# Nitrate-Polluted Waterbodies Remediation: Global Insights into Treatments for Compliance
## 4 Nitrate Removal Technologies in Waterbodies
### Transformation-Based Technologies
#### 4.2.1 Biological Denitrification  
Biological denitrification is a metabolic process by which bacteria, mainly under anaerobic conditions, biodegradable organic matter and use nitrate as terminal electron acceptor, leading finally to the inert gas N\({}_{2}\)[139]. Three groups of denitrifying bacteria are distinguished depending on which substances they use as an energy source: autotrophic bacteria (oxidise inorganic compounds), heterotrophic bacteria (oxidise organic compounds), or mixotrophic bacteria (oxidise both inorganic and organic compounds). Autotrophic denitrifying bacteria use H\({}_{2}\) gas or reduced sulphur compounds as the energy source (electron donors) and an inorganic carbon source, usually CO\({}_{2}\) or bicarbonate [20,140]. Most biological denitrification treatments are based on heterotrophic bacteria that need a source of organic carbon, such as methanol, ethanol, or acetic acid [141,142], but methods have also been developed in which a gaseous substrate as CO or CH\({}_{4}\) is used [143]. Heterotrophic biological water denitrification treatments have been applied on an industrial scale to a greater extent, mainly due to the faster rate at which this process is carried out. In the case of autotrophic denitrification, longer spatial times are required [144], which leads to bigger reaction volumes and considerably increased costs. Typically, during the process of biological denitrification, pre-treatment of the water is required, in which the nutrient carbon source and electron donor substances are incor  
Figure 5: Nitrate adsorption capacities of selected sorbents. PMZ [125], ACPP [126], ACSB [113], OTSB [127], MSS [128], LDH [129], MCN [130], HLA [131], SMZ [121], MCS [118], CSAB [100], CHB [132], AR [133], CMCN [130], ATW [134], FCN [135], MPS [136], CLC [106], AGN [137], AFC [138]. porated. Post-treatments are also performed at the outlet of the bioreactor to adjust water quality to its final use (Figure 6). Autotrophic denitrification is performed by bacteria as _Thiobacillus denitrificans, Thiomicrospira denitrificans, Azorcus denitrificans_, or _Bosea thiooxidants_[145; 146]. These microorganisms are mainly mesophiles with an optimum temperature between 25 and 35 \({}^{\circ}\)C. In this process, alkaline substances are added (HCO\({}_{3}\)\({}^{-}\), CaCO\({}_{3}\)) to neutralize the H\({}^{+}\) produced [147]. These bacteria also have the ability to use O\({}_{2}\) as electron scavenger for the oxidation of sulphur compounds. The final product obtained will depend on the oxygen concentration. At low O\({}_{2}\) concentrations (<0.1 mg O\({}_{2}\)/L), partial oxidation to S\({}^{0}\) occurs, while at high O\({}_{2}\) concentrations, SO\({}_{4}\)\({}^{2-}\) is produced [142]. To enhance nitrate removal, the presence of oxygen should be avoided since the bacteria will preferentially use oxygen as an electron acceptor. Under autotrophic conditions, it is critical that the proportion of nitrate ions and sulphur compounds is balanced in a stoichiometric ratio so that the reducing bacteria can operate properly [148]. When raw water contains high concentrations of nitrate, an effluent with high sulphate content is obtained. In this scenario, the generation of anaerobic zones could result in the formation of sulphides [149]. Heterotrophic denitrification processes that occur under anoxic conditions are the most investigated and most widely implemented. Recent research has mainly addressed the screening of microbes with high nitrate removal efficiency [150]. So far, a large number of heterotrophic bacteria, such as _Alcaligenes_ sp., _Pseudomonas_ sp., _Caracoocus_ sp., _Acinetobacter_ sp. and _Bacillus_ sp., have been reported [146; 151; 152].