From waste to added value product: towards a paradigm shift in life cycle assessment applied to wastewater sludge – a review

Abstract : This paper aims at highlighting considerations on the sludge status and subsequent Life Cycle Assessment modeling if a paradigm shift arose by moving from “waste” sludge to “product” sludge. Forty-four peer-reviewed papers of Life Cycle Assessment in the field of wastewater treatment have been critically reviewed to highlight the consequences of this paradigm shift on the Life Cycle Assessment method and to identify the impacts on the goal and scope step of Life Cycle Assessment questioned by the sludge status (“waste”, “waste-to-product” or “product” sludge). Results show that sludge is mainly considered as “waste-to-product” sludge. The “zero burden assumption” is always applied and sludge is never charged with an environmental burden associated with its production. This is not questionable when sewage sludge is considered as “waste” or “waste-to-product” sludge with energy recovery. However, this is becoming more debatable when sludge is considered as “product” sludge or as “waste-to-product” sludge with nutrient or material recovery.In such cases, the “zero burden assumption” is a no longer valid hypothesis as the sludge life-cycle needs to be fully considered. This can only be done if the sludge carries an upstream environmental burden. Allocating an environmental burden to the sludge appears as a challenging question requesting allocation factors between the sludge production and the water treatment for each step that generates sewage sludge.