Co-fermentation of municipal waste streams: Effects of pretreatment methods on volatile fatty acids production

Abstract

The effects of direct enzyme cocktail addition and other pretreatment methods on VFA production through co-fermentation of primary sludge and external organic waste were studied under initial alkaline (pH 10) and acidic (pH 5) conditions. Besides the direct enzymes addition, 24-h enzymatic, ultrasonic, ultrasonic+24-h enzymatic and thermal were the other pretreatment methods employed. Under the alkaline condition, direct addition of enzymes enhanced VFA production by 29–39%, attaining VFA yields of 364–633 mg COD g−1 VS throughout the experimental period, whereas there was 37–43% increase in VFA production under the acidic condition only at retention days 1 and 2, achieving VFA yield of 207 and 215 mg COD g−1 VS, respectively. VFA composition was dominated by acetic acid (54–67%) and propionic acid (31–46%) under alkaline and acidic conditions, respectively. Direct enzyme addition increased the percentage of caproic acid from 8-9% to 16–17%, under the acidic condition. While 24-h enzymatic pretreatment didn't show any improvement on VFA production, thermal, ultrasonic+24-h enzymatic and ultrasonic pretreatment methods increased VFA production by only 18, 14 and 4%, reaching maximum VFA yields of 512, 486 and 445 mg COD g−1 VS, respectively. On the other hand, these pretreatment methods significantly increased biogas production. Experiment without pretreatment achieved maximum biogas production of 392 NmL CH4 at day 9 while experiments with pretreatments achieved a maximum of 679–954 NmL CH4 in the order of no pretreatment < ultrasonic < thermal < 24-h enzymatic < ultrasonic+24-h enzymatic. The study gives insight into how pretreatment strategy can influence VFA production and composition.