- Reverse osmosis systems are used in a wide range of industrial applications for water treatment. The main application is industrial seawater desalination. Reverse osmosis not only filters dissolved salts from the water, but also removes contaminants and even nanoparticles (bacteria and viruses). The process is used to derive ultrapure water for power plants and pharmaceuticals production, as well as for treating potable water.
- Monitoring reverse osmosis systems requires inline measurement of the pH value, ORP and conductivity.
- Adding a variety of chemicals during the process prevents organic or inorganic deposits from building up on the osmosis membrane. Imprecise dosing can lead to membrane damage and biological growth as well. As a consequence, systems must be shut down and cleaned extensively.
Precise chemical dosing increases service life of filter membranes
The principles of osmosis and reverse osmosis
Osmosis is the diffusion of water molecules through a semi-permeable membrane that separates two liquids with different dissolved solids content. The process continues until both liquids have the same concentration of solids and osmotic pressure no longer affects the water molecules. On the other hand, all dissolved solids are removed in reverse osmosis in order to derive ultrapure water. The water is continuously pressed through the membrane with a pressure that must be higher than the osmotic pressure, filtering out all dissolved organic and inorganic solids. On the other side of the membrane, the resulting concentration of dissolved solids must be continuously discharged.
Conductivity shows contamination
Since water is conductive due to its dissolved salt content, conductivity measurements can determine its level of contamination. The lower the water’s conductivity, the higher its level of purity. This is why sensors that can measure low levels of conductivity are highly significant for controlling reverse osmosis processes. With a measurement range of 0.001–500 µS/cm, the SE 604 conductivity sensor guarantees adherence to the optimal values. In combination with a Stratos series transmitter, the SE 604 sensor from Memosens offers a cost-efficient solution for documenting the lowest levels of conductivity. It allows the intact state of semi-permeable membranes to be monitored.
Chlorine dosing based on ORP measurements
Chlorine, whose oxidative properties prevent bacterial contamination and the formation of algae, is added to the water. Overdosing will damage membranes and shorten their service life, therefore the aggressive properties of chlorine are neutralized by adding sodium bisulfite in a reduction-oxidation (redox) reaction. If too much reducing agent is added, however, biological organisms could flourish. To create the equilibrium required and avoid damage or premature wear to the membrane, the SE 565 ORP sensor measures the oxidation reduction potential (ORP) to determine the chemical concentrations required. The low-maintenance, steam-sterilizable sensor was developed for hygienic and ultrapure water applications and has a measurement range of -1500 mV to +1500 mV. It is ideal for use in media that contain sulfite and protein, and highly contaminated media.
pH value detects threat of lime deposits
Since lime can deposit and lead to negative effects on membrane function, it is necessary to monitor the lime content of process water using pH measurements. Both very high and low pH values indicate that lime has deposited on the membrane. The SE 558 pH sensor for ultrapure water applications was developed to measure media with very low conductivity starting at 10 µS/cm. For most reverse osmosis systems, it is the optimal solution for detecting deposits early on. The integrated temperature detector provides automatic temperature compensation for the pH signal during sensor calibration and operation.
Return on Investment
- The ability to measure micro conductivity values and the setpoint deviation of ORP and pH values is a key prerequisite for extending membrane service life and saving the cost of premature replacement
- The amount of chemicals such as chlorine and sodium bisulfite used can be significantly reduced when conductivity, pH and ORP are precisely recorded.
- Memosens sensors are precalibrated in the laboratory, eliminating the need for complicated on-site calibration under difficult conditions. As soon as the sensors are connected to a Stratos transmitter, their calibration data are automatically transferred to the device.