The TN removal efficiency was higher than 67% over the duration
of the testing period when the C/N ratio was ranged from 3
to 12 (Fig. 4). When C/N ratio was increased from 3 to 5, the TN
removal increased from 79% to 83%. NH4+–N including these taken
in the incoming stream and produced by heterotrophic bacteria
acted as substrate for nitrifiers. Nitrite and nitrate produced by nitrifiers
were consumed to N by denitrifiers. At a C/N ratio of 3, TN
removal efficiency maximized, which was probably due to the fact
that the rates for nitrification and denitrification maximized.
Denitrifiers obtain energy from organic carbon sources acting as
electron donors [24]. However, as heterotrophic bacteria are dominant
in the uptake of organic carbon sources at low C/N ratios, the
activity of denitrifiers was inhibited due to insufficient supply of
organic substrate. TN removal thus increased when more organic
carbon sources were available.