Nitrification processes are well known for certain problems in connection with transient build-up of nitrite ions. Moreover, some works have shown interest in controlling the build-up of this ion, particularly when treatment procedures for nitrogenous pollution are of the nitritation-denitritation type. With this in mind, we have carried out a programme of research to check the main factors responsible for the accumulation of this ion, i.e. [NH4]o, T°, pH, and dissolved O2. The main results highlight the key role played by the free form N-NH3 and by the temperature. This research thus provides answers to a number of practical questions and allows us to envisage setting up new procedures for the treatment of nitrogenous pollution by fixed cultures.

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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).

To keep overflows of raw effluent to a minimum during wet weather conditions, we investigated the ability of a secondary clarifier of an activated sludge treatment plant to accept hydraulic overloads without being washed out. The experiments, which were conducted on a full scale 8000 p.e. treatment plant, showed the feasibility of the project, and suggested some features, which were included in a one-dimensional model designed to study the behavior of the sludge blanket in the clarifier. This model takes into account the effect of convection currents, suspected to play an important part in the rising of sludge blankets. The sensitivity of the model to sludge settleability prevented its use with long times-series, unless a continuous recalibration was performed. Nevertheless, this model appears very interesting for a better understanding of the dynamics of the clarifier, as described by measured data. It could be used in relation with sensors to improve the operation of the treatment plant.

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