Mesophilic and thermophilic alkaline fermentation of waste activated sludge for hydrogen production: Focusing on homoacetogenesis

The present study compared the mesophilic and thermophilic alkaline fermentation of waste activated sludge (WAS) for hydrogen production with focus on homoacetogenesis, which mediated the consumption of H-2 and CO2 for acetate production. Batch experiments showed that hydrogen yield of WAS increased from 19.2 mL H-2/gVSS at 37 degrees C and pH 10-80.1 mL H-2/gVSS at 55 degrees C and pH 10. However, the production of volatile fatty acids (mainly acetate) was higher at 37 degrees C and pH 10 by comparison with 55 degrees C and pH 10. Hydrogen consumption due to homoacetogenesis was observed at 37 degrees C and pH 10 but not 55 degrees C and pH 10. Higher expression levels of genes relating with homoacetogenesis and lower expression levels of genes relating with hydrogen production were found at 37 degrees C and pH 10 compared to 55 degrees C and pH 10. The continuous experiment demonstrated the steady-state hydrogen yield of WAS was comparable to that obtained from batch experiments at 55 degrees C and pH 10, and homoacetogenesis was still inhibited. However, the steady-state hydrogen yield of WAS (6.5 mL H-2/gVSS) was much lower than that (19.2 mL H-2/gVSS) obtained from batch experiments at 37 degrees C and pH 10 due to the gradual enrichment of homoacetogens as demonstrated by qPCR analysis. The high-throughput sequencing analysis of 16S rRNA genes showed that the abundance of genus Clostridium, containing several homoacetogens, was 5 times higher at 37 degrees C and pH 10 than 55 degrees C and pH 10. (C) 2016 Elsevier Ltd. All rights reserved.