Ultrasonic-alkali method for synergistic breakdown of excess sludge for protein extraction

Abstract

Excess sludge is rich in proteins that can be recovered to prepare protein foaming agents and other products with high added value. To extract sludge proteins efficiently, more sludge proteins can be dissolved into the liquid phase by destroying sludge flocs and microbial cell structures. The commonly used alkali-thermal hydrolysis method has the disadvantages of high energy consumption and the additional Maillard reaction. Ultrasonic-alkali hydrolysis (UA) has been widely used as pretreatment for sludge anaerobic digestion, and the effect of UA on cell wall cracking gives it potential application prospects in sludge protein extraction. This study evaluated the potential of UA for sludge protein extraction, and the experimental conditions were optimized to explore the foaming performance of sludge proteins based on a single-factor experiment and orthogonal experiment. The results showed that under optimum hydrolysis conditions (pH of 12, sludge moisture content of 94%, ultrasonic power density of 1.7 W/mL, and ultrasonication duration of 24 min), the protein yield and protein extraction efficiency (RP) were 366.3 mg/g total solids (TS) and 81.4%, respectively. UA converted more than half of the proteins into polypeptides and thus promoted the foaming performance of the sludge proteins to meet the relevant standards for foaming agents. The dewatering performance of the hydrolysed sludge was improved by 93.4%, which was convenient for subsequent treatment. Further research illustrated that UA could shorten the treatment time of alkali hydrolysis (AH) by 36 times when achieving a similar protein yield, and the protein yield produced by UA was 11.4 times greater than that produced solely by ultrasonic hydrolysis (US), indicating that a synergistic effect occurred when AH was combined with US.