Trickling Filter

A trickling filter is a fixed-bed nitrification unit in which culture water is distributed over a packed column of biofilm media — lava rock, structured plastic, or random packing — and trickles downward while air moves upward, giving passive oxygenation of the thin biofilm layer. It performs three functions at once: nitrification (ammonia to nitrite and nitrate), CO2 stripping by the air current, and a small degree of mechanical filtration. Typical hydraulic loading runs 20–200 m3/m2/day with empty-bed contact times of 1–4 minutes, and ammonia removal of 0.2–0.6 g NH4-N/m2/day depending on temperature and media surface. Because the media do not move, trickling filters are cheap to build and run (no aeration needed) but can clog with excess solids and biofilm growth, requiring periodic media rinsing. They are common in smaller flow-through and partial-reuse trout, tilapia, and ornamental systems, and increasingly appear as a first-stage roughing filter ahead of an MBBR in larger RAS. Compared with moving beds they are more compact vertically but less tolerant of variable hydraulic loads.