Impacts of molybdate and ferric chloride on biohythane production through two-stage anaerobic digestion of sulfate-rich hydrolyzed tofu processing residue.

Biohythane production through one-stage anaerobic digestion of sulfate-rich hydrolyzed tofu processing residue has been hampered by high HS production. Herein, two-stage anaerobic digestion was investigated with the addition of molybdate (MoO; 0.24-3.63 g/L) and ferric chloride (FeCl; 0.025-5.4 g/L) to the dark fermentation stage (DF) to improve biohythane production. DF supplemented with 1.21 g/L MoO increased hydrogen yield by 14.6% over the control (68.39 ml/g-VS), while FeCl had no effect. Furthermore, the maximum methane yields of methanogenic fermentation were 524.8 and 521.6 ml/g-VS with 3.63 g/L MoO and 0.6 g/L FeCl compared to 466.07 ml/g-VS of the control. The maximum yields of biohythane and energy were 796.7 ml/g-VS and 21.8 MJ/kg-VS with 0.6 g/L FeCl when the sulfate removal efficiency was 66.7%, and HS content was limited at 0.08%. Therefore, adding 0.6 g/L FeCl is the most beneficial in improving energy recovery and sulfate removal with low HS content.