Sewage Treatment : Nitrification and De-Nitrification

Sewage Treatment : Nitrification and De-Nitrification

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:

Ammonia is first oxidized to Nitrate

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.

This 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.

e.g. Extended Aeration System   or done more efficiently, using a separate nitrification reactor.

De-Nitrification

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 microorganisms.

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 Air Stripping Process

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