Nano zero-valent iron regulates the enrichment of organics-degrading and hydrogenotrophic microbes to stimulate methane bioconversion of waste activated sludge

Underlying mechanism of continuous nano zero-valent iron (nZVI) supply in functional microbes enrichment and methane bioconversion of waste activated sludge were investigated in a lab-scale continuous stirred tank reactor (CSTR). Results indicated that methane productivity strongly relied on nZVI dosage (R = 0.998, p (0.05) < 0.001) and leached Fe²⁺ concentration (R = 0.943, p (0.05) = 0.005). The higher nZVI dosage, the higher methane productivity and organic removal were achieved. The methane production rate reached 77.6 ± 9.7 mL/g-VS/d at 160 mg-nZVI/g-TS, increasing by 2.3 times than the phase without nZVI supply. The corrosion/dissociation of nZVI, except for serving as trace element and electron donors (H₂/[H]), increased system buffering capacity (2835–3410 mg CaCO₃/L), and decreased oxidation reduction potential from − 152 mV to − 240 mV. Such thermodynamically favorable environment facilitated the proliferation of organics-degrading bacteria (Firmicutes, Chloroflexi, Smithella, etc.) and hydrogenotrophic methanogen (Methanobacterium). As a result, more large organic molecules such as proteins and polysaccharides were hydrolyzed into short-chain fatty acids, thus enhancing the methanogenesis. Furthermore, the continuous supply of nZVI regulated the colloidal properties of sludge flocs by altering particle size distribution and zeta potential via flocculation effect of the released Fe²⁺ (136.4 ± 12.1 mg/L). The dewaterability of digested sludge was improved to a certain extent, with capillary suction time dropping to 84.8 ± 4.1 s. This study provided a more systematic understanding of the role of nZVI in promoting methane bioconversion and subsequent digested sludge dewatering and volume reduction, thus expanding the potential application of nZVI in real-world scenarios.