The physico-chemical pretreatments take part of the optimal working element within a sea water desalination plant facility. Once the sea water intake has been made in order to obtain the best possible sea water quality, when this fact is not possible to achieve, the pretreatment is the next step. The pretreatment is a main part for the perfect sea water desalination plant functioning before sea water enters the facility. There are mainly two different types of pretreatment: a) Physical Pretreatment. Its goal is the suspended matter removal by means of a "physical barrier" without adding any chemical reagent. . b) Chemical Pretreatment. A chemical reagent is added to remove chemical and biological foulings, avoiding membranes chemical structure deterioration. It occurs as a consequence of a chemical reactant addition that promotes the elimination of a compound whose presence in the water supply is undesirable. Each process is hereinafter described separately: (a) physical pretreatment. The following pretreatments stand out: (a) 1-Grinding and sieving: They are more typical of the purification and water reuse rather than in sea water desalination. However, in the desalination systems, materials quality differs, for obvious reasons, from those designed for brackish water. A further separation system is the roughing, whose objective is to retain voluminous and suspended solids are dragged by the water supply. There are several types of filtering beds: horizontal and vertical ones. (a) 2.-Filtering beds: They are in charge of retaining on the surface or in the bosom of a fluid particles containing a liquid differentiating, in his case, the filtration depth from the filtration surface. Within the filtering beds, it is worth differentiating the pressure filters from gravity filters. The first ones are enclosed in pressurized metal containers and do not need any additional pump to generate the required pressure to reach the main element of the facility. b) Chemical pretreatment. b1) Disinfection: Sodium hypoclorite is the most common chemical reagent used although other disinfecting substances can also be used instead. In small facilities, a dosifying pump can be used in order to do this but on the other hand in larger facilities an online dosification is the most usual one by means of a chlorine generating and dosifying unit. b2) pH regulator: Sea water pH is around 8.2 but, if the optimal conditions are required in order to suffer the physico-chemical process, it is necessary to drop its value at around 6.7. In order to do this, Sulphuric acid or any other one, can be used. There are countries such as Argelia, where transporting hazardous and dangerous substances is forbidden so it is necessary to use an alternative solution like carbon dioxid which can be generated by a paquet unit. b3) Coagulant dosing unit: The product to be injected is coagulant, whose representative star is ferric chloride. Its behaviour is similar to acid dispenser equipment although it differs in terms of materials. Its produces the molecule destabilization preparing it the sump with its consequent elimination at sand filters. b4) Flocculation dosifying unit: The product to be injected is normally polielectrolyte being similar to coagulant dosing equipment. It makes the unsteady particles precipitate in order to clean the inlet water to both cartridge filters and membranes. b5) Anti-scalant equipment: Its purpose is to prevent the presence of precipitates on the surface of membranes with their consequent life shortening. In order to do this, a system of sodium metabisulfite dosification can be used.