Abstract
Dairy manure (DM) is an abundant agricultural residue that is largely composed of lignocellulosic biomass. The aim of this study was to investigate if carbon derived from DM fibers can be recovered as medium-chain fatty acids (MCFAs), which are mixed culture fermentation products of economic interest. DM fibers were subjected to combinations of physical, enzymatic, chemical, and thermochemical pretreatments to evaluate the possibility of producing carbohydrate-rich hydrolysates suitable for microbial fermentation by mixed cultures. Among the pretreatments tested, decrystalization dilute acid pretreatment (DCDA) produced the highest concentrations of glucose and xylose, and was selected for further experiments. Bioreactors fed DCDA hydrolysate were operated. Acetic acid and butyric acid comprised the majority of end products during operation of the bioreactors. MCFAs were transiently produced at a maximum concentration of 0.17 mg CODMCFAs/mg CODTotal. Analyses of the microbial communities in the bioreactors suggest that lactic acid bacteria, Megasphaera, and Caproiciproducens were involved in MCFA and C4 production during DCDA hydrolysate metabolism.
Highlights
Raw dairy manure (DM) is an abundant organic waste stream produced at rates of hundreds of megatonnes per year (Fischer, 1998; Milbrandt, 2005; Nass, 2007)
Except for CAP, retained approximately 80% of the original weight in the solid fraction before enzymatic hydrolysis (Table 1), whereas the CAP process solubilized a large fraction of the DM fibers, with only 55% of solids remaining after pretreatment
Most notable of the lactic acid bacteria was the genus Lactobacillus, which significantly correlated with total carbohydrate utilization rates (Supplementary Table S6), dominated abundance in both DM bioreactor (DMB) and DMB2 (Figure 5), and whose ASVs were closely associated with all time points and explanatory variables (Figure 6)
Summary
Raw dairy manure (DM) is an abundant organic waste stream produced at rates of hundreds of megatonnes per year (Fischer, 1998; Milbrandt, 2005; Nass, 2007). A common route for DM management is storage (e.g., slurry tanks or lagoons) followed by land application. This is a convenient approach, but contributes to greenhouse gas (GHG) emissions (Aguirre-Villegas and Larson, 2017). AD is an established bioprocess technology to convert organic material in DM into biogas (i.e., methane, carbon dioxide, and other trace gases). Captured biogas can be combusted for heat and electricity or, if upgraded, used for injection in the natural gas grid or used as transportation fuel. In the US, such conversions (i.e., into electricity or natural gas) are considered renewable fuel per the incentivizing Renewable Fuel Standard program created under the Energy Policy Act of 2005 (United States of America, Senate and House of Representatives, 2005); their long-term economic viability is currently a suspected
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