Towards Zero Liquid Discharge in drinking water production

Towards Zero Liquid Discharge in drinking

water production

Towards Zero Liquid Discharge in drinking

water production

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,

in het openbaar te verdedigen op dinsdag 3 juni 2014 om 10:00 uur door

Sara SALVADOR COB

Chemical Engineer (Universidad de Valladolid)

Dit proefschrift is goedgekeurd door de promotor: Prof. dr G.J. Witkamp

Copromotor Dr. F.E. Genceli Güner

Samenstelling promotiecommissie:

Rector Magnificus, voorzitter

Prof. dr. G.J. Witkamp, Technische Universiteit Delft, promotor Dr. F.E. Genceli Güner, Technische Universiteit Delft, copromotor

Prof. dr. I. Ortiz, Universidad de Cantabria, Spanje

Prof. dr. ir. A.B. de Haan, Technische Universiteit Delft

Prof. dr. M.D. Kennedy UNESCO-IHE/ Technische Universiteit Delft

Prof. dr. ir. L.C. Rietveld, Technische Universiteit Delft

Dr. P.S. Hofs, KWR Watercycle Research Institute

Prof. dr. ir. L.A.M. van der Wielen, Technische Universiteit Delft, reservelid Dr. P.S. Hofs en Dr. Emile Cornelissen, KWR Watercycle Research Institute hebben als begeleiders in belangrijke mate aan de totstandkoming van het proefschrift bijgedragen. This work was performed in the TTIW-cooperation framework of Wetsus, Centre of Excellence for Sustainable Water Technology (www.wetsus.nl). Wetsus is funded by the Dutch Ministry of Economic Affairs, the European Union Regional Development Fund, the Province of Fryslân, the City of Leeuwarden, the EZ/Kompas program of the ‘‘Samenwerkingsverband Noord-Nederland’’ and by the Joint Research Programme of the Dutch Water Companies.

Printed by Gldeprint Drukkerijen - The Netherlands Cover design by Sara Salvador Cob

Original photo of Niagara Falls by Jorge Amor Río Original SEM images by Arie Zwijnenburg ISBN/EAN 978-94-6108-692-1

“The whole ocean is made up of single drops”

Chinese proverb

Nanofiltration (NF) and reverse osmosis (RO) are used to produce clean water, but also produce a concentrate which contains most of the contaminants. Discharging concentrate streams to the environment is hindered by regulations, which are becoming more strict, and by the desire of recovering every single valuable atom. Therefore, the minimization of the concentrate volume to almost zero, is required in order to make treatment of the concentrate feasible. Currently several research studies are being conducted to find smart zero liquid discharge strategies in water desalination. In this PhD work the feasibility of reaching very high recovery (which equals a very low volume of concentrate) in a system consisting of cation exchange (CIEX) pretreatment, NF and RO was studied. An introduction to this study is given in Chapter 1.

In Chapter 2 the studied pilot plant (CIEX-NF-RO) is described in detail. The main problem that arises in such a system when working at high recovery is scaling. Since bivalent cations are removed with the CIEX resin, silica scaling was expected to become limiting. Four experiments at different total system (NF-RO) recovery between 91 and 98% were performed, each lasting about three weeks, using tap water (from water treatment plant (WTP) Tull en ‘t Waal) with high silica content (20 mg/L as SiO2) to feed the system. The results showed that 94% total system recovery was the maximum recovery possible without severe silica and aluminosilicate scaling in the RO membrane. This was mainly determined by performing autopsies with SEM-EDX on the four RO membranes after each experiment. Monitoring of the membrane permeability was not conclusive to detect scaling, since the decrease was similar in the experiments at 91, 94 and 96% total system recoveries. In Chapter 3 the scaling layer in the four RO membranes was characterized with different analytical techniques. These analyses showed that in the four experiments, the scaling of the RO membrane was either amorphous silica or amorphous aluminosilicates. The amounts of Si and Fe in the membrane were also determined, showing that the amount of Fe only increased slowly with recovery, but the amount of Si increased very quickly above 94% recovery, indicating the onset of silica or aluminosilicate scaling.

The aim of Chapter 4 was to find out the best method to remove silica from solution and study the feasibility of designing a pre-treatment step within the system to increase the total recovery. Several methods were tested to remove silica from solution in different streams of the pilot plant (feed water, NF recirculation loop and RO recirculation loop): precipitation with FeCl3·6H2O, precipitation with AlCl3·6H2O, seeding with silica gel and the use of a strong basic anion (SBA) exchange resin. Both precipitation with AlCl3·6H2O and the use of a SBA exchange resin were able to remove most of the silica present in solution. However, either relatively high concentrations of residual aluminum (in colloidal form) or high pH resulted from the treatment. The most suitable option would be precipitation with aluminum in the RO recirculation loop, followed by ultrafiltration, to

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remove the remaining aluminosilicate colloids present in the solution after precipitation, as these would otherwise act as seeds from which silica scaling can start.

In Chapter 5 an alternative for the removal of silica from solution (Chapter 4) to increase system recovery was studied: the use of antiscalants to inhibit the growth of silica scaling in the RO membrane. The pilot plant was moved to WTP Linschoten, which has a slightly different water composition from the tap water from WTP Tull en ‘t Waal used previously (Chapter 2), but still has a high silica content (17 mg/L as SiO2). At WTP

Linschoten 98% recovery was possible during three weeks of operation, but not 99%, where

severe silica and ironsilicate scaling was found in the RO membrane. To try to achieve 99% recovery, two different general antiscalants were used, BD25 with a carboxymethyl inulin base and OSM96 with a phosponate base. Unfortunately, neither of the antiscalants allowed stable operation at 99% total recovery in the studied system. Probably, this was due to the presence of particulate iron in the feed water or the long residence time of the pilot plant, estimated at 1 hour.

To achieve zero liquid discharge, the remaining concentrate has to be treated. This has been studied in Chapter 6. Evaporation is the common technique to dewater concentrate streams. In the present study an alternative technology has been chosen: eutectic freeze crystallization (EFC). In principle, EFC allows the separation of aqueous solutions into pure water and pure salts. The two waste streams in the system, the RO concentrate and the spent regenerant from the CIEX resin, were treated with EFC in batch mode in order to prove the principle. Both pure ice and NaHCO3 were extracted from the RO concentrate by application of EFC. Both pure ice and NaCl·2H2O were extracted from the regenerant. The total system CIEX-NF-RO-EFC recovery could potentially be increased from 98 to 99.7% by the implementation of this technique within the system. These preliminary results showed the application of EFC as a promising technique to achieve near zero waste discharge in combination with RO membranes by the production of reusable ice and salt from waste streams.

Increasing the recovery to such a high level might have consequences for the rejection of organic micropollutants, such as pharmaceuticals. These compounds are often present in feed water, and need to be removed. Therefore, in Chapter 7 the rejection in the NF and RO membranes at 98% total recovery (85% recovery each membrane) of a cocktail containing 22 different pharmaceuticals was investigated. The measured rejection of the pharmaceuticals in the NF was, except for one pharmaceutical (sotalol), above 83%. In the RO the rejection was always above 99.1%. The system (NF+RO) rejection was on average 93%. This shows that the nearly ZLD configuration is a suitable treatment option for removing organic micropollutants from water.

