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.

Due to negative environmental effects of nitrogen discharge to recipients and increasingly stringent effluent standards, effective nitrogen removal is necessity. Biological methods are the simplest and cheapest way to treat wastewater; however, it may become an extremely expensive option when high influent nitrogen concentrations are measured and there is a lack of biodegradable organic carbon. Therefore, there is a great need to find new solutions and improve existing technologies. The deammonification is an excellent example of such a new process that requires considerably low amounts of organic carbon and oxygen in comparison to conventional nitrification/denitrification. The main objective of presented research was to investigate an Anammox process accompanied with autotrophic nitrification and heterotrophic denitrification in one rotating biological contactor (RBC). During the research period, it was possible to carry out the Anammox process in low temperature below 20 °C. Additionally, it was found that the process is insensitive to high nitrite concentration in the reactor, up to 100 g NO₂-N m⁻³, resulting only in a temporary decrease in removal rates. Furthermore, analysis of data indicated that the Stover-Kincannon model can be used for the description of ammonium and nitrite removal processes.

A biofilm system with Kaldnes biofilm carrier was used in these studies to cultivate bacteria responsible for both partial nitritation and Anammox processes. Due to co-existence of oxygen and oxygen-free zones within the biofilm depth, both processes can occur in a single reactor. Oxygen that inhibits the Anammox process is consumed in the outer layer of the biofilm and in this way Anammox bacteria are protected from oxygen. The impact of oxygen concentration on nitrogen removal rates was investigated in the pilot plant (2.1 m³), supplied with reject water from the Himmerfjärden Waste Water Treatment Plant. The results of batch tests showed that the highest nitrogen removal rates were obtained for a dissolved oxygen (DO) concentration around 3 g O₂ m⁻³. At a DO concentration of 4 g O₂ m⁻³, an increase of nitrite and nitrate nitrogen concentrations in the batch reactor were observed. The average nitrogen removal rate in the pilot plant during a whole operating period oscillated around 1.3 g N m⁻²d⁻¹ (0.3 ± 0.1 kg N m⁻³d⁻¹) at the average dissolved oxygen concentration of 2.3 g O₂ m⁻³. The maximum value of a nitrogen removal rate amounted to 1.9 g N m⁻²d⁻¹ (0.47 kg N m⁻³d⁻¹) and was observed for a DO concentration equal to 2.5 g O₂ m⁻³. It was observed that increase of biofilm thickness during the operational period, had no influence on nitrogen removal rates in the pilot plant.