Both reactors were operated with natural air convection at 30 °C in Rotundine temperature controlled room. Air circulation across sponge medium was facilitated through lateral openings located above each sponge layer. All openings (diameter of 4 mm) were kept open at the start-up phase to ensure nitritation and, in a step wise manner, gradually closed in order to reach low dissolved oxygen (DO) concentrations, i.e. below 2 mg/L, across certain sponge layers to attain partial nitritation. In some occasions during the experimental period, the mentioned lateral openings were partly re-opened to provide additional oxygen. To limit air circulation over the height of the reactor, the sponge layers had the same cross-sectional area as the reactor’s surface area. Synthetic wastewater was fed from the top of each reactor with a mole miniature water distributor (shower). To minimize the influent DO concentrations, the demineralised water, by far having the largest share of the synthetic wastewater, was periodically flushed with nitrogen gas. In addition, an oxygen scavenger water lock, containing sodium sulphite and cobalt chloride, was installed in the ventilation located on top of the demineralized water tank.