Caproic acid production through lactate-based chain elongation: Effect of lactate-to-acetate ratio and substrate loading

Substrate properties play a key role in promoting the caproate yield through lactate-based carbon chain elongation pathway. In the present study, the effect of lactate-to-acetate (LA/AA) carbon ratio (from 0.5 to 5.0) and substrate loading (in terms of substrate/inoculum ratio within the range 20–180 mmol-C/g-VSS) on caproate fermentation was investigated. Results showed that both caproate content and yield increased by increasing the LA/AA ratio up to 3.0, then decreased at higher ratios due to activation of acrylate pathway and dispersion of carbon flux at elevated lactate content. At the optimal LA/AA carbon ratio of 3.0, substrate loading lower than 100 mmol-C/g-VSS was beneficial for efficient substrate utilization with low caproate selectivity, while higher substrate-to-inoculums (S/I) ratio led to incomplete substrate utilization and dispersed carbon flow, which finally reduced the caproate yield. Thus, the highest caproate yield of 0.42 g-COD/g-COD and selectivity of 49.5% were recorded at LA/AA and S/I ratio ratios of 3.0 and 100 mmol-C/g-VSS, respectively. The present study further depictures the novel approach for caproate production with lactate.