Methane emission from stored dairy manure slurry and slurry after digestion by methane digester

To examine the effect of storage temperature on the emission of methane (CH₄) and carbon dioxide (CO₂) and on recovery of nutrients from raw dairy manure slurry (RS) and slurry digested in a methane digester (DS), both slurries were stored in closed 100 L steel tanks under psychrophilic conditions (5, 10, 15 and 20°C) for a 150 day period. As the storage temperature increased, total methane emission increased in both types of slurry. The amount of methane emitted per unit of volatile solids of the RS and DS was 0.19 L/g and 0.10 L/g, respectively. The respective carbon dioxide emissions were 0.20 L/g for RS and 0.12 L/g for DS at 20°C of storage temperature. At temperatures greater than 15°C, the methane concentration in the emitted gas remained more than 40% of the total gas. During the experimental period, in excess of 90% of the total Kjeldahl nitrogen in these slurries was recovered (91.4–93.7% for RS and 93.7–98.4% for DS) after storage, and ammonium nitrogen was recovered in excess of 100% (100.1–143.2% for RS and 106.7–143.2% for DS storage tanks) because of the mineralization of organic nitrogen in the influent. These results indicate that manure slurry characteristics and storage temperature have significant impacts on methane emission. It can be concluded that on typical farms located in northern Japan, methane emission from manure storage tanks during late fall, winter and early spring may be negligible, because of manure temperatures less than 10°C. During late spring, summer and early fall, methane emissions can be substantially reduced by using underground storage to maintain lower manure temperatures.