In Chapter 8, the results presented in this work are being discussed and some recommendations for future research in this topic are given. The outcome of this study indicates that the nearly zero liquid discharge concept is technically possible, with the right combination of techniques. The studied system could be applied for the production of

Nanofiltratie (NF) en omgekeerde osmose (reverse osmosis - RO) membranen worden gebruikt om schoon water te produceren. Naast het schone water wordt hierbij echter ook een concentraat geproduceerd waarin het merendeel van de verontreinigingen zit. Lozen van dit concentraat in oppervlaktewater is mogelijk, maar wordt onder invloed van steeds strenger wordende regelgeving steeds moeilijker. Daarnaast is er de wens om al de in het concentraat aanwezige waardevolle stoffen terug te winnen. Om de behandeling die hiervoor nodig is mogelijk te maken, is het nodig om het volume van het concentraat sterk te minimaliseren, tot dicht bij het extreem - nul lozing van water (Zero Liquid Discharge - ZLD). Op dit moment lopen er verschillende onderzoeken om slimme manieren te vinden om ZLD mogelijk te maken bij het ontzouten van water. In dit proefschrift is onderzocht hoe goed het mogelijk is om een hele hoge opbrengst te halen (wat gelijk staat aan een heel klein volume concentraat) in een systeem bestaande uit een kationenwisselaar (CatIon EXchange - CIEX) als voorbehandeling, gevolgd door NF en RO (op het NF concentraat). In hoofdstuk 1 is de introductie gegeven op dit onderwerp.

In hoofdstuk 2 is de gebruikte pilot-opstelling (CIEX-NF-RO) in detail beschreven. Het grootste probleem dat opkomt wanneer je bij een hoge opbrengst werkt is de zogenaamde scaling. Aangezien tweewaardig positief geladen ionen (bivalente kationen) door de CIEX worden verwijderd, was de verwachting dat in dit systeem scaling van silica (negatief of ongeladen) limiterend zou zijn. Om dit vast te stellen zijn vier experimenten uitgevoerd met drinkwater van waterproductiebedrijf (Water Treatment Plant - WTP) Tull en 't Waal als voedingswater waarin 20 mg/L silica (SiO2) zit. Elk experiment duurde enkele weken en vond plaats bij een verschillende hoge opbrengst over het NF en RO membraan van in totaal 91, 94, 96 en 98%. Hieruit bleek dat 94% opbrengst de hoogst haalbare opbrengst was zonder dat er problemen ontstonden met scaling in het RO membraan. Autopsies op het RO membraan met onder andere Scanning ElectronenMicroscoop (SEM) en gekoppelde EnergieDispersieve röntgenspectroscopie (EDX) toonden aan dat de scaling bestond uit silica- en aluminosilicaat. Het meten van de waterdoorlaatbaarheid van het membraan was voor alle opbrengsten vergelijkbaar en was dus niet gevoelig genoeg om de opkomst van de scaling in een vroeg stadium te detecteren.

In hoofdstuk 3 is beschreven hoe de lagen scaling uit de vier experimenten van het vorige hoofdstuk met nog meer verschillende technieken zijn geanalyseerd. Deze analyses stelden vast dat de scaling op het RO membraan bestond uit amorf silica of amorf aluminosilicaat. De hoeveelheden silica en ijzer in het membraan werden ook bepaald en liepen langzaam op van 91 naar 94% opbrengst, en snel van 94 naar 98% opbrengst. Dit geeft een indicatie dat de scaling begint bij een opbrengst van 94%.

Het doel van hoofdstuk 4 was om de beste methode te vinden om silica te verwijderen uit het water, en om de haalbaarheid van een behandelingsstap in het CIEX-NF-RO

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systeem te bestuderen. Het doel hiervan is om de opbrengst van het systeem verder te verhogen. Verschillende methoden voor silica-verwijdering zijn uitgeprobeerd op verschillende stromen uit de pilot-opstelling (voedingswater, water uit de recirculatielus en uit de RO recirculatielus). De geteste methoden waren: neerslaan met ijzerchloride (FeCl3·6H2O), aluminiumchloride (AlCl3·6H2O), kleine silica gel-deeltjes of het gebruik van een Sterk Basische Anionenwisselaarshars (SBA). De meeste silica is uit de oplossing te verwijderen door gebruik van aluminiumchloride of met de SBA. Beide methoden hebben echter een groot nadeel. In het geval van de aluminiumchloride blijft er relatief veel aluminium in oplossing achter (als kleine deeltjes), en in het geval van de SBA heeft de oplossing na de behandeling een zeer hoge pH. De handigste optie lijkt vooralsnog het verwijderen van silica met aluminiumchloride in de RO recirculatielus, gevolgd door utrafiltratie om de gevormde kleine deeltjes te verwijderen uit de oplossing. Dit is nodig omdat op deze kleine deeltjes anders aangroei van silica kan plaatsvinden, en daarmee de scaling sneller op gang kan komen.

In hoofdstuk 5 is een alternatief onderzocht voor de verwijdering van silica uit de oplossing (hoofdstuk 4): het doseren van antiscalants om de aangroei van silica scaling op het RO membraan te voorkomen. Voor deze studie is de pilot-opstelling verplaatst naar WTP Linschoten. Het drinkwater te Linschoten heeft een iets andere samenstelling dan het drinkwater van Tull en 't Waal dat daarvoor was gebruikt (hoofdstuk 2), maar heeft nog steeds een relatief hoge concentratie silica (17 mg/L SiO2). Op WTP Linschoten bleek het mogelijk de pilot gedurende drie weken bij 98% opbrengst water te laten produceren. Bij 99% opbrengst werd wederom scaling problematisch in het RO membraan, dit keer van silica en ijzersilicaat. In een poging om 99% opbrengst te halen zijn twee experimenten met twee verschillende antiscalants uitgevoerd. Beide antiscalants, BD25 (een relatief nieuwe op carboxymethyl inuline gebaseerde antiscalant) en OSM96 (een standaard antiscalant op fosfonaat-basis) waren helaas niet in staat om de scaling bij 99% te voorkomen. Dit kwam waarschijnlijk door de aanwezigheid van ijzer in de vorm van kleine deeltjes, of de lange verblijftijd van het water in de RO recirculatielus van de pilot (ongeveer 1 uur).

Om ZLD te halen is het nodig om het overgebleven concentraat te behandelen. Hoofdstuk 6 gaat daarover. Vaak wordt verdamping gebruikt om concentraat te ontwateren. Hier is een andere techniek gekozen: eutectische vrieskristallisatie (Eutectic Freezing Crystallization - EFC). In principe is het mogelijk met EFC een zoute oplossing in puur zout en puur water te scheiden. Zowel het concentraat als het regeneraat (dat vrijkomt bij de regeneratie van de CIEX) zijn behandeld met EFC om te laten zien dat het principe werkt. Puur ijs en NaHCO3 zijn gedeeltelijk uit het RO concentraat teruggewonnen door toepassing van EFC. Puur ijs en NaCl·2H2O zijn ook gedeeltelijk teruggewonnen uit het CIEX regeneraat. De totale opbrengst van het systeem (CIEX-NF-RO-EFC) kan waarschijnlijk door toepassing van EFC worden verhoogd van 98 tot 99.7%. Deze verkennende resultaten laten zien dat de combinatie van RO en toepassing van EFC een veelbelovende techniek is om dicht bij ZLD te komen.

het voedingswater, en worden bij voorkeur verwijderd. In hoofdstuk 7 is daarom de retentie van 22 geneesmiddelen over het NF en RO membraan onderzocht bij een totale opbrengst van 98%. De gemeten retentie over het NF membraan voor alle geneesmiddelen (behalve sotalol) was meer dan 83%. Over het RO membraan was de retentie altijd hoger dan 99.1%. De gemiddelde geneesmiddelenretentie voor het systeem (NF+RO) was 93%. Dit laat zien dat het goed mogelijk is in een ZLD systeem met NF gevolgd door RO organische microverontreinigingen te verwijderen.

