Pre & Post Treatment for Reverse Osmosis Plants

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Pre & Post Treatment for Reverse Osmosis Plants

Posted On : Sep-14-2011 | seen (578) times | Article Word Count : 1335 |

SECTION 1.0 PRETREATMENT

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.

C- FOULANTS

C.1 SCALE FORMATION

Scaling occurs if one or more of the following soluble salts which are sparingly exist in the feed water deposits on membrane surface:
1) The scaling calcium carbonate mainly comes from the precipitation of calcium carbonate; it results from a concentration of calcium and alkalinity in feed water.
The common solution to this is the reduction of feed water pH by injection of acid to destroy the alkalinity of the feed water, the alkalinity is converted to CO2 thus eliminating one of the components required for precipitation of calcium carbonate.
2) The second scaling problem is calcium sulphate. Sodium hexametaphosphate is the solution as well as a number of other available anti-scalant materials. With the use of anti-scalant, a precipitate that may occur in a minute will now take hours. This keeps the membrane surface free of that type of scalant.
3) Silica is the third and most difficult material if precipitated. The potential for silica scale depends not only on amounts of silica in the feed water but also on the temperature and pH. Silica solubility is affected by pH. If pH is above 8.0 or below 7.0 silica precipitations will be avoided. Also, decreasing recovery ratio or increasing feed water temperature both make silica scaling less likely.

C.2 BIOLOGICAL SLIME FORMATION

It is common with surface water that micro-organisms have a tendency to grow on most surfaces that are immersed in waters which do not contain biocides. The amount of growth is dependent upon factors related to the water supply such as the type and degree of microbial contamination, nutrients present, temperature, and intensity of water flow.
When membrane surfaces are coated with biological growth, organics or bacteria, it results in the formation of slimes. Salts rejected by the membrane are trapped in the slime layer and are not readily swept away by the feed water. If scale forms within that slime, it will blind the membrane surface, requiring periodic membrane chemical cleaning with biocides and detergents.
Slime can be avoided and controlled easily by feeding 0.5 ppm chlorine in the feed water, however, most synthetic membranes of the polyamide type degrade fairly rapidly when exposed to chlorine. Periodic secondary treatment with sodium bisulfite (NaHSO3) will limit the extent of biological growth.

C.3 SUSPENDED SOLIDS

Membrane can be fouled by large particulate. This can also result in plugging of feed water passages as well as coating of membrane with amorphous precipitates. Pre-filtration with multi-media filter and five micron sediment cartridge filter will generally control this problem.

C.4 COLLOIDS

Colloids are particles which are usually less than 1 micron. They pass through standard filters and can not be readily removed by ordinary sedimentation process. They are often present in surface water and are usually removed by coagulation followed by filtration. In large systems, coagulants such as alum (Al2[SO4]3) or ferric chloride (FeCl3), polymer and polyelectrolyte can be used with flocculation, sedimentation and filtration configurations.

In some systems, in-line coagulation and filtration can be used before the RO. Generally, there are three (3) kinds of colloids which cause fouling. They are:
1. Aluminum silicate class (clays)
2. Iron colloids form corrosion of carbon steel piping, pumps, filters, etc.
3. Aluminum hydroxide if alum is used for clarification.

All the above colloids are hydrophillic and they are usually in the range of 0.1 to 0.3 micron size. These colloids are held in suspension in the feed solution because of their electrical charge. During RO, the above colloids can agglomerate and foul membrane elements. Measures of colloidal fouing potential include silt density index (SDI) and turbidity. The turbidity of an RO feed should be less than 1 NTU.

C.5 METAL OXIDES

Iron fouling results from iron oxidizing out of solution or from corrosion in the system and piping. Selection of material is essential to eliminate this type of fouling. Generally, total iron of more than .05 ppm in the RO feed will dictate a need to control or remove. Iron fouling shows up as a gelatinous substance. If iron fouling is discovered early enough, it can be chemically cleaned. Aluminum hydroxide may exist in feed water subsequent to using alum in pretreatment. Its presence may alter the strategy of injecting acid as a means of controlling scale.

C.6 OIL AND GREASE DEPOSITION

Hydrocarbons and silicon based oils and grease will coat membrane surfaces and decrease the performance of the membrane significantly. Limit for oil and grease in RO feed water is .1 ppm.

C.7 SUMMARY

General strategies to keep in mind to eliminate scaling:
1. Reduce the product water recovery ratio to avoid exceeding solubility limits in feed.
2. Soften the water by ion exchange to remove polyvalent metal ion.
3. Feed acid to reduce pH and bicarbonate/carbonate level.
4. Inject appropriate antiscalant.

