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

Abstract

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.