Enhanced VFAs production from microalgal hydrolytic acidification with ultrasonic-alkali pretreatment

Cell disruption to break the rigid algal cell wall was essential to improve the utilization of resources enriched in microalgae. This study aimed to compare the effects of alkali, ultrasound, and the synergistic effect of ultrasonic and alkali on microalgae disruption and preliminarily investigated the synergistic disruption mechanism. The effect of the above methods on the disruption of microalgal cells was quantified by measuring protein and TOC release. Under the optimized conditions of the UA method (initial pH: 12.0, sound intensity: 6 W/mL, 40 min), the protein release was 360.42 mg/g TS, which were 5.6 and 7.1 times higher than those of alkali and ultrasound, respectively. EEMs spectroscopy showed that org-N compounds were mainly released during UA treatment of Chlorella pyrenoidosa (C. pyrenoidosa), with soluble microbial by-product-like compounds predominating. With free radical detection and particle size analysis, it was found that nutrient release occurred mainly during the microalgal bloom dispersion and individual microalgal micro-fragmented phases. The practical applicability of the synergistic strategy was also investigated. Microalgal hydrolytic acidification after UA pretreatment was suitable for VFAs production. Compared to the blank group, the peak VFAs production (392.5 mg COD/g VSS) increased by 200 % and the time reduced from 11 days to 9 days. These results indicated that synergistic disruption pretreatment was expected to improve the performance of microalgal biomass hydrolytic acidification.