Anaerobic Digestion of Low Rate Digesters in Temperate Climates

Low rate digesters are widely used in tropical and subtropical regions to produce energy from livestock manure. The fixed dome model is the most popular digester in China. These digesters are affordable, simple to operate and can produce the daily energy needed for a family from 5-10 pigs or cows. However, they often suffer from poor biogas production and potential digester failure during the seasonal transition from mesophilic (25-45°C) to psychrophilic temperatures (5-25°C ) in temperate climates.;Three investigations were conducted on the anaerobic digestion of diluted dairy manure in temperate climates at variable mesophhilic and psychrophilic temperatures to describe limitations and identify design and management improvements. The first study characterized the performance of a modified fixed-dome digester in temperate climates to identify factors that affect biogas production. The second study investigated the effect of the source of inoculum and the inoculum substrate ratio (ISR) on the methane production and stability during the start-up of low rate digesters. The final study diagnosed the effect of organic loading rates (OLR) on biogas production, and identified organic loading rates that prevent the acidification during transition from summer to winter temperatures. The first study made it clear that temperature and the organic loading rate were the key variables to be controlled in later studies attempting to improve low rate digester performance during colder temperatures. In the case of temperature, it is recommended to keep the digester temperature above 20°C. For the system and climate of this study, it was not possible to maintain this temperature with burial and insulation, and will necessitate supplemental heating to maintain temperatures above 20°C year round. Furthermore, the results demonstrate the need to reduce the organic loading rate during warmer months to make the digester less vulnerable to souring as temperature declines.;The second study demonstrated the advantages of using an adapted inoculum when starting a low rate manure digester. In the absence of adapted inoculum, an inoculum from another source will result in delayed performance. The use of an inoculum substrate ratio of 3 at 25 °C provides an optimal framework to start-up a low rate digester using dairy manure as a feedstock.;Results of the third study indicated that organic loading rates of 0.3 kg VS/m3 day for colder months of the year will prevent souring of digesters and allow farmers to begin production in warm weather without reinoculation. Digesters with organic loading rates greater than 1 kg VS/m 3 showed instability with a VFA/TIC ratio greater than 0.4 when temperature decreased below 15°C. This temperature was identified as a threshold below which an overloaded digester shows instability. However, the digesters with greater organic loading rates produced a greater amount of methane when totaled across colder and warmer periods. Therefore, a “run to fail” management approach could be used to maximize biogas and methane production, but will require periodic reinoculation when transitioning from colder to warmer temperatures. Conversely, digesters operated with a organic loading rate of 0.3 kg VS/m 3 day will produce less methane, but will operate year round without reinoculation.