Pool filtration removes suspended particles from water — fine particles that cause cloudiness, dead algae cells, and small debris. All three main filter types (sand, cartridge, DE) work on the same principle: passing water through a medium that captures particles.
Key Facts
- Sand filters capture particles down to 20–40 microns; cartridge filters to 10–15 microns; DE filters to 3–5 microns.
- Filter pressure above 8–10 psi over baseline indicates a dirty filter that needs cleaning.
- Pool water should turn over (pass through the filter) at least once every 8 hours.
- Running the filter is the most important single factor in maintaining clear water — chemistry alone cannot compensate for insufficient filtration.
How Filtration Works
Pool filters remove particles by forcing water through a filter medium that physically captures and holds particles as the water passes through. Larger particles are captured easily; finer particles require a finer filter medium or coagulation (via clarifier) to form larger clumps that the filter can capture. All filters have a maximum particle size they can capture and a maximum flow rate beyond which particles pass through without being captured. The filter must be sized correctly for the pump flow rate and the pool volume to function effectively.
Pressure and Run Time
Every filter has a pressure gauge. When the filter is clean, note the baseline pressure. As the filter captures particles, the pressure rises. An increase of 8–10 psi above baseline is the standard backwash or cleaning threshold — beyond this, the filter resistance is reducing flow rate and filtration efficiency. Run the pool pump long enough each day to achieve at least one complete turnover of the pool volume. For a 15,000-gallon pool with a pump delivering 50 GPM, one turnover takes 300 minutes (5 hours). Running 8 hours per day provides 1.6 turnovers, which is adequate for most pools.
Filter Maintenance
Sand filters: backwash when pressure rises 8–10 psi above clean baseline. Backwash by reversing water flow through the sand bed to flush captured debris to waste. Replace sand every 5–7 years. Cartridge filters: remove and rinse with a hose when pressure rises, and soak in degreaser monthly. Replace cartridge elements annually or when they no longer return to baseline pressure after cleaning. DE (diatomaceous earth) filters: backwash when pressure rises and recharge with fresh DE powder. Bump or backwash after major events. Annual disassembly and cleaning of the internal grids is recommended.
Examples
A 20,000-gallon pool with a 1.5 HP pump that delivers approximately 60 GPM needs a filter that can handle 60 GPM flow without channelling. A sand filter with a 24-inch tank is rated for approximately 60–80 GPM — appropriate for this pool. A 20-inch tank (rated 40–60 GPM) would be borderline — water would pass through faster than it can filter effectively during high-flow periods. When in doubt, choose the next size up — a slightly oversized filter is far less problematic than an undersized one.
Common Mistakes to Avoid
- Backwashing a sand filter without looking at the pressure gauge — visual inspection of the pool is not a reliable indicator of filter status.
- Running the pump for only 4–6 hours in summer when turnover calculation shows 8 hours is needed.
- Not noting the clean baseline pressure when a new filter is installed — without a baseline, there is no way to know when the 8–10 psi rise has been reached.
- Pool & Hot Tub Alliance — Pool & Spa Operator Handbook, 2022