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METHANE RECOVERY FROM ANIMAL MANURES: THE CURRENT OPPORTUNITIES CASEBOOK The covered lagoon digester has several merits. First, it has good potential for widespread adoption in the United States, especially in the southeast and southwest regions, because many dairy and swine facilities use hydraulic flushing to collect manure and anaerobic lagoons to treat waste. Second, covered lagoon digester O&M is simple and straightforward compared to complete-mix and plug-flow digesters. Third, the capital costs for this type of digester can be less than those required for the complete-mix and plug-flow types of conventional digesters. Hydraulic flushing creates a manure too dilute to be used in conventional reactors such as complete-mix or plug-flow digesters Covering an anaerobic lagoon and harvesting the biogas can be a simplified technology; however, the approach raises at least three significant concerns. A key issue is that digestion rate is dependent on temperature; therefore, biogas production varies seasonally if the lagoon is not externally heated. This means that methane production is greatest in the warm, summer months and lowest during the cooler, winter months. At the Randleigh Dairy, daily biogas production during the summer averaged 35% more than during the winter. This may make end-use applications more problematic than they are with conventional digesters, which have less significant seasonal variations in methane production. A second concern is that it can take an anaerobic lagoon as long as 1-2 years to achieve its “steady- state” biogas production potential. It is best to start a project of this type in late spring or early summer to take advantage of warm weather. Digesters that are started during cool months are subject to upset from overfeeding. Moreover, any anaerobic lagoon (covered or not) is impractical in areas with a high water table because of the potential for groundwater contamination. Lagoons built into highly permeable soils must be adequately lined to prevent groundwater contamination. The complete-mix, plug-flow for dairy, and the covered anaerobic lagoon are the only ones now 15 recognized by the USDA’s Natural Resource Conservation Service (NRCS) in the form of “National Guidance provided to States.” Other types of AD systems may have the technical and economic potential to process animal manures. 15 Complete-Mix Digester (Code Number 364i). Washington, DC: U.S. Department of Agriculture. NRCS (1996). Covered Anaerobic Lagoon (Code Number 360), Plug-Flow Digester (Code Number 363i) and 2-10PDF Image | Methane Recovery from Animal Manures The Current Opportunities Casebook
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