Comparative study of pretreatment methods for the reverse enzymatic hydrolysis of Chlorella and Spirulina microalgae for bioethanol production

The microalgae Chlorella and Spirulina are sources of raw materials for renewable bioethanol production. The conversion of microalgae into ethanol requires the breakdown of complex carbohydrates into glucose via hydrolysis. In this study, reverse enzymatic hydrolysis was performed via incubation with glucoamylase followed by alpha-amylase. The effects of acid and non-acid pretreatment on the glucose yield, surface morphology, and ethanol yield following fermentation were investigated. The proximate results of the dry microalgae powders show that S. platensis exhibited the highest carbohydrate content (52.52%), while C. vulgaris and C. pyrenoidosa contained slightly lower levels of 43.24 and 41.15%, respectively. The maximum glucose yields from S. platensis, C. vulgaris, and C. pyrenoidosa following acid pretreatment were 21.701, 17.632, and 15.126 g/L, respectively. Scanning electron microscopy showed that acid pretreatment damaged the microalgal cell walls to facilitate enzyme access during hydrolysis. The highest ethanol yields were achieved after 48 h of fermentation, 15.85, 11.74, and 11.33 g/L for S. platensis, C. vulgaris, and C. pyrenoidosa, respectively. Sulfuric acid pretreatment before hydrolysis significantly increased bioethanol production from the microalgae. The results also indicate that S. platensis exhibits a superior ethanol production potential than the other microalgae types examined herein.