Bicarbonate drives methane production in a bioaugmented anaerobic digestion of waste activated sludge: Liberating uronic acids inertized by Fe(III)/Al(III).

The limitation in the bioaugmented anaerobic digestion of waste activated sludge (WAS) has recently been revealed by the poor degradability of gelling organics inertized by metal ions, particularly the uronic acids bound with Fe(III) and Al(III). However, the method to liberate these organics by damaging the gelling structure remains scarce. For the first time, this work developed a sodium bicarbonate (SBC) based strategy to liberate the gelling organics bound by metal ions, based on the DFT calculation, promoting their conversion in a polygalacturonate-degrading consortium (GDC) based bioaugmentation of WAS digestion. The results demonstrated that dosing SBC, a main byproduct of biogas upgrading, released 442.5 ± 15.6 mg/L SCOD and 122.3 ± 8.4 mg/L uronic acids from WAS, increasing CH4 production by 38.2%. SBC enhanced hydrolytic activity by stimulating polygalacturonate (PGA) lyase (EC 4.2.2.2) from Paludibacter, restoring enzyme activities by 69.5% in PGA-Fe(III) and 27.3% in PGA-Al(III). Subsequently, SBC reactivated the methanogenic potential of the bound uronic acids, resulting in 3.9-fold and 2.7-fold higher CH4 production from PGA-Fe(III) and PGA-Al(III), respectively. These changes were accompanied by the enrichment of both hydrolytic bacteria and methanogenic archaea in GDC. The destruction of uronic acids-metal complexes was driven by HCO3- competition, which contributed 86%-96% to WAS solubilization, EPS release, and methanogenesis, compared to pH. Consequently, this study established SBC-induced liberation of inertized uronic acids as an innovative and efficient strategy for overcoming metal inhibition in the bioaugmented anaerobic digestion of WAS.