Testing low cost anaerobic digestion (AD) systems

Authors: LANSING, STEPHANIE - University Of Maryland; Mulbry, Walter

Submitted to: Biocycle
Publication Type: Trade Journal
Publication Acceptance Date: 11/15/2014
Publication Date: 11/30/2014
Publication URL: http://handle.nal.usda.gov/10113/61612
Citation: Lansing, S., Mulbry III, W.W. 2014. Testing low cost anaerobic digestion (AD) systems. Biocycle. 55(9):44-46.

Interpretive Summary: The U.S. Department of Agriculture, Agricultural Research Service (ARS) oversees the Beltsville Agricultural Research Center (BARC) located in Beltsville, Maryland. One component of BARC research focuses on manure treatment and the fate of manure nitrogen and phosphorus within the watershed. To evaluate the potential for low technology and low cost anaerobic digesters for small dairies, BARC and researchers from the University of Maryland installed six modified Taiwanese-model field-scale (FS) digesters near the original dairy manure digester. These small digesters are not intended as a practical model applicable for dairies. Instead, this size represents a compromise between the desire for small inexpensive units and the need for units large enough to mirror field conditions on farms similar to those encountered by full-scale digesters. Construction of multiple FS units enables researchers to conduct simultaneous replicated tests of digester operating parameters and inputs at the field scale. Initial tests focused on determining the performance and variability among the digesters. The FS units typically each produce about 1.5 cubic meters of biogas (about 1 cubic meter of methane) per day at 30°C. (Collected methane from the FS units goes to a household gas grill at the site.) This rate matched that of the dairy's large digester when normalized for loading rate. Average production rates were on the high end of values reported for dairy manure digesters. It is likely that use of solids-separated manure as the digester influent (rather than unseparated manure containing less degradable sawdust) was an important factor. The first study using the FS units tested the effect of three different inoculum amounts on digester start-up. In this study, duplicate digesters were filled using three inoculum amounts representing 25, 33 or 50 percent of the digester's liquid volume, respectively. Results showed that mean biogas production varied about 10 percent between replicates during the 4-month trial. Mean biogas production was not significantly different between treatments during the trial. However, biogas production varied considerably depending on the dairy's water use. Water use was relatively stable in cold weather but increased almost 2-fold during hot weather when water misters where used to cool the animals. The most recent study evaluated the effect of digester temperature on digester performance. Duplicate digesters were operated at 22°, 28° or 35°C (72°, 82° and 95°F, respectively). Results showed that mean methane production from digesters operated at 28°C was approximately 10 percent lower than that from digesters operated at 35°C. Methane production values from digesters operated at 22°C were roughly 30 percent lower than values from digesters operated at 35°C. Since digester heating represents an unwanted energy drain for digesters, operating digesters at 28°C could improve net energy values for small digester systems.

Technical Abstract: To evaluate the potential for low technology and low cost digesters for small dairies, BARC and researchers from the University of Maryland installed six modified Taiwanese-model field-scale (FS) digesters near the original dairy manure digester. The FS units receive the same post-separated liquid manure as the BARC digester. Each unit treats up to 42 gallons of manure daily, corresponding to the waste from approximately two lactating dairy cows/digester or 12 lactating dairy cows for the entire system. These very small units are not intended as a practical model applicable for dairies. Instead, this size represents a compromise between the desire for small inexpensive units and the need for units large enough to mirror field conditions on farms similar to those encountered by full-scale digesters. Construction of multiple FS units enables researchers to conduct simultaneous replicated tests of digester operating parameters and inputs at the field scale. Initial tests focused on determining the performance and variability among the digesters. The FS units typically each produce about 1.5 cubic meters of biogas (about 1 cubic meter of methane) per day at 30°C. This rate matched that of the dairy's large digester when normalized for loading rate. Average production rates were on the high end of values reported for dairy manure digesters. It is likely that use of solids-separated manure as the digester influent (rather than unseparated manure containing less degradable sawdust) was an important factor.