Impact of Dried Powdered Sludge Supplementation on High-Solid Anaerobic Digestion: Experimental, Modeling, and Economic Perspectives

High-solid anaerobic digestion (HSAD) of sewage sludge has gained attention as both an energy recovery process and a sustainable approach for handling hazardous waste. This study presents the use of dried powdered sludge (DPS) as a biological stress factor in improving the hydrolysis of HSAD of dewatered sewage sludge. Batch experiments with varied concentrations of DPS (1–20 g/L) and without DPS (control) were tested to examine the effect of DPS on hydrolytic enzyme activity, substrate solubilization, and cumulative methane production (CMP). Blank assays were performed to separate the intrinsic biodegradable contribution of DPS from net system responses. A two-phase experimental design was adopted: Phase 1 quantified the effect of DPS on hydrolytic enzyme activity and substrate solubilization, while Phase 2 assessed full anaerobic digestion performance through CMP and volatile solids reduction. The mechanistic basis of enhancement was further supported using a reduced anaerobic digestion model no. 1 (ADM1). Phase 1 experiments revealed that up to 10 g/L DPS addition (G3) in the digester stimulated substantial levels of increased protease, cellulase, and lipase activity by +92%, +50%, and +50%, respectively, which was in accordance with enhanced solubilization of complex organics as quantified by soluble chemical oxygen demand (COD). The hydrolytic enzyme activity measurements supported the hypothesis that DPS enhances hydrolysis by stimulating microbial enzyme production. Phase 2 experiments revealed that DPS addition (1–20 g/L) markedly improved HSAD performance over control, CMP rose by +4.8%–22.3%, and volatile solids were reduced by as much as 44.3%, confirming that DPS enhanced both hydrolysis and overall organic breakdown—key for making HSAD more efficient and sustainable. The hypothesis was also verified by using reduced ADM1. The kinetic parameters estimated through reduced ADM1 revealed that the hydrolysis, acidogenesis, and methanogenesis all enhanced with an increasing DPS dose of up to 10 g/L. However, additional DPS doses retarded the rates of the downstream stages of AD, likely due to inhibition through the accumulation of heavy metals. A cost–benefit analysis that included revenues from electricity generated from methane and the savings costs of sludge disposal showed an optimal DPS dose of 11.4 g/L, which is very close to the biological performance optimum (10 g/L). The combined experimental, modeling, and economic evaluation suggests that moderate DPS supplementation may provide a scalable and cost-effective strategy to improve HSAD performance without additional chemical or energy inputs, contributing to sustainable sludge management within a circular economy framework.