Cellulose represents a significant fraction of domestic wastewater, but it is not considered as a separate state variable in the conventional Activated Sludge Models (ASM). Cellulose is a very slowly degradable substrate that in traditional wastewater characterisation methods would be characterised partly as slowly biodegradable and partly as inert material. This can be problematic when the same model is used under different temperature conditions or different solid retention times. Also with the emerging attention for cellulose recovery, inclusion of this compound in models helps to assess the impact of cellulose recovery on operations and on operational costs. But also in membrane bioreactors, sieves are used in order to remove fibrous material, mainly cellulose. In this study a modification of ASM1 is proposed, where cellulose is introduced as a separate state variable and it is supposed to degrade with a first order hydrolysis rate. With the aid of this model, the effect of fine sieves is simulated using two alternatives, by either sieving the influent or the activated sludge. The model proved to be useful for operational purposes, and illustrates the need for including cellulose as separate state variable.