Sea water pretreatment

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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.

2 Comments

  1. French English Je suis totalement contre tout processus de désalement d'eau de mer. Toute production d'eau potable va générer une quantité à l'identique d'eaux usées qui seront rejetées dans l'eau de mer. Comme tous les systèmes de traitement des eaux usées sont totalement inefficace (performance épuratoire proche de zéro) une quantité très importante de pollution organique sera rejetée dans l'eau de mer. Cette pollution diffuse diluée dans le liquide tel que l'urée se retrouvera dans l'usine de désalement. Les processus de traitement du désalement génèrent des déchets qui sont rejetés en mer. Encore un cas de pollution. Le concept d'Assainissement Biologique des eaux usées traite les eaux usées sans les rejeter dans les milieux hydrauliques naturels, il recycle le reliquat d'eau en sortie du dispositif pour l'arrosage du jardin potager ou de toute autre production agronomique. Economie d'eau potable consommée pour l'arrosage de la biodiversité, dépollution des sols et des milieux hydrauliques naturels, préservation de cette ressource rare l'eau potable des nappes phréatiques. I am totally against any process of seawater desalting. Any water production will generate an identical amount of wastewater to be discharged into the sea water. Like all wastewater treatment systems are completely ineffective (purification performance close to zero) a very large amount of organic pollution is released into the seawater. This diffuse pollution in the diluted liquid such as urea will be in desalination plant. The desalination treatment processes generate waste that will be discharged into the sea. Another case of pollution. The concept of Biological Wastewater Treatment treats wastewater without rejecting them in natural water environments. It recycles the water remaining in the output of the device for watering the garden or other agricultural production. Economy drinking water consumed for irrigation of biodiversity, soil remediation and natural water environments, preservation of this scarce resource: drinking water aquifers.

    1 Comment reply

    1. Dear Jean Marius: I have the same opinion about the excessive operational costs involved in the desalination treatment process, but not on what you are saying about the wastes that are discharged into the sea. Before doing it, brine is diluted to an amount much lower than the original one, hence this problem is solved by using the marine outfalls. Additionally, nowadays there are some energy recovery devices (called ERD) which can reduce the specific energy consumption up to values around 3.0 kWh/m3 depending . Apart from that, not always the use of chemical reagents is advisable in order to improve the desalination plant functioning. Best Regards