Aluminum-phosphorus complex mediated anaerobic digestion of waste activated sludge: Physicochemical reconfiguration, interfacial thermodynamics, and microbial mechanisms

• Al-P enhanced sludge organic matter solubilization and methane production. • Al-P enhanced sludge disintegration via physicochemical reconfiguration of sludge. • The increase in interfacial interaction energy promoted the dispersion of sludge particles. • Al-P enriched both acetoclastic and hydrogenotrophic methanogens and their associated methanogenic pathways. • The enhancement of the CO 2 to methane pathway was the most pronounced. Aluminum-phosphorus (Al-P) compounds are byproducts of chemical phosphorus removal in wastewater treatment plants (WWTPs), resulting in their accumulation in waste activated sludge (WAS). However, their effects on the anaerobic digestion (AD) of WAS remain poorly understood. This study investigated the effects and underlying mechanisms of Al-P on WAS anaerobic digestion. The results showed that Al-P enhanced methane production, with increases of 19.16% and 48.09% at dosages of 100 and 500 mg Al/g TS, respectively. Mechanism analysis indicated that Al-P reconfigured sludge physicochemical properties by binding with Ca 2+ and Mg 2+ , releasing extracellular polymeric substances (EPS), reducing sludge particle size, and increasing the proportion of internal water, thereby promoting sludge dispersion and floc disintegration. Interfacial thermodynamic analysis indicated that Al-P modified sludge surface properties by increasing acid-base interaction energy and the flocculation energy barrier. The total interaction energy of sludge particles rose from -15.26 mJ/m 2 in the control to 85.55 mJ/m 2 at an Al-P dose of 500 mg Al/g TS, which quantitatively confirmed sludge disintegration. Such structural and thermodynamic reconfiguration elevated the activities of key enzymes involved in hydrolysis, acidogenesis, and methanogenesis. Microbial analysis revealed that Al-P addition enriched both acetoclastic and hydrogenotrophic methanogens, as well as syntrophic bacteria that support hydrogenotrophic methanogenesis. Analyses of functional genes and metabolic pathways further corroborated that Al-P promoted both acidogenesis and the methanogenesis. This study provides novel insights into the potential of Al-P as an effective interfacial regulator for enhancing methane production and promoting sustainable bioenergy recovery from WAS.