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Testing low cost anaerobic digestion (AD) systems

Stephanie Lansing, Walter Mulbry
BioCycle 2014 v.55 no.9 pp. 44-46
anaerobic digesters, anaerobic digestion, biogas, dairies, dairy cows, dairy manure, farms, lactation, liquid manure, methane, sawdust
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.