Lactate addition boosts valerate yields in granular mixed culture carbohydrate fermentation

Valerate is a valuable volatile fatty acid with multiple industrial applications that can be sustainably produced from biowaste using odd-chain elongation processes in open mixed culture fermentation (MCF). The selective production of valerate in MCF has not been well explored to date. This preliminary study aims to improve the valerate yields achievable in a lab-scale granular MCF from glucose, by exogenous addition of lactate. Constant operating conditions were maintained (HRT 20 h; 30 °C; pH 5.4) with increasing lactate:glucose ratios in the influent while the total substrate COD concentration remained constant at 36 g COD Lfeed-1. Increasing ratios led to higher valerate production, achieving a maximum concentration and yield of 12.7 g COD L-1 and 34 % COD, respectively, at a lac:glu molar ratio of 9.9:1. This yield is 65 % higher than obtained without lactate addition. The higher valerate yields were concomitant with a decrease in propionate, suggesting a propionate plus lactate pathway. The abundance of Megasphaera, well-known chain elongator, increased (up to 52 %) with increasing lac:glu ratios. In contrast, higher ratios (23.6:1) resulted in system collapse which was attributed to the low metabolic energy available from the low glucose fed, insufficient to sustain biomass growth and the expected high maintenance in a medium with high concentrations of undissociated acids.