Simultaneous sulfide removal and COD retention of organic sulfur via a pH-controlled MWNTs-assisted sulfidogenic process

초록

Sulfur-containing organic compounds are converted into hydrogen sulfide (H2S) and other volatile sulfur compounds during anaerobic digestion, limiting biogas valorization and necessitating pre-removal. This study investigated an acidogenic-cum-sulfidogenic process to enhance sulfur release from sulfur-containing amino acids (cysteine and methionine) while retaining chemical oxygen demand (COD) for downstream methanogenesis. Batch experiments showed complete degradation of cysteine and methionine within 2 h and 24 h, respectively, at pH 7.0. At pH 5.5, cysteine degradation required more than 8 h, and methionine remained incompletely degraded even after 120 h. However, at pH 5.5 supplemented with multi-walled carbon nanotubes (MWNTs), cysteine was fully degraded within 4 h and approximately 90% of methionine was degraded within 24 h. COD retention in the effluent reached 75% at pH 5.5 and pH 5.5 + MWNTs, representing a 50% increase compared to pH 7.0. Sulfur removal was highest at pH 7.0 (80%), which was 60% higher than at pH 5.5 and 15% higher than pH 5.5 + MWNTs. Notably, the pH 5.5 + MWNTs condition achieved balanced performance, with 75% COD retention and 70% sulfur removal. Microbial community analysis revealed enhanced enrichment of Deltaproteobacteria and Synergistia under pH 5.5 + MWNTs, with relative abundances of 23.49% and 16.13%, respectively. These values were 63% and 68% higher than at pH 5.5 and 16% and 28% higher than at pH 7.0. The enrichment of these taxa, associated with direct interspecies electron transfer, likely contributed to improved process efficiency. Overall, pH 5.5 with MWNTs supplementation provided an optimal balance between sulfur removal and COD preservation.

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