The highest-performance multimedia filter configuration: three stratified layers of anthracite, silica sand, and garnet delivering SDI < 3 for demanding reverse osmosis pre-treatment duties. Industry standard for borehole water, desalination pre-treatment, and boiler feedwater preparation.
The triple-media filter adds a dense garnet layer beneath the sand, extending the coarse-to-fine particle removal gradient to three distinct pore sizes. The result is the highest storage capacity, the longest filter runs, and the lowest SDI output of any granular media filter without the complexity of membrane pre-filtration.
ES 0.8–1.4 mm · S.G. ~1.5 · Depth 250–400 mm. Coarse capture layer. Removes large flocs, algae, and colloidal turbidity. Low density ensures correct stratification above sand after backwash.
ES 0.45–0.55 mm · S.G. ~2.65 · Depth 300–450 mm. Intermediate polishing. Removes particles 5–50 μm that pass through the anthracite. The main working layer for SDI reduction.
ES 0.2–0.4 mm · S.G. 4.0–4.2 · Depth 100–200 mm. Fine polishing to < 5 μm. High density keeps garnet below sand during backwash, even at high upflow rates. Directly governs final SDI.
SDI mechanism: The garnet layer provides the finest pore spaces in the bed and is responsible for reducing SDI from the 3–5 typically achieved by dual-media down to < 2–3. The critical parameter is garnet effective size: finer ES gives lower SDI but higher backwash pressure drop. Our engineers size the garnet layer to the target SDI and available backwash pressure at each site.
| Application | Feed Water | Target SDI Out | Typical SLR | Media Config. |
|---|---|---|---|---|
| Boiler feed (high pressure) | Borehole or mains | < 3 | 12–15 m/h | A 350mm / S 400mm / G 150mm |
| Desalination pre-treatment | Seawater / coastal borehole | < 3 | 10–15 m/h | A 300mm / S 450mm / G 200mm |
| Pharmaceutical RO | Mains or WFI loop | < 2 | 10–12 m/h | A 250mm / S 400mm / G 200mm |
| Industrial process water | River or reservoir | < 3–5 | 12–18 m/h | A 400mm / S 450mm / G 100mm |
| Food & beverage process water | Borehole or mains | < 3 | 12–15 m/h | A 350mm / S 400mm / G 150mm |
Borehole water commonly contains dissolved iron and manganese at concentrations that, even when present at seemingly low levels, can cause severe fouling of RO membranes. Oxidised iron precipitates as gelatinous ferric hydroxide that coats membrane surfaces and blocks spacers within days. Manganese dioxide is similarly problematic.
Critical thresholds for RO membranes: Fe < 0.05 mg/L and Mn < 0.02 mg/L before the RO feed. These are tighter than drinking-water standards (Fe < 0.2 mg/L, Mn < 0.05 mg/L). Where borehole iron exceeds 0.2 mg/L, a dedicated aeration + greensand or Birm / Filox media stage upstream of the triple-media filter is essential. The triple-media filter then polishes the remaining oxidised particles to achieve the membrane-safe threshold.
Dissolved Fe²⁺ and Mn²⁺ must be oxidised to their insoluble Fe³⁺ and MnO₂ forms before filtration. Options include atmospheric aeration towers, pressure aeration vessels, or chemical oxidation with chlorine, potassium permanganate, or ozone. The choice depends on iron/manganese concentrations and site constraints.
Manganese greensand (glauconite coated with MnO₂) acts as both a filter medium and an oxidising agent for residual dissolved manganese. Requires periodic regeneration with potassium permanganate. Suitable for Mn < 1.5 mg/L.
Catalytic iron and manganese removal media that do not require chemical regeneration — dissolved oxygen in the feed provides the oxidising capacity. Birm suits Fe/Mn removal at dissolved oxygen > 15% saturation. Filox handles higher Fe/Mn concentrations and lower DO.
Media options, duplex arrangements and a worked pre-RO example
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.
Our expertise spans multiple industries with sector-specific water treatment solutions.