What is RO and how does it work?The structure and production process of the membrane

A reverse osmosis system is built around its individual membranes.  Each membrane is a spiral wound sheet of semi-permeable material. Membranes are available in 2-inch, 4-inch, and 8-inch diameter with the 4- and 8-inch diameter sizes most commonly used in industry. The industry has accepted a 40-inch length as a standard size so that membranes from different manufacturers are interchangeable in equipment systems. One of the primary measurements of a membrane is its square footage. Membranes are available in the range of 350-450 square feet of surface area.

Semi-permeable membranes were first constructed using cellulose acetate (CA) but later the industry switched primarily to the use of a thin film composite (TFC) being placed on top of a stronger substrate. TFC membranes are primarily used today.

Reverse osmosis is a continuously operating treatment technology that uses pressure to pass source water through a thin membrane and thereby separate impurities from water.

RO works by reversing the principle of osmosis, the natural tendency of water with dissolved salts to flow through a membrane from lower to higher salt concentration. This process is found throughout nature. Plants use it to absorb water and nutrients from the soil. In humans and other animals, kidneys use osmosis to absorb water from blood.

In an RO system, pressure (usually from a pump) is used to overcome natural osmotic pressure, forcing feedwater with its load of dissolved salts and other impurities through a highly sophisticated semipermeable membrane that removes a high percentage of the impurities. The product of this process is highly purified water.

The rejected salts and impurities concentrate above the membrane and are passed from the system to drain or onto other processes. In a typical commercial industrial application, 75% of the feed water is purified. In applications in which water conservation is important, 85% of the feed water is purified.

An RO system uses cross-filtration, where the solution crosses the filter with two outlets: the filtered water goes one way and the contaminated water goes another way. To avoid buildup of contaminants, cross-flow filtration allows water to sweep away contaminant buildup and enough turbulence to keep the membrane surface clean.

The membrane assembly consists of a pressure vessel with a membrane that allows feed water to be pressed against it. The membrane must be strong enough to withstand whatever pressure is applied against it. Reverse-osmosis membranes are made in a variety of configurations, with the two most common configurations being spiral-wound and hollow-fiber.
Only a part of the saline feed water pumped into the membrane assembly passes through the membrane with the salt removed. The remaining "concentrate" flow passes along the saline side of the membrane to flush away the concentrated salt solution. The percentage of desalinated water produced versus the saline water feed flow is known as the "recovery ratio". This varies with the salinity of the feed water and the system design parameters: typically 20% for small seawater systems, 40% – 50% for larger seawater systems, and 80% – 85% for brackish water. The concentrate flow is at typically only 3 bar / 50 psi less than the feed pressure, and thus still carries much of the high-pressure pump input energy.
The desalinated water purity is a function of the feed water salinity, membrane selection and recovery ratio. To achieve higher purity a second pass can be added which generally requires re-pumping. Purity expressed as total dissolved solids typically varies from 100 to 400 parts per million (ppm or mg/litre)on a seawater feed. A level of 500 ppm is generally accepted as the upper limit for drinking water, while the US Food and Drug Administration classifies mineral water as water containing at least 250 ppm.

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