Water type and guidelines for RO system design

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ELSEVIER Desalination 139 (2001) 97-113

DESALINATION

www.elsevier.com/locate/desal

Water type and guidelines for RO system design

Samir EI-Manharawy ~', Azza Hafez b "Nuclear Geochemistry Department, Nuclear Materials Corp., Cairo, Egypt

Tel. +20 (2) 347-4822; Fax +20 (2) 345-2371; entail: [email protected] bEngineering Research Division, National Research Center, Cairo, Egypt

Received 15 January 2001; accepted 27 January 2001

Abstract

Water type is the cornerstone in reverse osmosis system designing. However, the systematic differentiation and identification of different natural waters are almost missing. In the RO industry all natural waters are usually grouped under two types: brackish ( 1000-15000 mg/l) and seawater (> 15,000 mg/l) as based on their total salinity. Only two RO membrane types are commercially available (i.e., B WRO and SWRO ), which are currently used for all waters regardless of the wide variation in their chemical nature, even those of similar salinity. Most of the recommended pretreatment methods given by the membrane manufacturers are normally based on a single water type, either brackish or seawater. In addition, the current use of the misleading saturation indices (LSI, S&DSI, etc.) that developed originally for heating boilers and exchangers, led to serious inorganic scaling problems. These generalized approaches have serious impact on the performance and economics of RO desalination. The cost of $2-3 per lm 3 of desalinated water is not surprising anymore, specially in the case of small-capacity plants. The present work concerns the critical importance of differentiation of natural waters in a systematic chemical classification that may provide assistance in RO system design, selection of membrane and proper pretreatment as well. From the experience gained in hydrochemistry and water analysis along the last 30 years, besides the extensive literature survey and the statistical analysis of more than 200 water analyses, the present authors were able to conclude a systematic water classification. The proposed classification is based on the earlier work of EbManharawy and Hafez as well as the real molar-concentration of the dissolved ion associations of the investigated water samples which covered a wide range of natural surface and ground water types. It was possible to identify four major water classes (<10, 150, 400 and >600 mM chloride ion), including 10 subclasses of different water types [<0.1, 0.2, 0.5, 1.0, 2.0, 5.0, 10, 15, 20, >20 (SO/riCO3) molar ratio]. These classes and types cover the whole spectrum fi'om salinity of -200 mg/1 up to -60,000 mg/l. More detailed subdivisions could be derived when necessary. In light of the obtained water molar classification, it was possible to set guidelines for inorganic scale prediction and the suitable chemical pretreatment for specifically each of the proposed water types. Technical recommendations for RO system design and membrane selection are also provided in detail.

Keywords: Water classification; Water type; RO system design; Molar ratio; Membrane fouling

*Corresponding author.

Presented at the European Conference on Desalination and the Environment: Water Shortage. Lemesos, Cyprus, 28-31 May 2001.

0011-9164/01/$- See front matter © 2001 Elsevier Science B.V. All fights reserved PII: s0011-9164(01)00298-3

98 S. EI-Manharawy, A. Hafez / Desalination 139 (2001) 97-113

1. Introduction

Water type is the cornerstone in reverse osmosis (RO) system designing. However, the systematic chemical identification and differentiation of different natural waters are almost missing. Therefore, the RO industry relied on a very simple approach - - that all natural waters could be grouped into two types: brackish (1000-15,000 mg/l) and seawater (> 15,000 mg/l) as based on their gross salinity. Only two RO membrane types are commercially available (i.e., BWRO and SWRO), which are currently used for all waters types regardless of the wide variation in their chemical nature, even those of similar salinity.

Due to the fact that reliable systematic water classification is actually absent, many serious impacts could affect the performance and economics of the RO system, such as: improper membrane type selection, under-design membrane configuration, and inadequate chemical pretreatment. As a result, many technical problems usually arise, especially inorganic scaling, which reduces productivity and shortens membrane life to a great extent.

It may be quite strange to mention that at the Red Sea area, where the salinity of groundwater could be higher than 50,000 mg/l, some RO plants used to "dilute" feed water with about 30% of the product water volume, this in order to overcome the severe sulfate scaling problem. The basic design of these plants was based on the selection of SWRO membranes with a recovery level around 35% and neglected the wide chemical differences between these existing cases and that of normal seawater type.

The pretreatment methods that are recommended by the membrane manufacturers are usually given in a general way, regardless of water chemical type and concentration. These described pretreatment methods are discussed individually, which may lead to serious confu- sion. For example, if a highly salty groundwater

contains the following ionic configuration m high concentration of Na, Ca, Mg, CI, SO 4, SiO2, PO4, Ba, Sr, Fe and Mn - - the collective list of chemicals and antiscalants that are necessary for each ion individually will be very long and contradictory, which definitely makes the situation more complicated. This is because of the provided chemical information which is usually based on the "ideal solutions" of pure salts rather than "true water mixture type".

On the other hand, the provided test conditions neglect the critical effect and inter- ference that may arise from the existence of other common and uncommon ion associations. For example, the SWRO membrane's testing conditions are based on a pure solution of 32,000 mg NaCI/I, while that of BWRO are given for a pure solution of 2000mg NaCI/I. One morel thing should be mentioned too, that the test recovery level is typically less than 10% for SWRO and <15% for BWRO membrane in most eases.

Such generalized approaches have a serious impact on the design, performance and economics of the projected RO system. The cost of $2-3 per 1 m 3 of desalinated water is not surprising anymore, specially in the cases of medium- to small-capacity plants which cannot afford a suitable chemical laboratory and professional chemists.

Systematic water classification will provide valuable assistance in categorizing the efforts and technologies in order to solve specific problems of anomalous water types as should be without mixing up all possibilities in front of the end- user. Also, this may persuade membrane inanufactures to develop additional specific RO membranes that could match with the require- ments of the different natural water types, or at least with the major water classes.

The aim of the present work is to develop a systematic water classification that could differentiate between the chemical nature of surface and ground waters, and in addition considers the relevant inorganic sealing potential

S. EI-Manharawy, A. Hafez / Desalination 139 (2001) 97-113 99

that could happen under the RO pressure dehydration process.

