Effects of mixing and sodium formate on thermophilic in-situ biogas upgrading by H2 addition

Abstract The effects of hydrogen (H 2 ) injection amount, mixing, and sodium formate on in-situ biogas upgrading performance and microbial community structure were investigated during thermophilic digestion of pig manure in a continuous stirred tank reactor with a working volume of 11.2 L and an organic loading rate of 2.0 g volatile solid (VS)/(L∙d). Continuous mixing was favorable for in-situ biogas upgrading. The relative methane content and methane yield were maintained at approximately 80% and 269 L/kg VS, respectively, under continuous mixing conditions with an H 2 injection rate of 0.66 ml/(min∙V Reactor ) and an H 2 :CO 2 ratio of 4:1. The added H 2 inhibited acetate production, but not propionate production, during acidogenesis. Sodium formate was more beneficial for the acetogenesis of propionate degradation and restored system stability. Strictly hydrogenotrophic methanogens ( Methanoculleus , Methanobrevibacter , and Methanobacterium ) were dominant in the thermophilic in-situ biogas upgrading system. The syntrophic relationship between syntrophic acetate-oxidizing bacteria ( Gelria ) and hydrogenotrophic methanogens was weakened after the addition of H 2 .