Wastewater sludge is currently considered as a waste but with new industrial practices and european regulation (End-of-Waste directive) considerate will be soon a valuable product. This paradigm shift must be followed by the consideration of environmental constraints to introduce this new product on market. Assessing the environmental performances of these future ‘marketable sludges’ can be processed with Life Cycle Assessment (LCA) but it first implies to take into consideration the whole life cycle of this new product from the raw material extraction up to the product end of life. How this can be reasonably done when waste LCA consider that the waste is free of any environmental load when entering the system, assumption often known as the “zero burden approach”? Giving a environmental load to “product sludge” consists in the resolution of several methodological questions which arise as the wastewater treatment plant will provide several coproducts : the clean water and the sludge, both having a merchandable value. This paradigm shift implies that the wastewater treatment plant has several function and multifunctionnality has to be resolved. Because sludge is a direct and indivisible production from water treatment, process subdivision cannot be achieved. In the same basis, system expansion can be barely used as there is no known alternative to the “wastewater treatment” if system is expanded to only keep the “sludge production” one. Finally, allocating an environmental burden to these two coproducts appears to be the best way to solve the induced multifunctionnality. As a consequence, allocation factors have to be developed to allocate an environmental burden both to the sludge produced and to the clean water released in the receiving media. This is quite challenging as it supposed (i) to model the sludge production both from primary treatement and biological treatment, (ii) to create new allocation factor based on these physical and biological processes as classical allocation factors cannot be used, (iii) to validate these allocation factors whatever the wastewater treatment plant configuration. This paper aims at presenting the allocation factors used to charge sludge with an environmental burden when sludge is from now considered as “product sludge”.