This paper describes the performance of a full-scale common effluent plant treating tannery wastewater, based on a single-sludge nitrification/denitrification process, with the objective of investigating the reasons for its instability. The plant was monitored over a 1.5-year period; operational parameters were evaluated daily and their impact on process rates assessed together with the characteristics of the wastewater sampled at different points in the treatment scheme. The results showed that instability problems and a periodic failure of nitrification/denitrification occurred due to a series of reasons, often persisting simultaneously and having an additive synergetic effect. Factors whose magnitude would be insufficient to affect the performance of a plant treating domestic wastewater, eg; small deviations from the optimal pH in nitrification or denitrification basins, a temperature decrease to 17degreesC or an increase of the influent nitrogen content, caused the plant's efficiency to drop from 100% to 40% in some periods. To ensure stable performance, the dilution rate, D, along with the sludge retention time were shown to be key parameters for the control of the process. Maintaining values of D considerably lower than critical theoretical values calculated from the specific growth rate of nitrifiers at a given temperature and sludge recycle ratio may prevent problems in the plant's performance, particularly when high fluctuations of the concentration of inflow solids make calculation of the sludge age uncertain and proper sludge management difficult. (C) 2004 Society of Chemical Industry.