Sand and Multimedia Filtration

Sand and multimedia filters remove suspended solids from inlet and recirculated water by depth filtration through layered granular media. A typical multimedia bed stacks anthracite (0.8–1.6 mm, low density, top layer), silica sand (0.4–0.8 mm), and garnet (0.2–0.4 mm, high density, bottom), giving effective removal down to roughly 10–20 µm at filtration velocities of 5–15 m/h, while simple single-sand beds at 3–8 m/h manage 20–40 µm. Sizing is driven by the solids load: a filter holding back 5 g/L of suspended solids on a 50 m³/h loop will need backwashing every 12–24 hours, and as media clogs, head loss rises from a clean 0.2–0.5 bar toward the 0.8–1.2 bar backwash trigger. Backwashing reverses flow at 30–45 m/h for 5–10 minutes to fluidise the bed and strip accumulated solids, consuming 1–3% of treated throughput; excessive backwash frequency indicates upstream pre-treatment is undersized. In marine systems biofilm and carbonate fouling gradually cement media, so beds need periodic air-scour assisted backwashes or media replacement every 3–5 years. These filters protect UV reactors, membranes, and fish gills downstream, and act as a stable substrate for nitrifying bacteria when run at low loading. Operators should log pressure differential and backwash frequency, since a jump in either is the earliest sign of a developing water-quality problem.