Application of CO2 Microbubble to Enhance Methane Production in Anaerobic Digestion of Food Waste

Anaerobic digestion (AD) is a key technology for treating organic solid waste. Besides producing a significant amount of energy, the overall cost of the treatment is relatively low. It makes the technology become technically and economically feasible for treating organic solid waste. Many efforts have been carried out to enhance the performance of anaerobic digestion, with the main goal of maximizing methane production at a lower cost.
One way to enhance the methane production is by adding CO2 into the digester. Many papers have reported the significant increase in the methane production after CO2 addition. Despite most hypothesis that exogenous CO2 provides an additional carbon source for the methane production, few have discussed how the system gaining the H2 gas, as CO2 cannot stand alone as the substrate for methane production, especially when no H2 is added from any external source.
This research clarifies how the addition of exogenous CO2 into the AD process boosts methane production. This process is then used to analyse the effectiveness of using CO2 microbubble to enhance AD with landfill leachate addition. A comprehensive analysis using physical and chemical parameters as well as microbial community analysis are discussed in this thesis. All the discussions and conclusions are drawn based on three experimental constraints applied in this research: 1) mesophilic treatment, 2) batch operational mode, 3) CO2 injection into the system through the use of microbubble technology.
Besides the observed methane enhancement after periodic dosing of CO2, a higher methane yield than the theoretical methane potential was observed. In the second study, landfill leachate is added into the medium with the aim to give additional micronutrients from a less utilized substrate. Landfill leachate is widely known to contain a significant number of toxic chemicals besides its essential trace elements. The injection of CO2 microbubbles boosts the biogas production. The highest increase is shown when CO2 microbubble is combined with landfill leachate addition. However, a significant decrease is shown by this treatment method using landfill leachate but without the CO2 microbubble injection.
In the last study, a microbial study is performed to observe how the CO2 microbubble may change the microbial community structure that leads to biogas enhancement. Even though it was expected that both bacterial and archaeal community structure could be revealed, the analysis can only identify the bacterial community. An additional primer that fits to the archaeal community should have been used for the PCR.
The results show that periodic injection of CO2 microbubbles into the system increase the methane yields and methane production rates by up to 196% and 400%, respectively. In this study, a higher substrate degradation rate (140%) was also observed in the early stage of the treatment after CO2 microbubble injection. This study concludes that the periodic dosing of CO2 microbubbles increase the methane production in several ways: 1) increase substrate degradation rate, 2) provide additional carbon source in the form of exogenous CO2, 3) alter the environment to a favourable conditions for the growth of the microbial community inside the digester. A favourable condition includes a toxicants reduction from the medium, which result a higher abundance of hydrolytic and acetogenic bacteria in the treatment with CO2 microbubble injection.