Anaerobic co-digestion of food waste and waste activated sludge for methane production: Evaluation of optimum ratio, microbial analysis, and kinetic modeling

This study aimed to evaluate the anaerobic co-digestion of food waste (FW) and waste activated sludge (WAS) in batch reactors (R1-R4) to determine the optimum ratio using the biochemical methane potential (BMP) test. A food-to-microorganism (F/M) ratio of similar to 2 was maintained in each reactor, and the result indicated that FW to WAS ratio of 50:50% in R1 was optimum, with the highest cumulative methane production of 5916 mL observed over 32 days. The cumulative methane production follows the order of R1 > R2 > R3 > R4, which is correlated with the reduction of volatile solid (VS) and soluble chemical oxygen demand (sCOD). The digestate sludge from R1 after methane production shows its potential for use as a biofertilizer based on the acceptable C/N ratio (13.25). In addition, the liquid digestate generated is rich in carbon, nitrogen, and phosphorus, making this a valuable source of liquid fertilizer. Microbial community analysis using 16S rRNA sequencing in R1 showed the dominance of Firmicutes at the phylum level and Lactobacillus at the genus level in anaerobic co-digestion. The cumulative methane production in various reactors was validated using different kinetic models based on co-efficient of determination (R-2). The Logistic Function model gives a best fit for R1 (R-2 = 0.9915 and methane potential = 5964 mL), whereas the Fitzhugh model gives a good agreement for R2 (R-2 = 0.9885 and methane potential = 3280 mL), R3 (R-2 = 0.9812 and methane potential = 1442 mL), and R4 (R-2 = 0.9721 and methane potential = 1177 mL). The finding of this study would help to scale-up the co-digestion of FW and WAS at an optimum ratio for methane production.