In hoofdstuk 8 zijn de resultaten uit dit proefschrift bediscussieerd, en aanbevelingen voor vervolgonderzoek gegeven. Dit proefschrift geeft aan dat het technisch mogelijk is om dicht bij het ZLD concept te komen, door de juiste combinatie van technieken. Het bestudeerde systeem kan worden toegepast voor de productie van drinkwater uit grond-, of oppervlaktewater met hoge concentraties van bivalente kationen, silica en/of organische microverontreinigingen.

Las membranas de nanofiltración (NF) y ósmosis inversa (OI) se usan para producir agua de alta pureza, pero también generan un concentrado o rechazo, que contiene la mayoría de los contaminantes separados por la membrana. La descarga de estas corrientes de concentrado al medio ambiente se está viendo impedida por la aparición de leyes más estrictas y por el deseo de recuperar todas las sales posibles presentes en las mismas. Por este motivo, es conveniente minimizar el volumen de las corrientes de concentrado al máximo, para que su tratamiento sea viable. Actualmente, se están llevando a cabo muchos estudios para encontrar estrategias inteligentes que impliquen descarga líquida nula en la desalinización de agua. En esta tesis doctoral se ha estudiado la viabilidad de alcanzar una alta recuperación (que implica un mínimo volumen de concentrado) en un sistema compuesto por una resina de intercambio catiónico (IC), seguido de NF y OI. En el capítulo 1 se expone la introducción de este estudio.

En el capítulo 2 se describe con detalle la planta piloto estudiada (IC-NF-OI). El mayor problema que surge cuando se trabaja a alta recuperación es la aparición de incrustaciones en las membranas. En este caso, como los cationes divalentes se eliminan en la resina de IC, el factor limitante esperado es la aparición de incrustaciones de sílice. En este estudio se han realizado 4 experimentos a una recuperación total de agua del sistema (NF-OI) entre el 91 y el 98%, con una duración aproximada de tres semanas, usando como alimentación agua potable (obtenida de la estación de tratamiento de agua potable (ETAP) Tull en ‘t Waal) con un alto contenido en sílice (20 mg/L como SiO2). Los resultados mostraron que una recuperación total del sistema del 94% era la máxima posible, sin que aparecieran incrustaciones severas de sílice y aluminosilicatos en la membrana de OI. Este resultado fue determinado tras realizar autopsias (análisis SEM-EDX) a cada una de las membranas de OI al finalizar los experimentos. La monitorización del descenso de la permeabilidad en las membranas no fue determinante para detectar las incrustaciones, ya que el descenso fue similar en los experimentos al 91, 94 y 96% de recuperación del sistema.

En el capítulo 3 se estudió con más detalle la capa de incrustaciones en las cuatro membranas de OI, tratando de caracterizarla con diferentes técnicas analíticas. Estos análisis mostraron que en los cuatro experimentos la capa de incrustaciones estaba formada por sílice o aluminosilicatos amorfos. Se determinaron también las cantidades presentes de Si y Fe en las membranas, mostrando que la cantidad de Fe se incrementaba lentamente a medida que aumentaba la recuperación del sistema, pero la cantidad de Si aumentaba muy deprisa cuando se trabajaba con recuperación del sistema por encima del 94%, indicando ese punto como el comienzo de las incrustaciones de sílice o aluminosilicatos.

El objetivo del capítulo 4 era tratar de encontrar el mejor método para eliminar sílice en disolución y estudiar la viabilidad de diseñar un pretratamiento dentro del sistema para intentar aumentar la recuperación del mismo. Se estudiaron diferentes métodos para

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eliminar sílice en disolución en las diferentes corrientes de la planta piloto (agua de alimentación, la recirculación de la NF y la recirculación de la OI): precipitación con FeCl3·6H2O, precipitación con AlCl3·6H2O, uso de semillas de gel de sílice y el uso de una resina aniónica de base fuerte. Tanto la precipitación con AlCl3·6 H2O, como el uso de la resina aniónica fueron capaces de eliminar la mayor parte de la sílice disuelta. Sin embargo en el primer caso hay que prestar atención al aluminio residual (en forma coloidal) y en el segundo caso, al alto pH de la corriente después del tratamiento. La opción más factible para eliminar la sílice dentro del sistema sería la precipitación con aluminio en la corriente de recirculación de OI, seguido de ultrafiltración para eliminar los coloides de aluminosilicatos resultantes después de la precipitación. Si no se eliminan, pueden actuar como semillas para la nueva formación de incrustaciones de sílice.

En el capítulo 5 se estudia una alternativa a la eliminación de la sílice para incrementar la recuperación del sistema: el uso de antiincrustantes para inhibir el crecimiento de sílice en la membrana de OI. Para llevar a cabo este estudio, la planta piloto se trasladó a la ETAP de Linschoten, que tenía una composición de agua ligeramente distinta a la ETAP de

Tull en ‘t Waal (capítulo 2), pero también con concentración alta de sílice (17 mg/L como

SiO2). En la ETAP de Linschoten, fue posible alcanzar una recuperación del sistema del 98% durante 3 semanas de operación. Sin embargo, cuando se incrementó la recuperación a 99%, aparecieron incrustaciones severas de sílice y silicatos de hierro en la membrana de OI. Para intentar alcanzar 99% de recuperación se usaron dos antiincrustantes diferentes, BD25 con una base de carboximetil-inulina, y OSM96 con una base fosfonada. Desafortunadamente, ninguno de los antiincrustantes permitió una operación estable del sistema al 99% de recuperación. Probablemente eso se debe a la presencia de partículas de hierro en el agua de alimentación o al largo tiempo de residencia de la planta piloto, estimado en una hora.

Para alcanzar una descarga de líquido nula, el concentrado sobrante tiene que ser tratado. Esto ha sido estudiado en el capítulo 6. La evaporación es una técnica común para eliminar agua en las corrientes de concentrado, pero en este estudio se ha elegido una técnica alternativa: la cristalización eutéctica por congelamiento (CEC). CEC permite separar soluciones acuosas en agua pura y en sales útiles. Las dos corrientes residuales generadas en el sistema, el concentrado de la OI y el regenerante usado de la resina de IC, fueron tratadas con CEC en régimen discontinuo. Del concentrado de OI se extrajo hielo puro y NaHCO3 y del regenerante, hielo puro y NaCl·2H2O. La recuperación total del sistema IC-NF-RO-CEC se podría incrementar potencialmente desde el 98 al 99.7%, gracias a la implementación de esta técnica dentro del sistema. Estos resultados preliminares mostraron que CEC es una técnica prometedora para alcanzar la descarga casi nula de residuos en combinación con membranas de OI, produciendo hielo y sales de las corrientes residuales.

El aumento de la recuperación en estos sistemas a tal altos niveles podría tener consecuencias para el rechazo de contaminantes orgánicos persistentes, tales como los

membranas de NF y OI de 22 farmacéuticos diferentes, a una recuperación total del sistema del 98% (85% de recuperación en cada membrana). El porcentaje de rechazo de los farmacéuticos en la membrana de NF obtenido experimentalmente fue superior al 83%, excepto para un compuesto, el sotalol. En la membrana de OI el rechazo fue superior al 99.1%. El rechazo conjunto (NF+OI) fue de media del 93%. Estos resultados muestran que la configuración estudiada de descarga líquida casi nula es viable para la eliminación de contaminantes orgánicos persistentes en el agua.

