed to pass the skimmer and pump baskets. If either basket breaks be sure to replace it as soon as possible or you may end up with a costly 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.

Summary/TLDR

• Filtration is vitally important to water quality as it removes dirt, debris, and small particles.
• Filters require regular cleaning.
• Filters are useless on their own and require water flow provided by a pump.
• Blockages need to be cleared to enable continued flow.
• Regularly empty skimmer baskets, pump baskets, check pool cleaner heads, and clean pool filters.
• When emptying the baskets it is important to be careful that no debris gets through to the pump impeller as that may cause blockages in the impeller vanes.