# Nitrate-Polluted Waterbodies Remediation: Global Insights into Treatments for Compliance
## 4 Nitrate Removal Technologies in Waterbodies
### Separation-Based Technologies
#### 4.1.3 Electrodialysis  
Electrodialysis (ED) is a widely recognised membrane-based technology for nitrate removal, similar to reverse osmosis, except that in this case, ions are transferred through a semi-permeable ion exchange membrane from a more concentrated solution to a less concentrated solution by providing an electric field [63; 64]. This is an electrically driven process. This technology has proven to be reliable, exhibits excellent accuracy, and is easy to operate [65].  
This separation process is carried out in a filter-press equipment consisting of thin chambers between a pair of electrodes that generate an electric potential difference that enables the movement of the ions (Figure 3). These chambers are formed using intercalated cation and anion exchange membranes; the former allow the passage to cations and the latter to anions. In this way, the electric flow causes some chambers to run out of ions (diluate) and other chambers to have an increasing number of ions (concentrate). The sequential unit of a conventional cellstack (cell pair), consists of an anion exchange membrane, a diluate, a cation exchange membrane, and a concentrate [66; 67].  
The effectiveness of nitrate exclusion is significantly affected by the cell voltage, which is a key factor in nitrate removal by ED, since it is the driving force for separation across the membranes [68]. When the membrane potential was upgraded from 10 V to 30 V, the nitrate concentration in the diluate stream gradually decreased (31% and 71%, respectively) in 240 min with CMX-AMX NEOSEPTA ion exchange membranes. As the membrane potential increases, more ions can be transported across the membrane, as the movement of ions is enhanced by the increased conductivity generated by the higher voltage [34]. The maximum current efficiency was attained at 20 V [69].  
The initial nitrate concentration is a parameter that also determines the efficiency of the ED process as the mobility of these ions is facilitated by increasing the ionic strength of the solution [65]. Temperature is another factor that affects the efficiency of the ED process. It has been reported that running at a steady voltage of 15 V, the nitrate exclusion effectiveness goes from 73% to 76.6% and 82.6% depending on whether operating at 15 \({}^{\circ}\)C, 25 \({}^{\circ}\)C, or 40 \({}^{\circ}\)C [70]. The positive impact of temperature on the processing efficiency is related to the fact that temperature reduces the viscosity of the solution, increases the velocity of the ions, while promoting membrane dilatation, which in general improves the permeation of ions through the membrane [66]. In the ED process, the balance of ions between the charged sites on the membrane surfaces is controlled by the pH of the solution. For nitrate removal, weak acidic media greatly enhance electrodialysis due to the no competition between NO3- ions and H+. In contrast, under alkaline conditions, OH- ions significantly block the NO3- movement, thereby reducing nitrate removal [71].  
Electrodialysis is a technology that combines the advantages of selectivity and low chemical demand to remove nitrate ions from waterbodies. It is also worth noting that ED requires significantly lower operating pressure than reverse osmosis, its membranes are longer-lived, and the plant size can be adapted easily to the user's needs [43]. There are some disadvantages such as the fact that, in some situations, a pre-treatment stage of the feed water may be necessary depending on the level and type of pollutants it contains. Moreover, the water obtained after the electrodialysis process may require pH regulation and the concentrate obtained from membrane washing may need specific disposal [51].