Nitrification-Denitrification

Nitrification is the first step in the removal of nitrogen by means of this process. Biological nitrification is the work of two bacterial genera: Nitrosomonas, which oxidize ammonia to the intermediate product nitrite, and Nitrobacter, which converts nitrite to nitrate. Nitrifying bacteria are sensitive organisms and are extremely susceptible to a wide variety of inhibitors such as high concentrations of ammonia and nitrous acid, low DO levels (< 1 mg/l), pH outside the optimal range (7.5-8.6), and so on.

Nitrification can be achieved through both suspended-growth and attached-growth processes. In suspended-growth processes, nitrification is brought about either in the same reactor that is used for carbonaceous BOD removal, or in a separate suspended-growth reactor following a conventional activated sludge treatment process. Ammonia is oxidized to nitrate with either air or high-purity oxygen. Similarly, nitrification in an attached-growth system may be brought about either in the same attached-growth reactor that is used for carbonaceous BOD removal or in a separate reactor. Trickling filters, rotating biological contactors and packed towers can be used for nitrifying systems.

Denitrification involves the removal of nitrogen in the form of nitrate by conversion to nitrogen gas under anoxic conditions. In denitrifying systems, DO is a critical parameter. Its presence suppresses the enzyme system needed for denitrification. The optimal pH lies between 7 and 8. Denitrification can be achieved through both suspended- and attached-growth processes. Suspended-growth denitrification takes place in a plug-flow type of activated-sludge system. An external carbon source is usually necessary for micro-organism cell synthesis, since the nitrified effluent is low in carbonaceous matter. Some denitrification systems use the incoming wastewater for this purpose. A nitrogen-gas-stripped reactor should precede the denitrification clarifier because nitrogen gas hinders the settling of the mixed liquor. Attached-growth denitrification takes place in a column reactor containing stone or one of a number of synthetic media upon which the bacteria grow. Periodic backwashing and an external carbon source are necessary in a system of this kind.