En el capítulo 8 se tratan los principales resultados de este trabajo y se dan algunas recomendaciones para futuras investigaciones en este tema. El resultado de este estudio indica que el concepto de la descarga casi nula de líquido es técnicamente posible, con la combinación adecuada de unidades. El sistema estudiado podría utilizarse para la producción de agua potable a partir de aguas superficiales o subterráneas con concentración alta de cationes divalentes, sílice o contaminantes orgánicos persistentes.

Summary vii

Samenvatting xi

Resumen xv

Chapter 1: Introduction 1

Chapter 2: Silica and silicate precipitation as limiting factors in high-recovery reverse osmosis operations

9 Chapter 3: Amorphous aluminosilicate scaling characterization in a reverse

osmosis membrane

33 Chapter 4: Silica removal to prevent silica scaling in reverse osmosis membranes 49

Chapter 5: Towards zero liquid discharge: stable 98% recovery in nanofiltration and reverse osmosis

67 Chapter 6: Three strategies to treat reverse osmosis brine and cation exchange

spent regenerant to increase system recovery

89 Chapter 7: Rejection of pharmaceuticals in a high water-recovery system

combining ion exchange, nanofiltration and reverse osmosis

117

Chapter 8: General conclusions and recommendations 141

Appendices 147

List of publications 163

Conference proceedings and book abstracts 165

Acknowledgements 167

Introduction

1.1

Water scarcity

In October 2011, according to the United Nations Population Fund [1], the world population increased to the astonishing number of 7 billion people [2]. This means, among others, an increasing demand in clean water. Technically, there is enough fresh water in the world, but, unfortunately, it is not evenly distributed over the continents. Water scarcity affects around 700 million people in 43 countries [3] and it is one of the main problems that our society has to face in this century. In Figure 1.1, the water scarcity situation along the world is shown.

Figure 1.1. Global physical and economic water scarcity around the globe [3].

Additionally, only about 2.5% of the total water in the world is fresh water, while salty water represents the rest. From this 2.5%, only 0.3% is readily accessible for ecosystems and human use [4], while the rest is frozen or it is underground (Figure 1.2).

With the increasing population and contamination of the water sources, more efficient methods are needed to produce drinking water. Conventional water treatments might not be enough to meet the required drinking water quality. Therefore, more extensive treatment techniques are probably needed to make safe drinking water.

Chapter 1

2

Figure 1.2. Total and fresh world water distribution.

1.2

Membrane filtration: NF and RO

One of these technologies is membrane filtration, which is a separation process where a feed water stream containing pollutants is split into a clean water stream, the permeate, and a concentrated stream, the concentrate. With high pressure membranes, nanofiltration (NF) and reverse osmosis (RO), high purity water is produced as many contaminants are removed in one single step. To achieve the separation, pressure is applied over the membrane, then the feed water is forced through the membrane and the pollutants are retained in the concentrate. The removal of the pollutants depends on the pore size of the membrane. NF is used to remove multivalent salts and small organic molecules, while with RO all salts and small organic molecules can be removed. In Figure 1.3 there is a schematic drawing of a spiral wound configuration RO module.

Figure 1.3. Schematic spiral flow membrane module [5].

However, the application of NF and RO membranes has three main drawbacks: energy consumption, the production of a concentrate stream and fouling [6-9]. NF and RO systems are energy intensive and they require and optimum energy design configuration. The energy

1

required for RO desalination ranges between 0.4-7 kWh/m3_{, depending on the salinity of} the raw water [10]. Ideally energy can be withdrawn from renewable energies, as solar or wind energy [6]. Regarding the discharge of the concentrate, regulations are becoming more severe [11] as concentrate is considered as a source of contaminants. This results in increasing difficulties to dispose concentrates into surface or ground water. If the concentrate cannot be disposed of in surface or ground water, it needs to be reused or treated in a sustainable way. There are three main approaches to reduce the load of contaminants to the environment:

a) Lowering the recovery, which lowers the concentration of the contaminants, avoiding the addition of conditioning chemicals, like antiscalants or acids [12]. b) Removing the problematic contaminants from the concentrate before disposal

[11].

c) Increasing the recovery to have the smallest amount of concentrate possible. Due to the possible scarcity of groundwater as a resource, and its corresponding extraction limits, increasing the recovery is preferable.

The ideal solution would be a concept without concentrate discharge at all. This is the basic idea of the zero liquid discharge (ZLD) concept: almost no liquid waste, where the valuable salts can be recovered and the rest can be treated as chemical waste in a sustainable way.

The main limiting factor for achieving the ZLD concept in reverse osmosis is scaling. Salts like CaCO3, BaSO4 and CaSO4 [13-15] are very common scalants of RO membranes. However, this can be solved by removing bivalent ions with a cation exchange resin (CIEX). Then, the next problematic foulant is silica.

In this research the nearly ZLD concept has been investigated, addressing in detail silica scaling.

1.3

Silica scaling

Silica is found in ground waters and surface waters typically in the range of 2-40 mg/L SiO2. Due to the limited solubility silica scaling is one of the risks in the high water recovery concept. The solubility of monomeric Si(OH)4, often called monosilicic acid, is about 117 mg/L in water at a pH of 7 and a temperature of 25ºC. In aqueous solutions sufficiently supersaturated with dissolved silica and, in the absence of a suitable crystallization surface on which the soluble silica might be deposited, the monomer polymerizes by condensation of the silanol groups [16]. The polymerization of silica in solution, which is pH-dependent, leads to silica precipitation. Polymerization favors the maximization of the number of Si-O-Si bonds and minimization of number of Si-OH groups. The resulting spherical particles continue to grow, by crystallization of more silica onto the surface of the particles and/or by Ostwald ripening, which is the growth of bigger

Chapter 1

4

For high recovery RO, silica scaling usually becomes a problem due to the presence of silica in most feed waters and the consequent concentration of silica in the membranes. It has been shown that to achieve high recoveries in RO, silica scaling is an issue [17] and that polymerization of the monomers is the key to the development of silica deposits [18]. Once the silica is deposited on the membranes it is difficult to remove it by cleaning the membrane. Therefore, it is preferable to prevent the scaling.

Two strategies to prevent the occurrence of silica scaling in RO membranes are either the removal of silica from solution [19-21] or the addition of dedicated silica antiscalants [22]. The two strategies have been investigated in this thesis.

1.4

Concentrate management

Even at high recoveries a waste stream remains with still a high water content. In principle this low volume waste stream could be transported to the sea. However, for large capacity plants this is still a serious challenge. Alternatively, the remaining concentrate might be dewatered. In literature evaporation is mentioned as an option to deal with the brine problem. In arid areas evaporation ponds are used [23]. In humid climates evaporation has to be realized by putting thermal energy into the process, using evaporative crystallizers.

Given the fact that evaporative crystallization is energy intensive and thus costly, it could be worthwhile to explore the possibilities of alternative technologies. Eutectic freeze crystallization (EFC) is an alternative technology that is capable of separating aqueous solutions into pure water and pure, solidified solutes. The energy required to separate the water as ice could be significantly less than that required to separate it by evaporation, indicated by the fact that the heat of fusion of ice (6.01 kJ/mol) is less than the heat of evaporation of water (40.65 kJ/mol) [24]. In a previous study [25] it was shown that the

1

energy cost to treat an industrial KNO3-HNO3-H2O process stream with EFC was 69% lower compared to evaporative crystallization. Ideally the salts produced can be sold and used in agriculture or industry.

