ANTI-SCALING STUDIES ON HIGH CaCO3 WATERS IN SPIRAL-WRAP MEMBRANE SYSTEMS

I GOLDIE*, M AZIZ**, AH ABOZRIDA** and RD SANDERSON* *Department of Chemistry and Polymer Science, University of Stellenbosch, Stellenbosch

**Department of Chemical Engineering, Cape Town University of Technology, Cape Town

Report to the Water Research Commission by

The University of Stellenbosch

in association with The Cape Town University of Technology

on the project

DEVELOPMENT OF IMPROVED LOCAL ANTI-FOULING SPIRAL-WRAP MEMBRANE SYSTEMS

WRC Report No. 1593/1/08 ISBN 978-1-77005-763-0

OCTOBER 2008

DISCLAIMER

This report has been reviewed by the Water Research Commission (WRC) and approved for publication. Approval does not signify that the contents necessarily reflect the views

and policies of the WRC, nor does mention of trade names or commercial products constitute endorsement or recommendation for use.

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EXECUTIVE SUMMARY Background and motivation As the use of membrane technology becomes an increasingly attractive option for addressing the

water needs of South Africa, it is important to frequently review and evaluate the technology status

to identify developments and innovations that may be applicable to the local industry.

This project therefore aimed to demonstrate the economic viability of appropriate membrane

technology tailored to the broader local membrane market. Different types of membrane modules

exist, but the spiral-wrap element type is the workhorse in the membrane world. It is also the first

design that can be used for all four membrane classes, namely RO, NF, UF and MF. The spiral-

wrap membrane was therefore the preferred configuration choice for this study.

A main focus of this project was the incorporation of feasible anti-fouling or anti-scaling

technologies in existing membrane plant designs. To accommodate all these design variables,

different spiral-wrap designs needed to be evaluated in terms of their advantages and limitations.

Such an evaluation had to be carried out both in theory (literature and consultations) and on pilot

scale (where possible) to identify improvements for incorporation into local spiral-wrap membrane

systems.

A large market exists for proven defouling technology, as large sums are spent by large industries

on the replacement of fouled membranes

Objectives One of the original objectives specified the design a new spiral-wrap production plant for MF, UF,

NF and RO membranes, with the flexibility to incorporate feasible current and potential future anti-

fouling technologies. During the course of the project this focus was changed because the local

spiral-wrap membrane market was not deemed to be economically viable based on local use of

such membranes and the commodity-like nature of this type of membrane. The corresponding

objective was therefore changed to the development of spiral-membrane systems and

subsequently the design of a production plant for spiral-wrap membranes fell away.

Hence the revised research objectives were the following:

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1 Perform a literature search on the local and international status of spiral-wrap membrane

R&D (inclusive of anti-fouling characteristics, manufacturing procedures and R&D

innovations).

2 Identify local innovations, as well as suitable international developments, and incorporate

them into a more efficient new spiral-wrap system design.

3 Evaluate some of the above innovations experimentally and compare the results with those

obtained with conventional membrane systems (current membranes systems prior to

innovation).

4 Provide guidelines for the implementation of these innovations by the appropriate local

South African membrane sector.

Methodology

A literature study was conducted to identify novel improvements to spiral-wrap membrane systems

that could potentially be implemented locally, with potential economic benefit to the end-user. A

number of such improvements were identified, namely:

The use of higher or modified (pulsed or reversed) flow rates to prevent layer

concentration,

The introduction of sponge balls or magnetised powder beads to physically scour the

membrane surface clean, therefore eliminating the need for plant shutdown for cleaning

purposes,

Shock-treatment of the membrane to ‘loosen’ and remove foulants,

Electromagnetic pulsing over the membrane element (in desalination applications),

Conditioning of the feed water to limit scaling (by using metals or metal alloys).

Of the above options, novel anti-scaling treatment technology was identified as having reached the

stage in the R&D process where it could be investigated as part of this project.

This project therefore entailed the use of metal ions and/or magnetic fields to inhibit scale

formation. Although these treatment techniques have found application in other fields of water

treatment, the application in the field of desalination remains largely unproven. These techniques

were consequently evaluated and verified on laboratory and pilot plant scale. A flat test cell was

calibrated and used for the laboratory investigation, while a 500 L/h pilot plant was used for longer

trail runs. In both instances commercially available anti-scalants were used as reference, while

untreated membranes were used for comparison purposes.

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Results and conclusions The membrane market today is a mature, multi-billion dollar industry, with well-established

manufacturers and suppliers, and a diverse number of end-users. The water treatment sector is the

biggest user of membranes, and this sector in turn is dominated by the desalination market, where

mainly reverse osmosis membranes are used for brackish water and seawater desalination. The

spiral-wrap membrane design accounts for nearly all RO membranes in use for desalination,

making it the most important membrane type on the market. Locally, the application of membranes

for water treatment has become a given and, as is the case internationally, the need for

desalination to augment existing water supplies has become the topic of many recent investigations

and publications. Again, the spiral-wrap membrane design is (and will probably remain) the design

of choice for the main applications.

The establishment of a local spiral-wrap membrane manufacturing industry will not be economically

viable in the foreseeable future due to major market entry barriers, the (small) size of the local

industry and the capital intensity of such an investment. It follows therefore that the logical focus of

R&D should be on the improvement of (current) membrane systems and in particular spiral-wrap

membrane systems so that its application becomes more economical. The desalination of

groundwater is already being undertaken on a limited scale locally, but all indications from water

authorities are that this option will be increasingly exploited in future. Thus the application of spiral-

wrap membrane systems used in such desalination applications was therefore targeted for this

investigation.

In areas along the South African West Coast, scale formation during RO desalination can become

problematic. The dosing of expensive anti-scaling chemicals is required as a scale preventative

measure. This necessary practice can probably be optimised in terms of anti-scalant type and

dosage rate, but it will remain problematic for operators and plant management in remote rural

locations.

It was shown, on laboratory and pilot plant scale, that the use of specific concentrations of Zn ions

can inhibit scale formation in RO feed water by changing the scale crystal structure. A number of

factors, such as the presence of other ions, the pH and Langelier Saturation Index can determine

the efficiency of this anti-scale treatment. It is also CaCO3 specific. No evidence could be found that

it will inhibit the formation of other forms of scale.

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The generation of magnetic fields to prevent CaCO3 scale through the use of magnetic treatment

devices on the feed line to the RO membrane is a controversial science. On laboratory scale a

distinct improvement in the prevention of flux-limiting scale was found, but this was not observed on

pilot plant scale. Literature seems to support the theory that magnetic field treatment may be

effective under very specific conditions of feed water composition, magnet exposure time and

recovery.

Capacity development One project team member (SA) and three students (one enrolled for MTech (Libyan) and two

enrolled for BTech (one SA and one Libyan).

The MTech student was actively involved in the design and other engineering aspects, thus

enabling him to be able to play a leading role in the future implementation of the technology.

Recommendations for future research

Users, and potential users, of membrane systems should be made aware that use can be made of

alternate anti-scaling measures in cases where calcium carbonate scaling may occur (such as the

use of Zn ions). Advantages include much lower running cost, the availability of Zn metal,

environmental acceptance and low operator exposure risks. It is however important that prior to the

implementation of such measures the system should be properly tested up to at least the pilot plant

scale, for several weeks.

Future research on the use of metal ions as anti-scalant should focus on using different dosing

techniques, such as the electrolytic preparation of such ions at a predetermined concentration.

Further studies to determine the limits of effectiveness of metal ion dosing are also recommended,

in efforts to obtain a more accurate description of the types of feed waters that can be treated.

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ACKNOWLEDGEMENTS

The research in this report emanated from a project funded by the Water Research

Commission entitled:

“DEVELOPMENT OF IMPROVED LOCAL ANTI-FOULING

SPIRAL-WRAP MEMBRANE SYSTEMS”

The Steering Committee responsible for this project consisted of the following persons:

Dr G Offringa Water Research Commission (Chairman)

Dr I Goldie University of Stellenbosch (Project leader)

Prof RD Sanderson University of Stellenbosch

Mr M Aziz CPUT

Prof D McLachlan University of Stellenbosch

Mr J Clayton Project Assignments

Mr A Theunissen Chemtoll

Prof JJ Schoeman University of Pretoria

Mr GH du Plessis Sasol

The financing of the project by the Water Research Commission and the contribution of

the members of the Steering Committee is gratefully acknowledged.

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This project was only possible with the co-operation of several individuals and institutions.

The authors therefore wish to record their sincere thanks to the following persons:

Prof RD Sanderson University of Stellenbosch

Prof Ed Jacobs University of Stellenbosch

Mr Andrew Theunissen Ikusasa Chemicals (Pty) Ltd

Mr Stephanus Victor Ikusasa Chemicals (Pty) Ltd

Mr Willie Coetzee Cape Town University of Technology

Mr Nic Faasen West Coast District Municipality

Mr Ben van der Merwe West Coast District Municipality

Mr Gerhard Olwage Cape Agulhas Local Municipality

Dr M Hurndall (meeting secretary) University of Stellenbosch

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TABLE OF CONTENTS

CHAPTER 1: INTRODUCTION AND OBJECTIVES

1.1 INTRODUCTION 1 1.2 OBJECTIVES 2 CHAPTER 2: SPIRAL-WRAP MEMBRANES: BACKGROUND INFORMATION,

MARKET OVERVIEW AND RECENT IMPROVEMENTS 2.1 LITERATURE STUDY 3

2.1.1 Definitions 3 2.1.2 Types of membrane separation 3 2.1.3 Membrane geometries 4 2.1.4 Membrane materials and membrane symmetry 5 2.1.5 Membrane elements 6 2.1.6 Spiral-wrap membranes 6 2.1.7 Membrane systems 10 2.1.8 Cleaning and storage of membranes 11 2.1.9 Scaling and fouling 11

2.2 MARKET OVERVIEW 12

2.2.1 Membrane manufacturers 12 2.2.2 Membrane specifications 13 2.2.3 Description of the membrane market 13

2.3 RECENT IMPROVEMENTS IN SPIRAL-WRAP MEMBRANES 16 2.3.1 Technological developments 16 2.3.2 Innovative concepts 17

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CHAPTER 3: IMPROVEMENT OF ANTI-SCALING BEHAVIOUR OF SPIRAL-WRAP MEMBRANE SYSTEMS

3.1 IDENTIFICATION OF POTENTIAL METHODS OF IMPROVEMENT

3.1.1 Background 18 3.1.2 Research approach 18 3.1.3 Selection criteria 18

3.2 LABORATORY STUDIES 20 3.2.1 Background 20 3.2.2 Equipment 21 3.2.3 Operating procedure (operation of test cells) 22 3.2.4 Monitoring of variables (laboratory-scale) 23

3.3 PILOT PLANT STUDIES 24 3.3.1 Background 24 3.3.2 Plant requirements 25 3.3.3 Plant selection 25 3.3.4 Pilot plant acceptance 32

CHAPTER 4: ANTI-SCALING INVESTIGATIONS

4.1 REVIEW OF TECHNOLOGY 33 4.1.1 Background 33 4.1.2 Scale forming potential 35 4.1.3 Prevention of scale 36

4.2 ANTI-SCALING TREATMENT USING METAL IONS 38 4.2.1 Background 38 4.2.2 Laboratory investigations 39 4.2.3 Pilot plant investigations 41

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4.2.4 Discussion of results 45

4.3 ANTI-SCALING TREATMENT USING MAGNETIC FIELDS 46 4.3.1 Background 46 4.3.2 Laboratory investigations 47 4.3.3 Pilot plant investigations 48 4.3.4 Discussion of results 50

CHAPTER 5: CONCLUSIONS AND RECOMMENDATIONS

5.1 CONCLUSIONS 53 5.2 RECOMMENDATIONS 55

REFERENCES 56 Bibliography 59

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LIST OF FIGURES Figure 1: Water flow in a membrane system.

