Performance and control of biofilm systems with partial nitritation and Anammox for supernatant treatment

Sammanfattning: Separate treatment of supernatant with dewatering of digested sludge with application of partial nitritation/Anammox process is assessed to be a cost-effective way to remove about 10-15% of influent nitrogen and, thereby, facilitate possibilities to reach required effluent requirements from the plant. The combined partial nitritation/Anammox process can be performed in two separate reactors or in one-stage. Both process options have been investigated in technical- and laboratory-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 to ammonium ratio (NAR) for the following Anammox step. Dissolved oxygen (DO) and pH value were identified as key factors for the partial nitritation process. The Anammox process could also be operated in a stable way. A high nitrite concentration, however, inhibited the process and the time for recovering the process at low nitrite concentration was about four months. Seeding of the partial nitritation reactor with Anammox bacteria (the recirculation of Anammox effluent to the nitritation reactor) turned out to be a simple and easy method to enable creation of an oxic-anoxic biofilm in one reactor. Studies have shown that such a one-stage system would be the best choice for full-scale implementation due to significantly higher nitrogen removal rates and easier operation. The partial nitritation process was found to be the rate-limiting reaction to perform the overall nitrogen removal.Measurements of conductivity and pH were suitable parameters for monitoring of the nitrogen reactions. A control and monitoring system was developed both for two-stage and one-stage technology. The system was mainly based on relationships between conductivity and inorganic nitrogen components, while in the one-stage technology measurements are used of both conductivity and pH and their relationships with inorganic nitrogen compounds. Alkalinity was an additional measured parameter suitable for process control and monitoring. Theoretically calculated values of conductivity were in good agreement with experimentally obtained results.