Reverse Osmosis Plants
As a membrane separation process, reverse osmosis provides an almost complete desalination of water.
Aqueous solutions have an osmotic pressure which is a solution property proportional to the salt concentration. Regular well water („freshwater“) has a osmotic pressure of less than 1 bar (105 Pa), seawater around 20 bar.
Reverse osmosis membranes are semi-permeable which means that the membrane selectively allows certain components, for example water, to pass through it while retaining others. Application of an external pressure which is higher than the osmotic pressure will raise the chemical potential of the water in the salt solution and cause a solvent flow to the pure water side. A separation between the pure water (permeate) and the concentrated dissolved components (concentrate) occurs.
The degree of permeate purity and the permeate flow (m³/h) depend on the quality of the membrane, the pump pressure, the design of the modules and the design of the plant.
On the concentrate side of the membrane the osmotic pressure and the tendency of building up deposits increases. Therefore a part of the concentrate is continuously discharged. For economic reasons, the costs for wastewater disposal which corresponds with the amount of discharged concentrate, must be minimized.
The membranes are sensitive to all types of deposits on their interface. To keep the permeate flow stable, a microfiltration and conditioning or softening of the raw water is necessary. Economic aspects determine which pretreatments to select. Dissolved salts are mostly unselectively retained. The high-quality composite-membranes allow a salt rejection of 98 to 99,5 %, depending on the membrane. Different quality requirements of the permeate determine the selection of the membranes. For a continous good quality of the permeate, the option of a conductivity controlled permeate recirculation can be offered. Furthermore the option permeate pressure can be chosen because reverse osmosis plants normally have no pressure on the permeate side.
Reverse-osmosis plants consist of spiral wound modules installed in GFK–pressure vessels, which fulfil the chemical and mechanical standards and the high standards of hygiene. The operating pressure for freshwater and brackish water never exceeds 16 bar, so that the piping is done in noncorrosive plastic (alternatively PP or PVC) or stailness steel. Seawater desalination-plants are produced in a seawater-resistant stainless steel design.
The operation and control of all functions is regulated by a programmable microprocessor controller which is equipped with a folio-keyboard and a basic programming memory (EEPROM). Pneumatically operated plastic diaphragm valves are installed in plants with a permeate capacity of 250 l/h or more.
WAT-reverse osmosis plants are fully wired, reliable, safe and easy to handle. They are completely mounted on noncorrosive coated steel frames. Reverse osmosis plants of the same capacity can be differently designed, depending on the space available for installation.
Areas of Application
Feedwater for airwashers and air humidifier
The requirements for the operation of airwashers and air humidifiers are continuously increasing, especially because of hygienic requirements and because of the sensitivity of technical equipment. The operation with salt reduced water is often required or at least recommended.
Salt reduced water also considerably increases the effectiveness of UV-disinfection.
Feedwater for cooling water systems
The decisive cost factor of the operation of recirculating cooling water systems is the consumption (wasteful consumption) of water and wastewater. Use of desalinated water allows an operation with a high thickening of the water before deposition occurs. If a blending device for mixing feed water with permeate is installed, an optimum between deposition tendency and corrosiveness can be achieved.
In closed cooling water systems, the best operation conditions (free of corrosion) can be achieved with desalinated water.
Feedwater for steam generation
Here as well, the decisive cost factor of the operation of boilers is the maximum thickening of the circulating water which can be achieved. The discharge of thickened water with high salt concentration should not exceed 5-10 % of the steam. Desalinated water not only allows a operation with considerable higher thickening of the water, but also avoids the corrosion caused by carbonic acid in the condensate system.
Autoclaves
By using desalinated water, all types of autoclaves can be operated free of deposition.
Reverse osmosis as a preliminary stage of ultrapure water
Reverse osmosis is a cost-effective desalination process. With a secondary mixed-bed ion exchanger, a reverse osmosis plant or an elctrodeionisation plant a water quality can be produced, which fulfils the requirements of ultrapure water. Depending on the choice of the secondary plant a conductivity up to 0.055 µS/cm can be reached.
Process- and Rinsing water
Reverse osmosis permeate also fulfils the requirements for process water or rinsing water used for surface treatment of metals such as pickling, metal cleaning, phosphatizing, chromating or galvanizing. It is also used in eloxal processes or as a pretreament for coating.
Drinking water and sanitary water
Well water with a high salt concentration, brackish water or seawater can be turned into water with high drinking water quality.
