The Reverse Osmosis is part of the processes of "Filtration on Membrane", that exploit the particular properties of the semipermeable membranes as selective barriers, allowing the passage of certain constituents present in the liquid and retaining other ones.
In normal conditions, interposing a membrane of this type between an aqueous solution and water, it establishes a difference of pressure (osmotic pressure) that pushes the solvent through the membrane in the direction of the solution (Direct Osmosis).
Instead, if you apply a pressure higher than the osmotic one on the solution with the highest concentration, it is obtained the separation of solvent from solute (Reverse Osmosis).
The system consists of a hydraulic pressurization group of the feeding water and some devices with membranes, called “vessels”, which under the effect of pressure cause to two different streams.
Dall’alimento si ottengono quindi un permeato (più diluito) ed un concentrato.
The foremost (orthogonal) that passes through the membrane and exit depleted in salts and organic substances. The latter (tangential) that carries away the accumulation of solids on the membrane surface. Thus, from the feeding water,a permeate (more dilute) and a concentrate are obtained.
The performance of a Reverse Osmosis plant are firstly related to the average flow of the permeate (permeate flow) and to its quality (rejection capacity of the membranes). Flow and rejection depend, in large measure, on the intrinsic properties of the used membranes (water permeability, impermeability to solids). They are also subject to the influence of the physical-chemical conditions of the process, such as pressure, temperature, pH, concentration and composition of the feeding water.
Any deviation of the parameters of design of the system, both due to choices of operators, both due to unforeseen external causes, inevitably leads to changes in the quality of the permeate and possible accelerations of fouling of the membranes. The main types of dirtness are fouling and scaling. The foremost is due to the presence of colloids or biomass and can be controlled with frequent washing cycles. The latter (scaling) is due to the formation of crystals and deposits of organic and inorganic compounds, produced by the overcoming of the solubility point of the various components, single or combined. Both these dirtnesses affect heavily the average life of the membrane and, for this reason, it is necessary that the feeding liquid has high quality characteristics.
To reduce the amount of fouling / scaling you can take the following precautions:
- Pre-treatment of the liquid to be treated. Its aim is reducing upstream SST, bacteria and constituents that may precipitate or damage the membrane. In addition to treatments such as softeners and filtrations, generally some antiscalant is dosed in line and the pH is adjusted chemically to contrast the phenomenon of precipitation;
- Washing of membranes, by flushing of water at low pressure to remove the solid material accumulated on the membrane surface;
- Chemical cleaning of the membrane (CIP). A chemical treatment is applied to remove those constituents that the water flushing is not able to remove.
This technology can be used for different purposes:
- Plants for sea water desalination;
- Softening plants (with a single stage);
- Demineralization plant (as a replacement to traditional and more expensive ion exchange filters);
- Plant for feeding of high pressure boilers (RO double pass);
- Treatment Plant of wastewater for their reuse in industrial processes by means of RO with multiple stages in series, for a production of permeate up to 94%.
The choice of using multiple stages in series, installing some boosters between one and another stage, enables to optimize the efficiency of the membranes, to reach the best profile of flow and pressure, and, at the same time, to reduce energy consumption.
The system works automatically, modulating the pressure and the flow rate in inlet of each stage through the use of inverters and in function of the set-point of the working pressure, set in the PLC. Through the use of instruments such as pressure transmitters, flow meters, pH, redox and conductivity meters, you can strictly control the process and optimize your system, improving energy efficiency and enabling the maximum useful life of the membranes.