When the C/N ratio was increased from 5 to 12, the TN removal
efficiency decreased from 83% to 67% (Fig. 4). This significant inhibition
of nitrification at high C/N ratios possibly results from competition
for electron donors among heterotrophic bacteria and
nitrifiers. Nitrification did not occur completely, the conversion
of nitrite or nitrate to nitrogen gas was not high, which lead to
low TN removal. C/N ratios larger than 7 was considered to be
proper for nitrogen removal from municipal sewage at atmosphere
pressure, removal efficiency maximized at 11.1, which was then
determined as the optimum value for the C/N ratio [25]. In this
study, high TN removal efficiency was achieved at much lower
C/N ratios, and 79% removal was achieved at a C/N ratio of 3, which
suggests that the demand of organic carbon for TN removal was
lower at PA than at atmosphere pressure aeration.