# Microbially Induced Desaturation and Carbonate Precipitation through Denitrification: A Review
## 4 Potential Applications of Denitrification-Based MICP Biotechnology
### MICP-Driven Applications
#### 4.1.3 Nitrate and Calcium Removal from Industrial Streams  
Various industrial wastewaters contain significant amount of nitrate, and unless properly handled, these streams possess a considerable threat against the environment [39; 62]. Numerous methods of removing nitrate from wastewater have been performed, such as microbial denitrification, catalytic denitrification, reverse osmosis, electrodialysis, ion exchange and chemical treatment [63]. Among them, microbial denitrification is a promising, cost-effective, and environment-friendly method because it is capable of practically and permanently removing nitrate from water, forming harmless end products [64; 65]. The application of microbial denitrification to remove nitrate from wastewater has been studied and reviewed in detail by many researchers [64; 65; 66]. In this paper, we focused only on high nitrate and high calcium (generated in the stainless-steel pickling process) containing industrial streams, and the removal of these two ions through denitrification-based MICP. [PERSON] et al. [39] investigated the use of seed sludge from the leachate treatment plant and from the sewage treatment plant for nitrate removal from calcium rich industrial streams. Microbial denitrification generates alkalinity (Table 1, Figure 2) in the form of CO\({}_{2}^{-}\) and HCO\({}_{3}^{-}\) ions. When the wastewater is rich in Ca\({}^{2+}\), the generated alkalinity leads to the precipitation of CaCO\({}_{3}\) minerals, as well as the other calcium compounds (i.e., CaF\({}_{2}\), Ca\({}_{3}\)(PO\({}_{4}\))\({}_{2}\)) by increasing the pH of the media. The process removes 90% to 96% of the calcium ions in the wastewater and enables reuse of the treated water as process water without causing significant crust formation in the pipeline network [39]. [PERSON] et al. [30] used two types of denitrifying bacteria, _Pseudomonas aeruginosa_ and _D. nitroreducens_, as well as the indigenous non-axenic microbial community of paper mill wastewater treatment plant, to evaluate the feasibility of exploiting denitrification-based MICP to remove Ca\({}^{2+}\) from paper mill wastewater. They reported that both strains and  
Figure 4: Revealing the simultaneous corrosion inhibition function of denitrification-based microbial self-healing concrete by comparing the evolution of 300 \(\upmu\)m wide cracks, embedded steel rebar surfaces and the open circuit potentials (OCP) of the rebars (**a,b**) in denitrification based microbial self-healing concrete; and (**c,d**) in ureolysis based microbial self-healing concrete. The dashed horizontal line (\(\cdots\)) represents the critical OCP value (\(-250\) mV) for Fe in the tested Cl\(-\) solution. Scale bars represent 1 mm (based on [PERSON] et al. [34]). the indigenous microbial community in the paper mill wastewater performed similarly with a CaCO\({}_{3}\) precipitation yield of 12.7 g CaCO\({}_{3}\)/g NO\({}_{3}\)-N at the end of four days of incubation. Those findings showed that denitrification based MICP can be an effective strategy to remove Ca\({}^{2+}\) from paper mill wastewater and the indigenous microbial species in already operating paper mill wastewater treatment plant can be exploited for such application. However, the addition of external nitrate was reported to be the drawback of this approach as the wastewater generated in the paper mill industry does not contain enough nitrate for effective removal of calcium. In order to optimize calcium and nitrate removal from industrial streams, and maximize microbial activity for generation of reusable treated water, one might consider combining high nitrate and high calcium wastewaters of different industries. Industrial wastewaters mostly contain hazardous contaminants such as heavy metals in high concentrations. Most of the conventional treatment technologies either use chemicals to trigger chemical precipitation of those pollutants or high-cost and energy intensive technologies such as electrolysis, ion exchange and membrane separation.