Figure 2: The filtration spectrum.

Figure 3: Direct and crossflow geometries in membrane applications.

Figure 4: Asymmetric membrane structure (in this case a TFC membrane).

Figure 5: Asymmetric polyamide membrane.

Figure 6: Simplified diagram of a single-stage membrane filtration plant with membrane

pressure vessel and two membrane elements.

Figure 7: Components of a spiral-wrap element.

Figure 8: Placement of more than one membrane element inside a pressure vessel.

Figure 9: Schematic representation of a two-stage RO plant.

Figure 10: The world membrane market in 2003.

Figure 11: Installed capacity of membranes worldwide (2005).

Figure 12: Groundwater quality map for South Africa (TDS).

Figure 13: Groundwater quality map for South Africa (hardness).

Figure 14: Osmonics test cell equipment used for laboratory evaluation of anti-scaling

measures.

Figure 15: Laboratory RO plant used for anti-scaling investigations.

Figure 16: Typical test cell calibration results

Figure 17: Measurements of variables taken during experimental investigations.

Figure 18: Simplified pilot plant design.

Figure 19: Illustration of the pilot plant used for trials.

Figure 20: Pilot plant testing at the University of Stellenbosch.

Figure 21: Rejection comparison between three different replacement membranes (tested

simultaneously).

Figure 22: Flux comparison between three different replacement membranes (tested

simultaneously)

Figure 23: Withoogte experimental setup: plant next to raw water feed tanks (left) and dosing

system used for synthetic feed water preparation (right).

Figure 24: Comparison of permeate flow results for pilot plant anti-scaling investigation carried

out at Withoogte (360 h).

Figure 25: Comparison of rejection values for pilot plant anti-scaling investigation carried out

at Withoogte (360 h).

Figure 26: Calcium–pH relationship in naturally occurring waters (SI = saturation index).

Figure 27: Concentration polarization in membranes.

Figure 28: Changes in flux after using different anti-scalant formulations.

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Figure 29: Percentage changes in flux after using different anti-scalant formulations.

Figure 30: Experimental setup used for Zn2+ dosing.

Figure 31: Flux comparison of two similar membranes before anti-scaling treatment.

Figure 32: Flux comparison of two similar membranes during Zn2+ dosing trials

Figure 33: SEM image of feed side of the membrane with no anti-scale treatment and with

Zn2+ as anti-scaling measure

Figure 34: Effect of MTD on flux in comparison with anti-scalants.

Figure 35: Test configuration used for magnetic treatment.

Figure 36: Comparison of flux values of two membranes over a 480-hour trial (one membrane

fitted with MTD).

Figure 37: SEM image of feed side of the membrane with no anti-scale treatment and with

MTD as anti-scaling measure

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LIST OF TABLES Table 1: Comparison of spiral-wrap membrane types

Table 2: Optimised operating parameters for the test cell used for evaluation of anti-scaling

measures

Table 3: Experimental parameters measured during laboratory investigations Table 4: Comparison of membrane elements used for commissioning of the pilot plant

Table 5: Comparison of Withoogte synthetic feed water with Bitterfontein feed water

Table 6: Normalised rejection and flux results of anti-scaling investigation carried out at

Withoogte (test time 300 h) Table 7: Characteristics of synthetic feedwater prepared for use in laboratory investigations

Table 8: RO performance results with Zn2+ on carbonate-scaling water

Table 9: Suiderstrand feed water analysis

Table 10: Flux changes (%) recorded in an anti-scaling investigation using Zn2+ at

Suiderstrand (compared to a reference membrane without any anti-scaling

treatment)

Table 11: Performance results from test runs on carbonate-scaling water

Table 12: Flux changes (%) recorded for a membrane treated with MTD at Suiderstrand (compared

to a reference membrane without any anti-scaling treatment)

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CHAPTER 1

INTRODUCTION AND OBJECTIVES

1.1 INTRODUCTION

As the use of membrane technology becomes an increasingly attractive option for addressing the

water needs of South Africa it is important to frequently review and evaluate the technology status

to identify developments and innovations that may be applicable to the local industry.

Municipalities are one of the important local water sectors. As sustainable and affordable household

water provision is one of their main responsibilities, there is an increasing interest in the application

of membrane technology to meet growing water needs. One such key area is desalination. Both

coastal and inland towns and cities can benefit from membrane desalination plants to overcome the

current and forecast shortfalls in supply.

This project therefore aimed to demonstrate the economic viability of selected membrane

technologies, to support the improvement of membrane systems for local use and, more

specifically, technologies that can make membrane desalination more cost efficient than they

currently are.

Overviews of the different types of available membrane modules are given, with the emphasis on

the spiral-wrap membranes. The spiral-wrap type is the workhorse in the desalination industry.

They are compact in design and plant investment and variable costs are relatively low when

compared to other types of membrane systems (e.g. tubular and hollow fibre). Spiral-wrap

membranes are also the only design that can be used for all four membrane classes, namely RO,

NF, UF and MF. The spiral-wrap membrane is therefore the preferred configuration choice for this

study. When choosing the specific design of a spiral-wrap membrane plant, a number of factors

need to be taken into account, such as application temperature limits, pressure limits, pH, feed flow

and feed viscosity.

Defouling and descaling are very important R&D areas in the membrane field. A large market exists

for proven defouling technology, as large amounts are spent on the replacement of fouled

membranes. An important focus of this project will be the flexibility of the membrane plant design to

allow the incorporation of feasible current and future anti-fouling technologies, such as infrasound

and nanomagnets, and the use of advanced chemicals. To accommodate all these design

variables, different spiral-wrap designs need to be evaluated in terms of their advantages and

2

limitations. Such an evaluation must be carried out both in theory (literature and consultations) and

experimentally, on pilot scale (where possible), in order to identify and create a final design for

incorporation into a local spiral-wrap membrane technology package, which will be the primary

research product of this study

1.2 OBJECTIVES

The original objectives of this research project were formulated as follows:

1. Perform a literature search on the local and international status of spiral-wrap membrane

R&D (inclusive of anti-fouling characteristics, manufacturing procedures and R&D

innovations)

2. Incorporate local innovations, as well as suitable international developments, into a more

efficient and novel spiral-wrap design.

3. Design a novel, pilot spiral-wrap production plant for MF, UF, NF and RO membranes, with

the flexibility to incorporate feasible current and potential future anti-fouling technologies

4. Evaluate a number of trial membrane modules and compare them with commercially

available membranes

5. Provide guidelines for the manufacturing of these improved spiral membranes for the South

African membrane industry

During the course of the project the initial focus on the improvement of membranes themselves was

changed because the local spiral-wrap membrane market was not deemed to be economically

viable based on local use of such membranes alone, while internationally no competitive advantage

could be found given the commodity-like nature of this type of membrane. The corresponding

objective was therefore changed to the development of spiral-membrane systems and

subsequently the design of a production plant for spiral-wrap membranes fell away.

Hence the revised research objectives were the following:

1 Perform a literature search on the local and international status of spiral-wrap membrane

R&D (inclusive of anti-fouling characteristics, manufacturing procedures and R&D

innovations).

2 Identify local innovations, as well as suitable international developments, and incorporate

them into a more efficient and novel spiral-wrap system design.

3 Evaluate some of the above innovations experimentally and compare the results with those

of conventional membrane systems (current membranes systems prior to innovation).

4 Provide guidelines for the implementation of these innovations by the appropriate local

South African membrane sector.

3

CHAPTER 2

SPIRAL-WRAP MEMBRANES: BACKGROUND INFORMATION, MARKET OVERVIEW AND RECENT IMPROVEMENTS

2.1 LITERATURE STUDY 2.1.1 Definitions

Membrane separation (or membrane filtration) can be described as the pressure- or

vacuum-driven process used for the separation of two aqueous phases by a synthetic

barrier that restricts the transport of various particulate and/or chemical species in a

specific manner. In membrane separation (also called membrane filtration), the volume of

raw water entering the membrane system is called the feed, the volume of water exiting the

system without passing through the membrane is called the concentrate (or brine or reject

or retentate), and the volume of water passing through the membrane is called the

permeate (or filtrate) (see Figure 1). The amount of permeate per time unit that passes

through a unit size of membrane area is called the flux, normally expressed in terms of

L/m2.h (Lmh).

Figure 1: Water flow in a membrane system.

2.1.2 Types of membrane separation

Membrane filtration can be classified into four classes, namely: microfiltration (MF),

ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO). Most of the applications of

4

membrane filtration are in the field of water treatment, where all four classes are employed.

The difference between these classes is best described according to which specific

substances can be removed. The appropriate choice of membrane is determined by the

specific application objective, and this is best illustrated by the filtration spectrum presented

in Figure 2.

Figure 2: The filtration spectrum.

Generally, as the pore size of the membrane decreases, more and/or smaller substances

can be removed from the feed, but higher operating pressures (normally implicating higher

operating costs) are required to achieve the separation.

2.1.3 Membrane geometries

Different membrane geometries exist for the different membrane applications. MF and UF

membranes normally have geometries allowing direct (or dead-end) flow, while NF and RO

systems nearly always employ crossflow (or tangential) geometries (Figure 3).

5

Figure 3: Direct and crossflow geometries in membrane applications.

2.1.4 Membrane materials and membrane symmetry

Different types of materials (mainly polymers) are used to manufacture membranes with

two types of symmetry, namely symmetric (isotropic) or asymmetric (anisotropic).

Symmetric membranes (Figure 4) are of uniform composition and the two sides of the

membrane have equal characteristics, while asymmetric membranes have a porous

structure that changes with depth, with the densest layer (on top) representing the actual

membrane. Most membranes used today have an asymmetric structure.

There are basically two types of asymmetric membranes, namely the older chemically

homogeneous phase-separation membranes, and the chemically heterogeneous thin-film

composite (TFC) membranes.

Figure 4: Asymmetric membrane structure (in this case a TFC membrane).

