A sequencing batch reactor treating the reject water produced from the dewatering of the anaerobic co-digestate of waste activated sludge and the organic fraction of municipal solid waste was monitored in order to assess the off-gas emissions that were produced. The gaseous emissions of nitrous oxide (N₂O), carbon dioxide (CO₂), methane (CH₄), ammonia (NH₃) and methyl mercaptan (CH₃SH) produced during the biological nitrogen removal via nitrite route were monitored. During experimental period 1 a volumetric nitrogen loading rate (vNLR) of 1.08 kg N/m³ d was applied, while in period 2 the vNLR was 0.81 kg N/m³ d. Nitrous oxide emissions decreased from 1.49% of the influent nitrogen load in period 1 to 0.24% in period 2. The higher dissolved oxygen (DO) concentration and the lower accumulation of nitrite in the reactor resulted in significantly lower nitrous oxide emissions in period 2. Some methane (5.8% of influent COD load) was emitted during period 1, as the lower DO concentration created micro-anaerobic conditions within the sludge flocs.