Optimization of biogas purification using anaerobic digestion of algae under ultrasonic pretreatment

The present study was undertaken to optimize CH 4 and H 2 concentrations and minimize biogas impurities during anaerobic digestion of spirulina microalgae. For this purpose, ultrasonic pretreatment factors including ultrasonic power and ultrasonic time were optimized. Glucose was added to digesters as a nutrient to improve the anaerobic digestion. Multivariate regression modeling and desirability analysis were integrated to determine the best substrate mixture and pretreatment factors. The regression analyses showed that the developed multivariate regression models could accurately predict the biogas composition. Furthermore, the developed models were successful in predicting biogas composition for future observations. CH 4 and H 2 concentrations of the feedstocks were improved with increasing algae content in the substrate mixtures. The desirability analysis was performed based on maximizing CH 4 and H 2 concentrations and minimizing CO 2 , H 2 S and CO concentrations. The highest desirability value (0.745) was obtained for a substrate mixture of 95.83 % algae and 4.17 % glucose, ultrasonically pretreated at 93.36 W for 13.5 min. These optimum conditions produced the highest CH 4 and H 2 concentrations (48.91 % and 8.19 %, respectively) and the lowest CO 2 , H 2 S and CO concentrations (9.83 %, 0.0019 % and 0.0344 %, respectively). The region of interest for the biogas composition, determined using the overlay method, showed that (i) substrate mixtures containing more than 50 % algae could achieve optimal biogas composition (ii) glucose could improve biogas composition when combined with strong ultrasonic pretreatment, and (iii) biogas composition was improved using an ultrasonic power lower than 160 W and an ultrasonic time shorter than 34 min.