Internal recycle ratio is an important parameter in anaerobic/anoxic/oxic (A(2)/O) wastewater treatment plant (WWTP) operation. An increase in this ratio decreases nitrate and nitrite concentration in the effluent and hence improves the nitrogen removal efficiency, even though the economical cost increases simultaneously. Determining the most favourable recycle ratio taking into account both considerations is an important item in A(2)/O WWTP operational optimisation. In this work, the effect of recycle ratio on nitrogen removal when using different influent nitrogen loads was tested in a pilot A(2)/O WWTP. Experimental results obtained show how increasing the internal recycle ratio from 0 to 5 produced a 12% increase in nitrogen removal. This increase was achieved by improving N-NOx removal by 9% with an increase in N-NH4+ removal of 3%. (C) 2003 Elsevier Ltd. All rights reserved.