# Microbially Induced Desaturation and Carbonate Precipitation through Denitrification: A Review
## 5 Challenges in Denitrification-Based MICP/MIDP Biotechnology  
Although denitrification-based MICP and MIDP biotechnology have been successfully demonstrated in many laboratory experiments and in several trials in the field, there are several challenges hindering the natural and commercial-scale applications of this technique. Table 5 summarizes the up-to-date challenges in upscaling of the approach as: (i) including the generation of harmful intermediates, (ii) environmental impacts, (iii) monitoring the remediation process, (iv) control of gas generation, (v) the low rate of CaCO\({}_{3}\) precipitation, and (vi) the homogeneous distribution of the treatment impact. The first challenge of this biotechnology is to avoid the accumulation of harmful intermediates by ensuring a complete denitrification reaction. Although the end product of denitrification is harmless nitrogen gas, three toxic intermediates, that is, nitrite, nitric oxide, and nitrous oxide, can accumulate when incomplete microbial nitrate reduction occurs [8]. The only exception to this is that the intermediate nitrite is functional as a commercial anodic rebar corrosion inhibitor in microbial self-healing concrete applications [34; 38]. Environmental factors, including pH, temperature, pressure, the concentrations of nutrients (electron donors/acceptors), and the abundance of operative microorganisms in the microbial community vary significantly in the natural soils. In contrast to laboratory experiments, in which most parameters can be controlled, these environmental factors are extremely complex and interfere with each other in natural soils. They affect the activities of the denitrifying bacteria and the generation and transportation of the denitrification reaction products. Thus, another challenge in the application of MICP and MIDP technologies is to design monitoring systems for field applications to quantify the influences of the complexities of these factors in natural soils and subsequent design of suitable microbial cultures for bioaugmentation of the relevant environment. Using nitrogen gas production for soil improvement such as liquefaction mitigation also involves potential challenges, including how to control the generation, distribution, and persistence of the nitrogen gas. [PERSON] and [PERSON] [51] reported that the gas bubbles formed during denitrification were not all retained in sand with a lower content of fines, resulting in the partial recovery of the degree of saturation without a continuous supply of nutrients. [PERSON] et al. [52] reported that gas bubbles were unstable in a 1 m high sand column under vertical and/or horizontal flow of groundwater. Moreover, an excess production of gas may induce cracks in the sand under low confinement conditions (e.g., shallow depths), which damage the sand structure and affect the sand column stability [25]. In terms of CaCO3 precipitation, denitrification-based MICP has a slower reaction rate than MICP through ureolysis, so it takes more time for the mechanical properties of soil to reach the desired values [8]. Ureolysis-based MICP has been reported to produce 6% CaCO3 (_w/w_) in a few days [90; 91], whereas denitrification-based MICP only generates an average of 1-3% CaCO3 (_w/w_) within a few weeks to several months [8; 20; 25]. Although slow precipitation rates seem like a drawback of MICP through denitrification, they enable maintaining microbial activity without occlusion of microbial cells with the precipitated CaCO3 crystals. Therefore, applying an optimized substrate regime and residence time can make denitrification based MICP more advantageous over ureolysis in long-term. However, there is no valid optimized procedure for field applications of MICP through denitrification, which remains as an obstacle before the transition of the concept into real life examples. The final challenge of using MICP technologies for practical applications is the struggle on obtaining a uniform treatment distribution. This is also valid for the other metabolic pathways such as ureolysis.