Pre & Post Treatment for Reverse Osmosis Plants

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A- INTRODUCTION Over the past three decades reverse osmosis has become a prominent method of solids liquid separation. Successful long-term performance of reverse osmosis (RO) systems primarily depends on three factors: proper pretreatment; adequate system design; and attention to operation and maintenance.

All these factors are important, but proper pretreatment is the basic foundation on which the successful design and operation of the RO plant is built. If untreated, almost all RO feed water streams will adversely affect the performance of an RO plant. RO System is, in its simplest interpretation, a cross flow filter capable of filtering out dissolved solids as well as larger species such as organics, heavy metals and colloidal particles.

A cross flow filter is one in which the liquid being filtered continuously passes over the filter surface. The filtrate passes through the filter surface while the impurities traverse the filter surface, becoming more and more concentrated, and exiting the filter as a concentrated waste stream. Ideally, the impurities are carried away in the concentrated stream rather than accumulated in the filter.

To increase the efficiency and life of reverse osmosis system, effective pretreatment of the feed water is required, Selection of the proper pretreatment will maximize efficiency and membrane life by minimizing fouling, scaling, and membrane degradation.

The net result of the proper pretreatment is as optimization of the product flow, product recovery, and salt rejection, all of which can be directly translated to operating costs. The importance of adequate pretreatment for water reverse osmosis systems cannot be over stressed.

This importance is perhaps more essential than the pretreatment requirements for ion exchange. With a marginal pretreatment system, periodic cleaning will restore most of the productivity. If pretreatment is inadequate, cleaning will be less effective in restoring the RO performance, and need for cleaning will increase.

Frequent cleaning should not be regarded as a substitute for proper pretreatment. It is imperative that pretreatment be considered as an essential part of the reverse osmosis systems. B- MEMBRANE FOULING The purpose of pretreatment is to eliminate or minimize the fouling potential of the feed water.

Selection of the proper pretreatment scheme for the feed water will ultimately depend on the feed water source and composition.. Membrane fouling is a complex phenomenon which involves several related but different effects. All fouling involves either trapping some type of materials within the reverse osmosis device itself or chemical deposits on the surface of the membrane.

The causes, symptoms and cures are different. Fouling results in changes of the basic membrane parameters of salt passage, pressure drop across the membrane and productivity or a combination of changes in these parameters. For an unfouled system operating at constant conditions, salt passage and pressure drop will remain essentially unchanged with time.

An increase in salt passage and increase in pressure drop usually indicates fouling. Foulants can be classified by several general categories: scale caused by sparingly soluble salts, deposition of metal oxides, large particulate, colloids, organic compounds, biological activity, and oil and grease. reverse osmosis systems water reverse osmosis systems reverse osmosis Ro system reverse osmosis system

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