Aerobic Nitrification in Sewage Treatment

Aerobic Nitrification in Sewage Treatment

	Aerobic Nitrification in Sewage Treatment - Ammonia is considered to be the major pollutant in sewage for two important reasons. Firstly, it is toxic to aquatic forms even in very low concentration. Secondly it exerts a high oxygen demand. For instance it can contribute upto 40% of the total BOD exerted. Moreover, much of the organic nitrogen present in sewage is in a form that can readily be converted to ammonia by heterotrophic bacteria, thus increasing the concentration of ammonia further. The concentration of ammonia in raw sewage is in the range of 30 mg per litre.
However, effluent standards impose the ammonia concentration in the range of < to mg per litre or even < 5 mg per litre in some cases. For these reasons the removal of ammonia during wastewater treatment is of considerable importance. Some of the ammonia present in sewage will be utilized by heterotrophic microorganisms to satisfy their nitrogen requirement. But in domestic sewage, which is typically a carbon limited medium, this route is not followed. Instead another major biological transformation, called nitrification occurs. In this process certain aerobic autotrophs, which use ammonia as a source of energy. Oxidize it to nitrate via nitrite. Thus nitrification occurs in two steps mediated by two different bacterial species.
There are certain limitations in this process. The bacterial species involved in nitrification have a very narrow pH range, i.e., between 7 and 8 and are highly sensitive to any change in the pH. If the concentration of hydrogen ions produced in the first stage of the process increases, it can cause significant reduction in pH. If the pH becomes sufficiently low, the nitrification process may be inhibited completely. Another problem with these nitrifying bacteria is that they are slow growing when compared with the heterotrophic bacteria in wastewater (activated sludge). In addition they are inhibited by a wide range of toxic organic and inorganic compounds as well as by low temperatures.
Another important point to note is that, although nitrification effectively neutralizes the oxygen demand exerted by ammonia it does not constitute an effective removal of nitrogen. Because ammonia is actually converted to nitrate. Although the nitrification reduces BOD and prevents toxic effects ammonia, nitrate and nitrite formed during the nitrification process still cause problems. Nitrite, when present in drinking water, can react with amines in the diet to form nitrosamines which are highly carcinogenic. Similarly, nitrate can cause methaemo globinaemia in young children which is commonly known as blue baby disease. Thus it is necessary to remove these soluble nitrogen compounds from the effluent and this is accomplished by the process denitrification.

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