Method for the biological treatment of nitrogen in effluents by nitritation

A method for the biological treatment of nitrogen in the form of ammonium in wastewater, by nitritation in a biological reactor, comprising:

• at least one step a of aerating the biological reactor containing the wastewater to be treated, wherein the aeration is controlled in order to maintain a concentration of dissolved oxygen in the reactor at an average value that is greater than or equal to 1 mg/L, so as to oxidize at least part of the ammonium into nitrites by the ammonia-oxidizing bacteria present in said reactor, characterized in that the method further comprises:
• at least one step b of eliminating at least part of the nitrites produced in step a by:
• a sampling b1 of water treated in step a outside the reactor, and/or by
• an anoxic biological transformation b2, steps a and b being carried out successively and cyclically so that the nitrite concentration in the reactor at the start of step a is less than 2 mg of N-NO2/L or at least two times lower than the concentration of ammonium in the reactor, and
• a step c of extracting, from the reactor, a fraction of the sludge resulting from steps a and b, per unit of time, calculated so that an effective age of aerated sludge of the reactor is obtained which is less than or equal to a theoretical minimum age of aerated sludge necessary for nitrification as defined by a decreasing exponential distribution as a function of the reactor temperature,
• such that the production of nitrates is minimized in comparison to the production of nitrites in the reactor by suppressing the activity of nitrite-oxidizing bacteria by means of said aeration in step a, said elimination of nitrites in step b and said effective age of aerated sludge in step c;
• and
• the steps a, b and c are repeated until the ratio of the concentration of nitrites to the sum of the concentrations of nitrites and nitrates at the end of the aeration step a is greater than 0.8.