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2.1.5 Membrane elements

Membranes are housed in elements or modules in membrane systems. Different types of

crossflow elements are available such as tubular, capillary, hollow-fibre, plate-and-frame,

and the spiral-wrap elements. The design criteria for such elements include the ability to

contain a high membrane packing density (high surface area), reliability, ease of membrane

or membrane element replacement, fouling control, and affordability (Gravel, 2002).

2.1.6 Spiral-wrap membranes

A spiral-wrap membrane element (also called a spiral-wound membrane, or ‘spiral’) can be

described as a membrane separation unit in which permeate is produced by the

pressurised crossflow of the feed over an asymmetric organic polymer membrane.

2.1.6.1 History

The spiral-wrap design was commercialised in 1967, using a cellulose acetate (CA) reverse

osmosis membrane (invented in 1960 by Loeb and Sourirajan). The design had a number

of layers of membrane material, folded over to create envelopes (see par. 2.1.6.6, Figure

7). Each envelope incorporated a fine medium inside to facilitate the permeate flow, and a

coarse medium or mesh outside to facilitate the concentrate flow. The concentrate flow

direction was straight through the spiral-wound element, while the permeate flowed inward

until it collected in a product tube. Spiral-wound elements are housed in pressure vessels

to accommodate the pressurised feed flow.

2.1.6.2 Market

The international membrane market is dominated by the desalination market, which

accounts for more than 80% of all membranes sold. Today, about 12 500 desalination

plants worldwide provide more than 34 million m3 of fresh water daily, with RO making up

an ever-increasing percentage (59% of the total new build capacity). Spiral-wrap elements

account for nearly 99% of all RO/NF applications in this market, with increasing use thereof

in the UF/MF market (Freedonia Group, 2006).

2.1.6.3 Strengths and weaknesses

Spiral-wrap membrane elements offer a number of advantages compared to other

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membrane types: they provide a relatively large membrane area per unit volume, the

separation process is inherently self-cleaning as it allows for the continuous removal of

contaminants, is energy-efficient, it can be used over a large operating range for all classes

of membrane applications, and costs per membrane area are relatively low.

Important weaknesses associated with the use of spiral-wrap elements include: sensitivity

to fouling, feed channels that can become blocked quite easily, backflush cleaning

(especially for the higher-pressure applications) that has not been successfully developed

yet, and fluid dynamics that have proved to be difficult to model.

2.1.6.4 Membranes

Conventional spiral-wrap membranes are made up of asymmetric layers of organic polymer

(e.g. polyamide or cellulose acetate, see par. 2.1.4), with the active membrane layer

(typically 0.1–1.0 m thick) responsible for the separation process The separation

characteristics of the membrane are determined by the nature or pore size of this ‘skin’

layer, while the mass transport rate is determined mainly by its thickness. It is supported on

a porous woven, non-woven or spun-bonded layer, about 100–200 m thick, that adds

mechanical strength to the membrane but has little effect on its separation characteristics

(see Figure 5).

Figure 5: Asymmetric polyamide membrane.

The membrane properties determine the application limits of the membrane element, and

the choice of membrane is often determined by its chemical and physical properties (e.g.

pH range, chlorine resistance limits) rather than flux-related characteristics.

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2.1.6.5 Cellulose vs polyamide membranes

Cellulosed acetate (CA), cellulose triaceteate (CTA) and crosslinked aromatic polyamide

(PA) membranes are commonly used in spiral-wrap applications. C(T)A membranes are

considered as the industry workhorses, while PA membranes best provide modern

rejection and flux requirements. A generalised comparison of some of the relevant

properties of these membrane materials for use in RO systems is given in Table 1 below.

(Sulphonated polysulphone membranes are more resistant to oxidation than PA, but they

must be used on softened brackish water to maintain salt rejection capacity.)

Table 1: Comparison of spiral-wrap RO membrane types (Note: Figures obtained from suppliers, and are to be viewed as relative rather than absolute)

CA CTA PA

Cost Low High

Max chlorine tolerance (mg/l) 1 3 <0.1

Typical temperature range (oC) 5–30 5–35 5–45

Typical application pH range 2–8 4–9 2–11

Bacterial attack resistance Poor Fair / good Good

Iron allowed in feed (mg/l) 1 1 0.1

Relative productivity 1 1 2

Relative cost 1 2 3

General 85–92 92–96 94–98

Calcium 93–98 92–95

Sodium 92–98 90–95

Magnesium 93–98 94–97

Manganese 96–98 92–96

Iron 96–98 92–96

Chloride 92–95 90–95

Fluoride 92–95 85–90

Nitrate 30–50 40–60 70–90

Examples of nominal

ion rejections in RO

application (%)

Sulphate 96–98 96–98

2.1.6.6 Spiral-wrap membrane elements

In order to use membranes in a separation application they must be installed as a

membrane element (par. 2.1.5) as part of a membrane filtration plant. One or more such

elements are normally placed inside a membrane pressure vessel, which is a cylindrical

9

tube that can withstand the feed pressure required to effect the membrane separation (see

Figure 6 for simplified membrane filtration plant layout).

Figure 6: Simplified diagram of a single-stage membrane filtration plant with a membrane

pressure vessel and two membrane elements.

A typical standard length of a membrane element is 40 inches (1016 mm), with a diameter

of 4 or 8 inches. (The use of inches instead of metric units is common practice in the

membrane industry). Smaller or more compact systems with shorter elements are also

available. Commercial elements can contain from one to more than 30 membrane ‘leaves’,

depending on the element diameter and element type (see Figure 7). The open side of the

leaf is connected to and sealed against the perforated central part of the permeate tube,

which collects permeate from all leaves. The leaves are rolled up with a sheet of feed

spacer between each of them, which provides the channel for the feed and concentrate

flow.

Figure 7: Components of a spiral-wrap element.

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During operation, the feed solution enters the face of the element through the feed spacer

channels in an axial direction and exits on the opposite end as concentrate. A part of the

feed (typically 10–20%) permeates through the membrane into the leaves and enters the

perforated permeate tube. The permeate tube acts as the pipe to allow the product water to

exit from the element. O-rings seal the feed water from the permeate water at each end.

The design of the feed spacer is very important, as it promotes turbulence to reduce fouling

by keeping particles suspended, preventing them from accumulating on the membrane

surface. Spacers are normally made from extruded diamond-mesh polypropylene. Varying

the thickness of the spacer can extend the application range of the membrane (thicker

mesh spacers reduce the effective membrane area but also reduce fouling).

2.1.6.7 Membrane pressure vessels

A pressure vessel is a tube that can withstand the feed pressure required to operate the

element. A number of membrane elements can be loaded in a single pressure vessel. To

allow permeate to exit the pressure tube, the permeate tubes of each element in a pressure

tube must be connected together in such a way as to not allow feed water to enter the

permeate stream. Figure 8 illustrates the placement of membrane elements inside a

pressure vessel.

Figure 8: Placement of more than one membrane element inside a pressure vessel.

(http://www.dow.com/PublishedLiterature/).

2.1.7 Membrane systems

A membrane plant is a complete treatment system incorporating membrane elements,

pressure pumps(s), piping, and pre- and post-treatment infrastructure. In water treatment,

pre-treatment can either be conventional or another class of membrane treatment (e.g. UF

11

preceding RO), while post-treatment will normally include disinfection.

RO and NF systems usually operate in a series of one or more stages. A single-stage plant

(Figure 6) consists of a pressure vessel with a number of membrane elements connected in

series. In a multi-stage membrane plant the concentrate from the first stage serves as the

feed to the second; the concentrate from the second stage serves as the feed to the third,

etc. Consequently, each successive stage of the array increases the total system recovery.

A simplified flow diagram for a two-stage RO desalination plant is given in Figure 9.

Figure 9: Schematic representation of a two-stage RO plant.

2.1.8 Cleaning and storage of membranes

Provision must be made for periodic cleaning of membranes. Normally cleaning is required

when the normalised permeate output rate drops by about 15% from the flow rate

established during the first 24–48 hours of operation, or when the salinity of the product

water rises noticeably. Membranes can be cleaned by the use of chemicals under pH

controlled conditions, as they normally cannot be disassembled for cleaning. Other

cleaning methods include depressurisation, air/water flushing and frequent backwashing,

depending on the membrane class. Failure to adhere to proper cleaning procedures can

cause the membranes to become clogged, causing irreparable damage that necessitates

their replacement.

2.1.9 Scaling and fouling

During operation, the layer of feed water next to the membrane surface (the boundary

layer) becomes increasingly concentrated with dissolved and suspended materials. This is

caused by permeate removal through the membrane when these impurities are left behind

12

near the membrane surface. These concentrations reach a steady level, depending on the

feed velocity, element recovery and permeate flux, causing a drop in flux, and/or loss in salt

rejection in the case of NF and RO. All these conditions require frequent cleaning (par.

2.1.8), which is expensive and can shorten the operational life of the membrane element

(Letz, 1996). Two important build-up phenomena are distinguished, namely scaling and

fouling.

2.1.9.1 Scaling

Scaling occurs when the membranes block due to the increase in concentration of

dissolved salts (normally carbonate salts) in the boundary layer to such an extent that the

solubility limit is exceeded and precipitation of the salts occurs on the membrane surface.

This phenomenon is discussed in detail in Chapter 3 of this report.

2.1.9.2 Fouling

Fouling occurs when membranes block because of suspended particles or because of the

growth of microorganisms on the membrane (Butt et al., 1997). Fouling by silt or carbon

fines can be noticed by the formation of brown or black material on the ends of the

membrane element, and is characterised by high flow and very poor salt rejection in RO/NF

due to the abrasive effects of the fine particles on the membrane materials. Colloidal fouling

stems from particles (e.g. clay or silica) accumulating on the surface of the membrane and

is very difficult to remove by conventional cleaning chemicals and methods. Iron fouling can

be noticed by red colouring on the membrane element ends. It causes low permeate flow

and poor salt rejection. Biofouling results in low flux but salt rejection is usually not affected.

Careful plant design can limit fouling problems, especially through the correct choice of

membrane elements. Higher feed rates tend to reduce membrane fouling. Pretreatment

(e.g. coarse filtration/MF/UF/flotation) can be used to prevent fouling in desalination

membranes. Chemical defouling with speciality chemicals can be used routinely to clean

specific types of foulants (e.g. colloids) in specific types of membranes.

2.2 MARKET OVERVIEW

2.2.1 Membrane manufacturers

After about a decade of intensive business refocusing, mergers and acquisitions, there are

13

currently about 40 commercial membrane manufacturers worldwide. The leading suppliers

of membranes are (in order of 2003 market share): Pall (13,4%), Millipore (6.4%),

Hydranautics (3.4%), Ionics (3.1%), GE Osmonics (2.5%), Dow Chemical Company (2.5%),

CUNO (2.5%) and Koch (2.2%). Hydranautics, GE Osmonics, Dow Chemicals (FilmTec)

and Koch manufacture and sell spiral-wrap membrane elements.

