The aim of the present work was to study the treatment of high strength nitrate waste (40000 ppm NO3 i.e., 9032 ppm NO3-N) by acclimatizing sludge initially capable of degrading dilute streams (100-200 ppm NO3-N). Sludge from an effluent treatment plant of a fertilizer industry was acclimatized for 15 d each at 1694, 3388, 6774 and 9032 ppm NO3-N in a 4 L sequencing batch reactor. Complete denitrification of extremely concentrated nitrate waste (9032 ppm NO3-N) using acclimatized sludge was achieved in just 6 h. During the acclimatization period, increase in nitrite peak value from zero to 5907 ppm NO2-N was observed, as the concentration was increased from 1694 to 9032 ppm NO3-N. Kinetic analysis of the nitrate and nitrite profile could reasonably support microbiological explanations for nitrite build up and changes in sludge composition. (c) 2006 Elsevier Ltd. All rights reserved.