METHOD FOR STARTING UP AND CONTROLLING A BIOLOGICAL PROCESS FOR AMMONIUM REMOVAL AT LOW AMMONIUM CONCENTRATIONS AND LOW TEMPERATURE THROUGH THE USE OF A TWO STAGE AUTOTROPHIC NITROGEN REMOVAL PROCESS

The invention relates to a method for starting up and controlling a biological autotrophic process for removing nitrogen contained in a wastewater stream at a temperature between 5 and 25°C and comprising an ammonium concentration equal to or below 110 mg N/L comprising the following stages, performed in separate units: (a) providing the wastewater stream to a partial nitritation reactor wherein a start-up of a partial nitritation takes place transforming the ammonium to nitrite in the presence of Ammonia Oxidizing Bacteria and oxygen while Free Nitrous Acid (FNA) is accumulated until reaching a value equal to or higher than 0.02 mg N/L; and (b) a second stage wherein a controlled effluent leaving the partial nitritation reactor and containing an ammonium to nitrite ratio from 0.7 to 1.0 is fed into one anoxic reactor wherein an Anammox process takes place transforming the ammonium and nitrite to nitrogen gas by an anoxic reaction performed by autotrophic denitrifying bacteria.