Swimming pool filtration plays a crucial role in maintaining clean and safe pool water. Recent outbreaks of cryptosporidiosis have underscored its importance. This organism cannot be eliminated by disinfectant alone and must be removed by filtration to prevent infection.

The filtration process is quite simple: water is drawn from the pool through outlets and a surface water draw-off system and directed to the top of the filter. The water passes through the filter media, usually sand, which traps contaminants and pollution. The filtered water then continues through the remaining components of the pool plant system.

Maintaining clear pool water is essential for user safety. Lifeguards must be able to see the depth of the water and spot a submerged casualty. If the water clarity falls below an established level, an Emergency Action Plan should be enacted. Admissions should be suspended and the pool cleared until the clarity reaches an acceptable level. The pool water clarity should be continuously monitored and the treatment system should provide clarity of no more than 0.5 nephelometric turbidity units (NTU).

Turbidity, or the presence of suspended colloidal or particulate matter, reduces pool water clarity. Identifying the source of excess turbidity is important for effective remediation. This could be pollution from bathers, external contamination, inadequate circulation or disinfection, or incorrect use of water treatment chemicals. Adequate filtration and backwashing, coupled with coagulation, are the likeliest remedies.

Swimming pool filters are typically designed with a vertical orientation and constructed using mild steel, stainless steel, plastic, or concrete. Mild steel is the most common material, and the vessel is lined with rubber or epoxy paint to prevent corrosion.

Properly selected and installed filters should last at least 25 years with appropriate inspection and maintenance. An annual inspection by a competent person is necessary to check for signs of physical wear or damage to the filter vessel and lining, as well as the condition of the media bed. An uneven or shallow bed, mud-balling, cracks, or fissures should be identified and rectified.

The filter media bed may need to be replaced every 5-10 years, depending on its condition during 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 sign of damage to the underdrain system.

Water flows downward through the filter, passing through the pores between sand grains. Pollution in the water is trapped in the sand bed layer through sedimentation, adsorption, and mechanical straining. The size of the sand grains used in swimming pool filters is usually 0.5-1.0mm, resulting in a pore size of approximately 50-70 microns. Anything larger than the pore size becomes trapped, while smaller particles may pass through unless they settle on the upper-facing surface of a sand grain or adhere to it through adsorption.

Sedimentation removes finer particles of pollution than straining, as the fine particulate matter settles on the upward-facing surfaces of the sand grains. However, as the amount of sediment increases, the space between sand grains decreases, causing the water velocity to increase. Further sedimentation cannot occur, and some sediment may be pushed further down into the filter bed due to the higher velocity. Adsorption, on the other hand, involves particles of pollution adhering to the sand grains through electrostatic charges. A sticky coating builds up, promoting further adherence of particles onto the filter media.