Parallel multimedia filter vessels operating in alternating duty/standby mode for continuous, uninterrupted pre-RO filtration. One vessel filters while the other backwashes — eliminating supply interruption for 24/7 boiler feed, process water, and critical RO duties.
A single multimedia filter vessel must be taken offline periodically for backwashing — typically for 15–25 minutes every 8–48 hours depending on raw water quality and flow rate. For most applications this interruption is acceptable. However, where the downstream process requires an absolutely continuous feed — steam boiler operation, 24/7 process water supply, or RO systems without feed tank storage — a duplex arrangement is essential.
Duplex principle: Two identically sized vessels are connected in parallel. Both vessels are simultaneously capable of handling the full design flow rate. Under normal operation, one vessel is on duty while the second is on standby (filtered and pressurised, ready to take over). When the duty vessel reaches its backwash trigger (differential pressure or timer), the standby vessel is automatically brought on-line, and the duty vessel is isolated for backwash. The transition takes < 30 seconds through automatic valve sequencing.
Automatic vessel changeover with no manual intervention and no interruption to downstream RO, boiler, or process supply. Essential for 24/7 manufacturing sites where boiler shutdown has significant production cost implications.
In addition to the duty/standby role during normal operation, the standby vessel provides immediate fallback capacity if the duty vessel develops a fault, requiring maintenance, or media inspection. Reduces site vulnerability to a single filter failure.
PLC-controlled valve sequence: isolation of duty vessel → slow opening of standby vessel inlet/outlet → confirmation of standby on-line → initiation of duty vessel backwash sequence. All within a pre-programmed routine requiring no operator action.
Changeover is triggered by differential pressure across the duty vessel reaching the backwash setpoint (typically 0.5–0.7 bar) or by a time-based timer (every 12, 24, or 48 hours), whichever occurs first. DP-based triggering is preferred as it reflects actual filter condition rather than elapsed time.
Vessel A on duty at full design flow. Vessel B on standby — full of filtered water, inlet and outlet isolation valves closed, pressurised to system pressure. PLC monitoring duty vessel differential pressure continuously.
Duty vessel A reaches DP setpoint (e.g., 0.5 bar). PLC initiates changeover sequence. Alarm to SCADA/BMS if integrated. Standby vessel B inlet valve opens slowly (30-second ramp) to avoid hydraulic shock to downstream RO.
Both vessels briefly in service (5–15 seconds) while outlet pressure equalises. Flow measurement confirms standby vessel is handling the full design flow. Duty vessel A inlet and outlet valves then close.
Backwash inlet valve opens. Backwash water (towns mains or filtered water) flows upward through Vessel A at 25–35 m/h for 15–20 minutes. Waste backwash water flows to drain via backwash outlet valve. Air scour may precede water backwash.
Fast-rinse cycle: filtered water flows downward through Vessel A to outlet-to-drain for 3–5 minutes, purging backwash water and resettling media. Turbidity or SDI probe in rinse outlet confirms acceptable water quality. Vessel A returned to standby status.
Next backwash event: Vessel B triggers, Vessel A takes duty. The system continuously alternates, giving each vessel equal service time and wear. Total elapsed cycle time between backwashes depends on raw water quality, flow rate, and SDI trigger setpoints.
Each vessel in a duplex system is sized for the full design flow rate, not half. This ensures either vessel alone can carry the duty during backwash of the other, and provides 2× total installed capacity for future expansion or when running both vessels in parallel on peak demand days.
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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