Improving the short-chain fatty acids production of waste activated sludge stimulated by a bi-frequency ultrasonic pretreatment

Short-chain fatty acids (SCFAs) are the most important intermediate in the waste activated sludge (WAS) fermentation process. This work explored a novel approach to improve the SCFAs production from WAS. Experimental results showed that the disintegration and acidification of WAS were enhanced markedly by using bi-frequency (28+40 kHz) ultrasonic pretreatment compared with monofrequency (28 kHz and 40 kHz) ultrasonic pretreatments. After 28+40 kHz ultrasonic pretreatment, the SCOD concentration increased from original 363 mg COD l ^-1 to 10810 mg COD l^-1 which was 1.53-fold and 1.44-fold of the values obtained with 28kHz and 40kHz ultrasonic pretreatments, respectively. The maximum SCFAs production reached 7587 mg COD l^-1 in the 28+40 kHz test which was respectively 1.25-fold and 1.31-fold of that in the 28kHz (6053 mg COD l^-1) and 40 kHz (5809 mg COD l^-1) tests. This was the highest SCFAs production obtained so far using WAS, pretreated by ultrasonic technology, as the renewable carbon source. SCFAs composition analysis revealed there was more acetic acid (3992 mg COD l ^-1, accounted for 52.6% of the total SCFAs) for the 28+40 kHz ultrasonic pretreatment which was beneficial to many subsequent bioprocesses.