Is phosphate fertilizer production from wastewater sludge a solution to reduce phosphorus depletion? A life cycle assessment perspective

Production of goods drastically leads to non-renewable resources depletion. To slow down this depletion, alternatives need to be found. Recycling waste in order to recover resources is one of these alternatives. However, such alternative will be sustainable only if resource recovery does not worsen resources depletion. As an example, phosphorus is present in a diffuse form in domestic wastewater and recovered in sewage sludge. With an appropriate treatment, phosphorus can be recovered in a mineral form and used as fertilizer in place of fertiliser from phosphate rocks, and so reducing phosphorus resource depletion. This paper aims at highlighting if dissipated phosphorus recovery by producing sludge-based phosphate fertilizer can be a suitable alternative to reduce phosphorus depletion. To ensure the environmental relevance of such sludge-based products, the production of sludge-based phosphate fertilizers were compared to the production of phosphate fertilizer from phosphate rocks using Life Cycle Assessment. Four scenarios were assessed: (i) phosphorus mineral fertilizer produced from phosphate rock mining (reference scenario), (ii) sludge-based phosphate fertilizer obtained from biological acidification, (iii) sludge-based phosphate fertilizer obtained from crystallization, (iv) sludge-based phosphate fertilizer obtained from Gifhorn process, and (v) sludge-based phosphate fertilizer obtained from sludge ashes with AshDec process. The functional unit (FU) is the production of 1 kg of P. As sludge management is oriented to reach a final marketable product with predictable characteristics in line with its future reuse, we allocated part of environmental burden of water treatment to sludge production according to the method proposed by Pradel et al (2018, 10.1016/j.jclepro.2017.10.112). Results show that sludge-based phosphate fertilizer production has highest impacts compared to the reference scenario. This is mainly explained by (i) the allocation of the water treatment environmental burden to sludge production, (ii) reagents and energy needed to concentrate the diffuse phosphorus leading in more important impacts bring back to the FU and an unequal balance between consumed resources and recovered phosphorus. In this paper, we also discuss the potentiality of reducing phosphorus depletion if diffuse phosphorus recovery rate is improved and the difficulties to assess phosphorus resource depletion in current Life Cycle Assessment characterization methods