Abstract
To convert wastes into sustainable liquid fuels and chemicals, new resource recovery technologies are required. Chain elongation is a carboxylate-platform bioprocess that converts short-chain carboxylates (SCCs) (e.g., acetate [C2] and n-butyrate [C4]) into medium-chain carboxylates (MCCs) (e.g., n-caprylate [C8] and n-caproate [C6]) with hydrogen gas as a side product. Ethanol or another electron donor (e.g., lactate, carbohydrate) is required. Competitive MCC productivities, yields (product vs. substrate fed), and specificities (product vs. all products) were only achieved previously from an organic waste material when exogenous ethanol had been added. Here, we converted a real organic waste, which inherently contains ethanol, into MCCs with n-caprylate as the target product. We used wine lees, which consisted primarily of settled yeast cells and ethanol from wine fermentation, and produced MCCs with a reactor microbiome. We operated the bioreactor at a pH of 5.2 and with continuous in-line extraction and achieved a MCC productivity of 3.9 g COD/L-d at an organic loading rate of 5.8 g COD/L-d, resulting in a promising MCC yield of 67% and specificities of 36% for each n-caprylate and n-caproate (72% for both). Compared to all other studies that used complex organic substrates, we achieved the highest n-caprylate-to-ncaproate product ratio of 1.0 (COD basis), because we used increased broth-recycle rates through the forward membrane contactor, which improved in-line extraction rates. Increased recycle rates also allowed us to achieve the highest reported MCC production flux per membrane surface area thus far (20.1 g COD/m2-d). Through microbial community analyses, we determined that an operational taxonomic unit (OTU) for Bacteroides spp. was dominant and was positively correlated with increased MCC productivities. Our data also suggested that the microbiome may have been shaped for improved MCC production by the high broth-recycle rates. Comparable abiotic studies suggest that further increases in the broth-recycle rates could improve the overall mass transfer coefficient and its corresponding MCC production flux by almost 30 times beyond the maximum that we achieved. With improved in-line extraction, the chain-elongation biotechnology production platform offers new opportunities for resource recovery and sustainable production of liquid fuels and chemicals.
Highlights
To provide sustainable fuels, fertilizers, and other chemicals, a paradigm shift is proposed toward resource recovery from wastes (Guest et al, 2009)
For the first time, promising mediumchain carboxylate (MCC) production was performed from a real organic waste without the addition of exogenous ethanol
We demonstrated that equal production rates of C8 and C6 can be produced with a reactor microbiome that treats a real organic waste at an ethanol-to-non-ethanol substrate ratio of ∼0.7
Summary
Fertilizers, and other chemicals, a paradigm shift is proposed toward resource recovery from wastes (Guest et al, 2009). Resource recovery technologies have been developed to treat many food processing and agricultural wastes (Angenent et al, 2004; Agler et al, 2011), including winery wastes (Makris et al, 2007; Mosse et al, 2011; Riaño et al, 2011). These same qualities suggest that winery wastes are promising candidate substrates for the production of sustainable fuels and chemicals via the carboxylate platform (Agler et al, 2011; Angenent et al, 2016)
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