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Reverse Osmosis & Desalination Pretreatment
Desalination demand expands as communities look to the sea
Fresh water is a finite resource experiencing unprecedented demand. As human population expands, societal requirements, industries and agriculture becomes more water intense, the strain on fresh water supplies increases. There simply isn’t enough fresh water to meet all the needs in many regions of the world.
Many nations, businesses and industries are looking to the sea and desalination to provide the fresh water to meet current and future demand. Advances in reverse osmosis (RO), the most frequently used technology for desalination, have improved the energy efficiency and the recovery ratio, making it more competitive as a technology for producing fresh water.
Raw seawater contains many particles, chemicals, and microorganisms that can foul reverse osmosis membranes, including turbidity components, oils, minerals, color, organic matter, soluble metals, and algae. Fouling reduces the flux through the membranes, reducing the recovery ratio, driving up operating costs by requiring frequent cleaning, and increasing energy costs by increasing the pressure required to maintain treatment. Cost effective operation of the reverse osmosis system requires the removal of these contaminants from the feed water with pretreatment.
Determining the silt density index (SDI) is one method for determining the fouling potential of feedwater to a reverse osmosis system. The SDI measures the rate of plugging of a 0.45 micrometer filter when fed with the test water at a constant pressure of 2 bar (30 psi or 206.8 kPa). The SDI number is the percent drop in the flow rate per minute averaged over a period of time, generally 15 minutes. While an SDI of less than 5 may be considered acceptable for a feedwater to reverse osmosis, substantial improvements in RO performance, both recovery ratio at a given energy expenditure (pressure) and run time before cleaning require an SDI of less than 3. Raw seawater can have an SDI of 8 to 10, or sometimes as high as 15. Therefore, effective RO operation requires pretreatment.
Xylem technologies provide treatment for broad range of fouling contaminants
Xylem offers different technologies for pretreatment of reverse osmosis flow - granular media filtration using Filterworx® from the Leopold® brand , and ultrafiltration membranes. Each targets a different set of fouling factors and can be used in series or individually depending upon the quality and composition of the raw seawater.
FilterWorx is advanced granular media filtration that effectively removes settleable contaminants like clays, colloidal silica, precipitated metal hydroxides, organics like humic and fulvic acids, and some levels of algae and bacteria.
Xylem offers a variety of options for ultrafiltration (UF) membrane pretreatment, including both immersed and pressurized filtration systems. UF produces consistently high quality effluent for feed water to RO systems, with SDIs consistently below 3.
Xylem’s water chemistry experts work with clients to evaluate their raw water quality and develop the configuration that meets the individual treatment goals. Designs must take into consideration feed water fluctuations, including seasonal variations such as the tendency for algal blooms. Xylem pretreatment technologies have proven reliability in treating raw waters with SDIs between 8 to 10, and excursions as high as 15. With the proper treatment options in place, SDIs can be consistently held to under 3, improving the operation of RO systems, reducing chemical cleaning and extending RO membrane life.
The demand for fresh water will continue to grow. With Xylem pretreatment options, RO desalination is more efficient and reliable.
Together we can do it. Let’s provide fresh water. Let’s solve water.