Volatile fatty acid (VFA) product spectrum as a function of the solids retention time (SRT) in an anaerobic granular sludge process

Abstract

Volatile fatty acids (VFA) may serve as building blocks for chemicals and polymers. A technology enabling high-rate VFA production from carbohydrate-rich water is the anaerobic granular sludge process. In this study the characteristics of an anaerobic granular sludge enrichment 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 various SRT of 1-2, 10-20, and 40-50 d. A low sludge volume index (SVI) of 11-44 mL·gTSS-1 allowed a high volatile suspended solid (VSS) concentration that reached 59 gVSS·L-1 . This high VSS concentration enabled a glucose consumption rates in the range of 1100 gCOD·L-1·d-1. Two product spectra were obtained: (i) an propionate:acetate mixture with a ratio of 2.05:1 (molpropionate:molacetate) produced at an SRT of 40-50 d; (ii) an acetate dominated product spectrum was obtained at 1-2 d and 10-20 d SRT (0.71-0.75 molacetate·molVFA-1). Overall a high VFA yield between 0.77-0.79 was obtained throughout all enrichments. This work demonstrates that high rate VFA production combining high yields and low solid concentrations in the effluent can be achieved. This works contributes to the implementation of waste-based production of VFA using anaerobic granular sludge.