2. Background

There are several models currently used in the fields of hydrochemistry and geochemistry for the chemical differentiation among the different types of natural waters, either surface or underground. Such models (e.g., Piper Diagram, Stiff Diagram, Wilcox Diagram, and Durov Diagram) were based mainly on the graphical projection of major ions in milli-equivalent where the total cations and the total anions are set equal to 100%. The main purpose of these diagrams is to show the relative clustering of data points to indicate samples that have similar chemical compositions. In other words, the available hydrochemical diagrams are useful in correlation, comparison and illustration of similarities and/or differences among natural waters rather than water classification. More details about these models and their uses can be found in Piper [1], Hem [2], Freeze and Cherry [3], and Driscoll [4].

The Kansas Geological Survey [5] provided six classes of water types that are predominant in ground waters in sedimentary aquifers: • Class 1: Ca-HCO3 and Ca,Mg-HCO3 or

Mg,Ca-HCO3 • Class 2:Ca-SO4 and Ca,Mg-SO4 • Class 3: Ca-CI • Class 4:Na-HCO3 • Class 5: Na-SO 4 • Class 6: Na-CI type

This classification describes the major cation- anion abundance in a water type. However, these water classes (or ion associations) could be found for several salinities and ionic strengths.

In agriculture, there is a simple irrigation water classification [6] which depends on the suitability of water salinity for irrigation purposes and as expressed in electrical conductivity (EC× 106) as follows: Class 1 (001000), Class 2 (1000o3000) and Class 3 (>3000). Such classification neglects the variable chemical nature of the different water types.

The industrial use of water is usually classified according to its hardness content (Ca + Mg as expressed in mg/l CaCO3). One of the popular classifications is given by Twort et al. [7] who concluded the following classification: soft (0-50), moderately soft (50-- 100), slightly hard (1000150), moderately hard (1500200), hard (2000300), and very hard (>300 mg/1 CaCO3).

E1-Manharawy and Hafez [8] investigated the possible relationships among some natural waters and the actual scales formed on the RO membranes. They found that there is a gradual positive trend between the molar concentration of dissolved scale-forming ions and the chemical nature of the formed scales. The study had shown that the molar ratios could be helpful in differentiating natural water types. In addition, it was possible to use these molar ratios in the prediction of scale formation and its physical and chemical nature as well. Some useful molar ratios such as (SO4/HCO3), (SO4/HCO3)*CI, (HCOJCI), (SO4*SiO2), [CI/ (SO4*SIOz)],

Table 1 Prediction of scaling potential by using (SO4/HCO3) molar ratio [8]

Category no. (SO4/HCO3) (in mM) Chloride level (mg/l) SO4 scaling potential CO3 scaling potential

4 >15 >20,000 High Low 3 10-15 >10,000 Medium high Medium 2 1-10 > 3,000 Medium Medium high 1 <1 < 3,000 Low High

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[(Ca* SO4) / SiO2], [(Ca* SO4) / (CI*HCO3* SiO2) ] were successfully applied for the prediction of inorganic scaling in a number of 33 running RO plants with capacities lower than 500 m3/d.

Four categories (Table 1) were concluded for the prediction of scaling potential inside the RO membrane system, as based on the relationship between the molar ratio SO4/HCO3 (in mM) and the corresponding chloride concentration (in mg/l) of the estimated brine (concentrate) composition.

3. Basic approach

In the present work, the authors were deeply convinced that the semi-quantitative modeling that resulted from the last study could be used for more detailed differentiation and classification of different natural waters. In order to expand the investigation range, it was necessary to dig in the available water analysis records that accumulated in Geochemica Laboratories (Cairo) during the last 5 years. The data collection procedure considered the following conditions: • Coverage of different local natural water

facies as possible, including the Nile water, Mediterranean Sea water, Red Sea water, brackish and salty groundwater.

• Water analysis should include the following: TDS, Ca, Mg, Na, K, CI, CO3, HCO3, SO4, SiO2, PO4, Fe, Mn,, Ba and Sr.

• Cations/anions equivalent balance difference should be less than 4-2%.

• Sampling basic information (i.e., location, depth, pH, conductivity and temperature) had been recorded in the field.

• Analyses had been carried according to the standard methods that given by APHA, AWWA and WPC [9].

Carbon dioxide gas analyses were not measured in most cases, and even when deter- mined in a few numbers of samples, the necessary field sample preservation was not

followed properly. Therefore the available CO2 data were considered unreliable and rejected.

From more than 1000 water analyses, 211 samples were carefully evaluated and selected for the present study. Molar concentrations of all cations and anions were calculated in mM, and the total mM was obtained for each sample. Molar concentration of an ion is normally calculated as follows:

MolesJl i ter = (mgJl i te r + gWion) / 1000mg/gra

where MW is the molecular weight of ions. For practical data handling, all molar concen-

trations could be expressed in milli-mole (raM), and are simply calculated as follows:

[mMio n = mgion/liter + gWion]

The calculated molar concentrations (mM) and their corresponding molar ratios were subjected to further numerical processing by using Excel mathematical software and its computerized graphical modeling. The volume of the processed data and the obtained possibilities are too large to be included in the present text.

4. Results and discussion

The processed data indicated the following parameters, considering that all of the investigated samples were free of both of hydroxyl (OH) and carbonate (CO3) anions.

Total dissolved solids (TDS, rag/l) could not be used as a basis for chemical classification because of the wide variation of different ion content in the natural waters. For example, the percentage of chloride ion (in rag/l), as compared with its TDS, could be as low as -8% and high up to -26% in two samples of similar TDS. The same is also true for other major ions as shown in Table 2.

