This article has the objective of showing the cause effect relationship between the dynamics of growth of autotrophic populations involved in nitrification and the uncontrolled accumulation of nitrite ions. This accumulation results in a disequilibrium in number or viability between the genera Nitrosomonas and Nicrobacter. This disequilibrium can be imposed, for example, by an inhibition of the activity of the genus Nitrobacter linked to the presence of free ammonia in the environment. The threshold of inhibition and the resultant degree of accumulation of nitrite depend both on the history of the sludge utilised as inoculum (mixed autotrophic population or enriched in one of two sources) and on the hydraulic regime of the reactor (completely mixed reactor for the activated sludges and piston reactor for the fixed cultures). These results enable us to better understand the behaviour of a nitrification reactor and to propose solutions either to avoid the accumulation of nitrites or to intensify this accumulation with the goal of proposing a new process of nitritation-denitritation via the nitrites route (nitrates shunt).

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