2019 Volume 98 Issue 8 Pages 186-193
The purpose of this research was to provide an accurate estimate of greenhouse gas (GHG) emissions during the storage of liquid digested slurry after separation (LDSS) produced by a dairy-manure-fed biogas plant in Hokkaido, Japan, in order to create a reliable gas inventory for life-cycle assessment (LCA) of the system. To the best of our knowledge, there have been no case studies measuring GHG generation by anaerobic digesters in such snowy cold regions. Therefore, a GHG measurement method adapted to snowfall and freeze-thaw conditions was newly developed; we measured the amounts of GHGs produced before and after the melting of frozen LDSS at the surface layer of the storage tank during the snowmelt period. The concentrations of CO2, CH4, N2O and NH3 in the LDSS storage tank (above ground, open-type, diameter: 44 m, capacity (effective volume): 6,839 m3 (5,471 m3)) were measured with a multi-gas monitor (infrared photoacoustic detector manufactured by INNOVA) using an openchamber technique. The measurements were conducted at the LDSS storage tank for a biogas plant in Ebetsu City, central Hokkaido, Japan from February 20 to April 3, 2018. This plant processes 29 m3/day, has a fermentation temperature of 40 °C, and hydraulic retention time (HRT) of 40 days. The results revealed that almost no GHG emissions were observed from the frozen storage tank. In addition, when the frozen LDSS began to melt, the GHG emission pattern showed diurnal fluctuations that generally moved in tandem with the ambient temperature. The GHG emissions greatly changed during the period immediately after the frozen LDSS began melting until it melted completely. After the frozen LDSS was melted, CH4, CO2, and N2O emissions were 113.0 mg CH4/m2/day, 88.8 mg CO2/m2/day, and 0.021 mg N2O/m2/day.