Co-Fermentation of Municipal Waste Streams : Effect of Pretreatment Methods on Volatile Fatty Acids Production

The effect of direct enzyme addition and other pretreatment methods on VFA production through co-fermentation of primary sludge and external organic waste was studied under initial alkaline (pH 10) and acidic (pH 5) conditions. Beside the direct enzyme cocktail addition, 24-hour enzymatic, ultrasonic, ultrasonic + 24-hour enzymatic and thermal were the other pretreatment methods. Under the alkaline condition, direct addition of enzyme enhanced VFA production by 29-39% throughout the experimental period, whereas there was 37-43% increase in VFA production under the acidic condition only at retention days 1 and 2. VFA composition was dominated by acetic acid (54-67%) and propionic acid (31-46%) under alkaline and acidic condition, respectively. Direct enzyme addition resulted in the increment in the percentage of caproic acid from 8-9% to 16-17%, under the acidic condition. While 24-hour enzymatic pretreatment didn’t show any improvement on VFA production, thermal, ultrasonic + 24-hour enzymatic and ultrasonic pretreatment methods increased VFA production by 18, 14 and 4%, respectively. On the other hand, the pretreatment methods significantly increased biogas production. Experiment without pretreatment achieved maximum biogas production of 392 NmL CH 4 at day 9 while experiments with pretreatments achieved maximum of 679-954 NmL CH 4 in the order of no pretreatment < ultrasonic < thermal < 24-hour enzymatic < ultrasonic + 24-hour enzymatic. The study gives insight into how pretreatment strategy can influence VFA production and composition