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

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 $6\mathrm{g}·{\mathrm{L}}^{-1}$ 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 $\mathrm{1,144}\mathrm{mL}·{\mathrm{gTSS}}^{-1}$ allowed for the high SRT and high volatile suspended solid (VSS) concentration that reached $59\mathrm{gVSS}·{\mathrm{L}}^{-1}$. This high VSS concentration enabled a glucose consumption rate of $\mathrm{1,100}\mathrm{gCOD}·{\mathrm{L}}^{-1}·{\mathrm{day}}^{-1}$ at an SRT of 40–50 days. Two product spectra were obtained: (1) a propionate:acetate mixture with a ratio of $2.05:1$ (${\mathrm{mol}}_{\mathrm{propionate}}:{\mathrm{mol}}_{\mathrm{acetate}}$) 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 ${\mathrm{mol}}_{\mathrm{acetate}}·{\mathrm{mol}}_{\mathrm{VFA}}^{-1}$). 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.