What the anthracite, sand and garnet layers actually do, how they re-stratify after backwash, and the ES / UC / SG numbers a competent tender should specify.
Effective Size, Uniformity Coefficient, Specific Gravity
A multimedia filter is not "sand with extra layers". It is a graded bed engineered so each layer captures the particle fraction the layer above missed. Selection of effective size (ES), uniformity coefficient (UC) and specific gravity (SG) determines whether the bed will stratify correctly after backwash and whether breakthrough or head-loss limits the run length.
Anthracite over Silica Sand over Garnet
| Layer | Media | ES (mm) | UC | SG | Depth (mm) | Role |
|---|---|---|---|---|---|---|
| Top | Anthracite | 0.85–1.10 | ≤ 1.5 | 1.4–1.7 | 400–500 | Coarse depth filtration, holds 70% of solids load |
| Middle | Silica sand | 0.45–0.55 | ≤ 1.5 | 2.6–2.7 | 250–350 | Primary clarification, micron-range capture |
| Bottom | Garnet | 0.20–0.30 | ≤ 1.6 | 4.0–4.2 | 75–150 | Polishing layer, prevents fine breakthrough |
| Support 1 | Coarse sand or fine gravel | 1.5–3.0 | – | 2.6–2.7 | 50–75 | Transition above nozzle plate |
| Support 2 | Gravel | 3–6 | – | 2.6–2.7 | 75–150 | Distributor cover, prevents fines bleed |
| Support 3 | Coarse gravel | 6–12 | – | 2.6–2.7 | 100–150 | Bottom layer over lateral underdrain |
Why the inversion works: after backwash the lighter anthracite (SG 1.5) settles slower than the denser sand (SG 2.65) and garnet (SG 4.1), so the stack re-stratifies coarse-to-fine top-down despite the descending size order. Get the SG ratios wrong and the bed mixes — channelling within hours.
ES, UC and SG — What Tenders Should Specify
The mesh size that passes 10% of the media by weight (d10). Sets the cut-off of what the layer captures. Too small → rapid head loss build-up; too large → breakthrough of fines.
Ratio d60/d10. UC ≤ 1.5 is the practical ceiling for filter sand; higher UC means a wide size range, which packs tightly after backwash and gives unstable run lengths. AWWA B100 sets UC ≤ 1.7 as the absolute limit.
Dictates how the bed re-stratifies after fluidisation. The ratio between adjacent layers must be tuned so they settle in the intended order — not by particle size alone.
The annual % mass loss to crushing and dissolution. Anthracite is the most friable (1–3 %/yr); garnet is the most stable (< 0.5 %/yr). Plan a top-up every 3–5 years.
AWWA B100 limits acid solubility to ≤ 5 % for sand and gravel. Critical for low-pH duties — pickle-liquor, mine drainage, citrus effluent.
Closer to 1.0 = round = lower head loss for the same ES. Crushed garnet (sphericity ~0.6) loses head faster than well-rounded river sand (sphericity ~0.85).
When the Standard Stack Is Not the Right Stack
| Combination | Layers | When to Specify |
|---|---|---|
| Dual media | Anthracite + sand | Lower Capital expenditure, conventional surface water; loses the garnet polishing for < 5 µm |
| Standard 3-layer | Anthracite + sand + garnet | Default for RO pretreatment, reuse polishing, tertiary effluent |
| Greensand / pyrolusite | Top: anthracite. Middle: manganese greensand. Bottom: garnet. | Iron, manganese, hydrogen sulphide removal — see aeration fundamentals for upstream oxidation |
| GAC cap | GAC on top of sand | Combined adsorption + filtration for taste/odour, microcontaminants — see GAC filter |
| Pumice top | Pumice + sand + garnet | Very high influent solids > 50 NTU; pumice (SG 0.8) carries more cake |
| Synthetic media | Polymeric beads + sand | Where extreme cake load demands the highest depth-filtration capacity |
Cross-Links into the MMF Topic Cluster
How freeboard, distributor selection and nozzle plates depend on the media stack chosen above.
Read MoreAir-scour and water-rinse rates calculated from the media SG and ES.
Read MoreWhere this media stack fits in pretreatment, reuse and tertiary trains.
Read MoreMudballs, channelling and bed cementation diagnosed from media core samples.
Read MoreBack to the main multimedia filter page and product range.
Read MoreConfirm the media stack on real influent before committing to a full-scale order.
Read MoreA re-bed costs more than the original media order. Send us your influent analysis, target turbidity and the next process stage — we will specify a stack that re-stratifies cleanly and runs the design length.
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