Integrated Intensification and Nutrient Recovery Strategies in Two-Stage Anaerobic Co-Digestion of Sewage Sludge and the Organic Fraction of Municipal Solid Waste: State of the Art, Engineering Challenges of Scale-Up, and Perspectives

Two-stage anaerobic digestion (TSAD) is an alternative to single-stage anaerobic digestion, separating hydrolytic–acidogenic and methanogenic phases to improve stability, organic matter degradation, and methane production. This review examines TSAD for co-digestion of sewage sludge (SS) and the organic fraction of municipal solid waste (OFMSW), emphasizing performance, scale-up challenges, digestate intensification, and nutrient recovery. TSAD can increase methane production by 25–50% compared with single-stage systems, while volatile solids removal can reach 87–93% depending on substrate type, temperature regime, hydraulic retention time, and organic loading rate. However, improvement remains variable and depends on substrate biodegradability, reactor configuration, and process control. Beyond methane recovery, the review highlights valorizing digestate as a secondary resource. Digestate post-treatment technologies, including thermal hydrolysis and steam explosion, report methane improvements from 26% to more than 300%, although energy demand and economic feasibility remain constraints. Nitrogen recovery technologies, including ammonia stripping and membrane contactors, can achieve efficiencies above 80–95% under optimized conditions, while phosphorus may be recovered through struvite precipitation, calcium phosphate recovery, or biochar-based pathways. Future TSAD development should integrate biological conversion, digestate recirculation, nutrient recovery, techno-economic assessment, and life-cycle evaluation to support circular, resource-efficient organic waste treatment systems.