# Nitrate-Polluted Waterbodies Remediation: Global Insights into Treatments for Compliance
## 4 Nitrate Removal Technologies in Waterbodies
### Separation-Based Technologies
#### 4.1.2 Reverse Osmosis  
Reverse osmosis (RO) is a membrane-based technology that is used to remove a large majority of pollutants from water by pushing the water under pressure through a semipermeable membrane. The membrane, with a pore size in the range of 0.1 and 1.0 nm, only allows movement of water molecules to pass through, but rejects movement of solute particles across it. This technology can be used to eliminate multiple inorganic and organic pollutants of waterbodies [37; 45]. Reverse osmosis is the most widely used technique in water desalination processes for both drinking water and water for agricultural irrigation [52; 53].  
This separation technique yields a premium quality water in which only a reduced number of volatile organic constituents remain. Nevertheless, RO has a relatively high energy demand, around 1.8-2.0 kWh/m\({}^{3}\)[54], because it requires a high pressure (>6 bar) to pass the polluted water through the semipermeable membrane. In addition, reverse osmosis generates a significant waste concentrate stream (reject brine), containing high concentration of nitrate, as well as other ions [55]. The reject flow stream can be up to 30% of the total treated water flow. This high rejection represents a serious problem in areas where water availability is scarce [20].  
The removal rate of contaminants by reverse osmosis depends on the material used for the membrane, pore size, applied pressure, and quality of the feed water. Under optimal working conditions, approximately 85 to 95% of the nitrate ions in waterbodies can be removed with reverse osmosis [56].  
The materials used to fabricate reverse osmosis membranes are of polymeric structure, differing in thickness and pore size. The typical polymeric materials used are cellulose acetate and polyamide [57]. In recent decades, significant progress has been achieved in membrane and module manufacturing, such as the development of thin-film composite polyamide membranes with high chemical compatibility, mechanically very resistant, and excellent stability over a wide pH range [58].  
To enhance the performance of the final process, nanofiltration (NF) and reverse osmosis (RO) can be coupled for nitrate removal in water with elevated salinity. The NF removes Na\({}^{+}\) and Cl\({}^{-}\) ions from the water over NO\({}_{3}\)\({}^{-}\), and the RO mostly eliminates NO\({}_{3}\)\({}^{-}\) ions from the nitrate-enriched NF permeate. This scheme can achieve a reduction of 91-94% of the total NO\({}_{3}\)\({}^{-}\) level in waterbodies [55]. Moreover, these hybrid NF/RO process minimizes the brine reject fraction, making it more sustainable and friendly to the environment [59].  
Among the drawbacks to be considered in membrane-based separation processes is fouling, caused by the deposition of foreign matter from the feed water on the active surface of the membrane, due to their contact with soluble matter, organic matter either as colloidal or suspended particles, resulting in decreased permeation flux and salt rejection [60; 61]. In the case of fouling by salt deposits, the phenomenon is known as inorganic scaling. The salts causing the most important inorganic fouling problems in reverse osmosis are calcium sulphate and calcium carbonate [62]. These concerns can be minimized with proper pre-treatment of the feed water, which will result in longer membrane life and lower operating costs.