title: Electrolytic extraction drives volatile fatty acid chain elongation through lactic acid and replaces chemical pH control in thin stillage fermentation.
abstract: BACKGROUND
  Volatile fatty acids (VFA) are building blocks for the chemical industry. Sustainable, biological production is constrained by production and recovery costs, including the need for intensive pH correction. Membrane electrolysis has been developed as an in situ extraction technology tailored to the direct recovery of VFA from fermentation while stabilizing acidogenesis without caustic addition. A current applied across an anion exchange membrane reduces the fermentation broth (catholyte, water reduction: H2O + e(-) → ½ H2 + OH(-)) and drives carboxylate ions into a clean, concentrated VFA stream (anolyte, water oxidation: H2O → 2e(-) + 2 H(+) + O2).
  
  
  RESULTS
  In this study, we fermented thin stillage to generate a mixed VFA extract without chemical pH control. Membrane electrolysis (0.1 A, 3.22 ± 0.60 V) extracted 28 ± 6 % of carboxylates generated per day (on a carbon basis) and completely replaced caustic control of pH, with no impact on the total carboxylate production amount or rate. Hydrogen generated from the applied current shifted the fermentation outcome from predominantly C2 and C3 VFA (64 ± 3 % of the total VFA present in the control) to majority of C4 to C6 (70 ± 12 % in the experiment), with identical proportions in the VFA acid extract. A strain related to Megasphaera elsdenii (maximum abundance of 57 %), a bacteria capable of producing mid-chain VFA at a high rate, was enriched by the applied current, alongside a stable community of Lactobacillus spp. (10 %), enabling chain elongation of VFA through lactic acid. A conversion of 30 ± 5 % VFA produced per sCOD fed (60 ± 10 % of the reactive fraction) was achieved, with a 50 ± 6 % reduction in suspended solids likely by electro-coagulation.
  
  
  CONCLUSIONS
  VFA can be extracted directly from a fermentation broth by membrane electrolysis. The electrolytic water reduction products are utilized in the fermentation: OH(-) is used for pH control without added chemicals, and H2 is metabolized by species such as Megasphaera elsdenii to produce greater value, more reduced VFA. Electro-fermentation displays promise for generating added value chemical co-products from biorefinery sidestreams and wastes.
authors.affiliation: Laboratory of Microbial Ecology and Technology (LabMET), Ghent University, Coupure Links 653, Building A, Room A0.092, B-9000 Ghent, Belgium.
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Andersen
authors.affiliation: Laboratory of Microbial Ecology and Technology (LabMET), Ghent University, Coupure Links 653, Building A, Room A0.092, B-9000 Ghent, Belgium.
authors.forename: Pieter
authors.initials: P
authors.lastname: Candry
authors.affiliation: Laboratory of Microbial Ecology and Technology (LabMET), Ghent University, Coupure Links 653, Building A, Room A0.092, B-9000 Ghent, Belgium.
authors.forename: Thais
authors.initials: T
authors.lastname: Basadre
authors.affiliation: Laboratory of Microbial Ecology and Technology (LabMET), Ghent University, Coupure Links 653, Building A, Room A0.092, B-9000 Ghent, Belgium.
authors.forename: Way Cern
authors.initials: WC
authors.lastname: Khor
authors.affiliation: Laboratory of Microbial Ecology and Technology (LabMET), Ghent University, Coupure Links 653, Building A, Room A0.092, B-9000 Ghent, Belgium.
authors.forename: Hugo
authors.initials: H
authors.lastname: Roume
authors.affiliation: Laboratory of Microbial Ecology and Technology (LabMET), Ghent University, Coupure Links 653, Building A, Room A0.092, B-9000 Ghent, Belgium.
authors.forename: Emma
authors.initials: E
authors.lastname: Hernandez-Sanabria
authors.affiliation: Laboratory of Microbial Ecology and Technology (LabMET), Ghent University, Coupure Links 653, Building A, Room A0.092, B-9000 Ghent, Belgium ; Centre for Sustainable Chemical Technologies, University of Bath, Claverton Down, Bath, BA2 7AY UK.
authors.forename: Marta
authors.initials: M
authors.lastname: Coma
authors.affiliation: Laboratory of Microbial Ecology and Technology (LabMET), Ghent University, Coupure Links 653, Building A, Room A0.092, B-9000 Ghent, Belgium.
authors.forename: Korneel
authors.initials: K
authors.lastname: Rabaey
keywords: Biorefinery; Carboxylate platform; Chain elongation; Electro-fermentation; Extraction; Membrane electrolysis