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.

Keywords: Biorefinery; Carboxylate platform; Chain elongation; Electro-fermentation; Extraction; Membrane electrolysis.

Fig. 1

Schematic of electro-fermentation and membrane electrolysis

Fig. 2

Control and experimental fermentation over time; total amount of carboxylates and total suspended solids. In the experimental fermentation, current was applied prior to the vertical dotted line. a The total amount of measured carboxylates. Note the experimental case includes the amount extracted. b Total suspended solids measured represented by the proportion of total suspended solids in the fermenter relative to that of the feed

Fig. 3

Carbon oxygen demand (COD) balance in the feed, control fermentation, experiment fermentation (applied current) and in the extractant, measured during steady state (day 6–24). Note the extractant is also considered in the experiment column. a Total amount of measured carboxylates and ethanol. b Total amount of all identified components (n = 9 for carboxylates, n = 4 for other components, per stream)

Fig. 4

Redundancy analysis highlighting the dissimilarities among the relative abundances of the bacterial communities in the control and experimental (applied current) cases. “Ctrl” represents the community of the control reactor, which was described in RD2, while “Exp” indicates the community of the experimental (applied current) reactor, included in RD1. The blue axis represents time (days) and CE (extent of chain elongation)

Fig. 5

Schematic of species with the top four greatest relative abundance and the proposed pathway of substrates, VFA intermediate and VFA product. a In the control case some acetate (C2) and propionate (C3) may be used as an intermediate by Megasphaera sp. b In the experimental (applied current) case the Megasphaera sp. can metabolize electrolytically generated H₂ to gain additional energy and through generating more reduced, longer chain VFA by lactate elongation