DE Filter: How Diatomaceous Earth Filters Work and How to Care for Them
A DE filter is the finest-filtering of the three common pool filter types, capable of trapping particles as small as 2-5 microns, finer than sand or cartridge filters. "DE" stands for diatomaceous earth, a soft, powdery sedimentary rock made from the fossilized skeletons of diatoms (microscopic algae). Coated onto a set of internal grids, DE forms a filter cake that strains the water to remarkable clarity.
How a DE filter works
Inside the tank is a set of grids (fabric-covered frames) or, in some designs, flexible fingers/tubes. You add DE powder through the skimmer, the pump pulls it into the tank, and it coats the grids in an even layer. As water passes through this coating, the fine porous structure of the DE traps very small debris. Oils, lotions, and organics don't get "filtered out" so much as they foul the DE cake and grids over time, which is why periodic deep cleaning matters (more below).
Recharging DE after backwashing
This is the single most important thing to understand about DE filters, and the one owners most often get wrong. When you backwash a DE filter, you flush out the old, dirty DE along with the trapped debris. That means the grids are left bare, so you must add fresh DE afterward to recoat them. After backwashing, add the amount of DE your manual specifies (often close to a full charge).
How much DE to add
The correct amount is based on your filter's square footage. Check the label on your filter or the owner's manual for the exact figure. A common rule of thumb is roughly one pound of DE per 5 square feet of filter area, but always follow your manufacturer's spec. DE is added by mixing it with water into a slurry and pouring it slowly into the skimmer with the pump running.
Routine maintenance
- Record your clean starting pressure right after a fresh DE charge. That's your baseline.
- Backwash when pressure rises about 20-25% above that clean pressure (or per your manufacturer). For example, a clean pressure of 12 psi means cleaning at around 15 psi. Then recharge with fresh DE.
- Do a full teardown and deep clean at least once per season (and at closing), or more often as needed after algae cleanup, heavy oils, or persistently high pressure. Remove the grids, hose them off, and inspect. Backwashing alone gradually leaves behind caked DE and oils.
- Inspect grids for rips. A torn grid (or a cracked manifold or worn standpipe o-ring) lets DE blow back into the pool, and you'll see a puff of white powder from the returns. Replace damaged grids.
- Use a filter cleaner/degreaser during teardown to strip oils and mineral scale that plain rinsing leaves behind.
Handling DE powder safely
Use only the filter/pool-grade DE specified by your filter manufacturer. It's engineered for filtration performance. Food-grade DE is a different product with different properties and isn't a substitute for what your filter manual calls for. Regardless of grade, avoid breathing the dust.
DE vs. sand vs. cartridge
- DE: Finest filtration (about 2-5 microns), the clearest water, but the most hands-on. Requires recharging after backwash and periodic teardown.
- Sand: Simplest to maintain, backwash to clean, but filters only to about 20-40 microns. See Sand Filter Care.
- Cartridge: No backwashing (rinse the cartridge), filtration typically around 10-20 microns (varies by model), moderate maintenance. See Cartridge Filter Care.
Disposing of used DE
Backwashed DE waste is a slurry that shouldn't go into a septic system or storm drain in many areas, so check local regulations. Some regions require backwashing to a designated waste line or a DE separation tank that captures the spent powder.