Long-Term Continuous Extraction of Medium-Chain Carboxylates by Pertraction With Submerged Hollow-Fiber Membranes.

Jiajie Xu,Bin Bian,Pascal E Saikaly,Largus T Angenent

doi:10.3389/fbioe.2021.726946

Abstract

Medium-chain carboxylic acids (MCCAs), which can be generated from organic waste and agro-industrial side streams through microbial chain elongation, are valuable chemicals with numerous industrial applications. Membrane-based liquid-liquid extraction (pertraction) as a downstream separation process to extract MCCAs has been applied successfully. Here, a novel pertraction system with submerged hollow-fiber membranes in the fermentation bioreactor was applied to increase the MCCA extraction rate and reduce the footprint. The highest average surface-corrected MCCA extraction rate of 655.2 ± 86.4 mmol C m−2 d−1 was obtained, which was higher than any other previous reports, albeit the relatively small surface area removed only 11.6% of the introduced carbon via pertraction. This submerged extraction system was able to continuously extract MCCAs with a high extraction rate for more than 8 months. The average extraction rate of MCCA by internal membrane was 3.0- to 4.7-fold higher than the external pertraction (traditional pertraction) in the same bioreactor. A broth upflow velocity of 7.6 m h−1 was more efficient to extract MCCAs when compared to periodic biogas recirculation operation as a means to prevent membrane fouling. An even higher broth upflow velocity of 40.5 m h−1 resulted in a significant increase in methane production, losing more than 30% of carbon conversion to methane due to a loss of H2, and a subsequent drop in the H2 partial pressure. This resulted in the shift from a microbial community with chain elongators as the key functional group to methanogens, because the drop in H2 partial pressure led to thermodynamic conditions that oxidizes ethanol and carboxylic acids to acetate and H2 with methanogens as the syntrophic partner. Thus, operators of chain elongating systems should monitor the H2 partial pressure when changes in operating conditions are made.

Highlights

Carbon recovery from organic waste or wastewater is attractive to achieve a circular economy as part of a sustainable development, because it reduces the cost of waste treatment and increases the recoverable chemical energy (Lu et al, 2018; Hao et al, 2019)
Chain elongation harnesses the potential of certain microbes in anaerobic fermentation biotechnology to generate medium-chain carboxylic acids (MCCAs, C6-C12) through a microbial fermentation pathway with an electron acceptor, such as short-chain carboxylic acids (SCCAs, C2-C5), and an electron donor, which can be obtained through the hydrolysis of organic biomass (Angenent et al, 2016; Xu et al, 2018; Daly et al, 2020) or added to the waste
Medium-chain carboxylic acids (MCCAs) extraction included two steps: 1) MCCAs transferring from the broth to the organic solvent; and 2) MCCAs transferring from the organic solvent to the extraction solution

Summary

Introduction

Carbon recovery from organic waste or wastewater is attractive to achieve a circular economy as part of a sustainable development, because it reduces the cost of waste treatment and increases the recoverable chemical energy (Lu et al, 2018; Hao et al, 2019). One of the biotechnology production platforms that is of interest for renewable chemical production is microbial chain. It is challenging to reach a high concentration of MCCAs in the microbial fermentation system due to the cellular toxicity of MCCAs (Zhu et al, 2020). The uncharged carboxylic acids disrupt the cell membrane and these acids with longer carbon chains up to eight carbon chains are more toxic due to the increased hydrophobicity of the carbon chain (Butkus et al, 2011; Harroff et al, 2017). The inline pertraction system for MCCA extraction is considered one of the best options for reducing cell membrane toxicity and endproduct feedback inhibition, enabling high MCCAs production rates (Michel-Savin et al, 1990; Roe et al, 2002; Červeňanský et al, 2019; Lambrecht et al, 2019)

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