Travail de fin d'études en chimie et bio-industries : Understanding the Fate of Organic Matter and Nutrients in New Typologies of Organic Sludge Fertilizers.

Abstract

Global population growth, urbanisation and intensifying pressure on natural resources have made the sustainable management of organic waste streams an increasingly urgent priority. The depletion of finite mineral resources and the decline of soil organic matter across European agricultural land have underlined the need to recover and recycle nutrients from waste rather than disposing of them – a need made more urgent by rising fertilizer costs and geopolitical instability in supply chains. In this context, the concept of a circular bioeconomy - in which waste streams are valorised as resources rather than discarded - has become central to EU environmental and agricultural policy. Sewage sludge, generated in vast quantities across Europe, represents both a significant waste management burden and an underexploited source of organic matter and plant-available nutrients. When treated appropriately, SS can return to agricultural land as a biofertilizer, closing nutrient loops and reducing dependence on synthetic fertilizer inputs. However, it also contains a complex mixture of contaminants - including heavy metals, pharmaceuticals, PFAS and microplastics - whose behaviour during treatment and following land application is not fully understood. The fate of organic matter, nutrients and pollutants during sludge treatment is a function of both the treatment process applied, and the chemical structure of the compounds involved. Biological treatments tend to stabilise organic matter and preserve nutrients. Thermochemical treatments achieve greater volume reduction and alter contaminant concentrations and their interactions with organic matter, thereby affecting pollutant bioavailability in soil. However, this comes at the cost of structural changes to organic matter itself and shifts in nutrient bioavailability. The impact of sludge application on agricultural soils depends on soil physicochemical properties, vegetation and crop type, application rate and environmental conditions. Evaluating SS-derived products for land application thus requires comprehensive, multicriteria characterisation of their physicochemical properties, agronomic benefits and associated risks. This study aimed to provide a comprehensive characterization of sludge and sludge-derived products with an emphasis on the bioavailability, bioaccessibility and complexity of organic matter and nutrients across these matrices. The main objectives are: (i) to elucidate the transformation pathways of organic matter and nutrients across various sludge-derived products, (ii) to identify the most impactful physicochemical and agronomical fate parameters affected by these transformations through multivariate statistical analyses, (iii) to elucidate the links between treatment processes and their corresponding end-products and facilitate a reverse-engineering approach, to optimize process performance and product quality.