Separate treatment of supernatant with dewatering of digested sludge with application of partialnitritation/Anammox process is assessed to be a cost-effective way to remove about 10-15% ofinfluent nitrogen and, thereby, facilitate possibilities to reach required effluent requirements fromthe plant. The combined partial nitritation/Anammox process can be performed in two separatereactors or in one-stage. Both process options have been investigated in technical- andlaboratory-scale pilot plants with moving-bed biofilm reactors (MBBR) filled with Kaldnes rings.Use of the two-stage process resulted in a very stable partial nitritation with a suitable nitrite toammonium ratio (NAR) for the following Anammox step. Dissolved oxygen (DO) and pH valuewere identified as key factors for the partial nitritation process. The Anammox process could alsobe operated in a stable way. A high nitrite concentration, however, inhibited the process and thetime for recovering the process at low nitrite concentration was about four months. Seeding ofthe partial nitritation reactor with Anammox bacteria (the recirculation of Anammox effluent tothe nitritation reactor) turned out to be a simple and easy method to enable creation of an oxicanoxicbiofilm in one reactor. Studies have shown that such a one-stage system would be the bestchoice for full-scale implementation due to significantly higher nitrogen removal rates and easieroperation. The partial nitritation process was found to be the rate-limiting reaction to performthe overall nitrogen removal.Measurements of conductivity and pH were suitable parameters for monitoring of the nitrogenreactions. A control and monitoring system was developed both for two-stage and one-stagetechnology. The system was mainly based on relationships between conductivity and inorganicnitrogen components, while in the one-stage technology measurements are used of bothconductivity and pH and their relationships with inorganic nitrogen compounds. Alkalinity wasan additional measured parameter suitable for process control and monitoring. Theoreticallycalculated values of conductivity were in good agreement with experimentally obtained results
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