Methane emissions from storage of digestate at a dairy manure biogas facility

A large portion of dairy farms use liquid manure storage systems. Liquid manure can be used to generate biogas which may mitigate greenhouse gas (GHG) emissions, like methane (CH4). Biogas is a mixture of methane, carbon dioxide (CO2) and small amounts of other gases; it can be captured and used to generate electricity and heat. Biodigesters capture the gases produced that would normally escape into the atmosphere from dairy manure storage. Biodigesters receive manure as an input and produce multiple products: biogas, electricity, heat, and digestate which is the organic material left over from anaerobic digestion. Digestate must be stored in large open storages until it can be applied to fields as fertilizer. During storage, CH4 can be produced, which partially offsets the GHG benefits of biogas; however, the amount of emissions from digestate are poorly understood. The purpose of the study was to quantify year-round CH4 emissions from a digestate storage tank, examine the relationship between emissions and its driving factors, and compare these results to a similar emissions dataset from untreated manure measured during one year before the biodigester became operational. This study took place on a commercial farm in Ontario where a biodigester system became operational in May 2012. A micrometeorological mass balance approach was used to measure CH4 fluxes. Total annual CH4 emissions from digestate were 1.0 kg m−3 y−1, which was 85% lower compared to untreated manure. Average monthly volatile solids (VS) mass in the storage tank were reduced by 32%, volatile solids represent the substances in the digestate that can be transformed into biogas. The annual CH4 emissions scaled by VS were 26 g kg−1 VS y−1 for digestate and 76 g kg−1 VS y−1 for manure, suggesting that VS in the digestate were less suitable for CH4 production (less digestible). It was found that digestate temperature was the main driving factor in CH4 emissions, and was more strongly correlated to emission production at all depths for digestate than untreated manure. This study demonstrated a reduction in facility scale CH4 emissions by co-digesting dairy manure and off-farm materials with a 60-day hydraulic retention time and subsequent solid liquid separation. These data are helpful for determining the environmental benefits of on-farm anaerobic digestion facilities.