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 Seawater - Pretreatment for Reverse Osmosis and Desalination Applications

About RO Membranes - Brief Overview

RO refers to a type of water purification process that uses semipermeable membranes which block passage of particles, bacteria, and colloids, and through which water molecules diffuse freely in relation to dissolved constituents (such as dissolved salts, silica, boron, and many organic impurities). Because of this special property, under the influence of an applied hydraulic pressure and at a pressure differential higher than the osmotic pressure of the source water, the membranes allow passage of water while excluding passage of water impurities. RO as a commercialized unit process for water purification has been available for over 50 years.

About Reverse Osmosis (RO) Membranes

An RO system is built around combinations of individual membrane modules, typically formed from sheets of membranes arranged in a spirally wound configuration. Spirally wound membrane modules (also called “membranes”) are available in a variety of sizes, with 4 inch and 8 inch diameter by 40 inch length. Reverse osmosis is used to produce highly purified water for drinking water systems, industrial boilers, food and beverage processing, cosmetics, pharmaceutical production, seawater desalination, and many more applications.

Why PRETREATMENT FOR Reverse Osmosis (RO) Systems?

The primary objective of pretreatment to any RO membrane system is to make the feed water compatible with the membrane. Inadequate membrane pretreatment results in high chemical cleaning costs, increased downtime, and permanent loss of performance with reduced membrane life. This is particularly true in seawater reverse osmosis (SWRO) desalination plants with open intakes where water quality parameters fluctuate. In many cases high salinity can also significantly reduce the efficiency of coagulation and flocculation to dual-media filters. Although, when utilizing Vortisand® high efficiency cross-flow microsand technology, the submicron performance yields higher quality water, thus minimizing the requirement for coagulation and flocculation.

The high efficiency cross-flow microsand technology was put to the test by an independent third party. Download the whitepaper comparing the Vortisand technology to traditional pretreatment media filters.

 Reverse Osmosis Pretreatment Systems WIll Improve Your Bottom Line!

​There are many benefits to implementing a reverse osmosis pretreatment strategy! Ensuring your feed water is compatible with your seawater RO system will be the number one tool to reducing the potential for RO fouling, ultimately improving the following;

Maintenance & Operations: How often do you replace your fine cartridge filters? Imagine an environment where a high efficency pretreatment filter would enable you to forget about replacing them... What if your RO system required cleaning once a year versus every few months? What would be your return on investment? Enabling the system to run for longer durations of time, uninterrupted, with minimal upkeep costs...

Membrane Life: Replacing your RO membranes is a costly activity, a high efficiency cross-flow microsand pretreatment system can help in increasing the longevity of your system. Enabling you to get more out of your system, while still experiencing peak levels of performance.

Membrane Failure & Plant Shutdown: Production costs, maintenance costs and capital equipment expenses of resolving an unexpected RO failure can become costly. Especially, when looking for an urgent/immediate solution.

Water Quality: There is no need to put your outlet water quality at risk. A reverse osmosis pretreatment system will help keep the integrity of your final product consistent, while reducing the cost of producing it.

 Top 3 Reverse Osmosis Concerns

​Seawater and RO focused facility maintenance managers often sample the water at both the inlet and outlet of an RO system, getting an understanding of the operations efficiency and performance. The data provided from these tests (SDI and turbidity) help tell a story, one that influences the longevity of the RO system and its components. Fouling is the key concern for RO systems; creating an environment that promotes fouling will quickly plug your RO membranes, yielding increased maintenance and operational costs, reduced membrane life, increased RO failure and routine plant shutdown.

Seawater Reverse Osmosis Fouling: RO fouling includes; biological fouling, particle fouling, colloidal fouling, organic fouling, mineral fouling and oxidant fouling. Fouling is a general term referring to the deposit and accumulation of scale, algae, suspended solids and insoluble salts on a surface. Such suspended particles, approximately 5 microns in size, are more likely to clog your membrane system, leading to fouling. Like scale, fouling will also cause rapid declines in efficiency levels and membrane life-cycle.

Silt Water Density Index (SDI): Silt is composed of suspended particles accumulating on the surface of the membrane. Common sources of silt include organic colloids, iron corrosion products, precipitated iron hydroxide, algae, and fine particulate. SDI testing is an accepted method for estimating the rate at which fouling will occur in water purification systems, especially those using reverse osmosis or nanofiltration membrane systems.

According to the ASTM Standard D4189, SDI measures the time required to filter a fixed volume of water through a standard 0.45µm pore size microfiltration membrane with a constant given pressure of 30 psi. The difference between the initial time and the time of a second measurement after normally 15 minutes (after silt-built up) represents the SDI value. So what do these values mean you might ask? Generally speaking, and this differs with each application; an SDI level below 1 can yield several years without fouling, a level below 3 can yield several months, between 3 to 5 you are experiencing frequent cleaning with fouling being a big concern, and finally any level above 5 is extremely concerning and immediate pretreatment is a must.

Learn how high efficiency cross-flow microsand technology yielded levels of SDI below 2 with an average of 2.4! The study was done by an independent North American engineering firm, Fortex Experts.

Turbidity: The cloudiness or haziness of a fluid caused by large numbers of individual fine particles that are generally invisible to the naked eye. The measurement of turbidity is one of many water quality tests. Turbidity solely measures the amount of suspended solids in a sampling of water, and just confirms that particles are present. It is important to note that turbidity and levels of SDI are not the same, and there is no correlation between the two measurements.

The most common measurement for turbidity is the Nephelometric Turbidity Units (NTU). There are several ways you can measure levels of turbidity in water, one being measuring the strength of a light source as it passes through a water sample.

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​Looking to improve your water treatment efficiencies and reduce your water treatment costs? Contact our team of water experts to learn how the Vortisand® systems can help meet your water objectives!