The second stage,
the denitrification biofilter, is placed below the degassing
chamber. The biofilter comprises a PVC pipe (0.3 m diameter,
45 l in volume) filled with commercial cotton wool (the kind used
in first-aid kits) and plastic beads packed in the manner shown in
Fig. 2. The beads in the column occupied approximately 26 l, and
the total cotton wool content was approximately 1.1 kg.