The process of microbially induced carbonate precipitation (MICP) by denitrification was investigated in relation to its potential use as a ground improvement method. Liquid batch experiments indicated that the substrate solution had an optimum carbon-nitrogen ratio of 1•6 and confirmed that combining nitrate reduction and calcium carbonate precipitation leads to an efficient conversion, at which the pH is buffered slightly below 7 and the accumulation of toxic intermediate nitrogen compounds is limited. Sand column experiments confirmed that the volume and distribution of the gas phase strongly depend on the stress conditions. The produced gas volume is inversely related to the pore pressure and can be predicted based on a mass balance analysis, assuming conservation of mass and using theoretical laws of physics. At low pore pressure, the gas formed and accumulated at the top of the column, whereas calcium carbonate precipitation occurred mostly at the bottom near the substrate inlet; an excess amount of gas was produced, which vented from the sand columns and induced cracks in the sand at low confining pressures, which negatively affected the sand-stabilising effect of the calcium carbonate minerals.

Microbially induced carbonate precipitation (MICP) through denitrification can potentially be applied as a bio-based ground improvement technique. Two strategies involving multiple batch treatments in a modified triaxial test setup were used to study the process efficiency. Both strategies aim to achieve 1 weight percentage (% by weight) of precipitated calcium carbonate (CaCO 3 ) and differ in number of flushes, hydraulic residence time, and substrate concentrations. In the experiment with few flushes and high substrate concentrations the microbial process was inhibited, only 0.28% by weight CaCO 3 was measured in the sand after 5 weeks of treatment. The regime with many flushes and low substrate concentrations stimulated microbial growth resulting in 0.65% by weight CaCO 3 within the same time period. Biomass growth and nitrogen gas production were stable throughout the experiment at low concentration, reducing the hydraulic conductivity of the sand, which eventually led to clogging. Precipitation rates up to 0.26% by weight/day CaCO 3 were observed. Applying a suitable substrate regime and residence time is important to limit inhibition and maintain the cell activity, allow for an efficient conversion, and generate a good precipitation rate.

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