2.2.2 Membrane specifications

Many spiral-wrap membrane elements are marketed either directly or indirectly by the

above manufacturers. Specifications, together with useful application information for these

elements, are freely supplied by the manufacturers or their agents. A specification for a

spiral-wrap membrane element available from a reputable supplier can contain information

such as the product name, membrane type, description (dimensional data), performance

values, and recommended use.

2.2.3 Description of the membrane market

2.2.3.1 Introduction

The establishment of the desalination membrane industry took place in parallel with the

development of membranes and membrane processes (Lonsdale, 1982). Today the

structure of this industry is quite heterogeneous as far as the size of the companies and

their approach towards the market is concerned. Some companies have concentrated on

the production of membranes only, while others are divisions of major chemical companies.

Companies offer the complete range of membrane types for different applications, ranging

from seawater desalination to purification of wastewater, and fuel cells. Some companies

buy membranes or elements as key components from one or several membrane

manufacturers and then design and build the actual plant; they often also operate it. Critical

success factors for companies involved in the application of membranes include credibility,

reliability, local presence and financial strength. Supplier loyalty normally acts as an entry

barrier to new entrants and, as a result, market participants tend to be well-established

companies with a wealth of experience in their relevant fields (Freedonia Group, 2006). In

addition to the sales of membranes and membrane elements, membrane manufacturers

also introduce specific application expertise into the market. A multitude of small

companies are active in market niches such as treating certain wastewater streams or

providing services to the chemical or food and drug industries.

14

2.2.3.2 Market size

The total world market for membranes has grown at about 8% over the past decade and

totalled about $5 billion in 2003. Figure 10 shows the world's demand for membrane

materials, by region, in 2003. Although the 'other world' segment accounted for only about

6% of total worldwide demand then, future growth rates in this area are projected to be

strong.

Figure 10: The world membrane market in 2003 (Freedonia Group, 2006).

The market for water treatment membranes is dominated by a relatively small number of

large companies (accounting for over 80% of all membranes sold to date). The biggest

volume share belongs to RO/NF, to which the desalination of seawater or brackish

groundwater contributes the most (see Figure 11). RO currently accounts for about 30% of

the world's installed desalination capacity, from large-volume seawater desalination plants

to small-volume household systems.

15

Figure 11: Installed capacity of membranes worldwide (2005) (Cooley et al., 2006).

2.2.3.3 South African market

The South African membrane market is of a capital investment plus

maintenance/consumables nature, which is more difficult to quantify than a normal

consumption based market. Membrane elements are normally supplied to local clients by

big multi-national companies, who may or may not have a local presence through an

agency (e.g. Dow, Osmonics, Koch). Companies that do have a local representation offer

membrane elements as a part of a much bigger product portfolio, i.e. no company sells only

membranes to the SA market. Many local manufacturers of membrane equipment also hold

agencies for international membrane suppliers, but even with a local agency, users of

membrane elements still import directly from overseas.

It is nearly impossible to quantify the size of the local membrane market by inspection of

import statistics, as there are a number of different and diverse import codes under which

membrane elements can be imported. The local market size is insignificantly small in

international terms, with no local manufacturer of spiral-wrap membranes. It is believed that

no local industry will currently be economically viable based on local use of such

membranes alone, given the commodity-like nature thereof.

16

2.3 RECENT IMPROVEMENTS IN SPIRAL-WRAP MEMBRANES

2.3.1 Technological developments

Huge R&D investments by membrane manufacturing companies over the past two decades

have seen a steady increase in the number of spiral-wrap membranes for specialised

applications available on the market. Research focus has been mainly on the development

of cheaper membranes with improved performance (Kurihara, 2004).

2.3.1.1 Improved membrane chemistry

Low-energy membrane elements require lower pressures in desalination applications,

resulting in lowering operating costs. These elements have large active membrane surface

areas, combined with an increased number of membrane ‘leaves’ to decrease the path

distance of the permeate to the permeate tube. Operating pressure requirements for these

membranes have dropped significantly over recent years, e.g. from 28 to 10 bar for

brackish water, and from 68 to 55 bar for seawater. High-rejection membranes, e.g. TFC

polyamide membranes, provide very good rejection at low pressures. High-flux

membranes, e.g. modern TFC membranes, offer more than double the productivity (i.e.

higher volume water production per unit of surface area) than earlier PA or CA membranes.

Modern UF spiral-wrap elements usually incorporate high-flux membranes. Low-fouling

membranes have emerged as a result of innovative TFC membrane chemistry

manipulation, resulting in new membranes with much smoother, fouling resistant surfaces.

(Traditional PA membranes have inherent surface roughness, which collects colloidal

matter from the feed, causing fouling.)

Progress is currently being made with modified polysulphone membranes (mPSF) to

increase salt rejection via formally induced charges on the membrane surface, but these

membranes do not reject salt to the same high degree as CA or TFC membranes, and they

still do not match their performance or cost. These membranes also require that the feed

must first be softened to remove all divalent ions.

2.3.1.2 Improved element design

Improved and patented membrane element design is another focus area in spiral-wrap

R&D (Nicolaisen, 2002). Specialised feed channel spacers have been developed, favouring

the use of the spiral-wrap design in an increasing number of applications. This innovation

17

made the use of spiral-wrap in UF en MF applications commercially viable. GE Osmonics

have developed a proprietary Full-Fit™ design that has eliminated the element cover and

brine seals, allowing higher feed flows. iLEC technology (Dow–FilmTec patented design)

achieves a direct, leak-tight connection between adjacent element permeate tubes,

reducing the number of sealing surfaces to a single, axially compressed O-ring.

2.3.2 Innovative concepts

With the major membrane manufacturers having consolidated and strengthened their

business focuses over the past few years, it has become increasingly difficult for new

manufacturers to enter this near-commodity market with their own versions of existing

membrane elements (high investment and market access barriers exist). A great amount of

research is however being conducted internationally and locally on the improvement of

existing spiral-wrap membrane systems, especially to reduce or prevent fouling on the

membrane layer. Examples of local research areas include (the first three examples below

are of projects in progress at Polymer Science at the University of Stellenbosch):

The use of higher or modified (pulsed or reversed) flow rates to prevent layer

concentration

The introduction of sponge balls or magnetised powder beads (such as polymeric

hematite beads with a permanent magnetic moment) or other particulate matter into the

feed to physically scour the membrane surface clean using magnetic fields, therefore

eliminating the need for plant shutdown for cleaning purposes

Shock-treatment of the membrane to ‘loosen’ and remove foulants, e.g. the use of

infrasound backpulsing, directly into the permeate space, to clean the membrane. (The

laboratory results to date show that flux values of over 80% of the clean water flux

value can be restored by this procedure. This has the advantage that a plant does not

have to be shut down and that there are no soaps to dispose of.)

Electromagnetic pulsing over the membrane element (in desalination applications)

Conditioning of the feed water to limit scaling (by using metals or metal alloys).

The most likely local business opportunity in the spiral-membrane field lies in the

commercialisation of an innovation that improves the performance or reduces the operating

cost of a membrane system that uses commercially available membranes. Due to the

widespread use of spiral-wrap membranes, such improvement(s) should probably be

sought for specific applications where this design is used.

18

CHAPTER 3

IMPROVEMENT OF ANTI-SCALING BEHAVIOUR OF SPIRAL-WRAP MEMBRANE SYSTEMS 3.1 IDENTIFICATION OF POTENTIAL METHODS OF IMPROVEMENT

3.1.1 Background

Potable water supply is under pressure worldwide, and South Africa is no exception.

Desalination has been identified as one of the technologies that can augment water supply

locally (du Plessis et al., 2006). In the South African context, desalination implies making

use of membrane technology, which almost certainly means the use of spiral-wrap

membranes.

Membranes and related processes perform satisfactorily in many modern applications, but

improved design concepts that provide better process control, which in turn can result in a

longer operational life or lower operating costs, are still highly desirable. All relevant

aspects of the feed/membrane interface are therefore actively studied. This has resulted in

innovative advances on both the molecular and engineering levels of the technology.

Defouling and descaling remain the main R&D focus areas in the membrane field.

3.1.2 Research approach

In Section 2 the commercial importance of the spiral-wrap design was shown. Given the

local membrane market environment described in par. 2.2.3.3, potentially profitable

investment opportunities seem to be limited to (1) manufacturing under licence from a

major manufacturer for a specific market sector, or (2) identification, development and

commercialisation of technically feasible innovations on the improvement of existing spiral-

wrap membrane systems, most likely in the field of membrane defouling and descaling.

Option (1) involves a strategic and commercial decision by industry, which is outside of the

scope of this project. Option (2) was consequently investigated further here.

3.1.3 Selection criteria

In order to determine which aspects of membrane desalination can be addressed within the

stated research objectives (par. 1.2) it was decided to investigate the running cost of

19

elements in an existing desalination plant. The Bitterfontein desalination plant of the West

Coast District Municipality was selected for this investigation, as this is one of the few

operational municipal desalination plants that have accurate operational data available. The

groundwater quality (see later in Table 5, Section 3) is also representative of large areas of

the West Coast, making the investigation relevant to future desalination projects in that

area. Figure 12 (Simonic, 2000) shows the South African groundwater quality map in terms

of TDS.

Figure 12: Groundwater quality map for South Africa (TDS).

An analysis of the cost factors show that typical chemical costs associated with chemical

anti-scale treatment to prevent scaling can amount to about R0.20/kL (2007 costs) of the

20

water desalination costs. It was consequently decided to focus on alternate anti-scale

techniques that could reduce this running cost component and potentially offer additional

advantages (see Chapter 4 for discussion on anti-scale treatment). Figure 13 (Simonic

2000) shows the South African groundwater quality map in terms of hardness, and also

where alternative anti-scale technologies may find application.

Figure 13: Groundwater quality map for South Africa (hardness).

3.2 LABORATORY STUDIES 3.2.1 Background

In order to predict, monitor and quantify the effectiveness of selected scale preventative

21

measures on brackish water desalination membranes, a laboratory study was undertaken

jointly by the Institute of Polymer Science (IPS) (University of Stellenbosch) and the

Chemical Engineering Faculty of CPUT (Cape Peninsula University of Technology, Cape

Town campus). This study was aimed at screening the identified anti-scaling measures

before proceeding to pilot plant trials to verify results on a larger scale.

3.2.2 Equipment

The laboratory-scale investigation was carried out using an Osmonics high-pressure test

cell shown in Figure 14. Figure 15 shows the complete experimental setup used for this

investigation.

Figure 14: Osmonics test cell equipment used for laboratory evaluation of anti-scaling

measures.

Figure 15: Laboratory RO plant used for anti-scaling investigations.

22

3.2.3 Operating procedure (operation of the test cell)

Standard experimental procedure as prescribed by the supplier of the test cell was used for

the operation of the cell. It was also used to calibrate the plant before and between

experiments. The test cell was calibrated with a standard 2000 mg/l NaCl feed solution and

a commercially available brackish water membrane (BW-30 from Dow–Filmtec) to verify its

performance parameters (rejection and flux) in accordance with supplier specifications. A

period of two hours was found to be sufficient for the membrane performance to be within

the specification values.

