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

The effects of direct enzyme cocktail addition and other pretreatment methods on VFA production through cofermentation 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.