Worked design example: duplex triple-media pressure filters treating borehole water for an RO system supplying four steam boilers. 24 m³/hr continuous duty, automatic backwash on differential pressure, uninterrupted supply 24/7.
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 TreatmentA manufacturing facility draws water from a borehole at up to 70 m³/hr at 9 barg. The borehole water feeds a 200 m³ raw water storage tank, which in turn supplies an RO plant protecting four steam boilers. The existing multimedia filter system has failed and is bypassed, placing the RO membranes under direct particulate fouling stress. A replacement duplex filtration system is required at the design flow rate of 24 m³/hr to restore protection ahead of the RO plant and extend membrane life. Site operates 24/7; boiler supply must not be interrupted during filter backwash. Installation target is prior to December 2026.
Raw water quality note: Without a full water analysis (turbidity, TSS, iron, manganese, hardness, pH, SDI), the conservatively safe media selection for a borehole pre-RO duty is triple-media (anthracite–sand–garnet). If iron or manganese is confirmed above RO-safe thresholds (Fe > 0.05 mg/L, Mn > 0.02 mg/L), a dedicated aeration and greensand or Birm stage upstream of the multimedia filters must be added to the design. A water analysis is the essential first step before finalising the filtration train configuration.
Each vessel in the duplex pair must handle the full 24 m³/hr design flow independently. Vessel cross-sectional area is determined by the selected surface loading rate (SLR):
| SLR Option | Required Area (m²) | Vessel Diameter | Vessel Height (s/s) | Notes |
|---|---|---|---|---|
| 10 m/h (conservative) | 2.4 m² | 1,750 mm | ~2,400 mm | Lower flux, longest runs, easier backwash |
| 12 m/h (recommended) | 2.0 m² | 1,600 mm | ~2,200 mm | Standard design point for triple-media borehole duty |
| 15 m/h (higher rate) | 1.6 m² | 1,430 mm | ~2,000 mm | Compact, shorter runs, more frequent backwash |
Recommended preliminary basis: Two GRP vessels, each 1,600 mm diameter × approximately 2,200 mm straight-side height, charged with triple-media (anthracite 350 mm / sand 400 mm / garnet 150 mm) on a graded gravel underdrain. Each vessel footprint approximately 1.6 m × 1.6 m; overall installation width with interconnecting pipework approximately 4.0–4.5 m × 2.0 m depth. Fits within the available 9 m × 4.5 m plant room, respecting the 2.5 m roller shutter door height.
Borehole pump delivers water at up to 70 m³/hr at 9 barg. Pressure-reduced at the filtration building to ~2 barg via a pressure-reducing valve (PRV). Flow controlled at 24 m³/hr by a motorised modulating valve on the filter inlet.
200 m³ raw water feed tank provides hydraulic buffer between the borehole pump and the filtration system. Level-controlled pump starts and stops. If iron or manganese is present, atmospheric aeration of the storage tank provides partial oxidation before filtration.
Vessels A and B in duty/standby configuration. Flow enters the duty vessel top, passes downward through anthracite–sand–garnet, exits at the base through the underdrain. DP transmitter across each vessel monitors bed loading. Changeover at 0.5 bar DP or 24-hour timer (whichever first). Backwash with towns water at 4 barg via a dedicated backwash header.
A 5 μm cartridge filter housing (or self-cleaning strainer) as a polishing safety net between the multimedia filter and the RO feed pump guards against any media fines migrating during backwash transitions. Not always required but recommended when SDI < 2 is the target.
Filtered water at SDI < 3 feeds the existing RO plant. Restored pre-treatment eliminates the current particulate fouling load on the RO membranes, restoring design flux and extending element replacement interval from the shortened life caused by the current bypass operation.
RO permeate (low TDS, low hardness) feeds four steam boilers via the existing downstream treatment train (typically dealkalisation or deaeration). Restored RO membrane protection reduces boiler scaling incidents and chemical consumption associated with variable TDS breakthrough.
Differential pressure transmitters across each vessel trigger backwash automatically at 0.5 bar ΔP. A secondary timer trigger (24 hr) ensures backwash occurs even on very-low-turbidity days when ΔP accrues slowly. Requires no operator intervention for normal operation.
Full PLC panel with HMI touchscreen provides local valve position indication, run-hours, backwash counts, and alarm history. Hardwired I/O or Modbus RTU/TCP available for integration with an existing Building Management System (BMS) or site SCADA. Remote alarm output for unattended overnight operation.
Automatic backwash systems with pneumatic valves and a backwash pump typically require three-phase supply for the pump motor. A single-phase control panel is possible if the backwash pump is omitted and towns water pressure alone drives the backwash — feasible at 4 barg where the vessel diameter is ≤ 1,600 mm.
Backwash waste (turbid water with suspended solids flushed from the bed) runs to a floor drain or drain channel adjacent to the filter installation. Backwash volume: approximately 1.5–3 m³ per backwash cycle at 24 m³/hr vessel size. Confirm local trade effluent consent covers this intermittent discharge.
Media options, duplex arrangements and worked application examples
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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