Revealing the effect of Al₂O₃ on sulfur transformation in anaerobic sludge process

Al₂O₃, one multifunctional adsorbent and dehydrator, was widely recognized as a practical and environmentally friendly additive that enhances fermentation efficiency and facilitates the recovery of resources from waste-activated sludge (WAS). However, its potential harmful effects on WAS fermentation, such as the generation of hydrogen sulfide (H₂S), have been previously overlooked. This study found that with the increase of Al₂O₃ dosage from 0 to 60 mg/g VSS, the maximum production of H₂S decreased from 371.60 ± 3.72 × 10^-4 to 303.36 ± 3.03 × 10^-4 mg/g VSS. The study on the transformation of sulfur-containing compounds has identified that the primary cause for lowering the formation of hydrogen sulfide (H₂S) is the inhibitory effect of aluminium oxide (Al₂O₃) on sulfate reduction. The mechanism analysis discovered that Al₂O₃ initially stimulated the functional groups and hydrogen bonding networks present in sludge EPS. This resulted in a 2.04 % rise in the content of C-C groups, a 7.78 % increase in the content of C-O-C groups, and a 4.24 % increase in the content of β-turn and α-Helix structures. This resulted in the fracturing of sludge EPS and the release of soluble metal ions such as aluminium, magnesium, and iron. The liberated metal ions facilitated the conversion of H₂S gas and dissolved sulfide into metal sulfide, hence contributing significantly to the reduction of H₂S gas emissions. Microbial community research revealed that the inclusion of Al₂O₃ enhanced the performance of methanogens (e.g., Methanothrix), but inhibited sulfate reducing bacteria (e.g., unclassified_c__Deltaproteobacteria). Additional examination of functional genes demonstrated that Al₂O₃ decreases the amount of functional genes involved in the hydrolysis of organic sulfur (such as MetQ, pepD, CDO1, yhdR, etc.). and sulfate reduction processes (sat, cysC, aprAB, dsrAB, etc.). These findings offer novel perspectives on the treatment of sludge using Al₂O₃ and could have substantial consequences for sludge treatment.