Enhanced anaerobic digestion of organic wastes by carbon-based conductive materials

Abstract

To reduce greenhouse gas emissions, anaerobic digestion of organic wastes that could build up the carbon circulation has been widely studied. However, there are some limitations during the anaerobic digestion of organic wastes such as the slower conversion of volatile fatty acids to biogas resulting the accumulation of organic acids, decrease of pH, and finally, inhibition of methane producing microorganisms in the reactor. To solve these problem, various studies have been carried out including the addition of alkalinity, organic loading control, and the process modifications. As another novel solution, direct interspecies electron transfer (DIET) which increases the efficiency of the whole process by promoting the symbiotic relationship between acidogenic bacteria and methanogenic microorganisms through the addition of conductive materials, has been recently proposed. However, there are still many un-revealed mechanisms in DIET, especially, during the degradation of organic acids. In this study, the impact of carbon-based conductive materials was investigated during the anaerobic digestion of organic wastes, specially focused on combined and separated acidogenic and methanogenic steps.

In chapter 3, three types of carbon-based conductive materials (multi-walled carbon nanotubes

MWNTs, powder activated carbon

PAC, and expanded-graphite

EG) were selected to compare the DIET potential during the anaerobic digestion. From the batch experiment, MWNTs were proven as the best conductive materials among three carbon-based materials with the more than 30% increased methane production rate than control. In addition, the optimum dose of MWNTs was 0.13 g MWNT / g VS in consideration of both increased methane production rate and shortened lag phase.

In chapter 4, the effects of MWNTs to the composition of volatile fatty acids (VFAs) and methane production were monitored as the increase of glucose concentration from 5 g COD/L to 15 g COD/L at the fixed alkalinity of 5 g-$CaCO_3$/ L. As expected, methane production rate was increased in the reactor with MWNT (40 % to 155%) compared to the reactors without MWNTs as the organic loading increased from 5 g COD/L to 15 g COD/L. Interestingly, the increased methane production rate was highest at 15 g COD/L and the pH measurement showed that the recovery of pH was accelerated when MWNTs were added. More specifically, lactic acid was accumulated up to 8.9 g COD/L in the reactor without MWNT at day 5, while it was not detected in the reactor with MWNT. Accordingly, acetic acid concentration in the reactor with MWNT was 4.2 g COD/L higher, and butyric acid concentration was 4.5 g COD/L lower than those of the control. Therefore, above results showed that the addition of MWNTs enhanced the degradation of glucose to the final form of acidogenic products, and led to the accelerated methane production by DIET. To verify the impact of MWNTs to the acidogenic step, the experiment was repeated at the suppressed methanogenic condition by the addition of BES (Sodium-2 bromoethanosulfonate), which could selectively inhibit the methanogenic microbial activity. As a result, although the concentrations of total organic were similar in the reactors with and without MWNTs, the composition was significantly differed. In the presence of MWNTs, the concentration of acetic acid was higher by 614.0 mg COD/L and that of butyric acid lower by 576.8 mg COD/L than the control (without MWNTs). From above results, it can be concluded that the addition of MWNTs was not only enhanced the conversion of VFAs to biogases but also the acidogenesis.

In chapter 5, conversion rates of the individual organic acid (acetic, lactic, propionic, and butyric acids) to biogases were investigated in the absence and presence of MWNTs to verify the enhanced acidogenesis. As expected, conversion rates of lactic, propionic, butyric acids were increased with the addition of MWNTs by 42%, 54%, and 155% than those without MWNTs, respectively. Interestingly, these VFAs are known as thermodynamically non-spontaneous when converted to acetic acid. Thus, it can be hypothesized that the addition of conductive materials such as MWNTs promoted the more efficient energy transport generated from methanogens to acidogenic bacteria, which would enable the degradation of lactic, propionic, and butyric acids to acetic acid. It can be supported by the conversion of acetic acid to biogas. During the production of biogas from acetic acid, acetoclastic methanogen plays major role, thus, DIET by MWNTs should not accelerate the methane production. The results were clearly shown that there was not any noticeable changes in acetic acid degradation and methane production rates. From above results, MWNTs can accelerate acidogenic processes, which degrade high molecular weight VFAs to acetic acid.

In conclusion, the addition of carbon-based conductive materials such as MWNTs can increase not only the methane production rate as reported, but also accelerate the acidogenic conversions. Thus, the addition of conductive materials would be beneficial for the stability and economic feasibility of anaerobic digestion processes