Response surface methodology analysis of anaerobic syntrophic degradation of volatile fatty acids in an upflow anaerobic sludge bed reactor inoculated with enriched cultures

Abstract

Anaerobic oxidation of volatile fatty acids (VFAs) as the key intermediates is restricted thermodynamically. Presently, enriched acetogenic and methanogenic cultures were used for syntrophic anaerobic digestion of VFAs in an upflow anaerobic sludge bed reactor fed with acetic, propionic, and butyric acids at maximum concentrations of 5.0, 3.0, and 4.0 g/L, respectively. Interactive effects of propionate, butyrate and acetate were analyzed. Hydraulic retention time (HRT) and acetate oxidizing syntrophs and methanogen (hydrogenotrophs) to syntrophic bacteria (propionate- and butyrate-oxidizing bacteria) population ratio (M/A) were investigated as key microbiological and operating variables of VFA anaerobic degradations. M/A did not affect the size distribution and had little effect on extracellular polymer contents of the granules. Granular sludge with close spatial microbial proximity enhanced syntrophic degradation of VFAs compared to other cultures, such as suspended cultures. Optimum conditions were found to be propionate = 1.93 g/L, butyrate = 2.15 g/L, acetate = 2.50 g/L, HRT = 22 h, and M/A = 2.5 corresponding to maximum VFA removal and biogas production rate. Results of verification experiments and predicted values from fitted correlations were in close agreement at the 95% confidence interval. Granules seemed to be smaller particles and less stable in construction with an irregular fractured surface compared to the original granules.