Evaluation of anaerobic digestate for greenhouse gas emissions at various stages of its management

Abstract

Digestate has certain amount of plant nutrients and organic matter and can be used as organic fertilizer. However, it has significant amount of methane formation potential, and could contribute to the emission of greenhouse gases (GHGs) and hence need to be managed carefully. The main purpose of this study was to find environmentally suitable options for digestate management. In this study digestate from a continuously operated pilot-scale thermophilic dry anaerobic digester was continuously monitored for its characteristics like TS, VS, C and N. It was found that the C/N ratio of digestate was 15–20 for most of the study period, which is safe range for its application to agricultural land without further treatment. Based on the characteristics, GHG emission potential of digestate was determined at its different phases of handling/management (raw digestate, stored digestate, stored-cured digestate) and net GHG emission was found for five proposed scenarios of digestate management. Results showed that raw digestate had maximum GHG emission potential of 139 g CO2-eq/kg waste compared to stored digestate and stored-cured digestate having 125 and 80 g CO2-eq/kg waste respectively. Similarly, storage of digestate for 2 months was found to emit 10% of its total GHG potential. Moreover, net GHG emission was found minimum in the digestate management scenarios having land application of digestate.