From waste to product: a paradigm shift in LCA applied to wastewater sewage sludge

International audienceSince the last 20 years, wastewater treatment plants are become more effective in releasing a good quality water in the ecosystem due to stringent European directives aiming at maintaining and improving the aquatic environment (Directive 91/271/EEC, Directive 2000/60/EC). As a consequence, huge amount of wastewater sewage sludge are produced each year. This sludge is currently considered as a waste but new industrial practices and european regulation (End-of-Waste directive) are ongoing to consider sludge as a valuable product. In Life Cycle Assessment (LCA), boundarie are well drawn up between LCA of products and LCA of waste management. Indeed, LCA of products takes into consideration the whole product life cycle from the raw material extraction up to the product end of life with a functionnal unit defined in terms of system’s output (i.e. the product). LCA of waste management considers one or several wastes entering the system from the moment it becomes a waste by loosing value up to the moment it regains value or leave the waste management system as an emission. Specificities of this type of LCA are twofold (1) a functionnal unit defined as a system’s input (the waste to be treated) and (2) the assumption that the waste is free of any environmental load when entering the system, assumption often known as the “zero burden approach”. So there is an important gap between the two types of LCA. If the sludge is considered as a product and no more as a waste, what are the consequences for the LCA performing ? Indeed, such a paradigm shift will affect the way to do LCA in the wastewater field. Several methodological questions arise as the wastewater treatment plant will provide several coproducts : the treated water and the sludge. How to allocate the impacts between these two coproducts? Which allocation factors choose to affect an environmental load to the sludge? As a consequence, such methodological developments will enable stakeholders and water compagnies to highlight (i) the environmental interest to produce renewable energy ressources (such as sludge-based biomethane) compared to non-renewable energy ressources (such as fossil methane) or sludge-based fertilisers compared to mineral ones, (ii) the impact of water treatment on the environmental load affected to the sludge and so improve the whole environmental impact of the wastewater treatment plant. This paper aims to present the methodological aspects faced by LCA when dealing with this paradigm shift