Two-stage denitrification system (A). In stage 1, water enters a degassing chamber (B) where the dissolved oxygen is removed. Thereafter, it flows into a denitrification
biofilter (C), where cotton wool serves as the primary carbon source. The dark region in the cotton-bead packing regime (inside lower oval) depicts the region occupied by the
porous plastic beads. The lighter region represents the area filled with cotton.