Table 2 shows optimised operating conditions used at the onset of each experiment. Figure

16 shows typical results from a calibration test run.

Table 2: Optimised operating parameters for the test cell used for evaluation of anti-scaling

measures

PARAMETER VALUE UNITS

Effective membrane area 0.01377 m2

Volume of feed tank 150 L

Membrane type Dow–Filmtec BW-30 TFC

Cell pressure 10–15 bar

Flow rate 1000 mL/min

Pump frequency 35 Hz

Calibration feed concentration 2000 mg/L NaCl

23

RO TEST CELL CALIBRATION 10 OCT 200715 bar, 2000 mg/L NaCl

94

94.5

95

95.5

96

96.5

97

97.5

98

98.5

99

99.5

0 20 40 60 80 100 120 140 160 180 200

TIME (min)

REJ

ECTI

ON

(%)

.

30

31

32

33

34

35

36

37

38

39

40

Q (L

mh)

Rejection flux Q

Figure 16: Typical test cell calibration results.

3.2.4 Monitoring of variables (laboratory scale)

A number of experimental variables were measured and recorded during each experiment.

Table 3 and Figure 17 present these parameters and the points of measurement.

Table 3: Experimental parameters measured during laboratory investigations Feed water

(preparation tank)

Feed water

(from RO feed tank) Permeate

pH

Conductivity

Alkalinity

Temperature

pH

Conductivity

Temperature

Feed pressure

Feed flow rate

Conductivity

Permeate flow rate

Calculated: TDS,

Langelier Saturation

Index (LSI)

Calculated: TDS Calculated:

Flux, rejection, TDS

24

Figure 17: Measurements of variables taken during experimental investigations.

On completion of each experiment, the feed side of the membranes were analysed by

scanning electron microscopy (SEM) at different magnifications to determine if any

changes on the surface could be observed.

3.3 PILOT PLANT STUDIES

3.3.1 Background

Pilot plant evaluation of membrane elements and systems are carried out to determine

whether a process configuration is feasible for a specific feed water source. In this case,

the aim was to evaluate the descaling influence of modifications made to the feed water on

the membrane. A suitable pilot plant should therefore have a compact design that allows for

the accurate monitoring and recording of membrane parameters during operation, and for

the ability to comparatively demonstrate the performances and the robustness of the

membranes under investigation. The functional requirements of this pilot plant are

25

described below

3.3.2 Plant requirements

The pilot plant design had to allow for the parallel evaluation of selected commercial

membrane elements under identical environmental and feed water quality conditions. This

meant that, ideally, pressure vessels had to have the option to be operated in parallel,

allowing for the simultaneous testing of different types (or modifications of) spiral elements

under conditions of similar feed pressures. Provision for a facility for additional pre-

treatment was also required. Figure 18 shows drawing of a simplified pilot plant design and

layout.

Figure 18: Simplified pilot plant design.

Feed pressure flexibility was required so that a wide range of membrane desalination

applications could be evaluated, preferably at between 5 and 70 bar. Accurate

measurement and logging of specified plant parameters (e.g. flow rates, temperatures,

electrical conductivity, and power consumption) were required.

3.3.3 Plant selection

A mobile pilot plant belonging to Ikusasa Chemicals (Figure 19), a local company involved

with water treatment, was made available for further evaluations based on the criteria set

out in the previous paragraph. This evaluation process entailed two stages: a first stage to

determine the plant’s conformance to the above criteria, followed by a trial phase to

determine experimental conditions for anti-scale investigations.

26

Figure 19: Illustration of the pilot plant used for trials.

3.3.3.1 Evaluation of pilot plant

Commissioning of the pilot plant was carried out from March to April 2007. Three brackish

water RO membranes from different suppliers (but with similar operational specifications)

were chosen to determine whether performance differences between them could be

measured and recorded. Some properties of these membranes, as provided by the

suppliers, are compared in Table 4 below.

Table 4: Comparison of membrane elements used for commissioning of the pilot plant

(from suppliers’ specifications)

MEMBRANE

Membrane

area

(m2)

Flux

(Lmh)

Typical salt

rejection

(%)

Used in RSA?

A 7.2 52.7 99.0 Yes

B 7.9 48.0 99.5 Yes

C 8.0 47.4 99.7 No

Testing was carried out by varying the feed pressure and feed water TDS. Sodium chloride

(NaCl) dissolved in dechlorinated municipal water was used as feed water source. The

plant was run in either (a) recirculation mode, i.e. both permeate and concentrate were fed

back into the feed tank (resulting in a constant feed water TDS), or (b) only the concentrate

was piped back into the feed reservoir (resulting in an increasing feed water TDS). No heat

exchanger was used in the feed tank or on the plant, thereby allowing the feed water

temperature to increase with time in order to be able to collect rejection and flux data over a

27

wider temperature range. Figure 20 shows the pilot plant testing unit used at the University

of Stellenbosch.

Figure 20: Pilot plant testing unit used at the University of Stellenbosch.

A number of desalination parameters were monitored during the trial runs, such as feed

and permeate flow rates and quality, operating pressures, as well as power consumption.

Rejection was calculated and compared to the manufacturers’ specifications. The rejection

of individual elements was determined by isolating the specific element and then measuring

that particular permeate flow. The graphs below summarise some of the results, from which

it can be seen that there are small differences between membrane performances.

28

AVERAGE REJECTION COMPARISON

98.20%

98.40%

98.60%

98.80%

99.00%

99.20%

99.40%

99.60%

MEMBRANE A MEMBRANE B MEMBRANE C

REJ

ECTI

ON

(%)

.

Figure 21: Average rejection comparison between three different replacement membranes

(tested simultaneously).

FLUX COMPARISON

25.0

26.0

27.0

28.0

29.0

30.0

31.0

32.0

33.0

34.0

1000 1500 2000 2500 3000 3500 4000 4500 5000

TDS

FLU

X (L

mh)

MEMBRANE A MEMBRANE B MEMBRANE C

Figure 22: Flux comparison between three different replacement membranes (tested

simultaneously at ambient temperatures).

29

3.3.3.2 Pilot plant evaluation of anti-scalants

Pilot plant trials to study the effect of commercially available anti-scalant materials on

membrane performance were carried at the West Coast District Municipality’s Withoogte

water treatment plant, near Moorreesburg. The experimental setup used is shown in Figure

23. Two organophosphate anti-scaling materials for the prevention of CaCO3 scale were

used: the one has been in use for many years at the Bitterfontein desalination plant (“P”),

while the other is a potentially more cost-effective substitute (“V”). Feed water was

synthetically prepared, based on the feedwater used for the Bitterfontein RO plant, but with

a higher scaling potential to accelerate scaling on the membranes (see composition of

water in Table 5). Three similar brackish water membrane elements were evaluated in

parallel: the first element received feed water without any anti-scalant treatment, the

second element was dosed with P according to existing Bitterfontein practices, and the

third element was dosed with V according to supplier recommendations.

Figure 23: Withoogte experimental setup: plant next to raw water feed tanks (left) and

dosing system used for synthetic feed water preparation (right).

30

Table 5: Comparison of Withoogte synthetic feed water with Bitterfontein feed water

Element Units Withoogte synthetic feed water

Bitterfontein feed water

Cl mg/L 2200 2032

Alkalinity mg/L 625 185 Hardness mg/L 760 1100

Ca hardness mg/L 400 432 Mg hardness mg/L 360 679

pH 6.91 6.40 Conductivity mS/m 654 700

pHs - 7.2 7.2 Turbidity NTU 1.19 0.35

HCO3 mg/L 381 113 Ca mg/L 160 173 Mg mg/L 86 63

TDS mg/L 4186 4400 SO4 mg/L 450 406 LSI - 0.26 –0.75

The results obtained after completion of 360 hours of operation are summarised in Table 6,

and Figures 24 and 25.

Table 6: Normalised* rejection and flux results of anti-scaling investigation carried out at

Withoogte (test time 300 h)

RESULTS MEMBRANE 1

(no anti-scalant)

MEMBRANE 2

(with anti-scalant P)

MEMBRANE 3

(with anti-scalant V)

Flux decrease

(%) 42 16 22

Rejection change

(%) 0.82 0.49 0.44

* using FTNORM® from Dow

31

NORMALISED PERMEATE FLOW WITHOOGTE

0.100

0.120

0.140

0.160

0.180

0.200

0.220

0.240

28-M

ay-2

007

07-J

un-2

007

17-J

un-2

007

27-J

un-2

007

07-J

ul-2

007

17-J

ul-2

007

27-J

ul-2

007

06-A

ug-2

007

16-A

ug-2

007

DATE

Per

mea

te F

low

(L/h

)

NONE P V Poly. (V) Poly. (P ) Poly. (NONE)

Figure 24: Comparison of permeate flow results for pilot plant anti-scaling investigation

carried out at Withoogte (360 h).

NORMALISED REJECTION WITHOOGTE RO STUDY

98.0%

98.5%

99.0%

99.5%

100.0%

28-M

ay-2

007

07-J

un-2

007

17-J

un-2

007

27-J

un-2

007

07-J

ul-2

007

17-J

ul-2

007

27-J

ul-2

007

06-A

ug-2

007

16-A

ug-2

007

DATE

REJ

ECTI

ON

(%)

NONE V P

Figure 25: Comparison of rejection values for pilot plant anti-scaling investigation carried

out at Withoogte (360 h).

32

All membranes showed a significant decrease in flux after 360 hours (more than the

industry standard of 10% drop, which would normally require operator intervention such as

cleaning). The onset of the sharp decline in flux (>10%) occurred nearly simultaneously for

all three membranes (after about 200 hours), whereafter the rate of flux decline started to

differ noticeably. The membrane without anti-scalant treatment showed the sharpest drop

in flux. A difference in flux between the two membranes with anti-scalants was noticed: the

P dosing resulted in a smaller decrease in element flux. As the feed water had a very high

scaling potential, optimisation of the anti-scalant dosing conditions could have a big

influence on the scaling rate. No significant changes in rejection were measured over the

test period

3.3.4 Pilot plant acceptance

The results presented in the preceding paragraphs showed that the pilot plant was indeed

suitable for performing comparative tests on spiral-wrap membranes of similar

characteristics, and that the available data capturing equipment was sensitive enough to

record performance differences between membranes. It was therefore decided to use this

pilot plant to investigate other anti-scaling measures.

33

CHAPTER 4

ANTI-SCALING INVESTIGATIONS

4.1 TECHNOLOGY REVIEW The motivation for the study into anti-scaling measures that can limit or eliminate the need

for anti-scaling materials in brackish water RO desalination was provided in Chapter 3. In

this chapter two potential alternatives to conventional anti-scaling materials are considered,

discussed and evaluated.

