4.2 Filtration operation & backwashing

Filtration Fundamentals

Filtration removes suspended particles that disinfection alone cannot handle. Three filter media dominate public pools: high-rate sand, cartridge, and diatomaceous earth (DE). Sand strains debris through a bed of specially graded sand; cartridge filters trap dirt in a pleated polyester element; DE coats internal grids with a fine powder that filters down to a few microns. Whatever the media, the filter only works while water is flowing at the correct rate and the media is clean.

Filtration Rate

Filtration rate is the flow passing through each square foot of filter surface area, expressed in gpm per square foot (gpm/ft²). Each media has a maximum design rate:

• High-rate sand: up to about 15 gpm/ft² (some units rated to 20)
• Cartridge: about 0.375 gpm/ft² (roughly 1 gpm per 2.7 ft²)
• Diatomaceous earth: about 2 gpm/ft²

Worked example: a high-rate sand filter tank is 24 inches in diameter, so its radius is 1 foot and its surface area is π × radius² = 3.14 × 1² = 3.14 ft². At the maximum 15 gpm/ft², the filter can process 3.14 × 15 = 47 gpm. If your turnover calculation demanded 83 gpm, this single tank is far too small; you would need a larger tank or multiple filters. Running a filter above its rated gpm/ft² drives dirt straight through the media and ruins clarity, so filtration rate directly limits how fast you can turn the water over.

Filter Run Cycles and Pressure

A clean filter has a baseline (clean) pressure read on the influent gauge; note it every time you service the filter. As the media captures dirt, resistance rises, the pressure gauge climbs, and flow drops. The filter run is the time between cleanings. The standard rule is to backwash or clean when pressure rises 8 to 10 psi above the clean starting pressure (some manufacturers instead say when flow falls below the required turnover). Waiting longer chokes flow and lengthens turnover; cleaning too early wastes water and chemicals.

Backwashing Sand and DE Filters

Backwashing reverses the flow of water through the filter, lifting trapped debris off the media and flushing it to waste. The basic procedure:

1. Turn off the pump before moving any valve.
2. Set the multiport valve to BACKWASH.
3. Restart the pump and run until the sight glass runs clear, usually 2 to 3 minutes.
4. Turn off the pump and set the valve to RINSE; run 20 to 30 seconds to reseat the sand bed and clear the lines.
5. Turn off the pump, return the valve to FILTER, and record the new clean pressure.

Cartridge filters are never backwashed; you remove the elements and rinse them with a hose. DE filters must be recharged after every backwash: mix fresh DE powder into a slurry and pour it into the skimmer so it coats the grids evenly, using roughly 1 pound of DE per 10 square feet of grid area. Skipping the recharge leaves bare grids that will not filter.

Filter Startup and Air Relief

Before returning a filter to service, open the air-relief valve on top of the tank and run the pump until a steady stream of water, with no air, escapes, then close it. Trapped air is compressible and can cause the tank lid or clamp band to fail under pressure, which is a serious safety hazard. Always confirm the pump is off before moving a multiport valve, and never start the pump with the valve in the Closed position.

Multiport Valve Positions

Media Care and Replacement

Filter media does not last forever. High-rate sand gradually rounds off and channels, so it is typically replaced every 5 to 7 years; if backwashing no longer restores flow, suspect worn or channeled sand that lets water tunnel straight through. Cartridge elements are periodically soaked in a cleaning solution, rinsed, and replaced once the pleats will no longer clean up. DE grids should be inspected for tears that let powder blow back into the pool. Handle DE only with good ventilation, because the dry powder is a respiratory irritant; add it slowly and wear a dust mask when charging the filter.

Measuring Flow

You cannot manage what you do not measure. A flow meter installed on the return line shows actual gpm; compare it to the turnover-based target from section 4.1. Divide measured gpm by filter area to confirm the filtration rate stays within the media's limit. A steadily falling flow with rising pressure means the filter needs cleaning; low flow with normal pressure points elsewhere, such as a clogged pump basket, a closed valve, or a suction-side air leak. Logging clean pressure, dirty pressure, and flow at each service turns filtration from guesswork into a documented, repeatable routine.