When the obtained ions content (in rag/l) were converted into their real molar concentration (in

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Table 2 Analytical results of some selected samples that show the wide variation of their ion association content (%) of similar water salinities (mg/l)

Sample no. TDS, mg/l CI, % HCO3, % SO4, % Ca, % Mg, % Na, % K, %

# 144 233 12.01 48.03 9.44 9.01 4.29 13.30 2.14

# 34 255 27.46 17.26 13.34 9.02 3.92 18.43 3.14

# 142 613 9.48 54.78 3.43 8.67 4.41 14.88 0.82 # 156 682 26.38 30.77 8.94 3.81 1.76 26.08 1.17 # 183 674 7.57 47.95 11.28 13.96 3.71 10.I0 0.74 # 94 3,673 45.60 4.93 13.12 4.60 3.21 26.90 0.41

# 98 14,487 55.19 0.96 7.01 1.76 3.23 30.84 0.70 # 118 45,186 55.80 0.27 7.20 1.84 3.69 29.89 1.22

# 01 58,365 49.75 0.30 13.90 0.97 3.35 30.66 0.94

Table 3 Molar concentrations (mM) of some selected samples as resorted in descending order

Sample no. CI HCO3 SO4 Ca Mg Na K Total

# 19 846.21 2.90 84.12 40.72 80.54 760.55 14.07 1830.03 # 113 795.91 2.46 40.85 11.40 59.15 722.79 15.96 1649.14 # 114 767.28 3.03 44.49 12.65 71.86 677.38 12.74 1590.00

# 27 726.54 4.87 32.04 13.92 56.44 644.98 10.18 1489.43 # 35 641.69 5.82 32.84 33.21 100.82 437.84 8.06 1260.69 # 77 449.73 6.85 24.00 8.71 45.04 390.47 8.47 935.08

# 40 39.55 6.47 12.07 6.31 4.73 48.37 0.36 118.49 # 206 25.59 7.42 19.81 14.67 9.09 25.32 0.77 103.31 # 49 11.26 8.80 1.44 3.77 3.33 10.31 0.26 39.78

# 06 3.81 12.70 0.58 1.95 0.90 13.79 0.18 34.52 # I0 2.28 7.39 0.86 2.62 1.60 4.52 0.20 20.03 # 192 1.13 2.03 0.09 0.72 0.41 1.26 0.15 5.92

# 82 0.85 1.93 0.56 0.55 0.49 2.13 0.15 6.80

mM) , the si tuation changed, and the general f r ame o f a sys temat ic classif icat ion becomes much clearer. Table 3 presents some examples o f different types and origins, as expressed in milli- Moles (mM), and sorted in descending order according to their chloride mola r concentrat ions.

It was clear that the chloride ion is controll ing the solubil i ty o f b icarbonate and sulfate ions to a

great extent. General remarks could be noticed as follows:

• High chloride waters are usual ly depleted in bicarbonate and enriched with sulfate.

• Modera te chloride waters contains some bicarbonate and relat ively high sulfate.

• L o w chloride waters contains relat ively high bicarbonate and low sulfate.


Table 4 The relationship bet'ween CI and (SO4/HCO3) molar ratio of some selected water samples of different salinities

Sample no. CI (SO4/HCO3) Sample no. CI (SO4/I-[CO3)

# 19 846.21 29.01 # 88 26.15 4.23 # O1 819.13 29.11 # 134 24.68 4.24 # 113 795.91 16.61 # 137 21.44 2.09 # 84 775.83 13.20 # 64 20.59 1.24 # 114 767.28 14.68 # 50 19.61 0.36 # 118 711.23 16.93 # 03 18.19 0.82 # 29 698.03 15.90 # 18 13.65 0.31 # 95 687.64 15.79 # 153 12.61 0.68 # 116 683.92 11.58 # 69 12.55 0.41 # 81 675.12 12.97 # 138 12.05 0.27 # 100 620.65 11.69 # 49 11.26 0.16 # 210 559.61 12.06 # 02 10.72 0.14 # 71 550.01 12.40 # 48 10.52 0.11 # 127 170.72 12.28 # 06 3.81 0.05 # 78 122.82 8.73 # 148 1.66 0.04 # 20 108.63 9.56 # 142 1.64 0.04 # 157 90.86 6.52 # 99 1.44 0.15 # 202 60.65 5.08 # 105 1.35 0.37 # 33 53.62 5.80 # 108 1.27 0.08 # 12 26.40 3.63 # 09 1.13 0.04

• Very low chloride waters contains low bicarbonate and low sulfate.

The relationship between chloride (mM) and the molar ratio (SO4/HCO3) is more indicative and could be sorted systematically as illustrated in Table 4.

Some discrepancies can be noticed, which might be due to the fact that the investigated natural waters are of different sources and of mixed origins (i.e., open system), which make the gain and/or loss of some ions are quite expected during the hydrologic cycle. This is in addition to the major problem of the uncertainty of alkalinity data, which need more discussion.

Atmospheric CO2 gas is highly soluble in water (@10"= 0.2318, @ 2 0 ° = 0 . 1 6 8 8 , @ 3 0 ° =

0.1257 and @40 ° = 0.0973g/100g at 760mm)

[10]. In natural waters, the CO 2 gas content is normally found in the <10 to more than 150 mg/l range; due to its acidity nature, it has a powerful neutralization effect on the alkalinity content (i.e., OH, CO3 and HCO3) in water. The dissolv- ing mode of CO2 gas is chemical in part rather than physical; this means that it reacts directly with alkalinity. In other words, atmospheric CO2 gas could change the chemical nature of any water in just a short time.

Therefore, the accurate determination of dissolved acidic COz gas in water is practically difficult, and sometimes impossible. The instan- taneous gain and/or loss of dissolved CO2 gas is very easy, even during sampling and transpor- tation. The necessary precautions and renormalization are not available in most cases. The theoretical estimation of dissolved CO2 gas,

S. EI-Manharawy, A. Hafez / Desalination 139 (2001) 97-113 103

in light of the pH value, is not convenient and may lead to erroneous results and predictions. Indeed, this fact represents a technical difficulty for any proposed classification due to the critical importance of alkalinity as a major component.

[n nature, most o f the surface and ground waters are missing their carbonate content due to the effect of atmospheric CO2 gas, and the rest of unreacted bicarbonate is predominating. Conse- quently, the given data of bicarbonate could be considered as "in-time equilibrium concentrations", which are definitely not the same as in the original waters at their original location.

This unavoidable situation must be considered when using of the present water classification, or any other classification. However, the expected deviation at the high-chloride waters will be minimized due to the depletion of natural bicarbonate under the effect of high chloride concentration.

