Reverse osmosis

=== Pretreatment ===
Pretreatment is important when working nanofiltration membranes due to their spiral-wound design. The material is engineered to allow one-way flow. The design does not allow for backpulsing with water or air agitation to scour its surface and remove accumulated solids. Since material cannot be removed from the membrane surface, it is susceptible to [[fouling]] (loss of production capacity). Therefore, pretreatment is a necessity for any RO or nanofiltration system. Pretreatment has four major components:
* Screening solids: Solids must be removed and the water treated to prevent membrane fouling by particle or biological growth, and reduce the risk of damage to high-pressure components.
* Cartridge filtration: String-wound [[polypropylene]] filters are typically used to remove particles of 1–5&nbsp;[[micrometre|µm]] diameter.
* [[Dosing]]: Oxidizing [[biocide]]s, such as chlorine, are added to kill bacteria, followed by bisulfite dosing to deactivate the chlorine that can destroy a thin-film composite membrane. [[Biofouling]] inhibitors do not kill bacteria, while preventing them from growing slime on the membrane surface and plant walls.
* Prefiltration [[pH]] adjustment: If the pH, [[hardness]] and the [[alkalinity]] in the feedwater result in scaling while concentrated in the reject stream, acid is dosed to maintain [[carbonate]]s in their soluble [[carbonic acid]] form.
:CO<sub>3</sub><sup>2−</sup> + H<sub>3</sub>O<sup>+</sup> = HCO<sub>3</sub><sup>−</sup> + H<sub>2</sub>O
:HCO<sub>3</sub><sup>−</sup> + H<sub>3</sub>O<sup>+</sup> = H<sub>2</sub>CO<sub>3</sub> + H<sub>2</sub>O
* Carbonic acid cannot combine with calcium to form [[calcium carbonate]] scale. Calcium carbonate scaling tendency is estimated using the [[Hard water|Langelier saturation index]]. Adding too much [[sulfuric acid]] to control carbonate scales may result in [[calcium sulfate]], [[barium sulfate]], or [[strontium sulfate]] scale formation on the membrane.
* Prefiltration antiscalants: Scale inhibitors (also known as antiscalants) prevent formation of more scales than acid, which can only prevent formation of calcium carbonate and [[calcium phosphate]] scales. In addition to inhibiting carbonate and phosphate scales, antiscalants inhibit sulfate and fluoride scales and disperse [[colloid]]s and metal oxides. Despite claims that antiscalants can inhibit [[Silica Formation|silica formation]], no concrete evidence proves that silica [[polymerization]] is inhibited by antiscalants. Antiscalants can control acid-soluble scales at a fraction of the dosage required to control the same scale using sulfuric acid.<ref>{{cite journal|author=Malki, M.|title=Optimizing scale inhibition costs in reverse osmosis desalination plants|journal=International Desalination and Water Reuse Quarterly|year=2008|volume=17|issue=4|pages=28–29 |url=https://www.membranechemicals.com/conference-papers/}}</ref>
* Some small-scale desalination units use 'beach wells'. These are usually drilled on the seashore. These intake facilities are relatively simple to build and the seawater they collect is pretreated via slow filtration through subsurface sand/seabed formations. Raw seawater collected using beach wells is often of better quality in terms of solids, silt, oil, grease, organic contamination, and microorganisms, compared to open seawater intakes. Beach intakes may also yield source water of lower salinity.