Environmental and social sustainability assessment of a circular process for the valorisation of sewage sludge ash and mining by-products into bio-based fertilisers

Phosphorus (P) recovery from sewage sludge ash (SSA) represents an interesting solution to P supply concerns. While techno-economic assessments show promising outcomes, the environmental and social performance of these recovery processes remains insufficiently explored, hindering the market uptake of recovered P in bio-based fertilisers. This work investigates the environmental impacts of producing four commercial granular fertilisers representative of the Italian fertilizer market in 2024 (i.e., ENERGEO CV, ENERGEO CV TOP, LITHOZINC and PHEOSCOR). Furthermore, a social hotspot analysis (SHA) was conducted to account for the social implications of raw materials supply (e.g., P, Mg, S, Ca, Cl) in fertiliser manufacturing. Impacts were evaluated for a Business-As-Usual (BAU) scenario, in which fertilisers are based on mineral P (i.e., phosphorite), and a Circular Economy scenario (CE), in which phosphorite is partially replaced with P recovered from SSA via wet chemical extraction and co-precipitated with calcium hydroxide or low-grade magnesium oxide mining by-product (LG-MgO). Raw material supply covered from 56% to 98% of total environmental impacts across all fertilisers, except for ENERGEO CV – BAU and ENERGEO CV TOP – BAU, where core processes were the main contributors to specific sub-categories. LITHOZINC – CE and PHEOSCOR – CE showed similar or enhanced performances compared to the corresponding BAU formulations, indicating potential benefits in employing LG-MgO as precipitant. The supply chains of raw materials exhibited a medium-high social risk in the assessed categories, with Egyptian phosphorite extraction posing the greatest concerns for workers and local communities, and the innovative solution potentially improving the social performance of the fertiliser manufacturing process.