The effect of blade angle of turbine impellers on anaerobic digestion efficiency in stirred digesters

Highlights: • Suitable TIBA and t_mix were determined based energy efficiency and digestion quality. • Increase N_s from 30 to 100 rpm, reduced t_mix<1.2 times but increased the P_w > 37 times. • Suitable t_mix, determined by image processing method, can affect CO and H₂S compounds. • TIBA of 60° had the least CO and 30° had the most CH₄ related to removal slurry properties. • The efficiency-based new model showed TIBA = 30°, t_mix = 5min had the highest efficiency. -- Abstract: In this study the mixing quality of turbine impellers with blade angles of 30°, 45°, and 60° was studied on the anaerobic digestion efficiency using a new model based on mixing time, energy consumption, biogas compounds, and slurry characterizations. Study on the mixing process using a single indicator system based image processing technique showed a mixing for 5 and 10 min and pausing for 5 h. The results showed that (i) the impeller with blade angle of 30° and mixing time of 5 min consumed the least energy whereas produced the most CH₄ amounts associated with more TS, T.VS, C, and N degradation, (ii) the impeller with blade angle of 60° produced the least CO amounts associated with more COD and BOD removal, and (iii) the suitable mixing time decreased the CO and H₂S production. The results confirmed that the significant effects of the impeller blade angle and mixing time on biogas compounds were because of changing the slurry characterizations during the mixing process. Using the new model for efficiency evaluation, the impeller with blade angle of 30° in mixing time of 5 min had the highest efficiency among the researched cases documented its favorite for better digestion in higher efficiency.