Enhancing the recovery of complex amino acids from excess sludge via low-intensity ultrasound-assisted enzymatic hydrolysis

Abstract

The recovery of high-value products such as complex amino acids from activated sludge is a crucial alternative for the recycling of excess sludge, although several challenges still need to be addressed, especially in terms of extraction efficiency and product quality. Herein, a promising approach involving low-intensity ultrasound-assisted enzymatic hydrolysis (UEH) was utilized to obtain complex amino acids from sludge protein. Compared with the values obtained for single enzymatic hydrolysis (EH), after a 20-minute pre-treatment at ambient temperature using low-intensity ultrasound (0.15 W/mL), the enzyme activity increased by 28.7 %, and the degree of protein hydrolysis (DH) correspondingly rose by 93.3 %. The application of ultrasound during subsequent EH resulted in a decrease in DH, with the extent of the decrease increasing over time. This suggested that the primary cause was conformational changes of enzymes rather than mass transfer. Moreover, both the Michaelis–Menten model (R2 > 0.99) and protein hydrolysis model (R2 > 0.96) accurately simulated EH and UEH, indicating that UEH exhibits superior enzyme–substrate affinity and a higher rate of protein hydrolysis. Furthermore, after separation and purification, the total content of seven essential amino acids in the dry material exceeded that in traditional feed sources such as soybean meal. Additionally, the safety test and energy consumption analysis demonstrated that UEH has good potential for the extraction of complex amino acids from excess sludge.