In the present work a method for the treatment of design specifications in the simulation of complex solids processes has been developed. Adding a design specification to the flowsheet of a given solids process means that the solid product should fulfill a certain requirement. For example, its Sauter mean diameter should be kept within a certain interval or the mass fraction of particles above a certain threshold should be kept below a certain limit. The method developed here is not only able to handle product specifications but also other stream properties of the process. These properties can be either distributed or concentrated. Even dependently distributed properties e.g. a particle size-dependent contamination can be handled. The treatment of design specifications has been implemented as an extension of the flowsheet simulation system SolidSim. Practical applications of the implemented extension are illustrated with three examples: (i) a simple process for the preparation of brown coal, (ii) a process for the treatment of gravel and sand and (iii) a complex process for the mechanical separation and dewatering of harbor sediments contaminated with heavy metals. (C) 2008 Elsevier B.V. All rights reserved.