Composting as a Circular Pathway for Secondary Phosphorus Recovery from Salmon Farming Sludge

Recovering phosphorus (P) from organic waste streams is essential to reduce dependence on non-renewable phosphate rock and advance circular nutrient management. Salmon farming sludge is a nutrient-rich aquaculture residue with high resource recovery potential, although its direct agricultural use is limited by poor stability and sanitary concerns. This study assessed composting as a valorization pathway for transforming salmon sludge into a safer secondary P resource. Composting was conducted for 120 days in 200 L rotary-axis composters, and changes in P fractions, microbial biomass P, enzymatic activities, organic acids, and microbial communities were monitored. Composting promoted the redistribution of P from recalcitrant pools toward more labile fractions, especially H2O-P and NaHCO3-P, improving the fertilizer value of the final product. These shifts may be associated with microbial succession and biochemical activity, while potentially pathogenic microorganisms declined. Overall, composting offers a circular solution for phosphorus recovery, waste valorization, and fertilizer substitution in aquaculture–agriculture systems.