Diameter, bed depth, freeboard, distributor, nozzle-plate underdrain, pressure code — the design decisions that determine whether a multimedia filter runs to its sizing assumptions or fails inside year one.
Diameter, Bed Depth, Freeboard, Internals
A multimedia filter is a pressure vessel filled to a specified depth with graded media, with internal distributors that spread the inlet evenly across the bed and collect the filtrate through underdrain nozzles. Get the diameter wrong and hydraulic loading drives breakthrough; get the freeboard wrong and media is lost during backwash.
From Service Flow to Diameter
Typical multimedia filter service rate is 12–18 m/h (5–7 gpm/ft²). Conservative designs for variable influent stay at 10–12 m/h; aggressive designs on benign water (RO pretreatment after lamella clarification) can run 18–25 m/h. Higher rates trade run length for footprint.
Abed (m²) = Qservice (m³/h) ÷ vloading (m/h). For a 100 m³/h duty at 15 m/h: 6.7 m² → vessel ID 2.92 m. Round to a standard pressure-vessel diameter.
Total media depth typically 850–1000 mm including support gravels. L/D ratio (bed depth to ES) should be > 1200 to guarantee depth filtration rather than surface straining.
Freeboard above the static bed must accommodate full fluidisation during backwash — typically 50% of bed depth, so 450–500 mm. Skimping on freeboard sends media to drain.
Pressure-vessel shells come in standard diameters (1.0, 1.2, 1.5, 1.8, 2.0, 2.4, 2.8, 3.0, 3.2, 3.6, 4.0 m). Picking the next size up gives loading-rate headroom; picking smaller forces multiple vessels in parallel — sometimes the right answer for redundancy.
Pressure, Materials, Code
| Item | Typical value | Notes |
|---|---|---|
| Design pressure | 6–10 barg | Higher for boiler-feed pre-RO duties |
| Operating pressure | 2–5 barg | Influent pressure must cover bed head loss + downstream demand |
| Head loss, clean bed | 0.2–0.4 bar | Sets minimum upstream supply pressure |
| Terminal head loss | 1.0–1.5 bar | Backwash trigger threshold |
| Shell material | SS304L / SS316L / CS-rubber lined / GRP | Choice driven by influent chloride and disinfectant residual |
| Code | PED 2014/68/EU module H (UK/EU) or ASME VIII Div 1 (export) | U-stamp required for export to many jurisdictions |
| Head type | Ellipsoidal 2:1 or hemispherical | Hemispherical for D > 3 m to limit shell thickness |
| Nozzles | Inlet, outlet, backwash in/out, air in, drain, vent, manhole, sight glass | Manhole ≥ DN500 for media loading |
Distributor, Underdrain, Surface Wash
A baffle plate, hub-and-radial collector, or perforated dome that spreads incoming flow evenly across the bed. Velocity at any point ≤ 0.5 m/s to prevent surface scour of the top media. Maldistribution is the single most common reason for premature breakthrough.
False bottom carrying 30–50 nozzles per m², each slot width < 0.3 mm (smaller than the support gravel). Distributes backwash water and air evenly, collects filtrate without preferential paths. Plate flatness < 3 mm over the diameter is critical.
Slotted laterals laid in support gravel. Lower Capital expenditure than nozzle plates but harder to inspect and easier to plug. Used on D > 3.6 m where nozzle-plate flatness becomes difficult.
Separate header at the bottom of the bed introducing 50–80 Nm³/h·m² of air during backwash. Breaks up surface mudballs and dislodges trapped solids. Without air scour the backwash relies on water alone and run lengths shorten.
Rotating arm 50–150 mm above the bed delivering 6–12 bar surface jets during backwash. Disrupts crust formation on the anthracite. Common on dual-media beds with no air scour.
Two sight glasses (top of bed, mid-bed) and three sample taps (inlet, mid-filter, outlet) let the operator see fluidisation, take core samples and diagnose problems without opening the vessel.
When Each Configuration Is the Right Choice
| Aspect | Pressure filter (closed vessel) | Gravity filter (open bay) |
|---|---|---|
| Driving head | Pumped, 2–5 bar | Static, 0.3–1.5 m of water |
| Footprint | Small — vessels in row | Large — concrete bays |
| Operator visibility | Closed; rely on instruments | Open bed visible from walkway |
| Typical flow per unit | 10–500 m³/h | 500–5000 m³/h |
| Best fit | Industrial, packaged, prefab, RO pretreatment | Municipal DWTP, very high flow |
| Capital expenditure/m³ | Higher per unit, lower civil | Lower mechanical, much higher civil |
| Operating expenditure | Backwash pumping cost | Lower pumping, more operator hours |
Industrial default: packaged pressure-filter skids, two or three in parallel for redundancy, FAT'd in our workshop and delivered as a connect-and-go unit. See containerised systems for the full prefab approach.
Cross-Links into the MMF Topic Cluster
Anthracite, sand and garnet specifications that drive vessel sizing.
Read MoreAir-scour and water-rinse rates set the freeboard requirement.
Read MoreWhere the filter sits in pretreatment, reuse and tertiary trains.
Read MoreVessel-design choices that cause field problems and how to fix them.
Read MoreBack to the main multimedia filter page.
Read MoreWe CFD-validate distributor geometry for evenness < 5% variance across the bed.
Read MoreSend us the design flow, peak factor, influent turbidity and downstream process — we will return a vessel datasheet and skid GA within two weeks.
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