A sequencing batch reactor (SBR) and a chemostat SHARON continuous reactor were operated to develop the biological nitrogen removal via nitrite to treat real reject water with 800-900 mg of NH4+- N L-1 at laboratory scale. Methanol was added for denitrification in both reactors because of the lack of readily biodegradable chemical oxygen demand in reject water. An 8 h SBR cycle was operated with three internal aerobic/anoxic periods, maintaining the pH at 7-8 to control nitrite accumulation and alkalinity limitations in a 3 L tank, temperature was 32 C, the hydraulic retention time was 1 day, and the solid retention time was 11 days. The SHARON process was operated in a 4 L chemostat reactor at 33 C, where it was combined with denitrification in the same vessel with a total hydraulic retention time of 2 days using intermittent nitrification/denitrification periods of 1 h. Both systems were compared from the operational, kinetic, design, and economical points of view, giving a general conclusion that the SBR would be a slightly cheaper process (1.01 versus 1.28 euros kg(-1) of N) due to the higher volumetric reaction rates. On the other hand the SHARON/denitrification reactor would be a more stable and regular process when there are fluctuations and changes in the system.