5. Proposed water molar classification

From the gained experience in hydrogeochemistry and water analysis over the last 30 years, besides the extensive literature survey

Table 5 Classification of natural water types according to their molar concentrations and molar ratios

Class Type Chloride range (SO4/Alk.), mM Relative anion association

Class D: 10 600-1000 mM >20 Very high (~ 26,000 mg/l) [Sulfate end] Chloride 09 400-600 mM 15-20

(-21,000- -26,000 mg/l) Class C: 08 200-400 mM 10-15 High (-18,000- -21,000 mg/l) chloride 07 150-200 mM 5-10

(-7000- - 14,000 mg/l) Class B: 06 60-150 mM 2-5 Medium (-1800- ~7000 mg/l) chloride 05 10--60 mM 1-2

(-700- -1800 mg/l) Class A: 04 5-10 mM 0.5-1.0 Low (- 180- ~700 mg/l) Chloride 03 2.5-5.0 mM 0.2-0.5

(-90- - 180 mg/l) 02 1.5-2.5 mM 0.1-0.2

(-50--90 mg/l) 01 <1.5 mM < 0.1

(<50 rag/l) [Carbonate end]

Very low alkalinity/very high sulfate/very high chloride

Very low alkalinity/very high sulfate/very high chloride Very low alkalinity/high sulfate/high chloride Low alkalinity/medium sulfate/high chloride Low alkalinity/medium sulfate/ medium chloride Medium alkalinity/medium sulfate/ medium chloride Medium alkalinity/medium sulfate/ low chloride High alkalinity/low sulfate/low chloride High alkalinity/low sulfate/very low chloride High alkalinity/very low sulfate/very low chloride

Notes: milli-Mole (mM) = ion concentration (mg/l) + molecular weight (MW). {MW of chloride=35.45, OH = 17.01, CO 3 = 60.01, HCO 3 = 61.02 and SO 4 = 96.06) Alk. = alkalinity = OH + CO3 + HCO3 (in mM). (SO4/Alk.) = SO4 ion mM + alkalinity raM. The values in mg/l are provided for guidance only, and should not be used in calculations.

104 S. El-Manharawy, A. Hafez / Desalination 139 (2001) 97-113

and statistical data analysis of more than 200 water analyses, the present authors were able to devise a systematic water classification that is based on molar concentration and molar ratios as given in Table 5.

The proposed Water Molar Classification was based on the molar concentration of the chloride content (in raM) and the molar ratio of the dissolved sulfate/alkalinity (in mM) of the natural waters. In all the investigated cases, hydroxyl and carbonate ions were absent; therefore, the bicarbonate ion is the only alkalinity component that is considered here, i.e., (SO4HCO3). In case of the presence of OH and CO3 ions, the molar concentration of both should be added to the bicarbonate.

The present water classification consists of four major water classes and 10 water types: • Class A: Low chloride water (01,02,03,04) • Class B: Medium chloride water (05,06) • Class C: High chloride water (07,08) • Class D: Very high chloride water (09,10) Each of these water types can be further sub- divided when necessary. Several points could be observed in the proposed water classification:

1. The present water classification was based mainly on the molar concentration of the chloride ion content, which could be easily and accurately measured in the laboratory or even in the field. By means of chloride (in mM) only is it possible to locate the major water class to which it belongs.

2. In addition to chloride ions, sulfate and alkalinity (in mM) are both required to obtain the molar ratio (SO4/Alk.), and then locate the sub- classed water type.

3. The (SO4/Alk.) molar ratio could differentiate natural waters according to their actual and true contents of sulfate and alkalinity, and not as "expressed as CaCO3", which is chemically misleading.

4. The lowest (SO4/AIk.) molar ratio, i.e., Class A Type 01, represents the "carbonate end", while the highest ratio, i.e., Class D Type 10,

describes the "sulfate end" in the natural water range.

5. The chloride ion (in mM) is positively proportional with the SO4 ion (in mM) and reversibly with alkalinity (in mM) all over the range.

6. Equal amounts of sulfate and alkalinity could be normally found in a narrow chloride range (Type 04 and Type 05).

7. Alkalinity (usually as bicarbonate HCO3) has decreased markedly at a chloride level of -200raM (-7000mg CI/I) and at a chloride concentration of about 500 mM (- 18,000 mg CI/1) bicarbonate falls sharply and almost depleted.

8. Sulfate usually predominates at a chloride level higher than 400 mM (>14,000 mg C1/I), with no chance for the bicarbonate ion to exist in large amounts again. The effect of the dissolved chloride ion, as an uncommon ion, is mastering all ions association strictly.

9. At low chloride levels (<20 raM, <700 mg C1/1), carbonate and bicarbonate usually dominate over the sulfate ion. However, many anomalous cases have been currently recorded, and this could be attributed to the unavoidable effect of dissolved atmospheric CO2 gas on alkalinity as discussed before.

10. In some limited cases groundwater could originate from, or be mixed with, thermal fluids that traveled upward from great depths. Such waters are normally enriched with a lot of inorganic salts and minerals. Therefore, it is important to mention here that the present classification cannot cover thermal waters; it is only limited to natural meteoric waters.

6. Equivalent Water Salinity Classification

In a limited opinion survey on some people in the field, it was found that the proposed molar water classification is well accepted from qualified professionals and RO designers as well. However, it was clear that it could be difficult for others; therefore, it was a necessity to simplify

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Table 6 Classification of natural water types according to approximated salinity content

105

Class Type Approximate TDS range (mg/I) Proposed water type nomenclature

Class D: 10 38,000->60,000 Highly salty seawater Very high chloride 09 30,000-38,000 Seawater Class C: 08 15,000-30,000 Highly salty water High chloride 07 10,000-15,000 Medium salty water Class B: 06 4,000-10,000 Low salty water Medium chloride 05 1,200-4,000 Brackish water Class A: 04 800-1,200 Very fresh water Low chloride 03 500-800 Medium fresh water

02 200-500 Low fresh water 01 <200 Very low fresh water

Important note: This TDS water classification is proposed for general guidance only, and cannot replace the Molar Water Classification.

the proposed classification in terms of equivalent salinity (in mg/l), which is much more popular in practical daily uses. As discussed before, salinity (i.e., TDS mg/l) cannot describe the chemical nature and composition of a water type.

After several trials in order to combine both meanings in one, it was decided to keep the basic structure of the molar classification against the possible approximated equivalent TDS ranges in mg/l. The result is presented in Table 6.

7. Water type and RO scale prediction guidelines

First of all, when the RO scale problem is under discussion, the chemistry of concentrated water (or the brine) should be considered. Concentrated water results from the extraction of pure water (i.e., product permeate) from the saline feedwater, leaving more salt in the concentrate solution. The chemistry of concentrated water could be estimated from feed water analysis multiplied by the concentration factor (CF) as will be mentioned later.

