In this study a unified approach is defined for the design of single sludge pre-denitrification systems, based upon recently developed multi-component modelling of activated sludge. The "Braunschweig exercise" is adopted as a reference point, mainly for the critical review of different design methods proposed. Emphasis is placed upon the significance of process stoichiometry when using different parameters for substrate and biomass. Relationships proposed for the assessment of excess sludge production and oxygen requirements are evaluated in comparison with the unified approach. A new design methodology is defined and numerically tested with the basic data generated for the Braunschweig exercise. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.