Reverse Osmosis Plants

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Baikal TECH

Cairo, Egypt

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In the past few years, a method for treating saline water using reverse osmosis (R.O.) has been developed, which has resulted in obtaining water with a high degree of purity according to the specifications for drinking water and water used in the food and beverage industry of all kinds, as well as health and mineral water.

And before embarking on explaining the reverse osmosis process used for desalination, it is first necessary to provide this simple explanation of the osmosis phenomenon supported by some illustrative diagrams. This method primarily relies on passing the water to be treated through a semi-permeable membrane, resulting in a stream of purified water and a stream of concentrated saline water due to the separation of salts from the purified water. One of the membrane's characteristics is that it allows fresh water to pass through easily, but saline water passes through with difficulty. The reverse osmosis method is based on the theory of osmotic pressure.
Explanation of osmotic pressure theory:
When a saline solution and pure (fresh) water are separated by a semi-permeable membrane, the pure water passes through the membrane and dilutes the solution until the concentration reaches a certain level called osmotic equilibrium. The pressure that causes pure water to pass through the semi-permeable membrane to dilute the saline solution until osmotic equilibrium is reached is known as osmotic pressure.
And when the pressure of the saline solution is raised to a level much greater than the osmotic pressure, it is possible to reverse the flow of water through the semi-permeable membrane, meaning that the pure water contained in the saline solution will pass through the membrane towards the freshwater in the opposite direction of the osmotic pressure.
The semi-permeable membranes are in the form of a thin film or hollow fiber and have high permeability to fresh water. They can withstand the pressures of reverse osmosis processes, which can reach about 60 atmospheres for seawater, in order to overcome the osmotic pressure of seawater, which is 25 atmospheres. Reverse osmosis produces water of high purity according to drinking water specifications.
The reverse osmosis process is divided into four main stages:
The first stage: The pre-processing stage
The feed water flow is treated to align with the membrane operating conditions and to be free of solid particulates through Multimedia Filter sand filtration and Cartridge Filters, pH Adjustment, and Chemicals Dosing to prevent any scaling.
Phase Two: Pressure
During this process or stage, the pressure on the pre-treated water is raised to the appropriate level for the type of membranes and the concentration of dissolved salts in the water to be treated. The high-pressure pump raises the hydraulic pressure of the feed water to a sufficient level to overcome the natural osmotic pressure, with an increase enough to produce the required amount of fresh water. Thus, this pump provides the necessary pressure for water to pass through the membranes and retain the salts.

Phase three: Membrane separation
At this stage, the permeable membranes allow only fresh water to pass through, while dissolved salts cannot pass through and are redirected to the high-salinity discharge line. It is worth noting that a very small percentage remains in the fresh water flow, which is due to the imperfection of the membranes that allow these small percentages to pass through. The membranes come in several types, the most important of which are Spiral Wound membranes and Hollow Fine Fiber membranes. All these membranes are made from Cellulose Acetate, Aromatic Polyamids, or, as is the case nowadays, from Film Polymer compounds.
These membranes also remove more than 75 percent of salts, in addition to most types of organics, viruses, bacteria, and other chemical pollutants. The pore sizes of different types of membranes range from less than 10 angstroms to 100 microns.
Phase four: The stabilization or post-processing phase
Freshwater and membrane-produced water require pH adjustment, which is done by raising it from about 6.5 to 8.5.