Submitted to: Transactions of the ASAE
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/27/2005
Publication Date: 6/23/2005
Citation: Funk, P.A., Armijo, C.B., Hanson, A.T., Samani, Z.A., Macias-Corral, M.A., Smith, G.B., Riordan, J.T. 2005. Converting gin and dairy wastes to methane. Transactions of the ASAE. 48(3):1197-1201. Interpretive Summary: Combining the 8 million tons of waste generated by the cotton ginning industry each year with waste from the dairy industry contributes to biosecurity and energy security. Cotton gin byproducts have chemicals that disinfect dairy manure, resulting in a soil amendment that is safe to apply to field crops. This natural soil amendment is much higher in nitrogen than compost because it is produced in a sealed chamber. That means less air pollution, less flies and less odors, too. Furthermore, the methane gas produced using this novel process has much less carbon dioxide than that produced in plug flow reactors. The gas can be burned in an internal combustion engine or supplied to a fuel cell to generate electricity and heat. Distributed energy production reduces our dependence on imported fuels. And because this energy comes from renewable sources, producing it reduces global greenhouse gas emissions.
Technical Abstract: Alternatives to gin trash and manure disposal would benefit both the cotton ginning and dairy industries. Anaerobic digestion produces both methane gas and a class A soil amendment. Gin and dairy wastes were combined in a two-phase anaerobic system to determine the combinations of temperature, rewetting interval and mixture ratio that maximize potential methane gas production and minimize process completion time. No significant volatile fatty acid formation occurred after leachate pH approached 7.0, indicating process completion. This took approximately three weeks when temperatures were above 32° C (90° F), mixture ratios below 5:1 (gin to dairy waste, dry mass basis) and the solid phase was wetted twice daily. Ten percent of the mass was converted to soluble chemical oxygen demand (COD), which has potential for conversion to methane in the second phase. Larger scale trials with combined waste resulted in 87 m3 CH4 per dry ton of waste. Under mesophilic conditions the average product gas methane concentration was 72% (due to excluding the respiration gases of the acidifiers from the methane production reactor). Combining manure and gin trash results in 35% higher gas production per unit of digester volume compared to manure only. Seeding pasteurized combinations of manure and gin trash and manure and cellulose with E. coli O157:H7 in small scale single phase laboratory reactors confirmed the disinfecting property of gin trash. After 12 days E. coli O157:H7 was undetectable in the reactors containing gin trash and manure. The reactors containing cellulose and manure took 20 days to reach O157:H7 levels that were below detection levels by both selective media and polymerase chain reaction amplification.