How do swimming pool filters work?

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.

Filtration is an essential part of swimming pool water treatment and its importance has been emphasised in recent years due to several outbreaks of cryptosporidiosis, because this organism is not killed by the disinfectant in the pool water and therefore must be removed by filtration in order to prevent bathers becoming infected by it.

Filtration is a fairly simple process; water leaves the pool via the deep end outlets and the surface water draw-off system (deck-level, skimmer baskets, overflow channels etc.). It is piped to the plant room and gets directed into the top of the filter (or several filters in large pools), passes through the filter media (usually sand) where all the contaminants and pollution are trapped and the pool water comes out of the bottom and continues through the remaining components of the pool plant system.

Pool Water Clarity

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 Normal Operating Procedures), an Emergency Action Plan 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.

Filter Design and Construction

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

  • Mild steel
  • Stainless steel
  • Plastic
  • Concrete

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.

Inspection and Maintenance

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.

The basic sand filtration process

Water moves in a downward direction through the filter and gets passed through the pores between the sand grains. This is illustrated by the blue arrows.

Pollution within the water becomes entrained within the sand bed layer via processes of sedimentation, adsorption and mechanical straining. This is illustrated by the green areas.

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. With adsorption, very small particles of pollution adhere to the surface of the sand grains. This 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.