The effect of mixing rate and gas recirculation on biological CO2 methanation in two-stage CSTR systems

Abstract In-situ biological CO2 methanation (BM) in a biogas reactor by the addition of H2 is an attractive low-cost biogas upgrading process, as it does not require investment in a new reactor and can be incorporated into existing biogas plants. On the other hand, maintenance of stable reactor performance during in-situ BM is challenging due to factors such as high H2 partial pressure and CO2 depletion which may lead to an increase in pH. Thus, BM that uses two-serial connected continuous stirred tank reactors (CSTRs) could be an option. For such a process set-up, the second reactor acts as an upgrading reactor (UR) and a secondary CH4 producer for slow-degrading substrates such as straw and manure. In this study, improvement of the BM process was attempted by varying the mixing speed and gas recirculation to enhance hydrogen transfer to the liquid phase of the UR. The experiments showed that the mixing speed and gas recirculation had a significant effect on BM in CSTRs. CH4 production from BM was highest at 170 rpm, but the total CH4 production fell above 140 rpm due to reduced production of CH4 from the manure substrate. The CH4 production rate from CO2 and H2 conversion was further enhanced as the output gas was recirculated at 12.20 mL min−1. Further gas recirculation above 12.20 mL min−1 did not improve BM.