In the present study EFC has been explored to treat the RO concentrate and the spent regenerant from the CIEX resin, potentially increasing system recovery and producing pure water and pure salts.

1.5

Rejection of micropollutants

The presence of organic micropollutants in water sources, such as pharmaceuticals, pesticides, hormones, among others, has become more frequent in the past decades. These micropollutants might have a harmful effect for human health [27]. Therefore, close attention must be paid to their presence in drinking water sources. NF and RO are solid barriers against these substances but at high recoveries, rejection of micropollutants may decrease substantially [28]; therefore, the rejection of a selection of 22 pharmaceuticals by NF and RO at high recovery has been investigated in this PhD research.

1.6

Aim and scope of the research

The aim of this PhD research was to study the feasibility of the nearly ZLD concept in a system which could be used to produce drinking water from surface water or ground water. The system consists of a cation exchange resin (CIEX), NF and RO on NF concentrate.

In Chapter 2 we present the experimental results obtained with the pilot plant at different total system recoveries between 91 and 98%. The limiting total recovery which could be achieved in the system without problematic scaling was determined. The scaling formed in the RO membranes was also investigated. A detailed analysis of the deposits is shown in Chapter 3.

Chapter 4 presents several methods to remove silica from solution. Experiments were done first with synthetic water and then with water extracted from different streams of the pilot plant. The best method and conditions to remove silica within the pilot plant was determined.

In Chapter 5 the use of antiscalants to increase system recovery was investigated. Two different methods to test the efficiency of two general antiscalants against silica scaling were studied. These methods included laboratory and pilot testing.

In Chapter 6 treatment options for the waste streams generated in the pilot were explored. RO concentrate and CIEX spent regenerant were treated with EFC. With the application of this technique, the total system recovery can be potentially increased to nearly zero liquid discharge.

Chapter 1

6

Chapter 7 shows the rejection of a selection of 22 pharmaceuticals by the NF and RO membranes in the system at 98% total recovery. A model was used to predict the rejection of the pharmaceuticals by the NF membrane.

Finally, in Chapter 8 a general discussion and recommendations for future research are presented.

1

References

[1] United Nations Population Fund, Retrieved February 2014 from http://www.unfpa.org/.

[2] BBC, "Population seven billion: UN sets out challenges". October 26, 2011, Retrieved February 2014 from http://www.bbc.co.uk/news/world-15459643.

[3] United Nations, Retrieved February 2014 from http://www.un.org/waterforlifedecade/scarcity.shtml.

[4] UN Water, Retrieved February 2014 from http://www.unwater.org/statistics_res.html. [5] Wikipedia. Retrieved February 2014 from http://en.wikipedia.org/wiki/Membrane_technology. [6] L.F. Greenlee, D.F. Lawler, B.D. Freeman, B. Marrot, P. Moulin, Reverse osmosis desalination: Water sources, technology, and today's challenges, Water Research, 43 (2009) 2317-2348.

[7] A. Antony, J.H. Low, S. Gray, A.E. Childress, P. Le-Clech, G. Leslie, Scale formation and control in high pressure membrane water treatment systems: A review, Journal of Membrane Science, 383 (2011) 1-16.

[8] B. Van Der Bruggen, L. Lejon, C. Vandecasteele, Reuse, treatment, and discharge of the concentrate of pressure-driven membrane processes, Environmental Science and Technology, 37 (2003) 3733-3738.

[9] B. Van der Bruggen, M. Mänttäri, M. Nyström, Drawbacks of applying nanofiltration and how to avoid them: A review, Separation and Purification Technology, 63 (2008) 251-263.

[10] C. Fritzmann, J. Löwenberg, T. Wintgens, T. Melin, State-of-the-art of reverse osmosis desalination, Desalination, 216 (2007) 1-76.

[11] M.M. Nederlof, J.A.M. van Paassen, R. Jong, Nanofiltration concentrate disposal: experiences in The Netherlands, Desalination, 178 (2005) 303-312.

[12] P.J. Stuyfzand, K.J. Raat, Benefits and hurdles of using brackish groundwater as a drinking water source in the Netherlands, Hydrogeology Journal, 18 (2010) 117-130.

[13] D. Hasson, A. Drak, R. Semiat, Inception of CaSO4 scaling on RO membranes at various water recovery levels, Desalination, 139 (2001) 73-81.

[14] D. Hasson, R. Semiat, D. Bramson, M. Busch, B. Limoni-Relis, Suppression of CaCO3 scale deposition by anti-scalants, Desalination, 118 (1998) 285-296.

[15] Ś.F.E. Boerlage, M. D. Kennedy, G. Jan Witkamp, J. Peter van der Hoek, J. C. Schippers, BaSO4 solubility prediction in reverse osmosis membrane systems, Journal of Membrane Science, 159 (1999) 47-59.

[16] R.K. Iler, The chemistry of silica; Solubility, polymerization, colloid and surface properties, and biochemistry, John Wiley & Sons, Inc., 1979.

[17] A. Rahardianto, J. Gao, C.J. Gabelich, M.D. Williams, Y. Cohen, High recovery membrane desalting of low-salinity brackish water: Integration of accelerated precipitation softening with membrane RO, Journal of Membrane Science, 289 (2007) 123-137.

[18] J.S. Gill, Inhibition of silica-silicate deposit in industrial waters, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 74 (1993) 101-106.

[19] W. Bouguerra, M. Ben Sik Ali, B. Hamrouni, M. Dhahbi, Equilibrium and kinetic studies of adsorption of silica onto activated alumina, Desalination, 206 (2007) 141-146.

[20] M.B. Sik Ali, B. Hamrouni, S. Bouguecha, M. Dhahbi, Silica removal using ion-exchange resins, Desalination, 167 (2004) 273-279.

Chapter 1

8

[21] I. Latour, R. Miranda, A. Blanco, Silica removal from newsprint mill effluents with aluminum salts, Chemical Engineering Journal, 230 (2013) 522-531.

[22] E.G. Darton, RO plant experiences with high silica waters in the Canary Islands, Desalination, 124 (1999) 33-41.

[23] M. Mickley, Review of concentrate management options,

http://texaswater.tamu.edu/readings/desal/concentratedisposal.pdf, (2004).

[24] J. Nathoo, R. Jivanji, A.E. Lewis, Freezing your brines off: eutectic freeze crystallization for brine treatment, in: International mine water conference, South Africa, 2009.

[25] F.v.d. Ham, Eutectic Freeze Crystallization, Technical University of Delft, The Netherlands, 1999, pp. 122.

[26] EFC Separations, Retrieved February 2014 from http://www.efc.nl/the-efc-process/.

[27] C. Bellona, J.E. Drewes, P. Xu, G. Amy, Factors affecting the rejection of organic solutes during NF/RO treatment—a literature review, Water Research, 38 (2004) 2795-2809.

[28] A.R.D. Verliefde, Rejection of organic micropollutants by high pressure membranes (NF/RO), TU Delft, Delft, 2008.

