Drinking Water Treatment, Volume 4, Membranes Applied to Drinking Water and Desalination E-Book

Kader Gaid

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 448

Veröffentlichungsjahr: 2023

Beschreibung

Today, hundreds of millions of people drink contaminated water without knowing it. Yet water treatment technologies can effectively eliminate contamination and can supply urban and rural populations with safe drinking water in a secure way. For almost two centuries, the huge number of treatments available to guarantee water quality has grown alongside technological progress, the strengthening of industry norms and the reinforcement of consumer expectations. New treatment methods have been developed according to the advancement of knowledge and new sanitary regulations. This five-volume book sets out to clearly present the variety of treatments available along with their performance, limitations and conditions of use as well as ways to combine them to produce safe drinking water, which is a basic need essential to everyday life. The author shares his expertise acquired at Veolia, a company that is a world leader in water services and sanitation, desalination of sea water and the recycling of wastewater. Founded in France in 1853 to bring safe water to populations and to protect them from waterborne epidemics which ravaged cities, its history is intertwined with that of water treatment.

Leseprobe

Drinking Water Treatment 4

Membranes Applied to Drinking Water and Desalination

Kader Gaid

First published 2023 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:

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© ISTE Ltd 2023The rights of Kader Gaid to be identified as the author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s), contributor(s) or editor(s) and do not necessarily reflect the views of ISTE Group.

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Library of Congress Control Number: 2022947257

British Library Cataloguing-in-Publication DataA CIP record for this book is available from the British LibraryISBN 978-1-78630-786-6

17Microfiltration and Ultrafiltration

A membrane is a semi-permeable barrier that allows the selective separation of compounds that are present in water, under the effect of a pressure gradient. There are a large number of membranes on the market with different characteristics.

This chapter applies to ultrafiltration (UF) and microfiltration (MF) membranes that are used in drinking water, but can also be used for process water and tertiary wastewater filtration applications.

MF and UF are two techniques designed for solid/liquid separation through the removal of particles and macromolecules. MF is essentially aimed at the retention of particles between 0.1 and 5 µm in size, while UF is aimed at particles between 0.1 and 0.01 µm in size. They constitute barriers for suspended solids, colloids, bacteria and parasites. UF also eliminates viruses. They use working pressures of 0.2–1 bar for MF and in the range of 2–5 bar for UF.

There are different types of membranes, classified according to the type and size of the constituents they separate from the water. The best choice of membrane is not the most selective one, but the one that is sufficiently selective for the objectives set, and which offers the best operating and investment costs.

UF and MF membranes are porous membranes made from synthetic or ceramic polymers.

Figure 17.1.Classification of different species according to their size and membrane cut-off. For a color version of this figure, see www.iste.co.uk/gaid/watertreatment4.zip

17.1. UF and MF: cut-off

The cut-off is the molar mass of the smallest compound retained at 90% by the membrane. The lower the cut-off, the more the membrane is able to retain small molecules or colloids. Conversely, the higher the cut-off, the less the membrane is able to retain small molecules. In general, the cut-off is expressed in pore size (µm) for MF, while for UF, it is expressed in molecular weight in Dalton (or molecular weight cut-off [MWCO]), which is equivalent to 1 g per mole. The Dalton is equivalent to one-twelfth of the mass of a carbon atom 12, expresses the mass of a hydrogen atom and is equal to 1.66 × 10–27 kg. Thus, a protein with a molar mass equal to 75,000 g·mol–1 will have a mass equal to 75,000 Da or 75 kDa.

MF and UF are found in a fairly wide range of pore sizes: from 0.1 to 1 µm. Typically, MF membranes used in clean water (drinking water, process water) have a pore size between 0.05 and 0.2 µm, while UF membranes used for these applications have a pore size close to 0.01 to 0.03 µm or a MWCO of 80–200 kDa.

Figure 17.2.Cut-off point for microorganisms with filtration, microfiltration and ultrafiltration. For a color version of this figure, see www.iste.co.uk/gaid/watertreatment4.zip

MF and UF are often distinguished by the log virus removal. Using the MS2 phage as a reference, a membrane is accepted as UF when the log MS2 removal is at least 4 log (Figure 17.3).

UF has two characteristics that make it a particularly suitable unit operation for water treatment:

– On the one hand, the size of the pores allows the retention of bacteria and viruses while letting the dissolved salts pass through. This makes it possible to combine food safety and preservation of the mineral balance, both of which are essential for water intended for consumption.

– On the other hand, UF uses relatively low transmembrane pressures (TMPs) compared to nanofiltration or reverse osmosis. Consequently, this limits the operating costs associated with the energy consumption, related to the TMP.

