A common mistake: assuming that clear water means safe water. Clarity measures what your eye can see — but many pathogens are orders of magnitude smaller than anything your eye registers, and smaller than what standard pool filters can capture. Here's the scale.
The particle size chart
| Object | Size (microns) | Visible? |
|---|---|---|
| Fine beach sand | 100–2000 | Yes |
| Table salt grain | ~100 | Yes |
| Human hair | 50–70 | Yes |
| Pollen | 10–100 | Barely |
| Plant spore | 10–50 | Barely |
| Red blood cell | ~7 | No (microscope) |
| E. coli bacterium | 1–4 | No |
| Giardia cyst | 8–14 | No |
| Cryptosporidium oocyst | 3–6 | No |
| Staph bacterium | 0.5–1 | No |
| Virus (typical) | 0.02–0.3 | No |
What each filter type can remove
| Filter type | Practical micron limit | Catches | Doesn't catch |
|---|---|---|---|
| Sand (standard) | 15–20 | Hair, dust, larger debris, most pollen | Bacteria, Crypto, Giardia, viruses |
| Sand (with flocculant) | 5–10 | Most bacteria, some Crypto | Viruses, smallest particles |
| Cartridge | 8–15 | Most of what sand catches, plus larger bacteria | Most RWI pathogens |
| DE (diatomaceous earth) | 2–5 | Bacteria, Crypto, Giardia | Small viruses |
| DE (with coagulant) | ~1 | Nearly everything except viruses | Viruses |
Why this matters for RWI response
During a Cryptosporidium response, hyperchlorination does most of the kill — but the filter must remove the dead oocysts from the water column. A standard sand filter can't catch Crypto alone. Adding a polymeric clarifier or coagulant clumps the oocysts into larger particles the sand can catch.
This is why the CDC protocol for fecal incidents specifies replacing filter media or thoroughly chemical-soaking cartridges after the hold period — because oocysts accumulate in media that was never engineered to catch them, and can reinfect the pool when that media is disturbed later.
Clarity vs. safety
Clarity is measured by turbidity (typically NTU — nephelometric turbidity units). Commercial pool code often requires turbidity below 0.5 NTU or main drain visibility. But turbidity measures particles in the micron-plus range — the range your eye notices. Viruses and the smallest bacteria sail through even a crystal-clear pool.
This is why sanitizer residual plus filtrationis the two-layer system. Filtration removes what can be removed. Sanitizer kills what can't.
When filtration quality matters most
- High bather-load venues (splash pads, community pools): worth upgrading to DE or adding secondary disinfection (UV/ozone)
- Immunocompromised patient facilities (therapy pools): finest filtration plus secondary disinfection
- Post-incident recovery: temporary coagulant use to enhance removal