Rejection is defined as the degree to which dissolved solids are retained by the membrane. It is expressed in terms of percent of feed concentration and typical values for RO membranes are 98% or 99%. That means that 98% to 99% of dissolved solids are retained by the membrane.
The higher the concentration of dissolved solids, the greater the osmotic pressure of the solution and the driving pressure required to force the water flow inside the membrane in the reverse osmotic direction. As a consequence, typical working pressures for low salinity waters are 10-20 Bar, while typical working pressures for seawater RO are > 50 Bar.
Our reverse osmosis plants are typically designed for feeds with up to 3000 mg/l TDS and recovery rates between 60% and 80% . Our goal is always to minimise feed pressure and membrane costs while maximising permeate quality and recovery.
These RO units are pre-designed and factory assembled and tested to minimise installation, commissioning and start-up time. Using a feed water analysis as starting point the project design kicks-off based on specifications and use of the product water. In general all our systems will contain flow, temperature, conductivity and pressure transmitters, as well as control valves operated from an HMI/PLC control. Dry contacts are provided for pretreatment units or storage tanks and MODBUS communication is available for integration with other BMS or industrial interfaces.
In order to achieve high recovery rates for our brackish reverse osmosis plants, we use three-stage configurations for large systems or concentrate recirculation for smaller systems and pilot units. This way we can reach up to an 80% overall recovery rate while each membrane still works under its standard 10% recovery rate. Recirculation allows membranes in small units simulate concentration conditions applicable to the final element in an industrial installation with low scale hydraulics.
When ultra-pure water is required our two-pass reverse osmosis plants produce high quality water by pumping into a second reverse osmosis stage the produce water from a first stage.
When partial desalination with an acceptable level of water softening is the main goal, our reverse osmosis plants can be equipped with NF membranes characterised by a medium passage of monovalent salts and a very low passage of bivalents salts. As monovalent salts are the main contributor to osmotic pressure, these brackish reverse osmotic plants operate at a lower pressure, therefore with a lower energy need.
For irrigation purposes, typically only a medium desalination is required to align a bore or surface water quality with the crop salinity limit (typically less than 500µS/cm). For those cases our brackish reverse osmosis plants can integrate a bypass mode that mix a bypassing portion of the feed with the permeate.
Optional pre-treatments and post-treatments are available to remove metals (Fe, Mn, Al), turbidity, organic matter and bacteria or remineralising the product water. In general, reverse osmosis membranes are very sensitive to suspended solids so all our units are supplied with cartridge filters. However other pre-treatments might be required to extend the membranes life.
*(1) All data for information only and subject to change based on actual water analysis and final design parameters
When customers come to us looking for a Reverse Osmosis water treatment plant, one of the hardest questions to answer is, How Much Does a Reverse Osmosis Plant Cost?” The costs can be complex to estimate based on several factors like feed water chemistry or product water use but in general, and assuming that no pre-treatment is required other than a basic filtration, a 50m3/day will cost between $75,000 and $100,000.
Other important costs that will need to be considered are civil works, installation works, freight to site, and local permits.
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