# Microbially Induced Desaturation and Carbonate Precipitation through Denitrification: A Review
## 4 Potential Applications of Denitrification-Based MICP Biotechnology
### MIDP-Driven Applications
Occupancy of even a small volume of pore space by gas bubbles creates unsaturated conditions which significantly affect the hydromechanical behavior of the soil. Furthermore, the water flow in the unsaturated soil is affected by the growing crystals and varying pore size distribution [86; 87] and thus leads to a significant decrease in the hydraulic conductivity of soil [88; 89]. The reduction in bulk stiffness of the pore fluid, the Skempton's \(B\) parameter, and the \(P\)-wave velocity due to the gases generated during microbial denitrification was first demonstrated by [PERSON] and [PERSON] [51] who brought out microbial denitrification as a novel alternative for mitigating liquefaction of soils. [PERSON] et al. [52] reported that the in-situ formation of nitrogen gas bubbles through denitrification could reduce the saturation degree of sand to 80-95%. Additionally, their shaking table model tests revealed that upon desaturation of the soil through denitrification, the pore water pressure became lower, and the ground volumetric strain and the amount of settling were smaller than those of saturated soil. The results of [PERSON] and [PERSON] [54] also revealed that for 88-95% saturated soil, under compression and extension the recorded undrained shear strength ratio was more than two folds higher when compared to those recorded for fully saturated soil. Moreover, a small reduction in the saturation degree (S < 95%) leads to a transition in the undrained stress-strain behavior of the loose sand from static liquefaction to non-liquefaction. Later, [PERSON] et al. [53] detected the existence of biogas bubbles in microbially desaturated sand using computed tomography. The computed tomography images revealed that in treated sand,the gas was present as small pockets of pores and these pores were slightly bigger than the average size of the grains. In the studies of [PERSON] et al. [55; 56], MIDP by denitrification were used as a two-stage process to mitigate liquefaction by stimulation of denitrifying bacteria from natural sand. In Stage 1, the carbon dioxide and nitrogen gases produced by the denitrifying bacteria desaturated the saturated soil, thereby increased the cyclic strength of the soil (upwards of 40% improvement) and provided temporary mitigation [55]. Then, in Stage 2, approximately 1.5-2.0% (_w/w_) calcium carbonate precipitated over a period of one year, which increased the physical and mechanical properties (e.g., the strength, stiffness, dilatant behavior, and resistance to cyclic loading) of the soil resulting in a long-term mitigation due to the provided interparticle cementation, void ration reduction, and roughening of soil particles induced by MICP. These results indicated that desaturation and calcium carbonate precipitation via denitrification could notably mitigate the earthquake-triggered soil liquefaction [56]. Conventional ground improvements (e.g., solidification and reinforcement) for mitigation of soil liquefaction are energy-consuming and expensive, or not suitable for mitigating liquefaction potential beneath or near existing structures (e.g., densification, drainage and reinforcement). An alternative for mitigating liquefaction of soils is induced-partial saturation (i.e., desaturation) that can be conducted by injection of gas in saturated soil and entrapment of gas bubbles there. The advantage of desaturation over other mitigation methods will be its cost and energy effective implementation for new as well as existing structures. Microbe-induced desaturation by denitrification is a novel method, with desaturation via biogas generation providing short term mitigation and interparticle cementation via MICP providing long term mitigation of liquefaction. Overall, MIDP through denitrification shows promise as an environmentally friendly, and cost-effective ground improvement technique for liquefaction mitigation through desaturation via biogas generation and solidification via MICP.