From the outside, filtration looks simple. Do it for a living and it's physics, chemistry, maintenance cycles, and media science. This pillar is the engineering layer above the weekly filter-cleaning basics — filter media rate, what each media type can actually remove, and how to size a filter to a pool.
The size-what-you-catch chart
| Particle | Size (microns) |
|---|---|
| Table salt | ~100 |
| Human hair | ~70 |
| Pollen | ~25 |
| Red blood cell | ~7 |
| E. coli bacteria | ~4 |
| Cryptosporidium oocyst | ~4 |
| Virus | ~0.1 |
Sand filters catch down to ~20 µ. Cartridge to ~10 µ. DE to ~2–5 µ. This is why a perfectly working sand filter still can't catch Crypto — and why RWI response sometimes requires flocculant-enhanced filtration.
Filter media rate (FMR)
FMR is gallons per minute per square foot of filter area. It's the single number that tells you whether your filter is sized right. Published maximums:
- Sand: 3–15 gpm/ft² (low-rate vs. high-rate)
- Cartridge: 0.375 gpm/ft²
- DE: 2 gpm/ft²