EI-Manharawy and Hafez [8,11 ] proposed, for the first time, the use of molar ratios in RO scale

prediction. They discussed in detail the serious limitations of the currently used saturation indices [i.e., LSI (1936), RSI (1944) and S&DSI (1952)], which were specifically developed for the purpose of scale prediction from water under heating in thermal boilers and heat-exchanger equipment.

In light of the present Water Molar Classi- fication, it was possible to differentiate natural waters into specific chemical types. This will provide a useful tool to determine the possible scale type and scaling potential of a concentrated solution that comes in contact with the RO membrane surface as a result of pure water extraction, as presented in Table 7.

From field observation it has been found that carbonate scale potential is not linear as in the case of sulfate. The highest carbonate scaling potential lies at water type 05 and then decreases again towards the "carbonate end". This may be attributed to the limited effect of other anions (C1 and SO,) that are present in low concentration (i.e., Types 01, 02 and 03). At the upper range, the high concentration of chloride ion increases the solubility of carbonate and greatly limits its deposition.


Table 7 Guidelines of RO scale potential as based on Water Molar Classification

Class Type Chloride range (SO4/AIk.) mM Carbonate scale Sulfate scale potential potential

Class D: 10 600-1000 mM >20 Rare Extremely Very high (-26,000 mg/l) [sulfate end] high chloride 09 400--600 mM 15-20 Very low Very high

(~21,000- ~26,000 mg/l) Class C: 081 200-400 mM 10-15 Low Very high High chloride (-18,000- -21,000 mg/l)

07 150-200 mM 5-10 Medium High (-7,000--14,000 mg/l)

Class B: 06 60-150 mM 2-5 High Medium Medium (~1,800- ~7,000 mg/l) chloride 05 10-60 mM 1-2 Extremely high Medium

(-700- -1800 rag/l) Class A: 04 5-10 mM 0.5-1.0 High Low Low chloride (-180--700 mg/i)

03 2.5-5.0 mM 0.2-0.5 Medium Low (-90- -180 mg/l)

02 1.5-2.5 mM 0.1 - 0.2 Low Rare (-50- -90 mg/1)

01 <1.5 mM < 0.1 Rare Rare (<50 mg/l) [carbonate end]

Important notes: All figures above are strictly limited to ion molar concentrations of the concentrated solution after extraction of product fresh water. Molar chemical composition (in mM) of concentrated solution is estimated from the feedwater analysis multiplied with the concentration factor (CF), which is derived from the recovery (R), as follows:

m M ~ so4 = {mg/If~s04 + MWs04} * CF

where MW is the molecular weight of ion, CF = [1 + (l-R)], and R = (product rate, m3/h) + (feed rate, mJ/h)

The affinity of water type 05 for carbonate scaling under RO separation is remarkably high. Several recorded cases showed the full blockage of the RO elements with the fine white powder of CaCO3 had happened in less than 72 h from the start of the operation.

At the upper sulfate-end (i.e., highly salty seawater), it is quite normal to find big crystals ( -10x20mm) of hard solid gypsum (CaSO4. 21-120) inside the RO elements after a few weeks

of operation. The classical example of such high- sulfate water could be found in the highly salty groundwater at the Red Sea region where the TDS is in a range between 45,000 mg/l and 60,000 mg/l with a high (SO,/I-ICO3) molar ratio (>20).

The proposed water classification covers the sulfate and carbonate scales of Ca and Mg. However, the regular study of scale-formed samples generated from different water types

S. E1-Manharawy, A. Hafez / Desalination 139 (2001) 97-113 107

proved that Sr and Ba are always associated with calcium scales over the full range of proposed water t y p e s - in spite of the concentration of Sr and Ba which were quite below the saturation level in many cases. Such observation is in full agreement with the geochemical affinity concept of mineral formation theory [12]. There is no single reliable study that has found that both Sr and Ba could behave in another mode. Therefore, it is recommended grouping Sr and Ba in the same group as Ca, and no special prediction method is required.

Important note: As discussed earlier, natural waters could be subjected to gain and/or loss of some ions during their hydrological cycle; and as a result, some deviation from the proposed water molar classification would be expected. There- fore, when sulfate/carbonate scaling is under consideration, it is recommended relying on the (SO4/HCO3) molar ratio rather than chloride molar concentration.

The dissolved silica in natural water exists as monosilicic acid (H4SiO4). In this form, silica is generally unionized at the most natural pH level. Monosilicic acid attracts four additional water molecules beyond the two that make up part of the molecular structure in the hydrated state. Thus, the overall hydrated structure contains a total of six water molecules that probably play a significant role in its behavior in the RO process. It was observed that silica is usually detected in all of the investigated scales that formed inside the RO elements, in spite of its concentration in the feed water which is much lower than its theoretical saturation level (-120 mg/1 @ 25°C).

In addition, the deposition of heavy sulfate scale from the highly salty seawater in RO elements could not be explained by the solubility saturation theory. Moiler et al. [13] proved that the solubility of gypsum (CaSO4.2H~O) in brine waters greatly increases in the presence of high concentrations of NaCI and MgCI2, from about 0.015 M at an ion strength of 0.1 to about 0.45 M at IS 1.1. This may explain the anomalous high

804 content in normal seawater (-2700 mg/l) and in the salty seawater of the Red Sea (>4000 mg/l). If the solubility/saturation theory is applicable on the RO separation process, this will lead to minimizing the deposition of sulfate scale at the high chloride waters, which is not true.

Therefore, the present authors believe that the main controlling factor of the RO scale formation is closely related to the degree of "dehydration" of ions and molecules under the applied mechanical pressure. A hydrated sulfate ion contains four water molecules, the second after hydrated silica ion (=6 H20), and when subjected to the excessive extraction of pure water (i.e., recovered permeate), the "hydro-molecular stability" will be disturbed, and as a result deposition occurred.

Many field observations support this "dehydration concept". In several recorded cases where the sulfate scaling was very severe, it was possible to limit the sulfate scaling by just reducing the plant recovery level, for example from 35% to 26%, which is an actual case (TDS

54,000 mg/l) at the Red Sea area. So, it is quite safe to conclude: RO scale

potential is proportional to recovery. In other words, when the dehydration level increases, the scaling potential will be increased too, and vice versa.

