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The pH is a critical parameter in chain-elongating bioreactors, affecting: (1) the concentration of inhibitory undissociated carboxylic acids, which in turn affects the efficiency of product extraction; (2) the thermodynamics; and (3) the kinetics. Here, we examined the effect of five different pH levels (5.5 to 7.0) on n-caprylate (C8) production using an anaerobic sequencing batch reactor (ASBR) with continuous membrane-based liquid-liquid extraction (pertraction). We found that the product spectrum was directed by pH: mildly acidic pH (5-6) led to n-caproate (C6) production, while near-neutral and neutral pH (6.75-7) favored n-caprylate production. In particular, the pH of 6.75 led to the maximum values of volumetric n-caprylate production rate (75.6 ± 0.6 mmol C L−1 d−1; 0.06 g L−1 d−1) and n-caprylate concentration in the fermentation broth (420 mM C; 7.57 g L−1). Given that methane production remained low at near-neutral and neutral pH, we theorized that the high concentration of undissociated n-caprylic acid (5.71 mM C) inhibited methanogenesis. We then demonstrated such an inhibitory effect at neutral pH in: (1) microcosm experiments; and (2) the continuous bioreactor by adding methanogenic sludge. Furthermore, 16S rRNA gene sequencing analysis revealed that near-neutral and neutral pH led to more diverse microbial communities than at mildly-acidic pH. For the first time, we report predominant n-caprylate production in a microbiome at near-neutral and neutral pH conditions where methanogenesis was controlled by the inhibitory effects of undissociated n-caprylic acid. At the same time, extraction of this species occurred even at near-neutral and neutral pH.

Summary Acid whey is a Greek-yogurt waste stream and can be a resource to produce biofuel precursors. Our objective was to convert acid whey into medium-chain carboxylic acid (MCCA) oil with the open-culture carboxylate platform. Here, we developed a temperature-phased bioprocess with different anaerobic reactor microbiomes, performing thermophilic lactic acid production and mesophilic chain elongation, to produce MCCAs (C6–C9) from acid whey via lactic acid as an intermediate. For the lactic acid-producing bioreactor, we achieved a volumetric lactic acid production rate of 1,230 mmol C L −1 day −1 (1.54 g L −1 hr −1 ). For the chain-elongating bioreactor, we achieved a volumetric MCCA production rate of 111 mmol C L −1 day −1 and a volumetric n -caproic acid (C6) production rate of 81 mmol C L −1 day −1 (0.07 g L −1 hr −1 ). We converted a real waste stream into mainly MCCAs without the external addition of an electron donor.

Bioelectrochemical systems (BESs) employ extracellular electron transfer from bacteria that grow at electrodes. Due to biofilm and electrode limitations, industrial-scale applications require large electrode areas, and thus inexpensive electrode materials. Here, electro-spun polyacrylonitrile (PAN) and carbon nanofiber (CNF) were manufactured. In addition, the conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) was applied as a coating to these materials and to carbon cloth (CC). We tested these materials as electrodes by using physicochemical measurements, cyclic voltammetry, and bioelectrochemical growth-studies with Geobacter sulfurreducens. PAN is a nonconductive material without capacitance, but with PEDOT coating the conductivity and capacitance became sufficient to support electric current production in our BES. CNF outperformed CC in capacitance, but behaved similarly in our BES when normalized to projected surface area. With the PEDOT coating, CNF increased electric current production by 38% in our BES, while this was 64% for CC. When applied to a gold microfluidic electrode, electric current with G. sulfurreducens increased almost three-fold. PEDOT added considerable specific surface area to electrodes possessing a low surface area, but not with a high surface area such as CNF. This work demonstrates that electro-spun electrodes and PEDOT coating are a promising electrode alternative that can be readily implemented into existing BESs.

Acid whey is a Greek-yogurt waste stream and should be considered a resource to produce chemicals that can be used as biofuel precursors. Our objective was to convert acid whey into valuable medium-chain carboxylic acid (MCCA) oil with the open-culture carboxylate platform. Here, we developed a two-phase bioprocess with different anaerobic reactor microbiomes, performing thermophilic acidogenesis and mesophilic chain elongation, to produce MCCAs from acid whey via lactic acid as an intermediate. We achieved an MCCA production rate of 4.8 g chemical oxygen demand (COD) Lreactor-1 day-1 based on the volume of the chain-elongating bioreactor. The overall COD conversion efficiency for acid whey to MCCAs (C6-C9) was 53.1% with an MCCA specificity of 68.5% based on COD. n-Caproic acid (n-hexanoic acid [C6]) was the main product with a COD specificity of 49.8%. We converted a real waste stream into mainly MCCAs by chain elongation without the addition of electron donors.