# Microbially Induced Desaturation and Carbonate Precipitation through Denitrification: A Review
## 4 Potential Applications of Denitrification-Based MICP Biotechnology
### MICP-Driven Applications  
#### 4.1.1 Soil Reinforcement  
In many areas of the world, the in situ physical properties of soil (e.g., shear strength, stiffness, compressibility, permeability, and hydraulic conductivity) are often insufficient to meet the demands of human being. There are three methods of improving these soil properties, namely, mechanical compaction, chemical grouting, and biological techniques, such as biogrouting [57]. Among them, biogrouting based on MICP is considered to be a promising new technique because it is more environmentally friendly and cost-effective, and it does not mechanically/chemically disrupt the infrastructure [9; 20]. Over the past decade, many studies have evaluated the exploitation of MICP through denitrification for biomedical soil improvement under anaerobic conditions at the laboratory scale. [PERSON] [24] conducted soil column experiments and reported the CaCO\({}_{3}\) precipitation yield in the range of 1.3-10.6 g CaCO\({}_{3}\) (s)/g NO\({}_{3}^{-}\), in 2-40 days which resulted in weak cementation of sand particles (Table 3). This was the first study to demonstrate that microbial nitrate reduction may be a feasible mechanism for soil improvement. [PERSON] et al. [8] also reported the successful application of denitrification-based MICP for soil improvement by using _Castellaniella denitrificans_. They reported that about 6 g CaCO\({}_{3}\) (s)/g NO\({}_{3}^{-}\) was precipitated in seven days. After 100 days, the CaCO\({}_{3}\) content (by mass) at the top of the sand column was \(\sim\)10%, and it gradually decreased with depth to less than 1% at the bottom side. Furthermore, based on the observed conversion rate in their study, it was claimed that about 60 days would be enough to reach a target amount of 100 kg of CaCO\({}_{3}\) per cubic meter of soil, which was still far from the amount required for practical engineering applications. [PERSON] et al. [29] demonstrated that denitrification could be used to induce CaCO\({}_{3}\) precipitation in subsurface environments under anoxic and highly saline conditions at pressures of up to 8 MPa. In addition, they revealed that denitrification based MICP could be used for biocmentation of fine sand particles and coarse gravel. After that, some attempts were conducted to improve the efficiency of MICP through denitrification. [PERSON] et al. [25] used multiple substrate flushes for the total period of 65 days to obtain an average CaCO\({}_{3}\) content of approximately 1.1% in a sand column, and reported that regardless of the substrate type the most efficient microbial activity was achieved when the carbon to nitrogen ratio was about 1.6 (Table 3). Subsequently, [PERSON] et al. [33] further optimized the experimental process by increasing the number of flushes, lowering the substrate concentrations (e.g., 12 mmol/L Ca(CH\({}_{3}\)COO)\({}_{2}\) and 10 mmol/L Ca(NO\({}_{3}\))\({}_{2}\)), and decreasing the hydraulic residence time (2-3 days). They reported that under the optimized conditions, an average CaCO\({}_{3}\) content (by mass) of 0.65% could be achieved after five weeks of treatment, and precipitation rate could reach up to 0.26% CaCO\({}_{3}\) by weight/day. The treatment resulted in a distinct increase in the small strain stiffness, which indicated that denitrification-based MICP may be sufficient for practical soil improvement applications. However, a field application of biocmentation through denitrification has not yet been reported. Considering the scope of sustainable development and the transition towards a low carbon economy, in the field of geotechnical engineering, it is necessary to minimize the use of petrochemical-based products and/or environmentally hazardous chemicals, decrease the carbon emissions and avoid energy intensive treatment processes. These requirements created a great challenge in the field and provoked experts to reconsider the conventionally used ground improvement techniques in terms of their sustainability.