Pressure multimedia filtration protecting reverse osmosis membranes from particulate fouling. Single, dual, and triple-media configurations; duplex duty/standby; automatic backwash by differential pressure or timer. Designed for borehole, river, and mains feed water.
Single-grade pressure sand filters for reverse osmosis pre-treatment. Turbidity and TSS removal, automatic backwash, GRP or CS vessels. SDI reduction to < 5 for moderate-quality feed water.
Dual-media pressure filters combining anthracite and sand for improved particle capture, higher flux rates, and longer filter runs ahead of reverse osmosis. SDI < 5, flows 1–500 m³/hr.
Triple-media pressure filters (anthracite / silica sand / garnet) delivering SDI < 3 for critical RO pre-treatment. Ideal for borehole water, seawater pre-treatment, and pharmaceutical-grade process.
Duplex duty/standby multimedia filter systems for uninterrupted 24/7 pre-RO filtration. Alternating backwash between parallel vessels eliminates supply interruption for boiler feed, process, and.
Treating a borehole or groundwater supply? See our full approach — aeration and catalytic multimedia filtration for iron and manganese, ultrafiltration for SDI and RO pre-treatment, and disinfection for potable supply — with example arrangements by application.
Borehole TreatmentReverse osmosis membranes are sensitive to suspended solids. Unfiltered or poorly filtered feed water causes rapid fouling of the feed spacer, increasing differential pressure across the RO array, reducing permeate flux, and shortening membrane life. The industry-standard measure of particulate fouling potential is the Silt Density Index (SDI):
SDI target before RO membranes: < 5 for standard spiral-wound elements (acceptable) — < 3 preferred for long membrane life — < 2 for tight schedules or critical supply where maximum runtime between cleaning is required. Borehole water can present SDI of 5–20+ without pre-treatment, making multimedia filtration essential.
Removing particles > 5–10 μm before the RO system prevents spacer blockage, reduces cleaning frequency, and extends membrane element life from 2–3 years to 5+ years.
Well-designed triple-media pressure filters consistently achieve SDI < 3 on groundwater, surface water, and mains feeds with turbidities up to 10–20 NTU without coagulant. Higher turbidities may require upstream coagulation.
Differential-pressure-triggered or timer-initiated backwash restores filter capacity in 10–20 minutes. Duplex arrangements maintain continuous supply during backwash — essential for 24/7 boiler feed or process duties.
Media type is selected based on raw water turbidity, SDI requirement, and downstream RO operating pressure. A triple-media bed (anthracite–sand–garnet) gives the best SDI reduction per unit vessel area; dual-media offers a efficient middle ground.
Borehole supplies commonly contain dissolved iron (> 0.2 mg/L) and manganese (> 0.05 mg/L). These precipitate on RO membranes and scale filaments, causing irreversible fouling. Specialist oxidation and greensand or Birm media remove both prior to the RO stage.
GRP (glass-reinforced plastic) pressure vessels are the most common choice for corrosive or borehole water duties. Lined carbon steel suits larger diameters (> 1,800 mm). Stainless steel is available for pharmaceutical or food-grade requirements.
| Media Type | Layers | Typical SDI Out | Max. Flux (m/h) | Run Length | Best Application | Iron/Mn? |
|---|---|---|---|---|---|---|
| Single-Media Sand | 1 | 3–6 | 8–12 | 8–16 h | Low-turbidity mains or borehole, SDI < 5 acceptable | No |
| Dual-Media (A+S) | 2 | 2–5 | 12–18 | 12–24 h | General pre-RO duty, moderate turbidity | No |
| Triple-Media (A+S+G) | 3 | < 3 | 12–20 | 16–36 h | Critical pre-RO duty, demanding SDI, borehole water | No |
| Greensand / Birm | 1–2 | 3–5 | 8–12 | 8–16 h | Iron & manganese removal from borehole water | Yes |
| Triple-Media + Greensand (staged) | 4 (2 vessels) | < 3 | 8–12 | 8–16 h | Borehole with Fe/Mn AND demanding SDI requirement | Yes |
A = Anthracite | S = Sand | G = Garnet. SDI and run length are indicative — actual values depend on raw water quality. Pilot testing recommended for critical applications.
Multimedia pressure filters are sized on surface loading rate (SLR), the volumetric flow per unit of vessel cross-sectional area (m³/m²·h, commonly written as m/h). This governs the upward velocity of water through the bed and the penetration depth of particulates into the media layers.
| Flow Rate (m³/hr) | SLR 10 m/h (conservative) | SLR 15 m/h (standard) | SLR 20 m/h (high-rate MM) |
|---|---|---|---|
| 10 | 1.0 m² → 1,130 mm dia. | 0.67 m² → 920 mm dia. | 0.50 m² → 800 mm dia. |
| 24 | 2.4 m² → 1,750 mm dia. | 1.6 m² → 1,430 mm dia. | 1.2 m² → 1,240 mm dia. |
| 50 | 5.0 m² → 2,520 mm dia. | 3.3 m² → 2,050 mm dia. | 2.5 m² → 1,785 mm dia. |
| 100 | 10 m² → 3,570 mm dia. | 6.7 m² → 2,920 mm dia. | 5.0 m² → 2,520 mm dia. |
| 200 | 20 m² (2×1,785 mm) | 13.3 m² (2×1,460 mm) | 10 m² (2×1,260 mm) |
Media bed depth: Total media depth is typically 900–1,200 mm for dual-media and 1,000–1,400 mm for triple-media. Vessel straight-side height allows for the media bed plus freeboard (30–40% of bed depth) for backwash expansion, underdrain laterals, and inlet distribution. Overall vessel height is typically 2.0–3.0 m for standard industrial duties.
Backwash flow must fluidise the entire media bed to 30–50% expansion, suspending accumulated particles so they can be flushed to drain. The backwash rate depends on media type and water temperature (which affects kinematic viscosity and therefore the minimum fluidisation velocity).
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.