High-rate conversion of lactic acid-rich streams to caproic acid in a fermentative granular system

Lactic acid-driven chain elongation enables upgrading low-value organic streams into caproic acid. Recently, volumetric production rates over 0.5 g L-1 h(-1 )have been reported for carbohydrate-rich streams in expanded granular sludge bed (EGSB) reactors. However, many target streams contain mixtures of carbohydrates and lactic acid, and little is known about their impact on product profile and microbial ecology, or the importance of carbohydrates as substrate to achieve high rates. This manuscript investigated varying glucose-to-lactate ratios and observed that decreasing glucose-content eliminated odd-chain by-products, while glucose omission required acetic acid addition to support lactic acid conversion. Decreasing the glucose-content fed resulted in decreasing amounts of granular biomass, with the disappearance of granules when no glucose was fed. Lowering the HRT to 0.3 days while feeding only lactic and acetic acid likely triggered re-granulation, enabling the highest lactic acid-driven caproic acid production rates reported thus far at 16.4 +/- 1.7 g L-1 d(-1).

Automated summary

Lactic acid-driven chain elongation allows for the conversion of low-value organic streams into caproic acid. This study investigated the impact of varying glucose-to-lactate ratios and found that decreasing glucose-content eliminated unwanted by-products. The study also observed that decreasing glucose-content led to a decrease in granular biomass, but re-granulation was triggered by lowering the hydraulic retention time and feeding only lactic and acetic acid, resulting in the highest reported lactic acid-driven caproic acid production rates.