Different internal flows from a wastewater treatment plant were investigated for their usefulness as internal organic carbon source in the treatment of reject water with high ammonium loading, such as reject water from anaerobic digestion of sewage sludge (700-800 mg of NH4+-N L-1). This was done using a nitrification/ denitrification process via nitrite on a laboratory scale sequencing batch reactor working at 32 C and 8 h cycle length. The hydrolyzed primary sludge was found to be the only one feasible for denitrification. When using the primary sludge, the reactor worked with an average biomass concentration of 2700 +/- 250 mg of VSS L-1, obtaining a specific ammonium uptake rate of 17 mg of NH4+- N g(-1) of VSS h(-1), a specific nitrite uptake rate of 38 mg of NO2--N g(-1) of VSS h(-1), and a total nitrogen removal of 0.7 kg of N (day m(3))(-1). The use of that internal organic carbon source leads to a cost reduction of 0.2-0.3 euro kg(-1) of N removed.