A new approach to perform partial nitritation–anammox in a single tank was investigated. The tank incorporated a mesh (opening size 1.0 × 1.2 mm) as permeable barrier to create two distinct reaction zones (aerated and anoxic). The study reports on the operation and optimization of a 13 L laboratory scale reactor to treat sludge reject water with an NH4-N concentration of ∼750 mg·L−1. Performance throughout 250 days at increasing nitrogen loading rates is presented. The maximum loading rate applied was 1.5 kg NH4-N·m−3·d−1 at a hydraulic retention time of 12 h. Typical composition of the effluent was ∼50 mg·L−1 NH4-N; <5–10 mg·L−1 NO2-N and ∼60 mg·L−1 NO3-N. The corresponding average N removal rates were 85% for total nitrogen and 90% for NH4-N, respectively. Process control was very simple. It comprised only regulation of the aeration rate to maintain a pre-set pH (7.1) in the nitritation zone. Performance data clearly indicate that the investigated reactor configuration offers distinct advantages over currently installed processes. It demonstrated high robustness without the need for sophisticated process control. Apparently, the use of a permeable mesh to establish different reaction conditions in a single reactor provides new features of high potential.