Density stratification is what makes a multi-media bed work. After backwash each layer re-forms by specific gravity, heaviest at the bottom, without a support grid carrying the load. Garnet at SG 3.9–4.1 sits below silica sand at 2.6–2.7 and anthracite at 1.4–1.7, which is why it can be the fine layer and the ballast at the same time.
| Layer | Specific gravity | Typical effective size | Function |
|---|---|---|---|
| Anthracite | 1.4–1.7 | 0.85–1.10 mm | Coarse solids, some organics |
| Silica sand | 2.6–2.7 | 0.45–0.55 mm | Main filtration |
| Garnet | 3.9–4.1 | 0.20–0.35 mm | Fine filtration and bed ballast |
| Support gravel | 2.6 | 2–5 mm | Distributes flow at the nozzle plate |
Values are typical for municipal and industrial multi-media vessels. Your vessel drawing and the media supplier’s sieve curve govern.
Three numbers decide performance
- Effective size (d10) — sets the finest particle the bed can hold and the head loss it costs.
- Uniformity coefficient (d60/d10) — sets how much the layer stratifies internally on backwash. AWWA B100 caps it at 1.7; 1.5 or below is easier to keep clean.
- Bed depth to ES ratio — below about 1,000, breakthrough comes early and backwash frequency climbs.
Uniformity coefficient is the number buyers skip. At UC 1.7 the layer holds grains from d10 up to nearly 1.7×d10. On backwash the fine fraction lifts first and re-settles on top, so the top of the garnet layer gets finer with every cycle and the bottom gets coarser. Filtration rate then drifts with bed age, which is usually blamed on the media supplier rather than on the specification.
Backwash is where installations fail
Heavier media needs a higher backwash velocity for the same bed expansion. Dropping garnet into a vessel sized for sand, without recalculating, gives one of two outcomes: a bed that never fully cleans, or one that lifts into the anthracite and never separates again.
| Layer | Target expansion | Note |
|---|---|---|
| Anthracite | 25–35% | Lifts first; the first sign of over-rate |
| Silica sand | 15–25% | Reference layer for most designs |
| Garnet | 5–15% | Little visible expansion is normal — do not chase sand figures |
Control backwash by expansion, not by a fixed flow rate. The same flow that expands sand 20% moves garnet by a few percent only.
Replacing silica sand in an existing vessel
- Check the support nozzles and gravel: a heavier layer loads them harder.
- Recalculate the backwash pump head — the required velocity is higher.
- Check freeboard: a deeper or denser bed needs more space to expand into.
- Review the air-scour sequence; air first, then water, or the layers mix at the start of every cycle.
What to put in the purchase specification
- Effective size and uniformity coefficient, with the sieve method named.
- Specific gravity and bulk density — they are different numbers and both get used.
- Acid solubility to AWWA B100 or EN 12910.
- Full sieve curve per batch, not a single mesh figure.
- Packaging: 25 kg bags or 1,000 kg jumbo, and how the layer depth is calculated from tonnage.