8. Water type and RO membrane selection guidelines

There are not many options regarding this aspect. Two main types of membranes are commercially available, i.e., BWRO and SWRO. Only the limit of application between the two types may need some clarification.

Generally speaking, the selection of an RO membrane is critically dependent on two main factors, the first is the water-type, and the second is the desired recovery (i.e., productivity). Theoretically, the mechanical pressure (Pm~h) that requires reversing the natural osmotic


pressure (Pore) of a solution is about 0.76 bar for each 1000 mg/1 TDS. As the water flows through the membrane and salts are rejected, a boundary layer is formed near the membrane surface in which the salt concentration exceeds that in the bulk solution, and higher osmotic pressure is generated. This is known as the concentration polarization (CP). The effects of CP are: increasing osmotic pressure, reducing net driving pressure differential across the membrane, reducing product flow rate, and increasing scale potential. In order to compensate for these effects, higher pressure should be applied.

BWRO are highly sensitive to the effect of the CP at higher pressure, as it is normally operated at a pressure lower than 15 bar. In addition, there are no practical limits between the brackish and salt water type, for example; and from the commercial point of view, brackish water could be any water of salinity between 2000 and 10,000 mg/l, or may higher in many other cases. Such misleading advice had resulted in serious technical and economical implications for the RO industry.

Therefore, it is more reasonable to identify the term "brackish water" on the basis of its "concentrate TDS" at a given recovery level rather than that of feedwater concentration. Table 9 presents some given probabilities of brackish and salty waters at different corresponding recovery levels.

As shown, the concentration factor (CF) of the desalted solution could reach up to five times when 80% of pure water was extracted from the original feedwater. The CP will excessively increase the load; consequently, higher pressure will be required to overcome the resulting osmotic pressure, and so on. Most of the commercially available BWRO membranes cannot withstand such excessive pressure and will rapidly leak more salt into the produced permeate.

On the other hand, the present "water molar classification" is able to define the brackish water

as that which contains dissolved solids between 1200 and 4000 mg/l (Class B, Type 05).

From the field investigation and experience gained, it has been found that the commercial available BWRO membranes perform satis- factorily up to a concentrate TDS around 15,000rag/1. Over this boundary the BWRO membrane will run on the edge of fouling and scaling, and an SWRO membrane must be considered.

The related factor that affects membrane selection is the applied mechanical pressure. Theoretically, the net driving pressure required to overcome the effects of osmotic pressure (-0.76 bar/1000 ppm TDS) and associated CP that resulted from the dehydration of feed water (i.e., the concentrated brine) can be calculated as follows:

Pmech = (TDSf**d/1000) x CF x Poem x F~,~

where Pmeoh is the required mechanical pressure, TDSfeed is the feedwater concentration in mg/1, CF = the concentration factor = [ 1/(1 - Recovery)], Posm is osmotic pressure (-0.76 bar), and F~,~ is an empirical factor (-1.4) that covers reversal effects arising from CP, fouling resistance and other friction losses.

From field investigations it had been found that the reversal factor (F~,rs) ranges from 1.25 up to 1.50, depending on TDSb~e, membrane life, temperature, CP and fouling on the membrane surface. Generally, most of the investigated RO plants indicated that Fr~,~ was around 1.4 after 1 year of operation.

For example, the concentration of a brine resulting from 0.3 recovery of normal seawater (-35,000 mg/1, water type #09) is about 50,000mg/l, while at 0.5 recovery the brine concentration will be about 70,000 mg/l. In the first case the required mechanical pressure is about 53 bar, and 75 bar for the second. On the other hand, the highly salty seawater with a TDS of-55,000 mg/l (water-type # 10) will produce a

S. EI-Manharawy, A. Hafez / Desalination 139 (2001) 97-113

Table 9 The relationship between feedwater salinity and recovery level

109

Brine concentration (mg/1) at different recovery levels

Recovery level (R), % Concentration factor (CF) Feed TDS = 10,000 mg/l Feed TDS = 9,000 mg/l Feed TDS = 8,000 mg/l Feed TDS = 7,000 mg/l Feed TDS = 6,000 mg/l Feed TDS = 5,000 rng/l Feed TDS = 4,000 mg/l Feed TDS = 3,000 mg/! Feed TDS = 2,000 mg/l Feed TDS = 1,000 mg/l

80 70 60 50 5.00 3.33 2.50 2.00

14,000 ~: '::' "~:' 12,000

10,000 ~ , 13,320 -- 10,000 8,000 15,000 10,000 7,500 6,000 10,000 6,660 5,000 4,000 5,000 3,330 2,500 2,000

Note: The shaded figures are subjected to rapid scaling on the BWRO membrane.

brine of -79 ,000 at recovery 0.3, which needs a pressure of about 84 bar at least. At recovery 0.5 the estimated required mechanical pressure should be 117 bar in order to overcome the osmotic pressure, CP and possible fouling resistance.

Recently, there is a new trend to develop an ultra-high-pressure membrane type that can operate under a pressure of 100 bar or even more. I f this could happen, the desalination of highly salty seawater will be economically feasible.

9. Water type and RO pretreatment guidelines

In this respect it may be useful to list some basic chemical information gained from laboratory and fieldwork experiences and may provide assistance in selection of the proper water pretreatment method, not only in the RO field but also in any other industrial applications: • Removal of iron and manganese (<l.0g/l

total) is essential. • Softening (i.e., hardness removal) is highly

recommended in all cases and under any capacity.

• Carbonate solubility increases with acidity (pH <7) and decreases with basicity (pH >7).

• Sulfate solubility increases with basicity (pH >7) and decreases with acidity (pH <7).

• Silica solubility increases with basicity (pH >7) and decreases with acidity (pH <7).

• Iron solubility increases with acidity (pH <7) and decreases with basicity (pH >7).

• Hydrochloric acid reacts with Ca and Mg salts to form soluble chloride salt.

• Sulfuric acid reacts with Ca and Mg salts to form insoluble sulfate salt.

• Degasification is required when a large amount of CO2 gas is expected due to acidification.

• Solid carbonate residue could be easily removed by medium acidification.

• Solid sulfate residue is insoluble in diluted acids; it requires strong hot concentrated mineral acids mixture.