4.1.1 Background Scaling is often described in terms of hard water. Hard water is water that contains

dissolved minerals, like calcium, magnesium and silica. The relative amounts of the

different types of crystals are variable and can be altered by changing the balance of

minerals in the water, the temperature, and other factors. For example, if cubic CaCO3

crystals predominate then a hard scale will develop, but if the CaCO3 crystals are mainly

needle-shaped then either a soft scale forms or the crystals remain suspended in the water.

The most common scalants encountered are CaCO3 (calcium carbonate) and to a lesser

degree CaSO4 (calcium sulphate). Naturally occurring waters contain high concentrations

of soluble bicarbonate salts in dynamic equilibrium with carbonic acid (dissolved carbon

dioxide) and insoluble carbonate salts. Seawater, representing 98% of blended natural

waters on the surface of the earth, typically contains 410 mg/L Ca2+ ions and 143 mg/L

HCO3– ions. The carbonate scaling potential in groundwater is mainly due to CaCO3

crystallization, since it is the first carbonate scale to form under concentration or

basification. Figure 26 illustrates the Ca–pH relationship in equilibrium (Mons, 2006):

34

Figure 26: Calcium–pH relationship in naturally occurring waters (SI = saturation index)

(Mons, 2006).

Lee and Lueptow (2003) indentified two pathways for scale formation on and in

membranes: surface (heterogeneous) crystallization and bulk (homogeneous)

crystallization. In surface crystallization, flux decline results from the blockage of the

membrane surface by lateral growth of the scale deposit on the membrane. In bulk

crystallization, crystals form in the bulk solution sediment on the membrane surface,

leading to a flux decline. As concentration polarization increases (see next paragraph),

scale formation occurs more through surface crystallization. However, other factors affect

the crystallization process, such as pH, temperature, and the presence of other metal ions.

Although the spiral-wrap membrane design is efficient it is however very difficult to clean

due to the narrow feed channels and the mesh spacer, which can result in dead areas once

a degree of fouling has been established. Low-solubility salts may reach a supersaturation

level on the membrane surface, resulting in scale deposition on the membrane. This

deposition can cause a reduction in product water flow and changes in the salt rejection

characteristics of the membrane. This phenomenon at the membrane surface is due to a

concentration gradient that occurs at the separating surface as product water continuously

passes through the membrane leaf, leaving behind an ever-increasing level of dissolved

and suspended solids, creating a boundary layer – an effect known as concentration

polarization (CP) (see Figure 27).

35

Figure 27: Concentration polarization in membranes.

(http://www.yale.edu/env/elimelech/Conc-Polarization/sld005.htm)

Scale formation can reduce the effectiveness of the spacing material separating the

membrane envelopes and cause a reduction in the flow turbulence. This will result in an

increase in the CP at the membrane surface. Supersaturation is the main driving force for

scale formation, and this can be considerably augmented by the superimposed effect of

CP. Theoretical models exist that allow for the prediction of local solute concentration

polarization, permeate water flux, filter geometry, and membrane properties (Kim and

Hoek, 2005).

4.1.2 Scale forming potential

The Langelier Saturation Index (LSI) is a calculated number used to predict the calcium

carbonate stability of water; that is, whether water will precipitate, dissolve, or be in

equilibrium with calcium carbonate. The LSI and Stiff and Davis stability index (S&DSI) are

used by some RO membrane manufacturers to guide the use of feedwater treatment

chemicals (anti-scalants), where:

LSI < 0 [scale will not form: < – 0.5 equals corrosive water]

LSI = 0 [solution at equilibrium]

LSI > 0 [scale may form; > + 0.5 equals high scaling potential]

and

LSI = pH – pHs [pH is the actual value, and pHs is the saturation pH of

the feed]

= (pK2 – pKsp) + pCa2+ + pTA

36

[K2 is the second ionization constant of carbonic acid,

Ksp is the apparent solubility product constant of

CaCO3, and TA is the total alkalinity as the molar

concentration of HCO3- in the brine].

The above index loses its accuracy at TDS values above 4000 mg/l (Marwan et al., 1995).

At higher values the S&DSI can be used to calculate the scaling potential, as follows:

S&DSI = pH – pCa – pTA – K

where:

pCa is the negative log of the calcium concentration (expressed as molarity);

pTA is the negative log of the total alkalinity concentration (expressed as molarity);

K is a constant whose value depends on the water temperature and ionic strength.

Many more equations and programs for the estimation of scaling potential in RO systems

exist (Al-Shammiri et al., 2005).

4.1.3 Prevention of scale

A certain degree of scaling control can usually be achieved by optimization of operating

conditions, with specific reference to factors exerting an influence on CP (Saad, 2004). The

use of anti-scalants has however become standard industry practise. An ideal anti-scalant

for RO membranes should offer the following features (Ghafour, 2002):

(1) It should be compatible with, and should not foul, the RO membranes.

(2) It should be approved by institutional agencies and drinking water authorities for

the production of potable water.

(3) It should provide effective control of calcium carbonate up to high LSI values.

(4) It should provide effective control of calcium sulphate, barium sulphate, strontium

sulphate and calcium fluoride.

(5) It should preferably effectively control silica.

(6) It should exhibit good tolerance to aluminium, iron and manganese oxides.

(7) Its use should result in the maintenance of membrane surfaces by dispersing

particulate foulants.

(8) It should be effective in feed water over a wide pH range.

(9) It should exhibit high stability in the feed water over a wide temperature range.

(10) It should have no adverse effects over extended periods of use.

37

(11) It should be effective for bio-growth control, within its formulation, to protect against

biological fouling.

(12) It should be compatible with coagulants and/or coagulant aids (polyelectrolytes)

used in the pre-treatment stage as incompatible materials may cause membrane

fouling.

Alkaline scaling (e.g. calcium carbonate) is controllable by maintaining the pH of the feed

water below 7.5. For example, sulphuric acid, which is freely available and inexpensive,

can be used to control the LSI. Low pH, however, can cause system corrosion, which in

turn can be an additional source of fouling. When sulphuric acid (typically 93% strength) is

used for pH control the potential for calcium sulphate scale is created. Furthermore, when

natural waters are heavily buffered, large amounts of acid will be required to reduce the pH

through conversion of bicarbonate ions to carbonic acid and CO2. (Acid injection is required

when CA membranes are used.) Normally the use of an anti-scalant will provide a higher

recovery than use of acid alone. Acid addition may not be very cost effective due to the

capital costs of the accompanying infrastructure.

Anti-scalants with broad activity spectra are available today and, when properly chosen,

can efficiently, and simultaneously, control calcium carbonate, calcium sulphate, strontium

sulphate, barium sulphate and calcium fluoride scales, as well as inorganic foulants

resulting from iron, aluminium and reactive silica present in any given water or wastewater

at very low dosages, typically below 10 mg/L (Ning and Netwig, 2002). The use of an

effective anti-scalant will inhibit scale formation in feed water with LSI values of up to +2.6

(Van der Hoek et al., 2000). The ideal scale inhibitor should be able to control deposits due

to both calcium carbonate at high pH and calcium carbonate at low pH (and temperature).

Care must be taken however as some biodegradable anti-scalants will enhance biofouling

due to the large surface area available, even if the anti-scalant solution itself is not exposed

to microbiological contamination. This effect can manifest itself in an increase in the mass

transfer coefficient (MTC).

The industry standard for scale control treatment is sodium hexametaphosphate (SHMP). It

acts by inhibiting the growth of CaCO3 seed crystals. A study by Butt et al. (1997) revealed

that traditional H2SO4/SHMP treatment was superior in terms of water quality and water

output when compared to the use of modern advanced inhibitors (on an equivalent cost

basis). Its limitations include poor solubility of SHMP in the plant, and its hydrolysis to

sludge, forming orthophosphate. It can also act as a stimulant for microbial growth (Ning

and Netwig, 2002). Other anti-scalants include sodium tripolyphosphate (STPP) and

38

sodium polyacrylate (POA) (Al-Rammah, 2002).

Significant advances have recently been made in the development and use of organic anti-

scalants. Advanced inhibitors are now available that are non-corrosive, have a long shelf

life, and are claimed to be technically and economically competitive with H2SO4/SHMP

treatment. Different anti-scalants are selected on the basis of the simultaneous control of

calcium, strontium and barium sulphate, calcium fluoride, reactive silica, iron, aluminium,

and heavy metal scales, where acidification of feedwater is not necessary. Organo-

phosphonate-based anti-scalants do not result in biofouling. A scale inhibitor, polyamino

polyether methylenephosphonate (PAPEMP), described by Gill (1999), is claimed to be

able to overcome all the shortfalls of the technology currently employed for controlling

scale/deposit in water desalination operations.

Softening as pre-treatment replaces calcium and magnesium (scale-forming ions) by

sodium (a non-scaling ion). Suitable calcium alkalinity (100 mg/L CaCO3) can be added to

the product water before use.

Two other methods of scale prevention also exist, namely the use of trace amounts of

metal ions in the feed and the use of external magnetic fields (Baker et al., 1997; Lisitsin et

al., 2005). These two options are discussed in detail below.

4.2 ANTI-SCALING TREATMENT USING METAL IONS

4.2.1 Background

Anti-scalants are surface active materials that interfere with precipitation reactions in three

primary ways: threshold inhibition (supersaturated solutions), crystal modification (formation

of soft non-adherent scale) and dispersion (ionic charge separation) (Tlili et al., 2003).

Other studies have indicated that metallic ion impurities, notably Zn2+ ions, can significantly

hinder CaCO3 precipitation from hard waters, and alter crystal morphology (Smith et al.,

2003). Recent studies demonstrate a substantial potential for suppressing scale formation

in hard water heating systems by trace concentrations of Zn ions (Lisitsin et al., 2005). This

causes a substantial increase in induction time and induces the formation of calcium

carbonate in the aragonite rather than the calcite form (Smith et al, 2003). The formation of

aragonite rather than calcite occurs when the Zn/Ca concentration ratio is greater than 0.06

x 10-3. The presence of Zn2+ ions thus induces scale suppression effects substantially

similar to those of organic anti-scalants. It offers clear advantages, namely ease of dosage

39

(e.g. as Cu–Zn alloy), environmental friendliness, and the product water meets drinking

water criteria. Initial experimental results indicate that trace amounts of Zn can induce a

marked beneficial scale suppression effect within a certain range of water compositions.

4.2.2 Laboratory investigations Synthetic feed water with a high scaling potential was prepared according to the procedure

used by Lisitsin et al. (2005). Table 7 tabulates the feed water characteristics. A new

brackish water membrane type was used for each test run. The procedure described in par.

3.1.3 was followed to condition the membranes before running the experiment.

Table 7: Characteristics of synthetic feedwater prepared for use in laboratory investigations

Description Units Values Ca2+ mg/L 220

Mg2+ mg/L 10

Na+ mg/L 140

HCO3- mg/L 370

SO4- mg/L 40

Cl- mg/L 390

TDS mg/L 1140

pH - 7.96

pHs 6.6

Alkalinity mg/L 352

LSI - 0.750

Temp. C 22–24

Experiments were carried out in duplicate, using a once-through mode, i.e. the concentrate and

permeate were not recirculated. An 8-hour test period was sufficient to effect severe scaling on

the test membrane, and this time period was used to evaluate the effect of Zn++ as anti-scalant.

