Effect of Initial Microbial Diversity on Anaerobic Digestion of Fatty Acids and Biodegradable Plastics

Abstract

The use of biodegradable products has increased dramatically and still can cause a significant problem despite their eco-friendly label. To date, long-chain fatty acids (LCFAs) and bioplastics are considered as one of the most problematic compounds that exist in biodegradable waste. Thus, anaerobic digestion (AD) can be one of the alternatives to mitigate the problem. However, LCFAs and bioplastics can be hard to process in AD, since many microbes failed to break down these molecules. Moreover, rather than spending more energy and resources for more complex processes, the discrepancies related to the communities needs to be clarified first in mesophilic AD of LCFAs and bioplastics to achieve better process efficiency. Thus, employing different inocula can be one of the solutions for better understanding on AD using certain substrates. In the first chapter, the effect of eight different initial inocula on the performance and microbial dynamics of methanogenic communities during AD of palmitic acid (PA), stearic acid (SA), and oleic acid (OA) were observed based on the average process performance, microbial diversity, and key communities. The highest maximum production rate per lag phase ratios Rmax/λ were 8.67·10-3 L/L·day2, 6.76·10-3 L/L·day2, and 1.05·10-2 L/L·day2 for PA, SA, and OA treatments, respectively. There was a decrease in the average biodiversity index D at the endpoint of each substrate treatment. Microbial analysis results showed Methanoculleus and Methanosarcina as the key archaeal genus in the AD of PA, SA, an OA. In the second chapter, the impact of four different inocula on AD of polylactic acid (PLA) and polybutylene adipate terephthalate (PBAT) were analyzed according to their performance parameter analysis and biodegradability analysis. The highest maximum biogas production BPmax were 0.41 L/L and 0.40 L/L for PLA and PBAT treatments, respectively. There was a significant positive relationship between biogas potential value ThBG and disintegration degree D of the batch test. The increase of ThBG can be predicted by the decrease of pH and the increase of total volatile fatty acids (TVFA), CO2, CH4, and D in the system. AD of PBAT showed significantly higher potential rate (0.23%/day) compared to the previous existing studies. The results of this study provide useful information on the effect of initial inocula on the treatment of biodegradable waste. The underlying information about the performance parameter of each inoculum and key archaeal genus for AD from LCFAs as well as biogas potential from AD of bioplastics on each inoculum can be used to select appropriate seed for start-up of lab-scale AD process treating biodegradable waste that mostly contains LCFAs and bioplastics.