Effect of the Ca2+ concentration on anaerobic digestion and microbial communities of granular sludge

The effects of substrate Ca2+ concentration were studied relative to the anaerobic digestion and microbial community structure of anaerobic granular sludge. In the treatment of papermaking wastewater by anaerobic granular sludge, the cumulative gas production was maximum at a Ca2+ concentration in the substrate of 120 mg/L, whereas it was the lowest at a concentration of Ca2+ in the substrate of 4000 mg/L. A high Ca2+ concentration in the substrate (≥1200 mg/L) will cause the pH of the fermentation broth to decrease during the fermentation process, which is not conducive to the anaerobic fermentation of the methanogenic process, resulting in unsatisfactory anaerobic digestion of the granular sludge. In addition, when the Ca2+ concentration was below 200 mg/L, the abundance of the important bacterial family Ruminococcaceae (rumen bacteria) in the anaerobic fermentation hydrolysis stage was drastically reduced and methane gas production increased. When the Ca2+ concentration was above 200 mg/L, the abundance of Anaerolineaceae (Anaerobic Streptomyces), which supplies organic acids, was substantially reduced. The methane gas production decreased as the Ca2+ concentration increased. Thus, the results showed that when the concentration of Ca2+ was above 200 mg/L, the methanogenic activity of granular sludge decreased.