A dual-media filter puts a coarse, light layer of anthracite over a finer, heavier layer of silica sand. Solids are captured through the depth of the bed instead of only at the surface, so the filter runs longer between backwashes and handles higher loads than sand alone.
Typical dual-media bed
| Layer | Media | Effective size | Depth |
|---|---|---|---|
| Top | Hydro anthracite | 0.9 – 1.2 mm | 300 – 500 mm |
| Middle | Filter sand | 0.45 – 0.6 mm | 300 – 600 mm |
| Support | Graded gravel | 2 – 4 layers | 75 – 150 mm per layer |
These are common starting points from water treatment practice. Every filter should follow its designer’s or OEM’s specification.
Effective size and uniformity coefficient
Effective size (ES, or d10) is the sieve opening that 10% of the media by weight passes. It controls how fine a particle the bed can capture and how much head loss it creates.
Uniformity coefficient (UC) is d60 divided by d10. A UC close to 1 means grains of similar size, which gives more open pore space, longer runs and cleaner stratification after backwash. Filter media is commonly specified with a UC of 1.5 or lower.
Keeping the layers separate
After each backwash the bed must settle back with anthracite on top. That depends on the two media having the right size and density ratio. A common rule of thumb is that the anthracite effective size should be about two times the sand effective size, given anthracite’s lower density of about 1.4 – 1.5 against 2.65 for silica sand.
Adding garnet: multimedia filters
Adding a layer of garnet (specific gravity about 4) below the sand creates a coarse-to-fine bed in the direction of flow, so the finest polishing happens at the bottom. See our multimedia filter sets.
Signs a bed needs replacing
- Turbidity breakthrough soon after backwash
- Mud balls, cracking or channels in the bed surface
- Media loss into the underdrain or the treated water
- Anthracite worn finer than its original grade
Work out quantities with our filter media calculator.