Sewage Treatment : Nitrification
Typical wastewater influent can contain 85 mg/L total
Nitrogen. Though conventional treatment can remove 20 - 30 % ,
Nitrification-Denitrification can remove 70 - 90%
Ammonia Nitrogen is the most reduced nitrogen compound found
in wastewater. This compound can be converted to Nitrogen by biological
processes.This process is done in two (2) steps:
is first oxidized to Nitrate
is reduced to molecular Nitrogen
The organisms responsible for
nitrification are chem-autotrophic bacteria, nitrosomonas and nitrobacter.
These are aerobic bacteria and therefore need free oxygen to work.
Ammonia Nitrogen can be biologically oxidized by
chem-autotrophic bacteria to nitrates if molecular oxygen is present:
NH4 + 3/2O2 -- Nitosermense-- >
NO-2 + 2H+ + H2O
NO2- + ½ O2 --- Nitribacter
-- > NO3-
These reactions require a great supply of oxygen. Contact time
in secondary treatment may be sufficient to convert organic nitrogen to ammonia
nitrogen but not sufficient to convert ammonia nitrogen to nitrates.
reaction consumes about 4.6 mg of O2 7.1 mg alkalinity per mg ammonia nitrogen.
Under favourable conditions this process can be accomplished
in combination with carbonaceous removal in secondary systems.
Extended Aeration System or done
more efficiently, using a separate nitrification reactor.
Nitrate is reduced to nitrogen gas by the same facultative,
heterotrophic bacteria involved in oxidation of carbonaceous material.
Denitrification occurs when
oxygen levels are depleted and nitrate becomes the primary oxygen source for
The process is performed under
anoxic conditions, when the dissolved oxygen concentration is less than 0.5
mg/L, ideally less than 0.2.
When bacteria break apart nitrate
(NO3-) to gain the oxygen (O2), the nitrate is reduced to nitrous oxide (N2 O),
and, in turn, nitrogen gas (N2).
For the process to proceed, the
bacteria needs a carbon source. This can be obtained from carbon within the
waste or a small amount of primary effluent can be added. Alternatively, an
external source of carbon can be provided (Methanol).
After leaving the anoxic tank, the wastewater is aerated for
10 to 15 minutes to drive off the Nitrogen gas and add oxygen
to the wastewater before sedimentation
The process consist of
converting the amm liquid in air
The gaseous phasehase
NH3NH4+ exist and together the inequilibrium aqueousand the p dominance of any
one is dependent on pH and Temperature. A pH of >11 is required for complete
conversion to NH3