The aim of this study was to investigate the effect of varying limited ammonium concentrations on the heterotrophic denitrification process responsible for biological nitrogen removal. The system performance was evaluated with gradually decreasing ammoniumnitrogen concentrations from 20 mgNH4 + -N L -1 to inexistent ammonium inlet at 100 mgNO3 -L -1 . Results of this study indicated that the total nitrogen removal efficiency reached maximum level at influent ammonium-nitrogen of 5 mg L -1 due

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... the absence of residual ammonium at the end of the reaction, although varying ammonium concentrations did not noticeably affect the nitrate removal efficiency. Additionally, nitrate consumption rate had a tendency to increase with the limitation of influent ammonia and the nitrate consumption rate reached maximum level at operational condition where ammonium was not present, corresponding to 85.4 mgNO3 --N gMLSS -1 h -1 . The maximum ammonium consumption rate have attained with influent ammonium-nitrogen of 5 mg L -1 , being 18.4 mgNH4 + -N gMLSS -1 h -1