3.2 Filters (sand, DE, cartridge)
Key Takeaways
- Filtration removes suspended particles that chlorine cannot destroy; the three common types are sand, diatomaceous earth (DE), and cartridge.
- High-rate sand filters run about 15 gpm/ft², trap roughly 20–40 microns, and are backwashed when pressure rises about 8–10 psi above the clean baseline.
- DE filters give the finest filtration (about 1–5 microns) but must be recharged with fresh DE through the skimmer after every backwash.
- Cartridge filters do not backwash — you hose the pleated element clean and eventually replace it — which saves water in conservation areas.
- Watch the pressure differential: a clean filter reads low pressure and high flow, and a rise of about 8–10 psi over clean signals it is time to clean or backwash.
Why Filtration Matters
Circulation carries dirt from the pool to the filter, and filtration physically strains out the suspended particles — dust, body oils, pollen, fine debris — that chlorine cannot destroy. Sanitizer kills microorganisms and oxidizes waste; the filter removes the cloudiness. Together they keep water clear and safe. Particle size is measured in microns (millionths of a meter); the human eye can just detect a particle around 35 microns, so a filter that removes finer particles produces visibly clearer water. The three types the CPO exam expects you to know are high-rate sand, diatomaceous earth (DE), and cartridge.
The Three Filter Types
High-Rate Sand
A high-rate sand filter passes water downward through a bed of specially graded #20 silica sand. Particles lodge in the tiny spaces between the sand grains, and the media traps debris down to roughly 20–40 microns — the coarsest of the three. Design flow for a high-rate unit is about 15 gpm per square foot of filter area. As dirt accumulates, resistance rises and the pressure gauge climbs. Backwash when the gauge reads about 8–10 psi above the clean starting pressure: reverse the flow so it lifts and flushes the trapped debris out to waste, then return the valve to filter. Sand is typically replaced every 5–7 years. Pros: simple, durable, inexpensive, low maintenance. Cons: the coarsest filtration, and backwashing wastes water — including the chemically treated pool water sent to drain.
Diatomaceous Earth (DE)
A DE filter coats a set of grids (fingers) with diatomaceous earth, a fine powder of fossilized diatom skeletons. The powder forms a cake that strains the finest particles — down to about 1–5 microns, giving the clearest water of the three. Design flow is low, around 1.5–2 gpm/ft². When pressure climbs about 8–10 psi over clean, you backwash; but because backwashing strips the DE cake off the grids, you must then recharge (re-coat) the filter by adding a fresh slurry of DE through the skimmer. Periodically the grids are pulled and the old cake hosed off, and spent DE is disposed of per local rules. Pros: finest filtration and sparkling clarity. Cons: the most maintenance, recharge required after every backwash, and DE powder is a respiratory irritant — wear a dust mask when handling it.
Cartridge
A cartridge filter passes water through a large, pleated polyester element whose folds provide a big surface area, trapping particles to roughly 10–20 microns. Because flow per unit of area is modest, cartridge systems use large-surface elements. Crucially, cartridge filters do not backwash. When pressure rises about 8–10 psi over clean, you shut off the pump, remove the cartridge, and hose it clean (soaking it periodically to remove oils); a worn element is simply replaced. Pros: no backwashing, which conserves water and suits drought or water-restricted areas, gentle on plumbing, and no sand or DE to handle. Cons: cleaning is a hands-on chore, cartridges eventually need replacement, and filtration is not as fine as DE.
A facility in a water-conservation district needs a filter that never sends treated water to the drain during cleaning. Which type best fits, and how is it maintained?
Reading the Gauge and Backwashing
The single most important filter skill is watching the pressure differential. Record the clean pressure right after servicing the filter — this is your baseline. When the gauge later climbs about 8–10 psi above that clean baseline, the filter needs attention, whether that means backwashing (sand and DE) or hosing (cartridge). Never wait for the flow to crash: excessive head starves the whole system, wastes pump energy, and can shorten equipment life. Remember the pattern — a clean filter reads low pressure and high flow, while a dirty filter reads high pressure and low flow. After a sand or DE backwash the pressure should drop back near the clean baseline; if it does not, the media may be channeled, caked, or due for replacement (or, for DE, not yet recharged).
Because DE loses its filter cake during backwashing, always recharge it with the correct amount of fresh DE through the skimmer before returning the pool to normal operation — running a DE filter with bare grids lets fine dirt pass and can damage the fabric. For sand, avoid over-backwashing, which simply wastes water without improving clarity.
Filter Comparison
| Filter type | Typical micron range | Design flow rate | Cleaning method | Pros | Cons |
|---|---|---|---|---|---|
| High-rate sand | ~20–40 microns | ~15 gpm/ft² | Backwash at +8–10 psi | Simple, cheap, durable, low upkeep | Coarsest filtration; backwashing wastes water |
| Diatomaceous earth (DE) | ~1–5 microns | ~1.5–2 gpm/ft² | Backwash, then recharge DE via skimmer | Finest clarity of the three | Most maintenance; DE is a respiratory irritant |
| Cartridge | ~10–20 microns | Low per ft² (large element) | Hose clean / replace (no backwash) | No backwash, saves water, gentle on plumbing | Manual cleaning; element must be replaced |
Use the table as a quick decision aid: choose sand for durability and simplicity, DE when water clarity is the top priority, and cartridge where conserving water and avoiding backwash discharge matter most.
The pressure gauge on a high-rate sand filter has risen from its clean reading of 12 psi to 21 psi, and flow through the returns has weakened. What does this indicate and what is the correct response?
After backwashing a diatomaceous earth (DE) filter, what additional step is required that is NOT needed on a sand filter?