Mahmood Mahmoodi-Eshkaftaki, Askar Ghani, Amin Lotfalian Dehkordi
The present study aimed to simultaneously optimize substrate mixture and ultrasonic pretreatment for enhancing bio-H2 and bio-CH4 production while evaluating associated changes in feedstock characteristics, including bulk phenolic-related indicators, total solids (TS), and total volatile solids (TVS), during anaerobic digestion. For this purpose, various algae-to-glucose substrate ratios (0-100%) were evaluated under different ultrasonic power levels (0.04, 0.2, 0.4, and 0.6 W/mL) and sonication times (5, 10, 20, and 30 min). An I-optimal experimental design combined with response surface methodology (RSM) was employed to evaluate the combined effects of substrate mixture and ultrasonic pretreatment on bioenergy production and changes in feedstock characteristics during anaerobic co-digestion. To optimize bio-H2 and bio-CH4 production, desirability analysis was integrated with the overlay method. Analysis of the developed models indicated that (i) ultrasonic pretreatment enhanced total solids (TS) removal; (ii) both algae and glucose positively affected total volatile solids (TVS) removal, although algae required ultrasonic pretreatment for optimal performance; (iii) algae and glucose individually increased total carbohydrate content (TCHO) and total tannin content (TC), while their co-digestion reduced these values; (iv) algae increased total flavonoid content (TFC); and (v) algae enhanced bio-H2 and bio-CH4 production, with further improvement achieved through ultrasonic pretreatment. The optimization results indicated that a substrate mixture of 100% algae, ultrasonically pretreated at 0.51 W/mL for 18.65 min, maximized both bio-H2 and bio-CH4 production while improving the quality of the output digestate. The overlay analysis further showed that optimal fermentation performance could be achieved using mixtures containing more than 47% algae, with ultrasonic power in the range of 0.18-0.6 W/mL and sonication time between 13 and 30 min.