Depth filtration, not sieving
The spaces between grains in a filter bed are far larger than the particles the filter removes — a sand bed with 0.5 mm grains reliably captures 20 µm particles, twenty-five times smaller than the gaps. Removal happens by depth mechanisms: particles collide with grains and stick, settle in the slow zones between grains, and progressively build up through the top few hundred millimetres of bed. That is why a media filter improves slightly as it ripens after backwash, and why it has a working capacity rather than a fixed cut-off.
Because capture depends on particles meeting grains, the whole design revolves around how fast the water moves through the bed.
Linear velocity: the number that sizes the vessel
Divide the flow (m³/h) by the vessel's cross-sectional bed area (m²) and you get linear velocity in m/h — the single most important number in media filter design. Each medium has a design service range: run faster and particles are dragged through the bed before they can attach; the filter passes solids while showing a perfectly healthy pressure drop.
| Medium | Typical service velocity |
|---|---|
| Filter sand (fine, single medium) | 10 – 25 m/h |
| Glass media / AFM | 10 – 25 m/h (per grade datasheet) |
| Catalytic iron/manganese media | 10 – 12 m/h |
| Granular activated carbon | 5 – 15 m/h (contact time governs) |
The practical consequence: vessel diameter is chosen by flow, not by volume. A 10 m³/h duty at 20 m/h needs 0.5 m² of bed — roughly a 30" vessel — regardless of how tall the vessel is. Undersized diameter is the most common fault in field-built filters: the flow fits through the pipework, so nobody notices the bed is running at double its design velocity.
Bed depth: the contact distance
Depth gives particles the travel distance in which capture can happen. Working minimums are around 600 mm for sand and glass, more for carbon where empty bed contact time (bed volume ÷ flow) is the real criterion. A deep bed also holds more solids between backwashes. Depth cannot rescue excessive velocity, though — the two numbers do different jobs, and both have to be right.
Freeboard: the room to clean itself
Backwashing lifts and fluidises the bed so trapped solids can escape; the bed expands 25 – 50% while this happens. Freeboard is the empty vessel height above the settled bed that makes the expansion possible. Fill a vessel to the top with media and the backwash flushes the finest grains — the most useful ones — to drain every clean. Lighter media (anthracite, some glass grades) expand more and need more freeboard. As a rule, fill vessels to roughly half or two-thirds with media, per the medium's datasheet, and record the fill so future top-ups keep the design.
The supporting cast
Under the bed, graded gravel support layers (or a false-bottom distributor) stop fine media escaping through the lower distributor and spread the backwash evenly across the bed. Skipping the gravel on a slotted-riser vessel is a classic shortcut that costs a media charge — the fines migrate into the distributor and away with the treated water.
Rule of thumb: diameter from velocity, depth from contact, freeboard from expansion. Check the linear velocity first on any misbehaving filter — nine times out of ten a "filter that doesn't work" is a bed being run too fast for its area.