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Part 2: What Membrane Performance Issues Impact RO System Operation?

By Jane Kucera

BASICS CLEANING FOULING MEMBRANES OPERATIONS pH REVERSE OSMOSIS

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Abstract

The major issues facing reverse osmosis (RO) systems is loss of membrane performance, which can manifest itself as a decline in salt rejection; or a reduction in permeate flow, typically requiring an increase in operating pressure to maintain permeate flow, or both. The causes for loss of membrane performance (excluding mechanical equipment issues), include membrane fouling, scaling, and degradation

Membrane fouling involves the deposition of particulates (suspended solids), including bacteria, onto the membrane surface and onto the feed channel spacer. Usual foulants include colloidal silica, clays, metal oxides (e.g., iron, manganese, and aluminum), and hydrogen sulfide, as well as carry over of feedwater pretreatment media (e.g., carbon fines, and softener resin). Fouling can also be caused by organics in the feedwater, which coat the membrane surface, and by true color, which irreversibly adsorbs onto the membrane polymer.

Fouling typically occurs in the lead element(s) of an RO system, with the exception of biofouling, which can occur throughout an RO system. The lead elements act as particulate filters, removing the incoming suspended solids and organics from the feedwater. Membrane fouling results in loss of permeate flow that is usually compensated for by increasing the feed pressure. Fouling of the feed channel spacer, results in an increase in pressure drop through the RO system. Fouling, and particularly biofouling, can also lead to scaling of the membrane that increases the amount of salt passing through the membrane relative to the water flux, and lowers the permeate quality.

Membrane scaling generally occurs when the concentration of soluble ions reaches saturation within the membrane module. Scaling usually occurs in the last element(s) of an RO system, as this portion of the system has the highest concentration of dissolved solids in the feed/concentrate stream. Usual scale formers include calcium carbonate, calcium phosphate, calcium fluoride, barium sulfate, and reactive (soluble) silica. Membrane scaling results in loss of permeate flow that is usually compensated for by increasing the feed pressure. Scaling also increases the concentration at the membrane surface, such that the apparent salt passage increases and the permeate quality decreases.

Membrane degradation occurs when the membrane is exposed to incompatible conditions. It can involve damage to the polyamide thin film, the polysulfone microporous support, or the fabric backing. The most common method of polyamide damage is because of exposure to free chlorine or other oxidizers. Abrasion can also occur because of carbon fines and other solids that may get trapped between the feed channel spacer and the membrane. Damage to the polysulfone layer is usually from exposure to chemical solvents and hydrocarbons. The fabric layer is susceptible to hydrolysis at pH extremes. Degradation can occur all through an RO system, but, sometimes, can damage the leading elements to a greater extent.

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