Dow Filmtec Sr90400 Membrane

The SR90-400 membrane is defined as a thin film composite membrane consisting of three layers: a polyester support web, a microporous polysulfone interlayer, and an ultra thin barrier layer on the top surface.

In the SR90-400 membrane the barrier has a negative surface charge which repels anions (negatively charged ions). Monovalent anion rejection (for example chloride ions) decreases with increasing salinity because the high concentration of positive cations starts to shield the negative surface charge at the membrane surface. Divalent ions (for example sulphate) have a high charge density such the shielding effect is negated and these ions are then preferentially rejected by the membrane barrier layer.

The membrane is constructed into a spiral wound element, and a system comprises of several spiral wound membrane elements installed inside a pressure vessel. Pressurised water flows into the vessel and through the channels between the spiral windings of the element. Elements are connected together in series within a pressure vessel. The feedwater becomes more and more concentrated and will enter the next element, and at last exits from the last element to the concentrate valve where the applied pressure will be released. The permeate of each element will be collected in the common permeate tube installed in the centre of each spiral wound element and flows to a permeate collecting pipe outside of the pressure vessel.

Feedwater Converted to Low Salt Content Water by Passage through Membrane \ Feedwater/Brine Channel Spacer

Product Water

Water Flow

Product Water Flow (After Passage through Membrane)

Feedwater Converted to Low Salt Content Water by Passage through Membrane \ Feedwater/Brine Channel Spacer

Water Flow

Product Water Flow (After Passage through Membrane)

Membranes

Permeate Channel Spacer

Membranes

Permeate Channel Spacer

Product Water

Product Water

Feed

Figure 1 Detail ofspiral wound thin film composite membrane module

Feed

Figure 1 Detail ofspiral wound thin film composite membrane module

In practice the membrane vessels are normally arranged in two stages with an approximate 2:1 staging ratio (ie twice the number of membrane vessels in stage 1 to stage 2). The concentrated reject water from the 1st stage membrane vessels is supplied as feedwater to the 2nd stage vessels where a further stage of treatment occurs. The low sulphate seawater (Issw) product water from the two stages is then collected to form the overall lssw product stream. With this system it is possible to increase the % recovery (or water conversion factor) which minimises the weight and size of the package.

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