Solid State Fermentation Of Switchgrass Mixtures: Experimentation, Modeling And Analysis

In this study, the aerobic microbial degradation of switchgrass mixtures was characterized based on biological, physical and chemical parameters both experimentally and via mathematical modeling. Highly-instrumented 50L reactors were designed to facilitate better process control, online measurements and robust sampling for high temporal and spatial resolution. Switchgrass was amended with dog food and nitrogen fertilizer in C/N ratios of 15 and 18, respectively with initial moisture contents of 60 to 75% and biodegraded for 64 to 96h. Temperature and effluent gas concentrations were monitored online during the process and collected samples were analyzed for pH, moisture content, organic acid concentration, substrate composition and microbial community dynamics. A rapid technique for the extraction and purification of DNA from compost samples was developed and optimized. Real time PCR and probe hybridization procedures were also optimized for monitoring microbial community dynamics. Probe hybridization was used to monitor changes in the bacteria, fungi and yeast populations; the data from which was used to derive empirical parameters for process modeling, and real time PCR was used to measure changes in the gene copy numbers of bacteria, fungi, lactic acid bacteria and Aspergillus spp. Reactors were run in triplicate to test for process reproducibility. The results revealed that the reactors reproduced results well and that profiles obtained from different runs were similar to each other and to other generally accepted profiles. Of three moisture contents: 60, 65 and 75%, the highest level of activity was seen in the 75% reactor, suggesting that this system may have been operating near optimal moisture levels. Reactor height had a significant impact on the temperature and moisture content profiles and the bacterial population was found to be the driving force for carbon degradation in the process. Modeling the process using empirically derived microbial growth kinetics, substrate degradation kinetics and traditional heat and mass transfer equations resulted in model predictions that were in good agreement with the experimental data. It was also determined that using nitrogen fertilizer increased the initial rate of substrate degradation and after 96h, an average of18% of lignin, 24% of cellulose and 26% of hemicellulose had degraded