A biotechnological approach for suspended solids removal in biogas slurry via microbially induced calcite precipitation (MICP)

This study investigated the application of the microbially induced calcite precipitation (MICP) technique for removing suspended solids from biogas slurry. A series of laboratory tests on suspended solids content and colority determination was conducted to evaluate the feasibility of this technique and the removal performance of suspended solids. Combined with tests on residual calcium ions concentration and micro scale, the effects of crucial factors (including urease activity and CaCl2-Urea content) and the mechanisms of suspended solids removal were explored. The results unequivocally demonstrated the effectiveness of the MICP technique in removing suspended solids, achieving optimum removal efficiencies of 91.67±1.68% for suspended solids and 96.50% for colority. The introduction of additional urease-producing bacteria via centrifugation-extraction increased the urease activity of bacterial solution and further enhanced the degree of MICP reaction, but undesirably led to a slight reduction in the removal efficiencies of suspended solids and colority due to the presence of bacteria themselves as suspended solids. Appropriately increasing the CaCl2-Urea content could offer a more adequate material supply to prolong the duration of the MICP process, significantly enhancing the removal efficiency. Nevertheless, the high salinity environment associated with excessive CaCl2-Urea addition (2.0 mol/L) would impair bacteria survival and impede the MICP process, negatively affecting suspended solids removal. The SEM-EDS analysis indicated that in the MICP process, suspended solids were tightly encapsulated by calcium carbonate (mainly calcite) to form strong immobilization aiding their gravity-driven removal. Due to the richness of organic components, they could be considered for reuse as soil amendments and fertilizers. This research provides an innovative insight into the removal of suspended solids from biogas slurry using a biotechnological approach. The MICP technique holds promise for practical applications due to its effectiveness and workability.