Pool Plant Courses - Blog

Filtration is an important element of effective pool water treatment. The basic principle is that the untreated water is passed through a filtering medium (such as a bed of sand). The water is able to pass through the gaps between the grains of sand (called ‘pores’), but anything larger than the pore size is trapped within the filtering medium.

A reduction in the clarity of the pool water is a risk to pool users. It is essential that bathers are able to assess the depth of the water and for lifeguards to see a casualty below the surface of the water. If the water clarity falls below a stated level (defined in the EAP), the EAP should identify the procedures for suspending admissions and clearing the pool until the clarity reaches an acceptable level (as a minimum, the ability to see the body of a small child if it were located on the floor of the pool in the deepest water). The clarity of the pool water should be constantly monitored.

The pool water treatment system should be capable of providing clarity of no more than 0.5 nephelometric turbidity units (NTU). Clarity is reduced by turbidity – colloidal or particulate matter in suspension in the water. It is important to know the source of excess turbidity – whether pollution from bathers, external contamination, inadequate circulation/turnover or disinfection, or incorrect use of water treatment chemicals – in case this can be dealt with directly. The likeliest remedy, however, is adequate filtration and backwashing, coupled with coagulation.

Swimming pool filters are usually designed to a vertical orientation and are made out of various types of material:

The most commonly used is mild steel. The inside surface is lined with either rubber or epoxy paint in order to protect the vessel from corrosion.

Filters that have been appropriately selected and installed should last at least 25 years with proper arrangements in place for inspection and maintenance. On an annual basis, the filters should be opened and inspected by a competent person. For most facilities, this will mean using an external contractor. They should be looking for signs of physical wear or damage to the filter vessel and lining and the condition of the media bed. Issues such as an uneven or shallow bed, mud-balling, crack, fissures etc. should be identified and rectified.

The filter media bed may need to be replaced every 5-10 years, depending on its condition during the routine annual inspections. This provides an opportunity to inspect the underdrains for damage and repair or replace as needed. Deposits of sand on the pool bottom can be a sigh of damage to the underdrain system 

In swimming pool filters, the size of the sand grains is usually 0.5 - 1.0mm. This results in a pore size of approx. 50 – 70 microns (1mm = 1000 microns). Anything too big to pass through the pores will become entrapped, anything smaller may pass through unless they settle on the upper-facing surface of a sand grain, or they stick to the surface of a sand grain via adsorption. 

Sedimentation is where fine particulate matter settles on the upward-facing surfaces of the sand grains. The process of sedimentation can remove finer particles of pollution than straining. As the amount of sediment increases, the amount of space in between sand grains (pores) decreases. This will cause the velocity of water through the filter to increase. Further sedimentation can then no longer occur and, due to the higher velocity, some sediment could get pushed further down into the filter bed.  

Adsorption is where particles of pollution adhere to the sand grains. It is not to be confused with absorption. The process is promoted by electrostatic charges within the particles (similar to a balloon 'sticking' to a wall). Once particles begin to adhere to the sand grains, a sticky coating builds up, which promotes further adherence of particles onto the filter media. 

The filtration rate is the rate (in metres per hour) at which the pool water moves down through the filter during normal operation. It is not to be confused with the flow rate, which is the rate (in cubic metres per hour) at which water is moving through the circulation system.

The filtration rate is calculated by dividing the flow rate by the surface area of the filter. For example:

Circulation Rate: 100 cubic metres per hour 

Filter Surface Area: 5.72 metres squared

Filtration Rate: 100m3 / 5.72m2 = 17.48m3/m2/hr

You may already have a flow rate meter fitted onto the circulation pipework in your plant room to tell you what the flow rate is. If you do not have one fitted, it is highly recommended that you get one fitted as soon as possible as it will be difficult to calculate the turnover time and filtration rate without having the key piece of data that a flow rate meter provides.

If you are trying to calculate your filtration rate, but don’t know the surface area of the filter, you can easily calculate what the filter surface area is by using the formula: r2 x pi. This formula means the radius squared multiplied by pi. See below for an example using a filter that is 2.7 metres wide: 

Width of filter: 2.7 metres (this is also the diameter) 

Radius: 2.7m     /     2         =    1.35m 

Radius Squared: 1.35m  x    1.35m =    1.82 m2 

Surface Area: 1.82m   x    3.14 (pi)   =    5.72 m 2

There are three categories of filtration rate and they are set out below:

Low Rate: up to 10 m3/m2/hr very good filtration, but requires a very large surface area

Medium Rate: 10 - 25 m3/m2/hr recommended for public pools

High Rate: 25 - 50 m3/m2/hr recommended for small domestic pools only because this rate is too fast to deal with pollution in public pools