In-situ CO2 sequestration and nutrients removal in an anaerobic digestion-microbial electrolysis cell by silicates application: Effect of dosage and biogas circulation

Abstract

For overcoming high CO2 content in biogas and high NH4+ and PO43− concentrations in digestate of anaerobic digestion - microbial electrolysis cell (AD-MEC), in-situ CO2 sequestration and nutrients removal were investigated in AD-MEC by combined application of wollastonite and magnesium silicate. Wollastonite of 20 g/L and magnesium silicate of 40 g/L was the optimal dosage for CO2 sequestration and nutrients removal in the AD-MEC, which decreased CO2 content from 11.5% to 7.8%, and removed NH4+ and PO43− by 47.4% and 92.1%, respectively, by forming calcite and struvite precipitates. Higher silicates dosage enhanced little in CO2 sequestration and nutrients removal probably owing to the limitation of proton inventory for Ca2+ and Mg2+ release. Biogas circulation slightly deteriorated mineral CO2 sequestration and nutrients removal in AD-MEC, but enhanced CH4 production from 164.8 mL CH4/g CODr to 261.5 ± 5.1 mL CH4/g CODr. Integrating biogas circulation with silicates application in AD-MEC resulted in the CH4 content reached 96.7% ± 0.4%. High-throughput sequencing revealed that Geobacter was the dominant bacterial genus, while hydrogenotrophic methanogens of Methanobacterium and Methanomassiliicoccus dominated in archaeal communities on both anode and cathode in the AD-MEC with silicates addition. In conclusion, the combined application of wollastonite and magnesium silicate is an attractive way to improve the performance of AD-MEC by achieving in-situ mineral CO2 sequestration and nutrients removal.