• Solid silica residue is insoluble in diluted alkalis; it requires hot concentrated NaOH or


hot concentrated hydrofluoric acid (HF). • EDTA-Na is a strong chelation agent that

combines with Ca, Mg and metals ions under different chemical conditions to form soluble salts. This is only true in the aqueous phase, but not in the solid residue phase.

• Iron solubility in diluted hydrochloric acid is much higher than its solubility in citric acid.

• Addition of acid/base chemicals together will result in other salt formations that may be deposited during the pretreatment process.

• The rate (or velocity) of chemical reactions varies widely; some are fast and others are quite slow. In all cases sufficient time and efficient mixing (i.e., homogenization) must be provided with great care.

• Chlorination is a biochemical reaction type. Bacteria cells require contact time of 40 min at least to be destroyed under a CI 2 concentration higher than 0.5 mg/l, regarding that efficient mixing is necessary.

• Dissolved chlorine is usually depleted to less than 0.5 mg/l in less than 3 h.

• Uncontrolled addition of sodium bisulfite may reduce and deposit dissolved silicates and phosphates.

• Inorganic phosphate antiscalant is superior over the polymer antiscalants. Either well disposal or sun drying ponds could avoid the possible environmental impact.

• Over-dosing of polymer antiscalants may reverse its effect (i.e., from dispersion to agglomeration again); the inversion point is around 5 mg/1.

• A well-designed mixing tank for proper chemical reaction is of vital importance for successful pretreatment in all cases and under any capacity. Direct injection in-line must be avoided.

Considering these useful chemical tips, good pretreatment is guaranteed. In addition, short notes about the recommended pretreatment of different water types follow:

1. The desalting of lower TDS water (Types 01, 2 and 3) are usually used for some industrial purposes. In spite of their high bicarbonate content, as compared with CI and SO4, the potential for scaling is quite low. Therefore, simple acidification with HC1 to pH-6.5 will be satisfactory.

2. Water Types 04, 05 and 06 are of great potential for carbonate scaling, but the presence of some considerable amount of sulfate ions may interfere and accelerate deposition. In many recorded cases aggressive acidification with HC1 to pH 4.5-5.5 was successful to some extent, and the RO membranes survived for about 2 years. In other cases, softening (resin or nanofiltration) of feed water provided the perfect solution, and the life of the RO membrane extended for more than 4 years.

3. From field observation and investigations, the available SWRO membranes could handle salty waters and normal seawater (Types 07, 8 and 9) without much difficulty, considering that recovery level is --40% for water type 07 and -30% for seawater. Higher recovery may lead to sulfate scaling in short time. Normal HC1 acidification to pH -6.0 with good phosphate antiscalants are satisfactory in most cases.

4. Water type 10 (highly salty seawater) is characterized by excessively high NaCI and anomalous high sulfate content. The addition of HCI acid will decrease the solubility of CaSO4 greatly. The available options are (a) softening, (b) alkalization instead of acidification, (c) lower recovery (-20%), (d) dilution with fresh water, (e) two-pass RO system, and (f) a combined RO/ thermal desalination system.

Highly salty seawater (Type 10) is a real challenge not only for RO membrane desalting but also for thermal distillation due to its excessive potential for sulfate scaling. This type of water usually originates either from the evaporation of normal seawater, as in the ease of the Arabian Gulf where TDS ranges between -45,000 to -48,000mg/l, or through geo-


enrichment with additional salts as currently happened in the Red Sea basin.

Red Sea water is characterized as high salinity water, which varies in concentration from -42,000 rag/1 at the north up to -45,000 mg/l at the mid-south. This is due to the strong effect of the active volcanic rifting tectonics that release huge amounts of hot brine and salt deposits into the middle bottom of the sea. On the other hand, the related underground water is generally of higher salinity too, which is due to the excessive leaching of the dominating salt formations (halite and anhydrite). The salinity of Red Sea groundwater could be higher than 65,000 mg/l at depths deeper than 150 m. At depths less than 60m, the salinity ranges between 45,000 and 55,000 mg/1.

The (SO4PrICO3) molar ratio of the Gulf Sea water is about 11, while the surface Red Sea water is about 14 and the related groundwater could be higher than 20. In comparison, the (SOJHCO3) molar ratio of the Mediterranean Sea water is about 3.5. This may explain the great difficulty of desalting Red Sea water and its associated groundwater as well.

It may be surprising to mention here that "dilution" of feed water with a part of fresh water is a current practice in many RO plants in the Red Sea area. About one-third of the produced fresh water is currently used for the dilution of the highly salty groundwater. The net recovery of fresh water is normally less than 20%.

On a limited laboratory scale, the present authors investigated the possibility of alkalization - - instead of acidification - - of highly salty seawater (Type-10) with a commercial sodium hydroxide solution to pH >9.0. The results indicated remarkable retarding in sulfate scale deposition, but on the other hand, heavy calcium carbonate sludge was deposited. It was found that the source of CaCO3 originated from the high carbonate content of the commercial NaOH solution, which reacted with the available calcium of the salty water. Some other experiments

were carried out by using commercial ammonia liquid for alkalization, and the preliminary result was encouraging indeed. This research is still under consideration, and valuable participation from other parties is welcomed.

Low-pressure nanofiltration (NF) membranes (<15 bar) are widely used for the removal of dissolved organics, bacteria and hardness ions from fresh and brackish water. Unlike conventional resin softening, which replaces Ca÷Mg with soluble sodium ion, NF separates Ca+Mg÷ HCO3+SO4 together with some NaC1.

The application of high-pressure NF (>40 bar) in hardness removal from highly salty seawater is still under techno-economic evaluation. Recently, Hassan et al. [14] published pilot test experi- mental data on hardness removal from 44,000 mg/l seawater with a NF membrane as a pretreatment step before the RO system. It was reported that a satisfactory rejection level was achieved (89% Ca, 97% Mg, 61% Na, 86% HCO3, 97% SO4 and 61% CI). Some doubts may arise about the feasibility of such configuration because of the possibility of transferring of the scale problem from the RO membrane to the nano membrane. In addition, it seems that high- pressure NF (-42 bar) is quite costly as compared with the conventional resin softening process.