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<h1>Pre & Post Treatment for Reverse Osmosis Plants</h1> Author: <a href='http://www.articleseen.com/profile_jamesarmstrong.aspx'>jamesarmstrong</a> SECTION 1.0 PRETREATMENT

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.

C- 	  FOULANTS

C.1 SCALE FORMATION

Scaling occurs if one or more of the following soluble salts which are sparingly exist in the feed water deposits on membrane surface:
1) 	The scaling calcium carbonate mainly comes from the precipitation of calcium carbonate; it results from a concentration of calcium and alkalinity in feed water.
The common solution to this is the reduction of feed water pH by injection of acid to destroy the alkalinity of the feed water, the alkalinity is converted to CO2 thus eliminating one of the components required for precipitation of calcium carbonate.
2)   The second scaling problem is calcium sulphate. Sodium hexametaphosphate is the solution as well as a number of other available anti-scalant materials. With the use of anti-scalant, a precipitate that may occur in a minute will now take hours. This keeps the membrane surface free of that type of scalant. 
3)   Silica is the third and most difficult material if precipitated. The potential for silica scale depends not only on amounts of silica in the feed water but also on the temperature and pH. Silica solubility is affected by pH. If pH is above 8.0 or below 7.0 silica precipitations will be avoided. Also, decreasing recovery ratio or increasing feed water temperature both make silica scaling less likely.

C.2 BIOLOGICAL SLIME FORMATION

It is common with surface water that micro-organisms have a tendency to grow on most surfaces that are immersed in waters which do not contain biocides. The amount of growth is dependent upon factors related to the water supply such as the type and degree of microbial contamination, nutrients present, temperature, and intensity of water flow.
When membrane surfaces are coated with biological growth, organics or bacteria, it results in the formation of slimes. Salts rejected by the membrane are trapped in the slime layer and are not readily swept away by the feed water. If scale forms within that slime, it will blind the membrane surface, requiring periodic membrane chemical cleaning with biocides and detergents.
Slime can be avoided and controlled easily by feeding 0.5 ppm chlorine in the feed water, however, most synthetic membranes of the polyamide type degrade fairly rapidly when exposed to chlorine. Periodic secondary treatment with sodium bisulfite (NaHSO3) will limit the extent of biological growth.

C.3 SUSPENDED SOLIDS

Membrane can be fouled by large particulate. This can also result in plugging of feed water passages as well as coating of membrane with amorphous precipitates. Pre-filtration with multi-media filter and five micron sediment cartridge filter will generally control this problem.

C.4 COLLOIDS

Colloids are particles which are usually less than 1 micron. They pass through standard filters and can not be readily removed by ordinary sedimentation process. They are often present in surface water and are usually removed by coagulation followed by filtration. In large systems, coagulants such as alum (Al2[SO4]3) or ferric chloride (FeCl3), polymer and polyelectrolyte can be used with flocculation, sedimentation and filtration configurations.

In some systems, in-line coagulation and filtration can be used before the RO. Generally, there are three (3) kinds of colloids which cause fouling. They are:
1.	Aluminum silicate class (clays)
2.	Iron colloids form corrosion of carbon steel piping, pumps, filters, etc.
3.	Aluminum hydroxide if alum is used for clarification.

All the above colloids are hydrophillic and they are usually in the range of 0.1 to 0.3 micron size. These colloids are held in suspension in the feed solution because of their electrical charge. During RO, the above colloids can agglomerate and foul membrane elements.  Measures of colloidal fouing potential include silt density index (SDI) and turbidity.  The turbidity of an RO feed should be less than 1 NTU.

C.5 METAL OXIDES

Iron fouling results from iron oxidizing out of solution or from corrosion in the system and piping. Selection of material is essential to eliminate this type of fouling.  Generally, total iron of more than .05 ppm in the RO feed will dictate a need to control or remove. Iron fouling shows up as a gelatinous substance. If iron fouling is discovered early enough, it can be chemically cleaned. Aluminum hydroxide may exist in feed water subsequent to using alum in pretreatment.  Its presence may alter the strategy of injecting acid as a means of controlling scale.

C.6 OIL AND GREASE DEPOSITION

Hydrocarbons and silicon based oils and grease will coat membrane surfaces and decrease the performance of the membrane significantly. Limit for oil and grease in RO feed water is .1 ppm.

C.7 SUMMARY

General strategies to keep in mind to eliminate scaling:
1. 	Reduce the product water recovery ratio to avoid exceeding solubility limits in feed.
2. 	Soften the water by ion exchange to remove polyvalent metal ion.
3. 	Feed acid to reduce pH and bicarbonate/carbonate level.
4. 	Inject appropriate antiscalant.

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