Study of microbiological and operational parameters in thermophilic syntrophic degradation of volatile fatty acids in an upflow anaerobic sludge blanket reactor

Volatile fatty acids (VFAs) are critical intermediates in anaerobic digestion. Temperature affects thermodynamics of syntrophic digestion of VFAs as well as solubilities and diffusion coefficients of intermediate metabolites. In this research interactive effects of propionic (HPr), butyric (HBu) and acetic (HAc) acids, hydraulic retention time (HRT) and methanogen to acetogen population ratios (M/A) were investigated as major microbiological and operating variables in syntrophic reactions at thermophilic temperature (55 °C). Experiments were carried out in upflow anaerobic sludge blanket (UASB) reactor inoculated with enriched acetogenic and methanogenic cultures. Central composite design (CCD) was used to design experiments and results were analyzed using response surface methodology (RSM). Corresponding to maximum VFA removals and biogas production rate (BPR), optimum conditions were found to be HPr = 1.9 g/L, HBu = 2.2 g/L, HAc = 2.5 g/L, HRT = 22 h and M/A = 2.5. Results of verification experiments and predicted values from fitted correlations were in close agreement at 95% confidence interval. Analysis of the results of thermophilic process showed that trends and interactive effects of different parameters as well as its optimum conditions were very similar and comparable with the previous study at mesophilic temperature (37 °C).