Zn++ in the form of ZnCl2, at the required concentration, was added to the stirred RO feed tank.

Two commercially available organophosphonate based anti-scalants were also tested at the

recommended dosage rates to obtain further reference results. The results of the above

investigation are summarised in Table 8, and in Figures 28 and 29.

40

Table 8: RO performance results with Zn2+ on carbonate scaling water

ANTI-SCALE MEASURE Rej. (%) Q change/h

Q change (%)

None 97.89 –0.70 –13.3 Zn 1 mg/L 98.66 0.00 0.0 Zn 2 mg/L 98.50 –0.14 –2.8 Zn 3 mg/L 98.56 –0.21 –3.7 Zn 4 mg/L 98.59 –0.21 –4.4

Anti-scalant V 2 mg/L 97.98 0.05 1.0 Anti-scalant P 2 mg/L 98.53 0.14 3.25

ACTUAL FLUX CHANGES WITH Zn IONS

30323436384042444648

1 2 3 4 5 6 7 8

Time (h)

Flux

(Lm

h)

None Zn 1 mg/L Zn 2 mg/LZn 3 mg/L Zn 4 mg/L Anti-scalant “V” 2 mg/LAnti-scalant “P” 2 mg/L

Figure 28: Changes in flux after using different anti-scalant formulations.

41

% CHANGE IN FLUX WITH DIFFERENT ANTI-SCALING FORMULATIONS

-16-14-12-10

-8-6-4-20246

Anti-scalant

% c

hang

e

Q (CHANGE) % -14 0 -2.5 -1.4 -1.2 0.5 3.25

None Zn 1 mg/L Zn 2 mg/L Zn 3 mg/L Zn 4 mg/L "V" 2 mg/L "P" 2 mg/L

Figure 29: Percentage changes in flux after using different anti-scalant formulations.

A similar investigation was carried out to investigate the potential of Zn2+ to inhibit calcium

sulphate scaling under the above test conditions. A near saturated solution of CaSO4 was

used as feed water. ZnCl2 was added to the feed at 2 mg/l Zn2+, while in the control

experiment no ZnCl2 was added. No observable distinction between the results of the two

experiments could be made, i.e. in both cases the flux drop was already more than 10%

within two hours of the start of the experiment. This result is in agreement with what is

reported in literature, namely that Zn2+ ions will only be effective as an anti-scaling measure

in feed waters with CaCO3 scaling potential (par. 4.2.1).

4.2.3 Pilot plant investigation

Pilot plant trials on groundwater were conducted on the pilot plant described in par. 3.3 at

Suiderstrand in the Cape Agulhas Municipality, as part of an ongoing desalination trial. The

feed water composition is tabulated in Table 9, while the test configuration for Zn2+ dosing

is given in Figure 32. Low-energy brackish water membrane elements were used for this

investigation. Evaluation was carried out continuously over a test period of 960 hours. Two

membrane pressure vessels, each with an element, received raw feed water from the

borehole, and the concentrate from these elements then served as feed water for the third

element. Two membrane pressure vessels, each with an element, received raw feed water

42

from the borehole, and the concentrate from these elements then served as feed water for

a third element (a 2-to-1 membrane configuration). The top and middle membranes were

used for comparative studies (one membrane was always without anti-scale treatment).

The operating pressure was 5.5 bar, and the recovery was maintained at 43%. Zn2+ was

metered out by dosing dissolved ZnCl2 at a rate of 4 mg/l, using a conventional anti-scalant

dosing pump, to the middle membrane. The membranes were washed (at low and high pH)

before and after this experiment, whereafter the performance of each membrane was

tested against the manufacturer’s specifications with a standard NaCl solution.

Table 9: Suiderstrand feed water analysis

Element Units Value Cl– mg/L 582

Ca hardness mg/L 282 pH 6.94

Conductivity mS/m 2145 Turbidity NTU 0.75 HCO3

- mg/L 196 Ca2+ mg/L 195 Mg2+ mg/L 87 TDS mg/L 1809 SO4

= mg/L 65 LSI - 0.1

43

Figure 30: Experimental setup used for Zn2+ dosing (container with dissolved zinc chloride

to the right of plant)

Figure 31 shows the performance of both membranes with no anti-scaling treatment. The

results obtained after Zn2+ treatment are summarised in Table 10 and Figure 32.

Table 10: Flux changes (%) recorded in an anti-scaling investigation using Zn2+ at

Suiderstrand compared to a similar (reference) membrane without any anti-scaling

treatment

TRIAL NR ANTI-SCALING MEASURE

USED ON TEST MEMBRANE

FLUX COMPARISON WITH

REFERENCE MEMBRANE

1 None (260 h) 2% lower flux than reference

2 Zn2+ ions (960 h) 20% higher flux than reference

44

FLUX CHANGES FOR Zn TRIALS

19.00

20.00

21.00

22.00

23.00

24.00

25.00

26.00

27.00

06 F

ebru

ary

2008

07 F

ebru

ary

2008

08 F

ebru

ary

2008

09 F

ebru

ary

2008

10 F

ebru

ary

2008

11 F

ebru

ary

2008

12 F

ebru

ary

2008

13 F

ebru

ary

2008

14 F

ebru

ary

2008

15 F

ebru

ary

2008

16 F

ebru

ary

2008

17 F

ebru

ary

2008

DATE

FLU

X (L

mh)

Test membrane (without Zn treatment) Reference membrane

Figure 31: Flux comparison of two similar membranes before anti-scaling treatment.

FLUX CHANGES FOR Zn TRIALS

19.00

20.00

21.00

22.00

23.00

24.00

25.00

17Fe

brua

ry20

08 21Fe

brua

ry20

08 25Fe

brua

ry20

08 29Fe

brua

ry20

08

04 M

arch

2008

08 M

arch

2008

12 M

arch

2008

16 M

arch

2008

20 M

arch

2008

24 M

arch

2008

DATE

FLU

X (L

mh)

Test membrane (Zn treatment) Reference membrane

Figure 32: Flux comparison of two similar membranes during Zn2+ dosing trials (the

reference membrane was not exposed to 2 mg/L Zn2+ dosing). (Note: Initial changes in flux can be explained by changes in RO recovery rate.)

45

4.2.4 Discussion of results This investigation focused on the prevention of scale in membrane desalination systems

where the feed water has a calcium carbonate scaling potential. Both the laboratory and

pilot plant results showed that the use of Zn2+ as anti-scalant on a spiral-wrap membrane

system results in improved flux performance compared to when no Zn2+ is used.

On laboratory scale it was shown that for feed water with a very high scaling potential

conventional anti-scaling chemicals do perform slightly better than Zn2+. This better

performance can possibly be attributed to these anti-scalants also preventing other forms of

scale formation (such as calcium sulphate scaling) while the Zn2+ is a specific anti-

carbonate scaling ion. The results do however indicate that the optimised use of Zn2+ can

be an option, especially where the occurrence of other inorganic scales is unlikely. SEM

analysis of the feed side of the membranes used in the laboratory investigation also

showed a much cleaner membrane surface when Zn2+ was used compared to when no

anti-scaling measure was used (see Figure 33).

Figure 33: SEM image of feed side of the membrane with no anti-scaling treatment (left)

and with Zn2+ as anti-scaling measure (right).

On pilot scale it was also found that Zn2+ dosing resulted in an increasingly better flux

compared to an untreated membrane with feed water that had only a slightly calcium

carbonate scaling nature. This test was carried out over nearly 1000 hours, at which stage

cleaning of the untreated membrane became necessary because of flux decline (par.

2.1.8), while the membrane with Zn2+ treatment still showed steady flux.

When the above results are compared to those reported in literature it is confirmed that

Zn2+ dosages of between 2 and 5 mg/l can suppress scale formation in scaling waters

46

(Table 9). Furthermore, according to literature, Zn2+ dosing will be more effective when the

LSI of the feed solution is below 1,4 and the pH below 8.2 (Lisitsin et al., 2005), which was

indeed the case in the above investigations. This is caused by the decrease in Zn solubility

at higher pH values. Another result that was confirmed was that an increase in [Zn2+] above

2 mg/L reduces the scale inhibition effect, resulting in a faster drop in flux. This is caused

by the feed becoming supersaturated with Zn salts (e.g. ZnCO3). The effect of this is that at

higher pH values and higher [Zn2+], Zn depletion will be accelerated, resulting in a partial or

total reduction in anti-scaling efficiency.

Other metal ions (e.g. Cu2+) may also have the same, or better, beneficial effect on scale

prevention, but Zn is preferable because of the relatively high permissible level (5 mg/L)

allowed in drinking water.

This technology, as a replacement for conventional anti-scalants, is gaining ground in both

research and commercial institutions (Hasson and Semiat, 2004), and the results of this

research indicate that this direction should be further exploited locally.

4.3 ANTI-SCALING TREATMENT USING MAGNETIC FIELDS

4.3.1 Background

Physical water conditioners treat water using magnetic, electrolytic, or other electro-

magnetic processes. These devices 'stabilise' the hardness minerals, which leads to a

reduction or prevention of the build-up of hard scale. There is usually no significant change

to the chemical composition of the hard water as nothing is removed; however, the

electrolytic and some magnetic types do add minute traces of metal (usually zinc or iron) to

the water. Conditioners alter the physical properties of the dissolved hardness minerals that

cause scale formation. Physical water conditioners do not remove the hardness minerals

and so they will not provide softened water. Many designs of MTDs (magnetic treatment

devices) are available. Some use electromagnets whilst others use an array of permanent

magnets. Physical water conditioners can be grouped according to whether or not they

require a mains electricity power supply. There is currently no standard for physical water

conditioners.

The study of physical water treatment devices is a controversial subject, with results

ranging from total success to total failure (Smith et al., 2003). To date, anti-scaling

magnetic treatment has proven to be unsuccessful in avoiding scale formation on

47

membranes operated at high recovery with saturated calcium carbonate solutions, but it

presents an alternative solution in cases where the membrane is liable to foul

predominantly with crystalline material (Baker et al., 1997). Limited evidence of the use of

magnetic pretreatment to prevent such scale formation does appear in the literature.

Laboratory studies have shown increased solution precipitation rates, crystal size and

morphology changes, both enhanced and retarded coagulation, and retention of the anti-

scaling properties of the water for hours or days following treatment (Baker et al., 1997).

Most researchers agree that the anti-scaling effect results from changes in crystallization

behaviour promoting bulk crystallization rather than the formation of adherent scale (Baker

et al., 1997), while the magnetic effect seems to be enhanced by supersaturation and a

high ionic load. In many of these studies the results have only been apparent under

dynamic magnetic treatment, i.e. when the solution moves sufficiently rapidly through the

magnetic field, which is nearly always orientated orthogonally to the liquid flow.

