Apa dan bagaimana sebenarnya Reverse Osmosis akan diulas dalam rangkaian artikel "Back to Basic" ini. Artikel ini dikumpulkan dari berbagai sumber untuk memberi penyegaran pengetahuan dari yang paling dasar sampai aplikasi teknis di pabrik. Bagian kedua ini kami ambilkan dari Wikipedia . 

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Reverse osmosis is a filtration process typically used for water. It works by using pressure to force a solution through a membrane, retaining the solute on one side and allowing the pure solvent to pass to the other side. This is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied.

Procedure

Formally, reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure.

The membranes used for reverse osmosis have a dense barrier layer in the polymer matrix where most separation occurs. In most cases the membrane is designed to allow only water to pass through this dense layer while preventing the passage of solutes (such as salt ions). This process requires that a high pressure be exerted on the high concentration side of the membrane, usually 2–17 bar (30–250 psi) for fresh and brackish water, and 40–70 bar (600–1000 psi) for seawater, which has around 24 bar (350 psi) natural osmotic pressure which must be overcome.

This process is best known for its use in desalination (removing the salt from sea water to get fresh water), but it has also been used to purify fresh water for medical, industrial and domestic applications since the early 1970s.

When two solutions with different concentrations of a solute are mixed, the total amount of solutes in the two solutions will be equally distributed in the total amount of solvent from the two solutions.

Instead of mixing the two solutions together, they can be put in two compartments where they are separated from each other by a semipermeable membrane. The semipermeable membrane does not allow the solutes to move from one compartment to the other, but allows the solvent to move. Since equilibrium cannot be achieved by the movement of solutes from the compartment with high solute concentration to the one with low solute concentration, it is instead achieved by the movement of the solvent from areas of low solute concentration to areas of high solute concentration. When the solvent moves away from low concentration areas, it causes these areas to become more concentrated. On the other side, when the solvent moves into areas of high concentration, solute concentration will decrease. This process is termed osmosis. The tendency for solvent to flow through the membrane can be expressed as "osmotic pressure", since it is analogous to flow caused by a pressure differential.

In reverse osmosis, in a similar setup as that in osmosis, pressure is applied to the compartment with high concentration. In this case, there are two forces influencing the movement of water: the pressure caused by the difference in solute concentration between the two compartments (the osmotic pressure) and the externally applied pressure.

Pre-Treatment

Pre-treatment is important when working with RO and nanofiltration (NF) membranes due to the nature of their spiral wound design. The material is engineered in such a fashion to allow only one way flow through the system. As such the spiral wound design doesn't allow for backpulsing with water or air agitation to scour its surface and remove solids. Since accumulated material cannot be removed from the membrane surface systems they are highly susceptible to fouling (loss of production capacity). Therefore, pretreatment is a necessity for any RO or NF system. Pretreatment in SWRO system has four major components:

Screening of solids

Solids within the water must be removed and the water treated to prevent fouling of the membranes by fine particle or biological growth, and reduce the risk of damage to high-pressure pump components.

Cartridge filtrationation - Generally string-wound polypropylene filters that remove between 1 - 5 micrometre sized particles.

Dosing of oxidizing biocides such as chlorine to kill bacteria followed by bisulfite dosing to deactivate the chlorine which can destroy a thin-film composite membrane. There are also biofouling inhibitors which do not kill bacteria but simply prevent them from growing slime on the membrane surface.

Prefiltration pH adjustment

If the pH, hardness and the alkalinity in the feedwater result in a scaling tendency when they are concentrated in the reject stream, acid is dosed to maintain carbonates in their soulble carbonic acid form.

CO3-- + H3O+ = HCO3- + H2O

HCO3- + H3O+ = H2CO3 + H2O

Carbonic acid cannot combine with calcium to form calcium carbonate scale. Calcium Carbonate Scaling tendency is estimated using the 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 RO membrane.

Prefiltration Antiscalants

Scale inhibitors (also known as antiscalants) prevent formation of all scales compared to 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, disperse colloids and metal oxides and specialty products exist to inhibit silica formation.

High pressure pump

The pump supplies the pressure needed to push water through the membrane, even as the membrane rejects the passage of salt through it. Typical pressures for brackish water range from 225 to 375 lbf/in² (1.6 to 2.6 MPa). In the case of seawater, they range from 800 to 1,180 lbf/in² (6 to 8 MPa).

Membrane assembly

The membrane assembly consists of a pressure vessel with a membrane that allows feedwater to be pressed against it. The membrane must be strong enough to withstand whatever pressure is applied against it. RO membranes are made in a variety of configurations, with the two most common configurations being spiral-wound and a hollow-fiber.

Reinmineralisation and pH adjustment

The desalinated water is very corrosive and is "stabilized" to protect downstream pipelines and storages usually by adding lime or caustic to prevent corrosion of concrete or cement lined surfaces. Liming material is used in order to adjust pH at 6.8 to 8.1 to meet the potable water specifications, primarily for effective disinfection and for corrosion control.

Disinfection

Post-treatment consists of stabilizing the water and preparing for distribution. Desalination processes are very effective barriers to pathogenic organisms, however disinfection is used to ensure a "safe" water supply. Disinfection (sometimes called germicidal or bactericidal) is employed to kill any bacteria protozoa and virus that have bypassed the desalination process into the product water. Disinfection may be by means of ultraviolet radiation, using UV lamps directly on the product, or by chlorination or chloramination (chlorine and ammonia). In many countries either chlorination or chloramination is used to provide a "residual" disinfection agent in the water supply system to protect against infection of the water supply by contamination entering the system.

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