Post digestion of biogas production residues at mid-range mesophilic temperature

A common way to store digestate from anaerobic digesters is in open air lagoons. The aim of this study was to investigate whether cooling of digestate before transfer to the storage prevents methane production. Furthermore, if methanogenesis is not prevented, to determine the potential maximum methane slip from an open air lagoon, supplied with heat exchanged digestate from a mesophilic co-digestion plant such as Linköping biogas plant. Results indicate that methane production is not terminated by cooling and that a high methane production can occur in open air lagoons if the conditions are advantageous. Furthermore, the results suggest that it can be worthwhile, from both an economical and an environmental point, to replace open air lagoons with closed post-digesting units. At 25 C, the methane slip from an open air lagoon could reach as high as 2.6% of the total methane production of a biogas plant, even when the volume of the open air lagoon is only one third of the digesters volume. The combination of low additional cost of production, with significant decrease in release of green house gases to the atmosphere, makes the implementation of post-digestion units at larger biogas plants attractive. Abstract