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Aerobic nitrification is followed by denitrification in an anoxic basin (no dissolved O2, nitrate acts as the O2 source). Here, NO3- is reduced by heterotrophic C-degrading bacteria to form N2. Denitrification is the only biological process in which inorganically bonded N is converted into N2. The presence of a C source (dissolved organic compounds) is vital for reduction of nitrate. The TKN:COD ratio in the inflow should be < 0.1. It is therefore favourable if the primary sedimentation stage is omitted or the primary sedimentation time is short, the waste water is previously acidified (putrid waste water) and inflow amounts and concentrations are equalised (e.g. mixing basin).

Detrimental factors are a high proportion of external water and mixed water inflow, a reduction in the easily degradable COD fraction in the sewer network , a long residence time in the primary sedimentation basin without acidification, pre-precipitation, a high TKN:COD ratio, a high proportion of undissolved organic compounds, introduction of dissolved O2, and COD and N profiles which differ over time.

A well-functioning denitrification stage can prevent operating problems such as scum in the final sedimentation basin . Denitrification should be measured for winter operation, as the temperature has a decisive effect on nitrification bacteria species and thus on denitrification.

5 C(H2O) + 4 NO3-+ 4 H+ --> 5 CO2 + 2 N2 +2 H2O

1 mol of acid is consumed per mol NO3-ND and 2.9 g O2/g NO3-N is released for C decomposition.

For nitrification and denitrification, of 1 g NH4-N to N2, therefore, 4.6 g--2.9 g=1.7 g O2 are consumed.

The extent to which NO3- is reduced depends on the prevailing milieu. High NO3- concentrations and a lack of H donors result in accumulation of NO2-, NO and N2O and are excreted by the bacteria .

Most denitrifying bacteria belong to the proteobacteria and are facultative aerobic organisms . Examples of denitrifying bacteria are Pseudomonas denitrificans, Paracoccus denitrificans, Thiobacillus denitrificans, Pseudomonas aeruginosa and Bacillus licheniformis.

 Denitrification capacity [kg/kg]:

            0.75:        Ratio of anoxic to aerobic respiration (empirical)

denitrification upstream simultaneous
VD/VAB denitrification capacity [kg NO3-N/kg BOD5]
0,20 0,07 0,05
0,30 0,10 0,08
0,40 0,12 0,11
0,50 0,14 0,14

With constant stabilisation, approx. 1.6 g N/(P x d) are removed from the plant with the sludge (approx. 15%). With dehydration of raw sludge and burning up to 40% N can be removed from the waste water via the sludge.



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