In such high salinity seawater (>50,000 mg/l) the use of the two-pass RO system may provide a suitable solution. In this system highly salty feedwater passes first through the SWRO (#1) membrane, and the brackish product (2000- 3000mg/l) is pumped again into a BWRO (#2) membrane; then part of product #1 is mixed with product #2 to produce water of TDS less than 500 mg/l. The low concentrated brine of #2 is returned to the first feed water to dilute it.

10. Water type and RO chemical cleaning guidelines

Some short recommendations are given, which are based on field and laboratory

112 S. El-Manharawy, ,4. Hafez / Desalination 139 (2001) 97-I 13

experience gained in RO flushing and cleaning:

10.1. Water class .4: high carbonate water

• Flush daily with acidified water (pH -5). Do not use neutralized product water alone for flushing; only use acidified water. Sulfuric acid could be used in this class, but as a rule pure-grade HCI is the best option. In the presence of good-quality HCI, there is no need to use citric acid at all.

• Reduce the period of acidic cleaning cycle (pH -4) to every 6-8 weeks.

• When silica fouling is suspected, add 1-2% non-ionic detergent to the daily acidic flushing solution. Never use NaOH + EDTA solution for silica removal; it has practically no effect on the formed silica scale.

• Never use alkaline cleaning or alkaline anti- sealants.

• On flushing and cleaning cycles, increase flow rate, and do not permit cross-flow; just close the permeate valve to allow laminar (tangential) flow. Cross-flow will force foulant particles into the membrane film, and it will be damaged shortly.

• With severe fouling, it may be necessary to soak the membrane element overnight in a HC1 acidic solution (pH -3).

10. 2. Water class B: high carbonate~sulfate water

In this water class bicarbonate usually dominates, but some considerable amounts of sulfate are expected. Acidification with HC1 is a must in this case. The same scheme described above could be applied here, but due to the possible sulfate scaling it is recommended to reduce the period of the acidic cleaning cycle (pH 4-5) to be every 4 to 6 weeks.

10.3. Water class C." sulfate~bicarbonate water

In this water class sulfate usually dominates,

but some minor amounts of bicarbonate could be expected. Acidification with HCI is a must in this case. The same scheme described above could be applied here, but due to the possible sulfate scaling it is recommended to reduce the period of the acidic cleaning cycle (pH --6) to be every 4- 6 weeks.

10.4. Water class D: high sulfate water

In this water class sulfate highly dominates, and acidification will increase the rate of sulfate deposition greatly. • Flush daily with alkaline water (pH ~9.5) by

using pure NaOH salt. Sulfate solubility increases with alkalinity and decreases with acidity. Be sure about the purity of NaOH salt from carbonate residue, and prepare fresh solution when necessary.

• Reduce the period of alkaline cleaning to be every 3-4 weeks.

• For the alkaline cleaning cycle use only NaOH + EDTA solution recommended by the membrane manufacturer.

• When silica fouling is suspected, add 1-2% non-ionic detergent to the daily alkaline flushing solution.

• On flushing and cleaning cycles increase the flow rate, and do not permit cross-flow; just close the permeate valve to allow laminar (tangential) flow. Cross-flow will force foulant particles into the membrane film, and it will be damaged shortly.

• With severe fouling, there is no suitable method to redissolve the formed solid gypsum crystals; replacement of the RO element is the only available option.

Finally, the authors would like to draw atten- tion to the fact that RO technology has achieved remarkable progress during the last 30 years while "RO chemistry" is far behind. The RO industry is still relying on some inherited theories from thermal distillation science. The difference


is so obvious. Thermal distillation is a low- pressure evaporation system, while RO separation is a high-pressure, dehydration, physicochemical process. It is highly recommended to study, investigate and understand this missing branch of science.

11. Conclusions

The current study has shown that natural waters could be chemically differentiated and classified into four major chemical classes and ten water types as based on their ion molar concentrations and ratios. The proposed "Water Molar Classification" is useful in understanding the chemical nature of the investigated water type and its possible behavior under the pressure- dehydration process that characterizes desalination by RO membrane technology. Inorganic scaling potential, as well as scale chemical nature, could be easily predicted in light of the present classification. It was possible to determine guidelines for the RO system design, membrane selection, pretreatment and chemical cleaning methods that are suitable for a specific water type.

References

[1] A.M. Piper, American Geophysical Union Trans- actions, 25 (1944) 914.

[2] J.D. Hem, Study and interpretation of chemical characteristics of natural waters. US Geological Survey, paper # 1473, 1970.

[3] R.A. Freeze and J.A. Cherry, Groundwater, Prentice Hail, Englewood Cliffs, N J, 1979.

[4] F.G. Ddscoll, Groundwater and Wells, 2nd ed., Johnson Filtration Systems, Saint Paul, Minnesota, 1986.

[5] Kansas Geological Survey, Hydrogeochemistry ofthe Dakota Aquifer. Annual Report FY89, Open-File Report # 90-27, 1989.

[6] C.R. Glover, Irrigation water classification system, College of Agriculture, New Mexico State Univer- sity, Guide A-116, 1998.

[7] A.C. Twort, F.M. Law and F.W. Crowley, Water Supply Engineering, 3rd ed., Edward Arnold, London, 1985, p. 208.

[8] S. EI-Manharawy and A. Hafez, Desalination, 136 (2001) 243.

[9] American Public Health Association (APHA), American Water Works Association (AWWA), and Water Pollution Control (WPC), Standard Methods for the Examination of Water and Wastewater, 18th ed., New York, 1992.

[10] R.H. Perry and D. Green, Perry's Chemical Engi- neer's Handbook, 6th ed., Mc-Graw Hill, New York, 1984, pp. 3-98.

[11] S. EI-Manharawy and A. Hafez, Desalination, 131 (2000) 173.

[12] K.H. Wedepohi et ai., Handbook of Geochemistry, Springer-Verlag, Heidelberg, 1970.

[13] N. Moiler et ai., Chemical models for optimizing geothermal energy production, University of California at San Diego, US Department of Energy, Reservoir Technology Report # 16, 1996.

[14] A.M. Hassan, A.M. Farooque, A.T.M. Jamaiuddin, A.S. AI-Amoudi, M.A.K. AI-Sofi, A.F. AloRubaian, N.M. Kither, I.A.R. AI-Tisan and A. Rowaili, Desalination, 131 (2000) 157.

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Water type and guidelines for RO system design