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Two solutions made up of a solvent (water) and a solute (salt) with different concentrations have a tendency to equalize their concentrations when separated by a semi-permeable membrane. The water will tend to flow across the membrane to the more concentrated solution. This phenomenon is termed osmosis. If an external pressure is applied to the solution of higher concentration, the solvent will be separated from the solute and flow across the membrane via reverse osmosis.

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Spiral wound membrane elements are the primary configuration used today in water treatment applications. Two layers of the semi-permeable membrane are glued on three sides to create an envelope around a porous permeate collector fabric. The open end of several envelopes are glued to a perforated permeate tube in a configuration that allows permeate in the permeate collector fabric to flow into the tube.

These envelopes, with a layer of plastic netting between each to maintain the feed stream spacing, are wrapped around the tube in a spiral arrangement. The pressurized feed water flow enters at the end of the spiral and flows axially through the channels created by the plastic netting. Some of the feed water passes through the membrane while the majority of the dissolved solids are rejected, that is, not able to pass through. The purified water flows into the permeate collector fabric and flows along the spiral path into the permeate tube.

The concentration of dissolved solids increases as the feed flow progresses through the length of the element and exits the other end.

Multiple elements in series and/or parallel arrangements, called arrays, are utilized to meet the requirements of a particular system.

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The amount of membrane area employed for a given output determines a vitally important design element known as the flux rate. The flux rate is defined as the gallons per day of permeate which passes through one square foot of membrane area (GFD). A high flux rate translates to less membrane area and a lower capital cost. However, a higher flux rate results in increased fouling of the membrane and higher operating pressure. Hence, a higher cost of operation due to both energy and the maintenance associated with more frequent cleanings, and most likely shorter membrane life.

Flux (gfd) =

the daily permeate flow that passes through one square foot of membrane surface.

Flux (gfd) =	permeate output (gall/d)
	No. elements X element area (sf)

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The flow of the feed water stream across the membranes surface is called the cross flow. The lowest cross flow for each element occurs at the reject end of the element and the lowest cross flow for each series of elements occurs at the reject end of the last element. The cross flow design parameter is therefore the reject flow from the last element in each series. Higher reject flow provides higher cross flow.

Proper cross flow rates impart a shearing force and turbulence across the membrane surface aiding in the effort to reduce fouling and cleaning. Higher cross flow rates often result in a system having more pressure vessels, additional piping and a pump with a higher flowing rate, i.e. higher capital cost. The advantage, however, is reduced maintenance associated with less frequent cleanings and most likely improved membrane life.