Effect of mixing intensity on hydrolysis and acidification of sewage sludge in two-stage anaerobic digestion: Characteristics of dissolved organic matter and the key microorganisms.

Mixing should be optimized in anaerobic digestion (AD) systems to achieve excellent biomaterials production in the sewage sludge (SS) management in wastewater treatment plant. AD depends on the coordinated activity of hydrolysis, acidification and methanogenesis. However, the effect of mixing intensity on characteristics of hydrolysis and acidification in AD of SS is still poorly understood. This study focused on the mixing intensity (30, 60, 90 and 120 rpm) effect on the characteristics of dissolved organic matter (DOM) and the key microorganisms in the hydrolysis and acidification of SS. Results showed that enhanced hydrolysis and acidification efficiency was obtained at mixing of 90 and 120 rpm (p < 0.05), while the maximum acetic acid (388 ± 21 mg/L) was produced at 90 rpm. Mixing at 90 rpm enhanced the release of protein and polysaccharide as well as humic acid. Further analyses of DOM molecular features revealed that 90 rpm led to the highest molecular diversity and easily biodegradable molecules (lipid and proteins/amino sugars), which contributed to the maximum hydrolysis and acidification efficiency. Firmicutes and Actinobacteria significantly increased with mixing intensity (p<0.05), and Chloroflexi and Fusobacteria were enriched at mixing of 90 rpm, which favored the hydrolysis of SS. The enrichment of Clostridium XI and Clostridium sensu stricto contributed to the acidification of DOM at 90 and 120 rpm. The results of this study can advance our knowledge about mixing intensity effects on the AD systems of SS. This research also showed how increasing mixing intensity to a relatively high speed can enhance the hydrolysis and acidification efficiency of SS.