Two different-sized commercially available MTDs with proven performance records in other

industry sectors were donated to the project by a European company and subsequently

used for this investigation.

4.3.2 Laboratory investigations

The same laboratory setup and feed water described in par. 4.2.2 were used for this

investigation, but here the MTD device was placed over the feed line adjacent to the test

cell. The results of this investigation are summarised in Table 11 and Figure 33 in terms of

the flux changes after 8 hours and the rejection value at that time. Results obtained with

commercially available anti-scalants at recommended dosage rates are also shown, for the

purpose of comparison.

Table 11: Performance results from test runs on carbonate scaling water

ANTI-SCALING MEASURE REJ (%) Q change/h

Q change (%)

None 97.89 –0.517 –14.0 With MTD 98.23 0.000 0.0

Anti-scalant V 2 mg/L 97.98 0.050 1.0 Anti-scalant P 2 mg/L 98.53 0.140 3.25

48

Percentage change in flux over 8 hours with MTD

-16

-14

-12

-10

-8

-6

-4

-2

0

2

4

6

Anti-scaling technique

Perc

enta

ge c

hang

e in

flux

(%)

Q (CHANGE) % -14 0 1 3.25

None With MTD “V” 2 mg/L “P” 2 mg/L

Figure 34: Effect of MTD on flux in comparison with anti-scalants.

4.3.3 Pilot plant trials Pilot plant trials on groundwater were conducted at Suiderstrand in the Cape Agulhas

Municipality over a period of 480 hours (see par. 4.2.3 for experimental setup and

feedwater analysis). The test configuration used for evaluation of the magnetic treatment is

shown in Figure 35. The MTD was tested by fixing the apparatus on the top membrane

feed line, as close to the pressure vessel as possible.

49

Figure 35: Test configuration used for magnetic treatment (MTD device visible just in front

of top membrane pressure vessel, below pressure gauge).

The results obtained are summarised in Table 12 and Figure 35.

Table 12: Flux changes (%) recorded for a membrane treated with MTD at Suiderstrand

(compared to a reference membrane without any anti-scaling treatment)

TRIAL NR ANTI-SCALING MEASURE

USED ON TEST MEMBRANE

FLUX COMPARISON WITH

REFERENCE MEMBRANE

1 None (260 h) –2% lower flux than reference

2 External magnetic field (480 h) 3% higher flux than reference

50

FLUX CHANGES FOR MTD TRIALS

19.0020.0021.0022.0023.0024.0025.0026.0027.00

29D

ecem

ber

2007 01

Janu

ary

2008 04

Janu

ary

2008 07

Janu

ary

2008 10

Janu

ary

2008 13

Janu

ary

2008 16

Janu

ary

2008 19

Janu

ary

2008 22

Janu

ary

2008 25

Janu

ary

2008

DATE

FLU

X (L

mh)

Test membrane (with MTD) Reference membrane

Figure 36: Comparison of flux values of two membranes over a 480-h trial (one membrane

fitted with MTD). (White arrow indicates date of cleaning of membranes).

No significant improvement could be observed in terms flux change over the test period

from the above results for this specific feed water. (Only a very slight improvement of the

treated membrane over the reference membrane was recorded.)

4.3.4 Discussion of results

On laboratory scale it was shown that for feed water of very high scaling potential, MTD

anti-scale treatment had a positive effect when compared to no treatment at all. MTD

treatment did however not perform as well as when anti-scalant materials were used. This

could possibly be attributed to the anti-scalants also preventing other forms of scale

formation (such as calcium sulphate scaling), while the MTD is a specific anti-carbonate

scaling apparatus. SEM analysis of the feed side of the membranes used in the laboratory

investigation confirmed a much cleaner membrane surface and a different crystal shape

and size when MTD anti-scaling treatment is used compared to when no anti-scaling

measure is used (see Figure 37).

51

Figure 37: SEM image of feed side of membrane, with no anti-scaling treatment (left) and

with MTD anti-scaling treatment (right)

No significant flux advantage resulting from the use of the MTD on pilot plant scale was

observed (where the feed water had a very low scaling potential compared to in the

laboratory investigation).

Only a few references to the use of MTD with spiral-wrap membrane systems exist in

literature, which makes it difficult to explain the above results. It is however known that

CaCO3 scale normally found on RO membranes comprises mainly the trigonal calcite

structure (Butt et al., 1997), but that the orthorhombic aragonite structure is favoured when

scale formation takes place in the presence of a magnetic field (Baker et al., 1997). It is

specifically the aragonite structure that is preferred in other applications of anti-scale

treatments, as it is easier to remove from piping and heating systems (Smith et al., 2003).

SEM and XRD analyses were used to determine that the presence of a MTD on the RO

feed water line results in a mixture of calcite and aragonite in the concentrate (Baker et al.,

1997). The effectiveness of the MTD was found to be a function of the feed velocity, i.e. the

higher the flow rate the shorter the exposure time of the feed to the magnetic field and the

more predominant the formation of calcite. Recirculation of the concentrate also favours

calcite formation. These effects were only noticeable with near-supersaturated feed

solutions. In the above experimental investigation the feed water used in the laboratory was

much closer to supersaturation than in the pilot plant, which may explain the recorded

effectiveness of the MTD only on laboratory scale.

52

It is concluded that a thorough investigation (including pilot plant trials) should precede the

use of MTDs on spiral-wrap membrane systems. When compared to the use of Zn2+

(discussed in par. 4.3) it appears that the effects produced by MTDs are far less

pronounced than with such metal ions (refer to Coetzee et al., 1998). These devices may

only work where CaCO3 is the predominant scale factor, and even then factors such as the

strength of the magnetic field and the feed water LSI will determine the potential suitability

of MTDs when compared to conventional anti-scalants (Baker et al., 1997). Factors

inherent to spiral-wrap membranes may also play a role, as the potential nucleation

environment in an element is completely different to that in the test cell due to the spacer

and the membrane design. Where it is possible to be used on a RO plant it should offer

many advantages in terms of running cost savings and operator friendliness.

53

CHAPTER 5

CONCLUSIONS AND RECOMMENDATIONS

5.1 CONCLUSIONS The following conclusions are made at the completion of the investigation into spiral-wrap

membrane systems with improved anti-scaling characteristics:

1 The membrane market today is a mature, multi-billion dollar industry, with well-established

manufacturers and suppliers, and a diverse number of end-users.

2 The water treatment sector is the biggest user of membranes and this sector is, in turn,

dominated by the desalination market, where it is mainly reverse osmosis membranes that

are used for brackish water and seawater desalination.

3 The spiral-wrap membrane design accounts for nearly all RO membranes in use for

desalination, making it the most important membrane type on the market.

4 Locally, the application of membranes for water treatment is accepted and, as is the case

internationally, the need for desalination to augment existing water supplies has become

the topic of many recent investigations and publications. Again, the spiral-wrap membrane

design is (and will probably remain) the design of choice for the main applications.

5 The establishment of a local spiral-wrap membrane manufacturing industry will not be

economically viable in the foreseeable future due to major market entry barriers, the (small)

size of the local industry and the capital intensity of such an investment. It follows therefore

that the logical focus of R&D should be the improvement of (current) membrane systems,

and in particular spiral-wrap membrane systems so that its application becomes more

economical.

6 The desalination of groundwater is already being undertaken on a limited scale locally, but

all indications from water authorities are that this option will be increasingly exploited in

future. Thus the application of spiral-wrap membrane systems was therefore targeted for

this investigation, with the specific aim of reducing costs.

54

7 In areas along the South African West Coast, scale formation during RO desalination of

groundwater can become problematic. The dosing of expensive anti-scaling chemicals is

required as a scale preventative measure. This necessary practise can probably be

optimised in terms of anti-scalant type and dosage rate, but it will remain problematic for

operators and plant management in remote rural locations.

8 Two developments in terms of anti-scaling treatment for RO membranes were investigated

as part of this project, namely the use of metal ions and/or magnetic fields to inhibit scale

formation. Although these treatment techniques have found application in other fields of

water treatment, the application in the field of desalination remains largely unproven. These

techniques were consequently evaluated and verified on laboratory and pilot plant scale.

9 A flat test cell was calibrated and used for the laboratory investigation, while a 500 L/h pilot

plant was used for longer trail runs. In both instances commercially available anti-scalants

were used as reference, while untreated membranes were also used for comparison

purposes.

10 It was shown, on laboratory and pilot plant scale, that the use of specific concentrations of

Zn ions can inhibit scale formation in RO feed water by changing the scale crystal structure.

In laboratory experiments it was shown that Zn2+ concentrations of 1–4 mg/L can inhibit the

onset of scaling (i.e. no flux decline due to scaling), while on pilot plant scale it was shown

that the addition of 2 mg/L Zn2+ can improve the flux by up to 10% (compared to when no

anti-scalant treatment is used). It was also found that several factors, such as the presence

of other ions, the pH and LSI can determine the efficiency of this anti-scalant treatment. It is

also CaCO3 specific, as no evidence could be found that it will inhibit the formation of other

forms of scale.

11 The generation of magnetic fields to prevent CaCO3 scale through the use of magnetic

treatment devices on the feed line to the RO membrane is a controversial science. On

laboratory scale a distinct improvement in the prevention of flux-limiting scale was found,

but this was not observed on pilot plant scale. Literature seems to support the theory that

magnetic field treatment may be effective (only) under very specific conditions of feed water

composition, magnet exposure time and recovery.

55

5.2 RECOMMENDATIONS The following recommendations are made at the completion of the investigation into spiral-wrap

membrane systems with improved anti-scaling characteristics:

1 The use of alternate anti-scaling measures in membrane plants should be considered in

cases where calcium carbonate scaling may occur. Advantages of such measures can

include lower running cost due to lower chemical costs, the availability of raw materials,

environmental acceptance and low operator exposure risks, all of which are of particular

importance to remotely located desalination plants, as is the case with municipal-owned

plants along the West Coast of South Africa.

2 Two anti-scaling measures were investigated, both theoretically and experimentally. It is

particularly the use of Zn ions that is recommended for evaluation by existing and new end-

users of membranes. The use of magnetic fields to inhibit scale formation is not

recommended at this stage, mainly because of the lack of scientific evidence to support it.

3 It is however important that prior to the implementation of any alternate anti-scaling

measure the system should be adequately tested, up to at least the pilot plant scale, for

several weeks. Careful monitoring of permeate and concentrate water quality as well as

plant performance over the test period should show the implementation potential of anti-

scaling measures, especially where historical data is available as reference.

4 Future research on the use of bivalent metal ions as anti-scalants should focus on more

plant-friendly dosing techniques than the preparation of metal salt solutions, for example,

the electrolytic preparation of such ions at a predetermined concentration. Further studies

to determine the limits of the effectiveness of metal ion dosing are also recommended, in

order to obtain a more accurate description of the types of feed waters that can be treated.

Metal ions other than Zn should also be evaluated (e.g. Cu2+).

56

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