Effect of high concentration of ammonium on production of n-caproate: Recovery of a high-value biochemical from food waste via lactate-driven chain elongation.

Abstract

Ammonium accumulation is inevitable during the fermentation of food waste (FW), challenging the application of chain elongation process upgrading FW into the high-value biochemical n-caproate, which is a medium chain carboxylate. This study is the first to investigate ammonium inhibition of lactate-driven chain elongation process. The short-term exposure of a Clostridium IV-dominated chain elongating reactor microbiome at an ammonium concentration of 1-4g L-1 linearly decreased n-caproate production by 25-80%. High levels of ammonium (≥5g L-1) could cause failure of chain elongation, shifting the product from n-caproate to propionate. The involved mechanisms revealed that ammonium reshaped the microbial community from Clostridium IV domination to Clostridium IV and Propionibacterium co-domination (based on 16S rRNA sequencing) and reduced the activities of key enzymes involved in the reversed β-oxidization pathway. We propose an effective strategy from our study, which is the first one to do in our knowledge, to upgrade raw FW without dilution to n-caproate: lowering the ammonium accumulation to 1.0g L-1 at the setup phase for adaptation and prolonging the hydraulic retention time (10days) during the operation phase for the colonization of chain-elongation bacteria. These findings lay a foundation for the implementation of the LCE process on FW, providing an alternative way to alleviate the global FW crisis.