Near-neutral pH increased n-caprylate production in a microbiome with product inhibition of methanogenesis

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