Figure 17.3.Log MS2 removal according to the cut-off of MF and UF membranes. For a color version of this figure, see, www.iste.co.uk/gaid/watertreatment4.zip

UF membranes with a low cut-off are mostly used in industrial applications (such as the food industry, ultra-pure water). However, there are some applications in drinking water where UF membranes close to nanofiltration are used to treat colored water, especially on units of small size.

Even though membranes are characterized with cut-off values (in µm or Da) in the data sheets given by the suppliers, a membrane does not have a uniform pore size, but rather a pore size distribution that is more or less tight. The pore size distribution is therefore as large as the advertised nominal pore size.

Figure 17.4.Example of pore size distribution curves (source VERI Veolia). For a color version of this figure, see www.iste.co.uk/gaid/watertreatment4.zip

The pore diameter can be obtained from the membrane cut-off (MWCO) and the following relationship:

With:

– dp: pore diameter (m);

– k: Boltzmann constant (1.38064852 × 10–23 m2·kg·s–2·K–1);

– µ: dynamic viscosity (1.31 × 10–3 kg·m–1·s–1);

– MWCO: membrane cut-off for a temperature of 25°C (298°K).

A membrane with a MWCO of 150,000 Da has a pore diameter of 11.5 nm. In contrast, an MF membrane with an MWCO of 1,000,000 Da has an estimated pore diameter of 100 nm. Veolia has a research and development center (VERI) which includes, among its many activities and applications, a technical expertise and assistance service specific to membranes. As such, the membranes on the market are listed, analyzed and tested for their physical, chemical and biological properties, as well as for their associated performance. Figure 17.4 is an example of the pore distribution curves on several membranes on the market. The definition of UF versus MF for a drinking water application is often taken in terms of virus removal.

Table 17.1.General characteristics of microfiltration and ultrafiltration membranes

Membranes

Operating modes

Types of separation

Cut-off characteristics (µm)

Permeate composition

Elements and compounds removed

Microfiltration

Hydrostatic pressure difference

Sieving

0.08–2

Dissolved substances

SS, turbidity, parasites, bacteria

Ultrafiltration

Hydrostatic pressure difference

Sieving

0.005–0.1

Dissolved substances < 100(PMkDa)

SS, turbidity, parasitic macromolecules, bacteria, viruses

17.2. UF and MF: materials

There is a wide range of membranes available today, depending on their material and intrinsic filtration capacity. MF and UF membranes can be made from a wide variety of materials that all have different properties. There are organic membranes and ceramic membranes.

Ceramic membranes are not currently used in drinking water applications because of their high cost. Therefore, only organic membranes are presented here. The membranes not only differ in the material used but also in their structure.

We distinguish isotropic membranes, called “symmetrical”, whose structural properties are constant throughout their thickness, whether they are dense or porous. We also distinguish anisotropic membranes, called “asymmetrical”, whose internal structure is different from one membrane to another.

Initially, the most used materials were cellulose acetate and polypropylene (PP). Nowadays, there are more and more polyether sulfone (PES)/polysulfone (PS) and polyvinylidene difluoride (PVDF) membranes. Here is a summary of the main properties of these materials.

17.2.1. Cellulose acetate

– It is naturally very hydrophilic.

– It is inexpensive raw material and relatively easy to manufacture membrane.

– It has low adsorption tendency (relatively stable permeability).

– It is relatively resistant to chlorine.

– It is sensitive to hydrolysis by acids and bases (pH between 4 and 8).

– It is sensitive to temperature (<35°C).

– It is subject to biological degradation.

17.2.2. Polypropylene

– It consisted of hydrophobic polymer, with relatively low permeability (limited ability to mix the polymer).

– It has good mechanical strength and flexibility of the fiber.

– It shows risk of membrane oxidation by chlorine.

– It has been widely used for the manufacture of MF membranes.

17.2.3. Polyacrylonitrile

– It is hydrophobic polymer by nature; it is modified to make it more hydrophilic.

– It has good chemical resistance (e.g. pH 1.5–11).

– It has moderate tolerance to chlorine (e.g. 100–200 ppm).

17.2.4. Polyether sulfone/polysulfone

– PES is a hydrophobic polymer by nature. It is modified with polymer blends to produce hydrophilic membranes.

– Membrane is quite easy to manufacture and different pore sizes can be easily obtained.

– It has good mechanical resistance.

– It has very good chemical resistance (e.g. pH 1–13).

