A one-stage system with partial nitritation and Anammox processes in the moving-bed biofilm reactor

The ability of bacterial cultures to create biofilm brings a possibility to enhance biological wastewater treatment efficiency. Moreover, the ability of Anammox and Nitrosomonas species to grow within the same biofilm layer enabled a one-stage system for nitrogen removal to be designed. Such a system, with Kaldnes rings as carriers for biofilm growth, was tested in a technical pilot plant scale (2.1 m³) at the Himmerfjärden Waste Water Treatment Plant (WWTP) in the Stockholm region. The system was directly supplied with supernatant originating from dewatering of digested sludge containing high ammonium concentrations. Nearly 1-year of operational data showed that during the partial nitritation/Anammox process, alkalinity was utilised parallel to ammonium removal. The process resulted in a small pH drop, and its relationship with conductivity was found. The nitrogen removal rate for the whole period oscillated around 1.5 g N m⁻²d⁻¹ with a maximum value equal to 1.9 g N m⁻²d⁻¹. Parallel to the pilot plant experiment, a series of batch tests were run to investigate the influence on removal rates of different dissolved oxygen conditions and addition of nitrite. The highest nitrogen removal rate (5.2 g N m⁻²d⁻¹) in batch tests was obtained when the Anammox process was stimulated by the addition of nitrite. In the simultaneous partial nitritation and Anammox process, the partial nitritation was the rate-limiting step.