Substrate loading and nutrient composition steer caproic acid production and biofilm aggregation in high-rate granular reactors

Abstract

Carbohydrate-rich waste-streams form attractive feedstocks for the production of caproic acid via microbial chain elongation. However, process upscaling and implementation is hampered by low volumetric production rates. Biomass retention with granular fermentation technologies enables high-rate production of caproic acid at scale, but feedstock characteristics enabling microbial granulation under chain-elongating conditions are unknown. Here, we characterised the feedstock requirements for caproic acid production in high-rate expanded granular sludge bed reactors by investigating the impact of substrate concentration and nutrient composition on caproic acid production and granular biofilm aggregation. First, thin stillage was used as model feedstock to demonstrate that low substrate concentrations (11–49 g COD L-1) do not affect product profile, maintaining a caproic acid selectivity of 47 ± 3%, and improve granular biofilm aggregation. High substrate concentrations (55–88 g COD L-1) reduced caproic acid selectivity in favour of butyric acid, potentially due to concomitant product toxicity, and negatively affected granulation. Second, the impact of nutrient composition on production and granulation was investigated by developing a synthetic medium. This demonstrated that complex nutrients (here supplied as tryptone) were required to steer the microbial community from producing mainly acetic acid, butyric acid and iso-butyric acid to the target caproic acid. However, granules were observed in the system regardless of the addition of tryptone. Overall, these results provide crucial insight to identify suitable feedstocks for granular caproic acid producing technologies, opening the door to high-rate production of caproic acid from various underexplored carbohydrate-rich streams.