– It has moderate tolerance to chlorine (e.g. 100–200 ppm per cleaning).

– It has good resistance to a wide range of temperatures (e.g. 1–125°C for the polymer, but may be limited to 40°C related to other module materials).

– It is commonly used for UF membranes with a tight pore size distribution.

17.2.5. Polyvinylidene fluoride

– PVDF is a low hydrophilic polymer.

– It has very good chemical resistance (e.g. pH 1.5–11).

– It has very high chlorine resistance (e.g. 500 or 1,000 ppm).

– It has good resistance to a wide range of temperatures (e.g. 1–40°C).

– It is very flexible material and mechanical resistance (ideal for applications with air scouring).

– It is mostly used for MF membranes, but also for some UF.

Figure 17.5.(a) Electron microscope view of a polypropylene MF membrane (0.2 µm) and (b) dimensions of the fibers. For a color version of this figure, see www.iste.co.uk/gaid/watertreatment4.zip

17.3. UF and MF: membrane types

UF and MF membranes come in different forms. Their dimensions and implementation are not standardized. It can sometimes be difficult to change membranes if the latter has not been considered at the beginning of the project. However, some membrane manufacturers can offer replacement products with the same dimensions that use the same equipment, when replacing membranes in a plant.

The most common type of membrane in the MF/UF drinking water market is the hollow fiber, because it is compact and its implementation is well adapted to this type of application (cleaning, frontal or tangential modes possible, etc.). These membranes correspond to cylinders of variable length and of very small diameter. The internal diameter can vary between 0.3 and 1.5 mm.

There are two types of hollow fiber membranes: single bore and multi-bore.

Table 17.2.Single bore and multi-bore hollow fiber membranes

Single bore hollow fibers

These are the ones proposed by most of the membrane manufacturers. They can be found in a pressurized or submerged configuration. They can sometimes have a support layer adapted for membranes that require a high mechanical resistance. The filtration can be done according to the type of modules: from the inside to the outside of the fibers or from the outside to the inside.

Multi-bore hollow fibers

In this configuration, each fiber has several internal channels, which provides a better mechanical resistance to the fiber. The material used is hydrophilized polyether sulfone. The filtration is only done from the inside of the fiber to the outside. This particular type of hollow fiber is proposed by very few membrane manufacturers.

The hollow fibers are grouped into bundles of several thousand fibers, and each bundle is placed in a cylindrical module. The ends of the fibers are sealed on both sides in a resin, leaving access to the feed water through the interior of the fibers. The feed water enters from the inside of the fibers and can only pass through the tiny pores along the fiber, while the particles remain inside.

The filtered water (or permeate) is isolated from the feed water compartments by the resin that was used to seal the fibers. It is then collected in a central tube and discharged through the ends of the modules.

Figure 17.6.Ultra-filtered feed water and water outlet points. For a color version of this figure, see www.iste.co.uk/gaid/watertreatment4.zip

The hollow fibers are supported in cylindrical housing modules for pressurized membranes, or in more or less open modules for submerged membranes. The fibers are fixed by an epoxy potting at their extremity, which ensures the physical separation between the feed side and the permeate side of the membranes.

In some cases, pressurized modules contain a central collector that requires gaskets to seal between the feed and the permeate.

Figure 17.7.Example of a hollow fiber module in an internal–external configuration with a central collector. For a color version of this figure, see www.iste.co.uk/gaid/watertreatment4.zip

17.4. UF and MF: implementation of membranes under pressure

The membranes are inserted into housings or pressure tubes, which are installed on a skid and pressurized with a booster pump. The pressure on the feed side of the module or pressure tube provides the necessary pressure gradient for the water to pass through the membrane pores.

Figure 17.8.Membranes in a pressurized configuration. For a color version of this figure, see www.iste.co.uk/gaid/watertreatment4.zip

17.4.1. Horizontal-vertical configuration

Depending on the application, it is possible to install pressurized or submerged membranes in a horizontal or vertical configuration.

Figure 17.9.Vertical (a) and horizontal (b) configuration for pressurized membranes. For a color version of this figure, see www.iste.co.uk/gaid/watertreatment4.zip

The MF-UF membranes under pressure are most often installed in a vertical configuration.

Some membrane suppliers offer a horizontal arrangement in pressure tubes with several modules per pressure tube. In some cases, this configuration provides a more compact solution in terms of floor space.

Inside the pressure tube, the membranes work in parallel. In the case of a four-element tube, the ultra-filtered water is fed and discharged on both sides. The modules have a bypass system surrounding the permeate collector, which supplies the central modules with raw water and discharges the dirty backwash water. Interconnecting parts between the modules ensure the hydraulic operation in the pressure tube.

