Volatile Fatty Acid Product Spectrum as a Function of the Solids Retention Time in an Anaerobic Granular Sludge Process

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Volatile fatty acids (VFA) may serve as building blocks for the production of chemicals and polymers. A technology enabling high-rate VFA production from carbohydrate-rich wastewater is the anaerobic granular sludge process. In this study, the characteristics of an anaerobic granular sludge process fermenting glucose was evaluated at different solid retention times (SRT). A lab-scale anaerobic sequencing batch reactor fed with 6 g·L−1 glucose was operated at a pH of 5.5 and at SRT values of 1–2, 10–20, and 40–50 days and operated in total for 215 days. A low sludge volume index (SVI) of 1,144 mL·gTSS−1 allowed for the high SRT and high volatile suspended solid (VSS) concentration that reached 59 gVSS·L−1. This high VSS concentration enabled a glucose consumption rate of 1,100 gCOD·L−1·day−1 at an SRT of 40–50 days. Two product spectra were obtained: (1) a propionate:acetate mixture with a ratio of 2.05∶1 (molpropionate:molacetate) produced at an SRT of 40–50 days; and (2) an acetate dominated product spectrum was obtained at 1–2 days and 10–20 days SRT (0.71–0.75 molacetate·molVFA−1). Overall, a high VFA yield between 0.77 and 0.79 was obtained throughout all enrichments. This work demonstrates that high-rate VFA production combining high yields and low solid concentrations in the effluent technologically can be achieved. This work contributes to the implementation of waste-based production of VFA using anaerobic granular sludge.