Exploring the limits of carbohydrate conversion and product formation in open mixed culture fermentation

The industrial large-scale application of open mixed culture carbohydrate fermentation processes requires a better understanding of their capacity limits and product spectra. This work aims to explore these limits through the operation of two lab-scale fermenters at increasing influent glucose concentrations in continuous mode and using constant operating conditions (HRT = 20 h; pH 5.4; 30 °C). In one fermenter, glucose concentration increases were slow, in order to determine representative product yields. In the other fermenter, increases were faster, aiming also at gaining other operational insights along with a consistency validation of product yields. Different glucose concentrations led to consistent product yields in both fermenters. Full glucose conversion limit was reached at 33 gCOD L_feed^-1, with 32 mM of undissociated acids present. After glucose fermentation saturation, up to 10.6 gCOD L^-1 lactate accumulation was observed. In both reactors, a glucose concentrations of 33 gCOD L_feed^-1 led to the formation of granular biomass, with which full glucose conversion resumed up to 38 gCOD L_feed^-1. In terms of products, at low glucose concentrations with only suspended biomass, the main products were acetate and butyrate, while at higher concentrations with granular biomass, the main products shifted to propionate and valerate. The similar microbial communities and products between both reactors suggest that a shorter startup time to full capacity loading is possible, which is relevant for large scale industrial implementation.