Silica and silicate precipitation as limiting

factors in high-recovery reverse osmosis

operations

This chapter investigated silica and silicate precipitation as limiting factors in high-recovery membrane operations. For this purpose a cation exchange pretreatment is proposed to reduce Ca2+_{, Ba}2+_{, Mg}2+ _{levels to prevent scaling of salts containing these ions during} subsequent NF and RO filtration, in which RO is fed with NF concentrate. In a pilot plant experiments were carried out at total (NF+RO) water recovery of 91, 94, 96 and 98% with locally available tap water which contains 20 mg/L of silica as feed water. Autopsy studies were performed with the RO membranes after each experiment in which the fouling layer was studied using SEM-EDX to determine the structure and the composition of the fouling deposits. A thin cake layer was observed which covered approximately half of the membrane surface after operating for 20 days at 91 and 94% recovery. At 96 and 98% recovery the fouling layer was thicker and completely covered the membrane surface. EDX analysis indicated that the fouling layer was mainly composed of Si, Al, Fe and O, most likely due to the presence of iron oxides, iron hydroxides, silica and aluminosilicates. To be able to work at these high recoveries for an extended period, further measures need to be taken to prevent silica and aluminosilicate scaling.

This chapter is based on:

S. Salvador Cob, C. Beaupin, B. Hofs, M.M. Nederlof, D.J.H. Harmsen, E.R. Cornelissen, A. Zwijnenburg, F.E. Genceli Güner, G.J. Witkamp., Silica and silicate precipitation as limiting factors

1-Chapter 2

10

2.1

Introduction

Membrane technology is increasingly used in drinking water treatment [1]. Both nanofiltration (NF) and reverse osmosis (RO) produce high quality water by removing pathogens, organic micropollutants, colloids, natural organic matter and salts. However, NF and RO have several drawbacks, such as membrane fouling and the production of a concentrate stream that has to be disposed of. Regulations for the discharge of the concentrate are becoming more stringent [2] as concentrate is considered as a source of contaminants. This results in increasing difficulties to dispose concentrates into surface or ground water. If the concentrate cannot be disposed of in surface or ground water, it needs to be reused or treated in a sustainable way. There are three main approaches to reduce the load of contaminants to the environment: a) lowering the recovery, which lowers the concentration of the contaminants, avoiding the addition of conditioning chemicals (e.g., antiscalants or acids) [3], b) removing the problematic contaminants from the concentrate before disposal [2], or c) increasing the recovery to have the smallest amount of concentrate possible. Due to the possible scarcity of groundwater as a resource, and its corresponding extraction limits, increasing the recovery is preferable. Option b is used to remove specific compounds such as iron and phosphates (which is an option in the presence of P-based antiscalants), but it might not be feasible to remove monovalent ions, which are the predominant contaminants.

In this research we are aiming to increase the recovery to very high levels (up to 98%). The amount of water lost as concentrate is minimized, and its volume can be further reduced by evaporation or eutectic freeze crystallization. One of the main problems when operating at high recovery is precipitation of salts also known as scaling. The main objective of this study is to investigate the limiting factor of silica and silicate scaling in RO after removing bivalent cations with a cation exchange (CIEX) resin prior to NF and RO to operate at high recoveries of 91-98%. Also we aim at determining at which recovery silica and silicate scaling become the limiting factors and characterizing the fouling layer by membrane autopsies, combined with SEM-EDX analyses. This is in order to develop strategies to prevent scaling at high recoveries.

Silica is found in ground water and surface water typically in the range of 2-40 mg/L SiO2. The presence of silica complicates desalination processes because of its high scaling potential [4-6] and the intricate chemistry of silica adds to the complexity of this problem. Silica has a low solubility of about 117 mg/L in water at a pH of 7 and a temperature of 25ºC. When its solubility limit is exceeded in the absence of a suitable crystallization surface on which the soluble silica might be deposited, the monomer polymerizes by condensation of the silanol groups [7]. The polymerization of silica in solution, which is pH-dependent, leads to silica precipitation.

Several investigations on high recovery membrane systems have been carried out [8-19]. Rahardianto et al. [8] reached a total recovery of 98% in a system composed of a

2

primary RO, after which the concentrate was treated with accelerated precipitation softening, followed by a secondary RO for treatment of the concentrate. Precipitation softening consisted of increasing the pH to 10.5 using NaOH and adding calcite seeds to accelerate crystallization. Most of the Ca2+ _{(92%), Ba}2+ _{(96%) and Sr}2+ _{(78%) were} removed, however only a small reduction was achieved for Mg2+ _{and silica (10-20%).}

Heijman [9] et al. showed in a small scale experiment that recoveries between 97 and 99% using nanofiltration were, in principle, possible, if the scaling components were removed from the feed water. They developed two different treatment schemes for surface and ground water, respectively. The surface water treatment concept consisted of ion exchange to remove the bivalent cations, followed by ultrafiltration, nanofiltration and granular activated carbon filtration. The membrane permeability was constant for the relatively short testing period of 3 days at 97% recovery, indicating a possible absence of scaling. The ground water concept consisted of precipitation at high pH, followed by sedimentation, weak acid cation exchange and nanofiltration. For a recovery of 99% the membrane permeability was constant for 11 days. However, the pressure drop started to increase after 7 days as a result of fouling of the feed spacers due to carry over from the crystallization step.

Bond et al. [20] showed a different approach for a system composed of two RO modules in which the primary concentrate was treated to reduce its scaling potential and then the concentrate was treated again in the second RO. The intermediate treatments investigated were the addition of NaOH and Ca(OH)2 to elevate the pH to precipitate calcium, barium and strontium, and the addition of Al2(SO4)3·14H2O and Na2Al2O4 in an attempt to precipitate the silica as aluminosilicate. The projected recoveries for five different sources of brackish water were between 94 and 98%.

Oren et al. [11] proposed a hybrid process combining reverse osmosis and electrodialysis reversal (EDR) operating on the RO concentrate to achieve a 97-98% total recovery. However, silica deposition could occur within the RO and the EDR when trying to increase the RO recovery to more than 75%.

Rahardianto et al [8], Bond et al [6] and Gabelich et al [13] showed that once the carbonate and sulfate based precipitants were removed, the limiting minerals were silica and aluminosilicates. Furthermore, Gabelich et al. [21, 22] showed that silica selective antiscalants were not effective against aluminosilicates.

In conclusion, high recoveries in NF and RO membranes seem to be feasible if the scaling salts are removed in pretreatment steps. This seems rather easy for bivalent ions, but silica seems to be one the most important remaining issues. The removal of silica is crucial and further investigations are needed. When the bivalent cations are removed before membrane treatment, silica and silica-derived precipitants become the limiting factors in the RO membranes at high recoveries.

Chapter 2

12

2.2

Materials and methods

2.2.1 Pilot plant

A pilot plant was constructed at KWR Watercycle Research Institute to carry out silica scaling research at high recovery RO (see Figure 2.1). The pilot system consisted of a main pump assembly, cation exchange (CIEX) resin column, and NF-RO skid.

The feed water was filtered with a 1 μm cartridge filter to remove particulate matter and prevent particulate fouling. As a first step a CIEX resin was used to remove bivalent cations like Ca2+ _{and Mg}2+ _{to prevent deposition of salts containing these ions at high recoveries.} The CIEX column was filled with approximately 50 L of a strongly acidic, gel-type CIEX resin in the sodium form with a capacity of 2 eq./L (Lewatit Monoplus S100, supplied by Caldic, Belgium). The CIEX resin was regenerated weekly with a 15% NaCl solution. It was followed by treatment by nanofiltration (NF), 90% of the concentrate was recirculated and 10% was subsequently treated in the second stage by reverse osmosis (RO) in which 98% of the concentrate was recirculated. In the NF membrane the silica was partially removed (about 50%). Therefore, higher recoveries could be achieved in the subsequent RO membrane, than using, for instance, an RO-RO system.

