Three measurements, three questions
All three describe undissolved material in water, but they answer different engineering questions and do not convert into each other.
Total suspended solids (TSS, mg/L) is the actual mass of particles, found by filtering and weighing a sample. It answers how much material will my filter have to hold? — the number that sizes solids-handling duty on borehole, river and process water.
Turbidity (NTU) is an optical measurement: how much the particles scatter light. It answers how dirty does it look, and is disinfection safe? Drinking water should sit at or below 1 NTU — above that, particles shield microorganisms from chlorine and UV. Turbidity is fast and cheap, which is why it is the routine monitoring number, but it says nothing reliable about particle mass or size: fine clay reads high while dense sand reads low.
Silt Density Index (SDI) is a plugging test: municipal-style water is pushed through a 0.45 micron membrane disc at 2.07 bar, and the test measures how quickly the disc blinds over 15 minutes. It answers exactly one question — will this water foul a reverse osmosis membrane? — and it is the only one of the three that answers it.
Why RO needs SDI, not turbidity
A water can pass the turbidity test and still fail SDI badly. Colloids — particles so fine they barely scatter light — sail through a nephelometer reading below 1 NTU, then blanket the lead RO element in weeks. Membrane manufacturers therefore specify SDI, not NTU:
| SDI (15 min) | Meaning for RO |
|---|---|
| < 3 | Good feed; low fouling rate expected |
| 3 – 5 | Acceptable; DuPont's stated maximum is 5, but expect more frequent cleaning |
| > 5 | Do not run RO on this water — fix pretreatment first |
If SDI is high, the fix is upstream: media filtration, ultrafiltration, or coagulation ahead of the filters — not simply finer cartridges, which blind quickly for the same reason the test disc does.
Practical numbers for filter selection
- Municipal water: typically < 1 NTU, SDI 2 – 5. A 5 micron cartridge ahead of sensitive equipment is usually sufficient.
- Borehole water: turbidity varies enormously and often spikes on start-up as the borehole surges. Sample after several minutes of pumping, and size sediment removal for the spike, not the steady state.
- Surface water: seasonal. Design for the wet-season worst case; a media filter that loafs in winter and copes in summer beats a cartridge bank that must be changed weekly.
Rule of thumb: turbidity protects disinfection (keep it under 1 NTU ahead of UV or chlorine); SDI protects membranes (keep it under 5, preferably under 3, ahead of RO); TSS sizes the equipment that has to catch it all. Ask for the measurement that matches the risk you are managing.