Recent years have seen the introduction of new and varied designs of activa
ted sludge plants. With increasing needs for higher efficiencies and lower
costs, the possibility of a plant that operates more effectively has create
d the need for tools that can be used to evaluate and compare designs at th
e design stage. One such tool is the operating space diagram. It is the aim
of this paper to present this tool and demonstrate its application and rel
evance to design using a simple case study. In the case study, use of the o
perating space diagram suggested changes in design that would improve the f
lexibility of the process. It also was useful for designing suitable contro
l strategies.