Improving medium chain fatty acid productivity using chain elongation by reducing the hydraulic retention time in an upflow anaerobic filter

•MCFA production rate using chain elongation was improved by more than 3 times.•MCFA selectivity remained above 80% at 4h HRT.•Caprylate concentrations above solubility of caprylic acid in water were achieved.•Extra carbon dioxide additions improved the chain elongation process.•Carbon dioxide could be used as indirect substrate for chain elongation. The objective of this investigation was to further increase the medium chain fatty acid (MCFA) production rate by reducing the hydraulic retention time (HRT) in an upflow anaerobic filter. The results showed that the volumetric MCFA production rate was increased to 57.4g MCFA l−1d−1, more than 3 times higher than previous work. Despite the lower MCFA concentrations at 4h HRT, the MCFA selectivity remained above 80%. Extra carbon dioxide additions and higher yeast extract concentrations were required to increase the MCFA production rate. More research related to substrates and (micro)nutrients in mixed culture continuous reactors needs to be performed to reduce yeast extract use in chain elongation.