Nowadays, as the effluent water regulations become more stringent, there is a need to treat wastewater in the most efficient manner and according to sustainability principles. One of the possibilities to meet this challenge is treatment of side streams, which are usually returned to the main influent of Wastewater Treatment Plants (WWTP) increasing the total load. Following processes occurring in natural ecosystems a new biological technology - combination of partial nitritation and Anammox processes - for treatment of nitrogen-rich supernatant coming from digested sludge dewatering has been developed. The first stage of the process is an oxidation of half of the ammonium to nitrite (partial nitritation process). The following stage - Anammox process - is an anaerobic oxidation of ammonium and nitrite nitrogen to dinitrogen gas. The process has been successfully tested in a technical-scale pilot plant with a continuous supply of supernatant at Himmerfjärden WWTP. Kaldnes rings were provided for biofilm growth. Almost two-year experiences in operation of the two-stage process have been presented in this paper. The results showed that a proper adjustment of dissolved oxygen (DO) concentration in the bulk liquid and a pH value drop in the partial nitritation reactor is essential to obtain the ammonium-to-nitrite ratio (NAR) in the effluent close to 1.3 as required for the Anammox process. It took four months to recover the Anammox bacteria activity after NO2-N inhibition.

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