Reverse osmosis is a technology that separates solvent from a concentrated solution by the application of pressure. The solvent passes through a semipermeable membrane from the concentrated solution to the dilute solution. Reverse osmosis leads to two flows: firstly, the practically pure solvent and, secondly, the starting solution made even more concentrated.
Rejected fractions from reverse osmosis membranes can be recovered in the process by simply discharging or by concentrating with other methods, e.g. via evaporation or crystallization plants, to obtain zero discharge.
It is an efficient, clean and compact technology which provides high quality water from brackish water and even sea water. It is also a great ally for the treatment of effluents that have to be reused. Increasingly technologically advanced membranes are being used which allow reasonable permeate flows to be obtained at lower and lower pressures.
Condorchem Envitech has extensive experience in the design of reverse osmosis equipment, which is essential for ensuring it is optimally adapted to the varied specifications of customers.
A wide variety of products are designed and built, especially for the following applications:
Production of drinking water from brackish water (between 1,000 and 10,000 mg/L of salts) or sea water (10,000 mg/L of salts). Condorchem Envitech equipment is capable of operating at pressures sometimes up to 5 times lower than conventional equipment (50-70 bar), which leads to a noticeable decrease in operating costs. The equipment can produce potable water flows of between 1 and 1,000 m3/day.
Depending on the geographical location, mains water may have a high content of salts and other substances that, although not harmful, confer a series of unpleasant organoleptic qualities. Condorchem Envitech’s mains water purification equipment produces water of excellent organoleptic quality.
Its innovative modular design is suitable for flows of 1-3 L/min for domestic applications, with a wide range of higher flow rates (60 – 14,400 L/min) for industrial applications.
In these specific cases, Condorchem Envitech has much experience in the implementation of customized projects, by designing, constructing and implementing (turnkey) reverse osmosis processes, according to specific standards, codes or requirements of the customer. Production from these facilities can vary from 200 L/h to any capacity the customer requires.
Osmosis is a natural phenomenon that tends to equalize the concentration of two solutions, one concentrated and the other diluted, when these are separated by a semipermeable membrane. To equalize the chemical potential on both sides of the membrane, a solvent flow through the membrane from the diluted solution to the concentrated solution occurs spontaneously. This flow is proportional to the concentration difference and stops when the concentrations are equal.
If a slight pressure is exerted on the more concentrated solution when the osmotic flow occurs through the membrane, the flow is reduced. And if the pressure exerted is increased, there comes a point at which the flow is zero. The exact pressure that stops the flow is the osmotic pressure and depends on the nature of the solute and concentration of the solution. At this point, if the pressure exerted is even greater, the flow is reversed and the solvent flows through the membrane from the concentrated solution to the diluted solution.
It should be noted that the mechanism by which the osmosis occurs is different to the ultrafiltration process, despite the fact that a membrane is used in both cases. Osmosis is based on diffusion of the solvent through the membrane; depending on its type, this means a wide variety of species cannot cross it despite their low molecular weight.
A solvent can be separated from the dissolved solutes with high efficiency by reverse osmosis, obtaining permeates with salt concentrations of the order of 1-5% of the initial concentration.
Semipermeable membranes selectively allow the passage of the solvent while retaining the salts, and are a key component in the process. Initially, they were made of cellulose acetate, but recently polyamide has been shown to be more efficient in controlling pore size and, therefore, permeability.
In general, reverse osmosis membranes are very poorly permeable to ions and electrostatically charged particles, whereas they exhibit very little resistance to the passage of dissolved gases (e.g. oxygen or carbon dioxide) and low molecular weight molecules with no electrostatic charge.
Membrane fouling is an important feature of reverse osmosis, and must be closely controlled as it can pose a threat to productivity. This is due to several causes, such as the precipitation of salts present in the feed, exceeding the solubility product in the concentrate, formation of colloidal sediments and other particles in suspension and the growth of microorganisms on the membrane surface.
The membrane cleaning technique depends on the composition of the feed, the type of membrane and the main cause of the fouling. Generally, membrane rinsing periods consist of alternating between high-speed cleaning solutions being circulated over the membrane surface and periods when the membranes are immersed in cleaning solutions. These are usually:
A good membrane cleaning programme lengthens its useful life and, in most cases, a feed pre-treatment process is desirable; this is often recommended to be filtration followed by ultrafiltration.