The membrane used for NF was NF-2540 (DOW filmtec) and the RO membrane was TW30-2514 (DOW Filmtec). Both membranes had an active thin-film layer of polyamide layered with polysulfone as a porous support layer and had a spiral wound configuration. The NF-2540 minimum MgSO4 rejection was given as 98.0% and the TW30-2514 stabilized NaCl rejection was given as 99.5%. Before use, the modules were rinsed with tap water for one hour to remove preservation liquids present in the membrane.

The temperature of the water, after the main pump, was controlled to 18±1°C by a cooling system including a TAEevo 031 cooler (MTA, Italy). The feed flow to the pilot was kept constant and controlled by the main pump, which was pressure regulated (manually). In the first circulation stage the circulation pump was set to 225 L/h to obtain an effective velocity of 0.046 m/s across the membrane. The effective velocity was calculated as follows [23]:

ε

⋅ ⋅ = h b V eff v (2.1)

Where V = feed flow; b=leaf width; h=spacer thickness; ε=porosity.

The NF permeate flow was controlled manually, by a valve. In the second circulation stage the circulation pump was set to 255 L/h to obtain an effective velocity of 0.052 m/s. The RO concentrate flow was controlled manually by a valve. The NF permeate and RO concentrate flow were checked and adjusted manually to the set point twice per day (except in the weekends). The feed flow and flows in both loops were logged every minute, together with the pressure before and after each membrane and the temperature and

2

conductivity before and after the RO. The rest of flows, pH, temperatures and conductivities were measured manually two times a day (except in the weekends).

Figure 2.1. Pictures of the pilot plant used in this research.

The recovery of the pilot as discussed here is the total water recovery of the two membrane stages: this does not include losses due to the needed water for regeneration of the CIEX resin (typically 1-2%).

The membrane permeability, normalized pressure drop (NPD) (feed-concentrate) and salt passage, based on electrical conductivity measurements, (EC) were monitored for both membranes to investigate the beginning of scaling and other types of fouling. These process parameters were normalized for a proper control of the pilot plant and to be able to compare results from different experiments with each other [24]. The experiments were conducted between 11 and 20 days. In this time significant scaling formation was observed.

2.2.2 Membrane autopsy

After each run a membrane autopsy was performed of the RO membrane. A visual inspection of the two leafs of the membrane was first carried out followed by the collection of 3 representative samples of 3x3 cm2 _{taken at different locations of the membrane (inlet,} middle, outlet). The samples were analyzed using a JEOL-6480LV Scanning Electron Microscope (SEM) (JEOL Company) equipped with Noran System SIX X-Ray microanalysis (EDX) system (Thermo Electron Corporation) to determine the structure and the composition of the fouling (scaling) layers. The samples were coated with a thin (10 nm) Au layer. An accelerating voltage of 6 kV was used for SEM observation and 10 kV

Chapter 2

14

for the EDX analysis. Membrane cross-sections were obtained by breaking the membranes after freezing in liquid nitrogen.

2.2.3 Water analysis

The feed water of the installation was tap water, which was produced from groundwater at water treatment plant Tull en ‘t Waal (Water Supply Company Vitens) by aeration and rapid sand filtration with addition of poly aluminum chloride and without post-chlorination. The quality of the water is given in Table 2.1. The water had an average temperature of 12 ºC and pH 8.12. This water was selected due to its high silica concentration (20 mg/L SiO2), which is in the range of typical surface and ground water.

Table 2.1. Feed water average quality and standard deviation. mg/L STDEV Ca2+ _{70 3.29} Na+ _{14 1.14E-02} Mg2+ _{5.86 3.34E-02} Al3+ _{0.02 6.91E-05} K+ _{1.25 1.94E-03} Fe3+ _{0.01 1.21E-02} Ba2+ _{0.02 1.31E-03} Cr2+ _{0.01 1.21E-02} Cu2+ _{0.05 4.08E-05} Sr2+ _{0.23 3.38E-04} B3+ _{0.01 6.78E-03} Cl- _{9.19 1.17} HCO3- 277 21 SO42- <2 1.12E-05 SiO2 20 7.90E-01 DOC 1.95 1.90E-01

The different water streams (feed water, CIEX treated (feed) water, the NF feed, NF permeate, NF concentrate, RO feed, RO permeate, and RO concentrate) were analyzed to determine the concentration of inorganic compounds including scaling salts and the rejection behavior of the membranes. With these analyses the mass balance along the system was calculated for different components.

Dissolved organic carbon (DOC) was analyzed in accordance with ISO 8245 and ISO 1484. Before preservation the samples were filtered using a 0.45 μm membrane filter.

Most of the inorganic compounds present in the different streams were measured with Ion Couple Plasma Mass Spectrometry (ICP-MS) (Elan 6000, from Perkin Elmer). The method used a single-point calibration for some elements and the response of other elements was calculated using average response factors. A quantitative method was also

2

used for Si, using several points of calibration. These analyses were conducted by the Laboratory of Materials Research and Chemical Analyses of KWR.

Chloride and sulfate concentrations, however, were measured with an in house method by Vitens Laboratorium B.V. (Leeuwarden, The Netherlands). HCO3- and CO3 2-concentrations were measured by pH titration, according to the NEN 6531.

The water after the CIEX column was monitored with a Ca2+ _{selective electrode to} determine the Ca2+ _{concentration of the feed water of the first circulation stage. Based on} these measurements and the need to prevent breakthrough of bivalent cations to the first circulation stage, the regeneration of the resin was done regularly with a NaCl solution at 15 wt.%.

2.2.4 Phreeqc calculations

Phreeqc-2 software [25] was used to calculate the saturation index (SI) of most of the sparingly soluble inorganic salts in water using the database Wateq4f. The SI is defined as:

        = sp K IAP log SI _(2.2)

Where, IAP is the Ion Activity Product and Ksp is the solubility product. When SI>0 the compound is supersaturated and there is risk of scaling [26]. 2.2.5 Projection calculations

To determine the maximum allowable system recovery we performed projection calculations with ROSA 72 software (Dow Chemical) for our system and water type. The membranes used in this research were not available in the program, since the software is designed for higher scale operations. Therefore, the membranes NF270-2540 and TW30-2540 were selected. The flows and number of pressure vessels were adjusted in order to have the same conditions as in our pilot plant. The water analyses from Table 2.1 were used as feed water. The softening of the water, by means of the cation exchange, was taken in account for the projection calculations.

2.3

Results

2.3.1 Projection calculations

Projection calculations were performed prior to our experiments. Based on these calculations it was expected that at 96% total recovery the silica concentration at the RO feed would be close to its solubility limit. Hence, above 96% water recovery, silica scaling

Chapter 2

16

considered. Therefore, silica or silica-derived scaling could occur even at lower recoveries. To confirm this, the pilot plant was operated at 91, 94, 96 and 98% total recovery.

2.3.2 Silica scaling experiments at different recoveries

2.3.2.1 Operation parameters and mass balance

Experiments were carried out with the pilot plant at different operational settings to achieve different total system recovery values (Table 2.2).

Table 2.2. Summary of the experiments performed in our system. Total Recovery (%) NF Recovery (%) RO Recovery (%) Feed Flow (L/h) Feed Pressure (bar) NF Flux (L/m2_h) RO Flux (L/m2_h) 91 69 70 65 10 17.26 19.94 94 76 74 69 11 18.24 17.80 96 78 83 88 17 23.75 21.30 98 89 83 122 17 38.98 16.90

During the experiments the EC, Membrane Permeability, and NPD were monitored for both membranes, with a focus on the RO membrane, due to its high potential for scaling. All the experiments were operated during 20 days, except the last experiment, which was stopped after 11 days.

