Anthracite over silica sand — a two-layer stratified bed delivering improved particle capture depth, higher surface loading rates, and longer filter runs than single-media sand. The standard configuration for general-purpose RO pre-filtration on moderate-quality feed water.
After backwash, density-graded media naturally restratifies so the coarser, lighter anthracite layer (effective size 0.8–1.4 mm, S.G. ≈ 1.5) sits above the denser, finer silica sand (0.45–0.55 mm ES, S.G. ≈ 2.65). This produces a coarse-to-fine pore gradient in the direction of flow, allowing larger particles to be captured throughout the full depth of the bed rather than exclusively at the surface.
ES 0.8–1.4 mm · UC < 1.7 · S.G. 1.45–1.55 · Depth 300–450 mm. Captures large flocs and colloidal particles in the upper bed. Low specific gravity ensures it stays above sand after backwash.
ES 0.45–0.55 mm · UC < 1.6 · S.G. 2.60–2.70 · Depth 400–600 mm. Polishing layer capturing fine particles < 5 μm. Support gravel (2–32 mm) on underdrain below.
Why coarse-to-fine matters: In a single-media sand filter, the finest sand grains migrate to the top of the bed after backwash, creating a thin surface mat that blinds rapidly. The depth of storage is largely unused. A dual-media bed exploits the full bed depth for solids storage, increasing the volume of particulate removed per filter run and reducing backwash frequency.
Dual-media beds are routinely designed at 12–18 m/h SLR compared to 8–12 m/h for single-media, reducing vessel size by 25–40% for the same flow rate and achieving a better capital cost per m³/hr treated.
Run length between backwashes is typically 2–3× longer than a single-media bed at the same SLR, reducing backwash water consumption and pump starts. On clean borehole water at 15 m/h, 24–48 hour runs are achievable.
Dual-media consistently achieves SDI < 5 on typical groundwater and moderate-turbidity surface water without coagulant. SDI < 3 is achievable on good-quality borehole feeds with turbidity < 1 NTU.
Upflow backwash at 20–30 m/h re-stratifies the two media layers cleanly due to their density differential. Air scour is recommended before the water backwash on feeds with organic content (> 2 mg/L TOC) that can bind fine particles to anthracite surfaces.
| Criterion | Single-Media Sand | Dual-Media A+S | Triple-Media A+S+G |
|---|---|---|---|
| Typical SLR (m/h) | 8–12 | 12–18 | 12–20 |
| SDI out | < 5–6 | < 3–5 | < 2–3 |
| Run length | 8–16 h | 12–48 h | 16–60 h |
| Capital cost (relative) | Low | Medium | Medium–High |
| Media replacement cost | Low | Low–Medium | Medium |
| Best application | Mains/clean borehole | General pre-RO | Critical pre-RO, variable quality |
Technology selection, SDI targets, hydraulic design, and backwash sizing for multimedia pre-RO filtration.
View OverviewSingle-grade sand filtration for lower-turbidity feeds where SDI < 5 is acceptable without multi-layer grading.
View PageAnthracite over sand for improved particle capture, higher flux, and longer filter runs between backwashes.
View PageAnthracite / sand / garnet for the best SDI reduction — the standard pre-RO media configuration.
View PageParallel vessel pairs for uninterrupted 24/7 operation where backwash must not cut the downstream supply.
View PageWorked example: borehole → duplex multimedia → RO → steam boiler feed at 24 m³/hr.
View ExampleOur process engineers will review your raw water analysis, flow rate, SDI targets, and downstream RO membrane specification to recommend the optimum media type, vessel size, backwash regime, and control philosophy.
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