Multimedia Filter

A multimedia filter is a vertical pressure vessel containing graded layers of media: a coarse gravel support at the base, a primary sand filtration layer in the middle, and an anthracite pre-filtration layer at the top — with optional garnet for fine polishing where a lower SDI is required. Because each media layer has a different density, the bed re-stratifies after every backwash with the coarsest, least-dense anthracite on top and the finest, densest sand and garnet below. This stratification means that larger particles are captured high in the bed, smaller particles are captured progressively deeper, and the full bed depth is utilised for filtration rather than just the top few centimetres. The result is longer filter runs between backwashes, higher throughput per unit area, and a finer effluent quality than a conventional single-media sand filter. Multimedia filters remove turbidity, suspended solids, and iron or manganese floc from water and wastewater.

Vessels are fabricated in FRP (glass-reinforced plastic) for capacities up to approximately 25 m³/hr and vessel diameters up to 2.4 m, providing excellent corrosion resistance for aggressive feed water or chemical service. Larger units and high-pressure applications use MS (mild steel) vessels with internal epoxy coating or SS 304/316 construction. Internal underdrain laterals at the vessel base ensure uniform flow distribution during both filtration and backwash. Backwash is performed in two stages: an air scour cycle first breaks up media compaction and loosens trapped solids, followed by a high-rate water backwash that flushes dislodged material out of the vessel. Backwash initiation is controlled either manually, on a fixed time interval, or automatically via differential pressure — the preferred option for unattended operation. Design face velocity is 7–12 m/hr.

Multimedia filters are an essential pre-treatment step before reverse osmosis systems, where they reduce feed SDI below 5 to protect expensive RO membranes from particulate fouling. They are equally used upstream of UF membranes where raw water turbidity is high, for ETP tertiary polishing to meet discharge standards, and as standalone clarification for cooling tower makeup water, boiler feed pre-treatment, and general process water. In many plants they are installed in series: a multimedia filter removes bulk suspended solids and turbidity, followed by an activated carbon filter for organics and chlorine removal, and then the RO or UF system. For single-media filtration requirements, see our pressure sand filter; for the next polishing step, see our activated carbon filter.