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Reverse Osmosis

The reverse osmosis treatment generally is applied to desalting of seawater and brackish water destined to human consumption, low pressure boiler feeding, feeding of ion exchange demineralization plants and other processing use.

The aim of the treatment by inverse osmosis is reduction of 95 - 98% of dissolved salts and removal of bacterial charge from clear (filtered) water. The process of reverse osmosis is based on the peculiarity of half-permeable membranes, which water (or other solvents) is passing through, while molecules are retained as well as the ions of the dissolved solids.

In normal conditions, when such a membrane is inserted between an aqueous solution and water, due to the pressure difference caused by difference on density the solvent is passing through the membrane to the solution (direct osmosis). The separation of the solvent from the solute can be obtained by means of large pressure difference due to the application on the more concentrated solution of a higher pressure than the pressure of the other side of the membrane (reverse osmosis).

The velocity of solvent passage results to be much higher as higher is the pressure difference. Also many dissolved solids are diffused through the membranes in the lower concentration side. The process of reverse osmosis allows the water purification and dissolved solids concentration operations without any change in status and with a low consumption of energy.

The separation process is obtained by means of a high pressure pumping of the dissolved solids solution to be treated; from membrane-type devices, called modules, two different flows under the influence of the pressure are obtained. The first flow (the orthogonal one) is passing through the membrane and at the outlet high reduction of salts and organic take place, while the second flow (the tangential one) is passing over the surface layer before the membrane.

Two final results are obtained from the feeding flow through the system: the so called product (more diluted) and the concentrate.

The performance of a reverse osmosis plant is related as first to the average capacity of product (the permeated flow) and to the concentrate (the rejected flow) of the membranes.

Permeated flow and rejected flow are mostly due to the peculiarity of the used membranes (permeability to liquids, impermeability to solids), but at the same time they are also affected by the chemical and physical conditions of the process, such as pressure, temperature, pH, concentration and composition of the feeding solution.

In other words the permeated flow:
Increases (or decreases) according to the temperature and it is connected to the viscosity. The variation index changes according to different types of modules, and it is usually supplied in a table. If the temperature of the feeding solution changes frequently, it is often necessary to adjust the running conditions; this is a disadvantage for the manually adjusted plants.

Increases if the running pressure increases: it not always convenient or possible to work at high temperatures; still, the pressure-resistance limits of the modules must be always taken into consideration.

Decreases if the concentration of the feeding solution increases: however, unlike the temperature, the relation between the two cannot be generalized.

Decreases over the time, due to progressive irreversible degradation of the membranes; when the permeated flow increases and its quality decreases, it means that the active layer of the membrane has been worn or oxidized.

The quality of the permeated flow depends mainly on the rejection of the membranes: the rejection is sized through the ratio between the difference in concentration between feeding solution and permeated flow and the feeding solution concentration. The rejection factor is dimensioned usually with reference to a single component like sodium chloride.

The rejection flow:
Increases when the pressure increases, as this affects the difference between the velocity of water and the velocity of solids through the membranes. Slightly decreases when the temperature rises.

Decreases over the time, due to the progressive surface fouling of the membranes it is affected by pH variation of the feeding solution. Generally little affected by variations of solids concentration, as long as such concentration does not increase remarkably near the surface of the membranes.
The quality of the permeated flow does not depend only on the rejection: even if the rejection is constant, the concentration of the permeated flow could increase or decrease according to the concentration variations of the feeding solution. As inside the modules (especially if placed in series) the concentration increases gradually from the inlet to the outlet, the permeated flow will get progressively worse. It is therefore clear that the actual performances of the plant in terms of productivity and quality (concentration) of the parameters depend also on the recovery factor (or conversion factor). Such a factor describes the quantity ratio between permeated flow and feeding solution concentration.

Whatever deviation of the recovery factor from the fixed value, due to either a specific decision of the operator or external reasons, will always produce variations in the quality of the permeated flow.

Although the flow is steady, an eventual worsening of the quality of the permeated flow (which is immediately recognized by an increase in the conductivity), can be due to variations in the concentration of the feeding solution concentration or in the recovery factor. These two causes are easy to detect and correct, but if worsening in quality depends on problems of fouling or chemical damage of the membranes, nothing can be done to solve the problem.

 

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