Reduction of odors and water pollution as well as potential energy generation (CHP) on large livestock operations have stimulated increased interest in anaerobic digestion systems. Since the mid-1970s, interest in methane generation technology in the United States has varied. Early anaerobic digestion in the United States had been farm-based with primary efforts to develop appropriate technology, which would require low initial capital cost, low operating

costs, low maintenance, little operator time, and minimum management skills (Jewell et al. 1979). Based on a simplified reactor design and reported advantages of lower capital and operational costs, the plug-flow digester has been the most readily utilized design compared with a completely mixed design for dairy manure. A variation of the plug-flow digester (especially when food wastes are added) is a horizontal mixed digester used to maintain solids in suspension. A new development is the design of a vertically mixed digester tank of either concrete or steel. The mixing is achieved by several methods, such as slow rotational paddles at several depths, submersible pumps or propellers, and side-entry circulating systems with the motive force outside the tank. Multiple tanks may be needed for larger farms and/or when food wastes and other organic materials are introduced as a co-digestion feedstock.

The U. S. EPA (2008) through its AgSTAR program estimated in December 2008 that there are 121 farm-scale digesters operating at commercial livestock farms in the United States (Table 4.2). Six of these installations are systems that provide manure treatment for multiple farms. In 108 of the 121 operational systems, the captured biogas is used to generate electrical power, with many of the farms recovering waste heat for the electricity generating equipment for on — farm use. The U. S. EPA estimates that these systems generate ~218,000MWh of electricity per year. The remaining 13 systems burn the biogas in boilers, upgrade the gas for injection into the natural gas pipeline, or simply flare the captured gas.