Mathematical Simulation Of Anaerobic Digestion Reactors

Much attention has been focused on anaerobic technology owing to its high organic removal ability and energy recovery (hydrogen and methane) from organic wastes. Mathematical model is a useful tool to explore anaerobic reaction mechanism and promote anaerobic technology. However, there is still lack of comprehensive mathematical models to describe anaerobic digestion under different conditions. In the present study, the mathematic models were established to describe different performances of anaerobic reactors, including start-up, free ammonia inhibition, dispersion characteristics of USAB reactors, hydrogen production in a granule-based UASB reactor and lignocellulose degradation by rumen microbes. After calibration and validation using different sets of experimental data, these models were used to simulate and predict the anaerobic reactor performance as well as the effects of different operational parameters. Main contents and results of the present study are as follows:1. A little informance about the kinetics of start-up process of anaerobic reactors. Hence, a mathematical model was developed to describe the start-up process of anaerobic reactors through integrating the linear phenomenological equation into Anaerobic Digestion Model No.1 (ADM1). The model was respectively calibrated and verified using two sets of experiment data from a UASB reactor fed with soybean processing wastewater and from a CSTR reactor fed with simulated municipal solid waste. The simulation values captured the experiment data well, demonstrating the developed model was rational and valid to describe the start-up process of anaerobic reactors. It was also found from sensitivity analysis that the acceleration coefficients were the most important parameters in the model.2. Both the biochemistry and dispeision have significant effect on UASB reactors performance. A comprehensive mathematical model was established to describe both the biochemistry and dispersion characteristics of UASB reactors. Three different experiment data were used for model calibration and verification. A good agreement was found between the simulated results and the measured data. Moreover, the simulated results from the established model were better than those from ADM1, demonstrating the dispersion characteristic of UASB reactors can not be neglectable. The model also suggested that both the organic loading rate and liquid upflow velocity had a significant influence on the dispersion characteristic of UASB reactors. Due to this dispersion characteristic, pH value in UASB reactors was gradually increased from bottom to top of reactors, and H2 inhibition only tended to appear at the bottom of UASB reactors.3.The hydrogen production from sucrose in a granule-based UASB reactor was investigated and the response surface methodology (RSM) with a central composite design was employed to optimize hydrogen production. A maximum hydrogen yield of 1.62 mol-H2/mol-hexose was obtained under optimum conditions of substrate concentrate (Sin) 14.5 g/L and hydraulic retention time (HRT) 16.4 h. The hydrogen yield was strongly individually dependent on Sin and HRT, while their interactive effect on the hydrogen yield was slight. Ethanol, acetate and butyrate were the main aqueous products and their yields all correlated well with Sin and HRT, indicating a mixed-type fermentation existed in this UASB reactor.4. Aquatic plants were usually used for bioremediation. The rapidly increasing aquatic plants may become a secondly contaminat. Anaerobic digestion of lignocellulose-rich aquatic plants (alligator weed as an example) by rumen microbes was investigated and a mathematical model, based on ADM1, was developed to describe this process. In the developed model, lignocellulose was fractionated into slowly hydrolysable fraction (SHF), readily hydrolysable fraction (RHF) and inert fraction. The hydrolysis process of SHF and RHF could be described as surface-limiting kinetics. The SHF hydrolysis was the rate-limiting step in the anaerobic degradation of lignocellulose. Furthermore, in order to describe substrate degradation and various VFAs production at different pH, the model was further developed by integrating a pH inhibition equation related with enzyme activity. The results of model verification, after model calibration, shown that the new mathematical model was suitable to describe anaerobic digestion of lignocellulose-rich aquatic plants (alligator weed as an example) by rumen microbes.