Abstract
Bacterial communities change in bulk solution of anaerobic digestion (AD) and bio-electrochemical anaerobic digestion reactors (BEAD) were monitored at each organic loading rate (OLR) to investigate the effect of voltage supply on bacterial species change in bulk solution. Chemical oxygen demand (COD) degradation and methane production from AD and BEAD reactors were also analyzed by gradually increasing food waste OLR. The BEAD reactor maintained stable COD removal and methane production at 6.0 kg/m3·d. The maximum OLR of AD reactor for optimal operation was 4.0 kg/m3·d. pH and alkalinity decline and volatile fatty acid (VFA) accumulation, which are the problem in high load anaerobic digestion of readily decomposable food wastes, were again the major factors destroying the optimal operation condition of the AD reactor at 6.0 kg/m3·d. Contrarily, the electrochemically activated dense communities of exoelectrogenic bacteria and VFA-oxidizing bacteria prevented VFAs from accumulating inside the BEAD reactor. This maintained stable pH and alkalinity conditions, ultimately contributing to stable methane production.
Highlights
As the demands for bioenergy to replace fossil fuel continues to increase, anaerobic digestion (AD) investigation becomes more important in order to produce biogas out of organic wastes [1,2,3]
For this reason, when food wastes are processed at a higher organic loading rate (OLR), an imbalance among hydrolysis, acidogenesis, acetogenesis, and methanogenesis may occur in the AD reactor
Initial pH 7.70 did not decrease but continuously increased to 8.31 during each period S1–S3; and alkalinity was maintained at a start-up stage around 7585 mg/L as CaCO3 and it continuously increased up to 13,939 mg/L as CaCO3 at S3 stage. This indicates that the bio-electrochemical anaerobic digestion reactors (BEAD) reactor can quickly stabilize pH and alkalinity during the initial operation and maintain the pH stably at a high OLR [10]
Summary
As the demands for bioenergy to replace fossil fuel continues to increase, anaerobic digestion (AD) investigation becomes more important in order to produce biogas (methane) out of organic wastes [1,2,3]. High-concentrated organic wastes such as food wastes frequently cause overload in the reactor because they consist of decomposable carbohydrates featuring acidity (pH level under 5.0) [5] For this reason, when food wastes are processed at a higher OLR (organic loading rate), an imbalance among hydrolysis, acidogenesis, acetogenesis, and methanogenesis may occur in the AD reactor. This causes frequent accumulation of volatile fatty acids (VFAs), and pH and alkalinity decrease, which largely affects the AD performance [6,7]
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