Synergistic Carbon Sequestration for Sustainable Sludge Conversion

Wastewater treatment plants (WWTPs) represent significant carbon emission sources, as their sludge treatment processes (e.g., anaerobic digestion) release substantial amounts of biogenic CO. Existing technologies generally lack effective integrated carbon capture. Concurrently, green tides cause severe ecological damage, yet their biomass remains an underutilized potential carbon sink resource. Therefore, this review synthesizes supercritical water gasification (SCWG), organic rankine cycle (ORC), and Ulva biochar sequestration into a synergistic framework. SCWG directly processes wet sludge at >374C/22MPa to co-produce hydrogen and liquid CO, eliminating energy-intensive drying. The exothermic reaction generates waste heat (30% of input energy), recovered by ORC to power carbon capture units, achieving operational self-sufficiency. Captured liquid CO is transported via containerized systems (65 bar/20C) to Ulva farms, where micro-bubble aeration (<50m) enhances dissolution for algal photosynthesis. Harvested Ulva undergoes pyrolysis (300500C) to form century-scale stability biochar, completing a carbon closed-loop. This integrated pathway reduces greenhouse gas emissions by 3.88.2 kg CO/kg dry sludge versus conventional methods, while policy mechanisms (carbon tax, subsidies) improve economic viability. This review indicates that the elimination of sludge drying, achievement of energy self-sufficiency, and establishment of a carbon closed-loop collectively provide a viable pathway for hard-to-abate industries, thereby advancing carbon neutrality progress.