During the four experiments there was no significant increase in the NPD (smaller than 0.5 bar), so it is not reported in this chapter. Therefore, no blocking of the feed channel seemed to occur.

The evolution of the relative salt passage (EC) in the RO membrane is shown for the four experiments in Figure 2.2. The EC passage decreased with time for all the recoveries.

In the following graph (Figure 2.3), the relative membrane permeability of the RO membrane is shown for the four experiments. The permeability decreased with time at the four recoveries. The decrease was higher at 98% recovery.

The initial permeability and salt passage were different for the four experiments. This was probably due to the fact that two different batches of membranes were used. One batch was used for the experiments at 91 and 94% recovery and the other batch was used for the experiments at 96 and 98% recovery. The various streams in the pilot (Figure 2.4) were analyzed to determine the water composition. In Table 2.3 the flow and water composition of the different streams at 98% recovery are shown. The raw water was fed at a flow of 200 L/h to the CIEX unit. The CIEX unit produced 122 L/h softened water, which was fed into the loop with the NF membrane. Due to recirculation of the NF concentrate, the feed flow

2

for the NF membrane was 226 L/h. The NF membrane produced 107 L/hpermeate. 15 L/h of the NF concentrate was fed into the loop with the RO membrane. Due to recirculation of the RO concentrate the feed flow for the RO membrane was 256 L/h. The RO membrane

Figure 2.2. Relative salt passage (EC) of the RO membrane at 91, 94, 96 and 98 % total recovery (initial EC values: 0.49, 0.52, 0.37, and 0.30 % at 91, 94, 96 and 98 % total recovery, respectively).

Figure 2.3. Relative permeability of the RO membrane at 91, 94, 96 and 98 % total recovery (initial permeability values: 0.55, 0.49, 0.36, and 0.42 ·10-11_{m-s·Pa at 91, 94, 96 and 98 % total recovery,}

Chapter 2

18

produced 12 L/h permeate and 2.2 L/h of concentrate was extracted from the pilot. The data of the experiments carried out at recoveries of 91%, 94% and 96% can be found in Appendix A.

Figure 2.4. Scheme of the pilot plant indicating the analyzed streams during the experiments. The rejection values of the NF and RO membranes for the different inorganic compounds were >79% for the NF (except Si that was >28%) and >95% for the RO.

The DOC was removed by 95% by the NF membrane and 98% by the RO membrane. The main content of the RO concentrate was sodium (> 4 g/L), bicarbonate (>10 g/L) and DOC (100 mg/L). In Appendix A the detailed rejection values for each experiment can be found.

The removal of Ca2+_{(aq) and Mg}2+_{(aq) in the cation exchange resin was never lower} than 98.8% during the operation.

Silica was expected to be the critical parameter in the performance of the system. The concentration of silica in the feed of the RO membrane should be, in principle, below the solubility limit to avoid scaling. The silica solubility in the RO feed stream was calculated for the average temperature during the experiment according to the correlation developed by Gunnarsson [27] for amorphous silica. The solubility of amorphous silica increases with pH, therefore, the value obtained was corrected for the pH [28].

The concentration polarization factor, β, was calculated according to the classical film model [29]:













=

m v

k

J

exp

β

(2.3)

2

T abl e 2. 3 A ve ra ge f low a nd c onc ent ra tions of the s tr ea m s a t 98 % r ec ove ry. 8 R O con ce n tr at e 2.20 2.31 _4,258 1.21 3.37 0.50 0.10 285 101 10,305 189 7 _RO p er m eat e 12 _0.01 14 _{0.00 0.02 0.00 0.00} <3 _0.25 40 _0.78 6 R O fe ed 256 2.50 _4,480 1.22 3.52 0.54 0.11 _302.5 102 _10,064 202 5 _NF re ci rc u lat ion 104 0.40 677 0.03 0.57 0.12 0.03 47.5 16 _1,537 17 4 _NF p er m eat e 107 0.01 33 0.00 0.03 0.00 0.00 4.5 _<0.20 87 18 3 _NF fe ed 226 0.23 391 0.02 0.35 0.06 0.01 29 8.43 955 25 2 P re tr ea te d w at er 122 0.06 121 0.00 0.10 0.01 0.00 10 1.93 317 20 1 F ee d 200 70 15 5.98 1.23 0.02 0.00 10 1.90 271 20 Q ( L /h ) C a ( m g /L ) N a ( m g /L ) M g ( m g/ L ) K ( m g/ L ) A l ( m g/ L ) F e ( m g/ L ) C l ( m g/ L ) D O C ( m g/ L ) H C O3 ( m g/ L ) S il ic a ( m g/ L )

Chapter 2

20

In Table 2.4 we present the silica solubility values calculated for the average T and pH, the silica levels measured with ICP-MS, the β factor, and the silica concentration corrected for the β. The measured value is for the RO loop, but at the active membrane layer the silica concentration is higher due to β.

Table 2.4. Overview of silica concentrations and solubilities in the feed of the RO and at the active membrane layer. Total Recovery (%) T (ºC) pH Silica solubility (mg/L) Silica ICP-MS (mg/L) Concentration polarization (β) Silica corrected with β (mg/L) 91 18.6 8.4 119 72 1.63 117 94 18.0 8.5 114 111 1.56 173 96 18.2 8.6 118 130 1.69 220 98 18.3 8.6 118 201 1.52 305

In the experiments at 94, 96 and 98% recovery the silica concentration at the active layer of the RO membrane was above the silica solubility, so silica scaling was expected at this recovery. However, silica can remain dissolved in water for long periods of time [4]. 2.3.2.2 Membrane Autopsies

Membrane autopsies followed by SEM-EDX analysis were performed with a virgin RO membrane and with the RO membranes obtained after the experiments at 91, 94, 96 and 98% recovery to determine the presence, structure and composition of the deposits.

Top view and cross section SEM images of the virgin membrane are shown for comparison (Figure 2.5). The polyamide membrane surface layers had a ridge and valley structure, typical for thin film composite (TFC) polyamide membranes [30].

On SEM images of the surface of the RO membrane from the experiment at 91% recovery, a deposition layer was detected at some areas of the membrane surface (Figure 2.6). Closer inspection at lower magnification showed that some of these spots were located near the position where the cross-sections of the feed spacers’ wires touched the membrane surface. In the parts of the membrane where there was no fouling, the structure of the membrane could be seen. We estimated on the basis of SEM micrographs that the membrane surface was covered for approximately 50% with a deposition layer. From the cross-section images we estimated that the thickness of this deposition was approximately 1-2 μm.

From the EDX analysis (Table 2.5) we observed that approx. 8% of the deposit layer was Si, furthermore < 4% of Fe, Al and Na were found. The predominant part of the deposit layer consisted of C, O and there was 3% of S. This high percentage of S found for the membrane fouled at 91% recovery, about 5/3rd _{of the value found for the virgin}

2

membrane, shows that largely the composition of the polysulfone support layer of the TFC membrane was measured.

Figure 2.5. SEM pictures of a virgin RO membrane, top view (left) and cross section (right).

Figure 2.6. SEM pictures of the RO membrane fouled at 91% total recovery, top view (left) and cross section (right).

Figure 2.7. SEM pictures of the RO membrane fouled at 94% total recovery, top view (left) and cross section (right). Notice the different magnification of the top view compared to Figures 2.6, 2.8