Evaluation and modeling of dark fermentation of crop and livestock organic solid wastes.

摘要

This study focuses on evaluating and modeling fermentative biohydrogen production from crop and livestock organic solid wastes (CLOSWs) in particulate form by dark fermentation under more practical conditions. The primary objectives of this research are to i) evaluate hydrogen production from dark fermentation of crop and livestock organic solid wastes using cattle manure as a model particulate substrate ii) establish practically feasible process conditions for biohydrogen production by dark fermentation and iii) develop, calibrate, and validate a model for fermentative biohydrogen production from particulate substrates.;In the experimental studies, batch tests were conducted in two stages. In the first stage, experiments were arranged in duplicates to assess biohydrogen production from cattle manure with and without sucrose as a supplement at three different sucrose:manure ratios, and, with and without external seed at two different seed:manure ratios. These experiments were conducted under ambient temperature, without any external nutrient addition, gas sparging, initial pH adjustment or pH control to make the hydrogen production process more practical and feasible.;Based on the results from first stage experiments, five more reactors were set up in triplicate in the second stage, to study the effect of manure:liquid ratio. Novel findings from the first stage experiments such as ability of cattle manure to provide hydrogen producing cultures, buffering capacity, and nutrient supplement were considered in setting up the second stage reactors. These reactors were also conducted under same experimental conditions as in the first stage.;A kinetic model for dark fermentation of complex particulate substrates was developed and validated with experimental data from stage two experiments, using cattle manure as a representative complex substrate. The model is based on the premise that degradable particulate components of cattle manure are composed of cellulose and hemicellulose, which are first hydrolyzed into soluble monomers, and then fermented to liquid and gaseous end products. Hydrolysis of the two particulate components is modeled by a surface-limiting rate equation, and the formation of end products is modeled following the Anaerobic Digestion Model (ADM1). The integrated model was calibrated using experimental data from one batch reactor and validated with dissolved COD, hydrogen, and volatile fatty acid data from four other batch reactors.;Predictions by this model agreed well with the temporal trends in the experimental data, with r2 averaging 0.85 for dissolved COD; 0.94 for total COD; 0.84 for hydrogen production; 0.84 for acetic acid; and 0.89 for butyric acid; quality of fit in the case of propionic acid was lower with r2 averaging 0.57. Sensitivity analysis was carried out to identify the more sensitive parameters in the model. Based on the results of these sensitivity studies, it is suggested that the parameters established in this study could be appropriate for modeling dark fermentation of cattle manure supplemented with sucrose under conditions similar to those reported in this dissertation.;This study also included net energy gain analysis of the fermentative hydrogen production process. Net energy gains realized in this study were compared against literature results to assess the effects of substrate concentration and fermentation temperature. From these comparisons, it can be seen that the net energy gains realized in this study are greater than those reported previously under different substrate concentrations and fermentation temperatures. Based on this comparison, it is concluded that the test conditions evaluated in this study are appropriate for biohydrogen production with positive net energy gain. It is proposed here that fermentative biohydrogen production studies should be compared on the basis of net energy gain rather than the typical yield-based comparisons, so that results from different test conditions such as substrates and temperatures used by researchers could be compared on a rational basis. Continuing the net energy gain analysis, conservative, theoretical energy calculations were performed to estimate the electrical energy that can be potentially produced from the effluents of the dark fermentation process. (Abstract shortened by UMI.)