Beyond Stand-Alone Technologies: Integrated Pathways for Energy, Nutrient and Carbon Recovery from Sewage Sludge

Sewage sludge (SS) management is progressively shifting from waste disposal toward resource recovery and circular bioeconomy approaches. While conventional technologies such as anaerobic digestion (AD), incineration, and land application remain widely adopted, increasing attention is directed toward processes capable of simultaneously recovering energy, nutrients, and carbon-based products. Most existing reviews evaluate biochemical, thermochemical, and electrochemical technologies as stand-alone options, whereas their integration within sludge biorefineries remains comparatively underexplored. This critical review examines how conventional and emerging technologies, including hydrothermal carbonization (HTC), pyrolysis, and bioelectrochemical systems (BES), can be combined to improve resource recovery. The literature was selected based on topical relevance and publication date, prioritizing recent studies on resource recovery and integrated treatment configurations. Particular attention is given to AD-HTC, AD-pyrolysis, and AD–BES. The comparison indicates that AD-HTC is particularly relevant for wet digestate because it avoids complete drying and offers opportunities for process-water recycling and nutrient recovery, whereas AD-pyrolysis provides energy-rich products and carbonaceous solids but requires greater attention to drying and heat integration. AD–BES can enhance methane or nutrient recovery, although energy performance depends strongly on external electrical input and operating conditions.