Kinetics of CaCO3 precipitation in an anaerobic digestion process integrated with silicate minerals

Integration of silicate minerals in the anaerobic digestion (AD) process has recently been proposed as a way to sequester CO2, neutralize the pH and produce biobased products such as biogas and biofertilizers. The objective of this study was to investigate the kinetics of CaCO3 precipitation in an anaerobic digestion process integrated with a silicate mineral (wollastonite).The first series of experiments studied the precipitation kinetics during the methanogenic phase by addition of calciumacetate to anaerobic batch reactors. CaCO3 precipitation initiated when the ion activity product of Ca2+ and CO32− exceeded calcium carbonate monohydrate solubility product (Ksp of 5.7 × 10−8 at 35 °C). Addition of calcite seed crystal at the start of the experiment showed that the precipitation could be initiated earlier by reducing the induction period. In these experiments, precipitation of CO2 in the form of CaCO3 showed an 52% increase in CH4 content of the biogas (71 ± 0.5% v/v CH4) compared to that of the control experiment fed with sodium acetate (53 ± 0.5% v/v CH4).In the second set of experiments, single-stage anaerobic batch digestions with 25 g/l wollastonite (63–125 μm) at two different substrate concentrations (5.5 g/l and 17.5 g/l insoluble starch) were studied. In these experiments the main processes of wollastonite dissolution and calcite precipitation occurred, however the limited separation of fermentation and methanogenesis phases resulted in a limited increase in methane content of the biogas. Although the precipitation was observed at lower supersaturation, the crystal growth followed the same pattern as in previous experiments with calcium acetate. In both sets of experiments, a self-regulating pH could be achieved in the system as the result of the presence of wollastonite which caused the pH to remain in the range of 5.7–7.9. Obtaining an improved biogas and a self-regulated pH in a digester contribute to the advantages of CO2 sequestration by means of silicate minerals in anaerobic digestion systems.