Since the vertical configuration allows for tangential filtration and features air backwash conditions (backwash, forward rinse, air scour and chemical backwash), it is recommended for variations in raw water quality throughout the year, due to its more robust operation and cleaning facilities. In addition, a horizontal configuration is only possible for water qualities below 10 NTU, especially when there is no pre-treatment.

Figure 17.10.Filtration mode in a horizontal configuration (Xiga for Cabot-Norit). For a color version of this figure, see www.iste.co.uk/gaid/watertreatment4.zip

Figure 17.11.Backwash filtration mode in a horizontal configuration (Xiga for Cabot-Norit). For a color version of this figure, see www.iste.co.uk/gaid/watertreatment4.zip

Figure 17.12.Recommended net flows for both configurations depending on turbidity. For a color version of this figure, see www.iste.co.uk/gaid/watertreatment4.zip

The horizontal configuration also has the disadvantage of making it more difficult to identify a module with broken fibers. The use of air is rather restricted to integrity testing and is not used to improve backwashing, due to the difficulty of evacuating air during re-filtration and the horizontal position of the modules.

In terms of footprint, for high production capacities and good feed water qualities (<1 NTU), the horizontal configuration is more advantageous due to larger standardized units. For example, with 55 m2 Cabot-Norit membranes, the footprint of a 120-module vertical unit is 41.4 m2, while the footprint of a 192-module horizontal unit is 16.7 m2.

The footprint is a parameter considered by the manufacturer, but the operating conditions are often the selective parameter. For example, membranes are chosen in a vertical configuration for small installations when powdered activated carbon (PAC) has to be added, if tangential filtration has to be implemented, if turbidity is variable (especially in direct raw water supply), or because the addition of a coagulant and integrity tests are easier to implement than for vertical modules.

The vertical configuration is therefore the most frequently used. It allows for more complex backwash sequences, if necessary with air and water phases, and thus for the treatment of more heavily loaded water. Furthermore, as the vertical modules are individually connected to the collector and the horizontal modules are loaded into pressure tubes, the allowable operating pressure for horizontal configurations is higher.

17.4.2. Submerged membranes

Submerged MF membranes are available in a vertical configuration with, for example, vertical modules for Evoqua’s CMF-S® (formerly known as Memcor) and Zenon modules.

Figure 17.13.Configuration of submerged membranes. For a color version of this figure, see www.iste.co.uk/gaid/watertreatment4.zip

The membranes are submerged in a tank (steel, HDPE, coated concrete, etc.) called a “cell”, and the filtration is done by sucking water through the membrane thanks to a pump placed at the permeate outlet of the membrane cell.

The CMF-S® cell uses hollow fiber membranes. Feed water flows into a cell containing submerged membrane modules and is drawn through the membrane fibers in an external–internal filtration direction. The microfiltered water exits from the top of the module and is led to a central collector. CMF-S® systems can be equipped with both PP and PVDF membranes.

A backwash system uses a combination of air and water backwash to remove the build-up of solids that have been deposited on the outer surface of the membrane. Backwash frequencies range from 15 to 60 min.

Figure 17.14.Membranes in submerged configuration. For a color version of this figure, see www.iste.co.uk/gaid/watertreatment4.zip

Chemical cleanings are programmed for the removal of organic matter, mineral elements and biofilm that have not been eliminated during the hydraulic and chemical backwashes.

The CMF-S® system is designed with an automated integrity test, known as the “pressure decay test” (PDT).

The washing efficiency is obtained using the following equation:

With:

– R1: resistance calculated after a previous wash (m–1 × 1012);

– R2: resistance calculated before a wash, after an operating time corresponding to a volume of filtered water (m–1 × 1012);

– R3: resistance calculated after the wash (m–1 × 1012).

Figure 17.15.Washing efficiency based on the resistance to fouling

17.5. Filtration modes: frontal or tangential

UF can be conducted according to two flow modes, depending on the quality of the water to be treated and the objectives to be reached: tangential filtration or frontal filtration.

In tangential filtration, the fluid flows parallel to the membrane, resulting in a shear which limits particle accumulation. This mode of operation produces water continuously. In this implementation method, there is therefore inevitably an inlet for the feed and two outlets: one for the permeate and the other for a flow of water which does not pass through the membrane, which is known as concentrate or retentate.

Figure 17.16.Frontal and tangential filtration. For a color version of this figure, see www.iste.co.uk/gaid/watertreatment4.zip

This filtration method limits clogging and deposit formation by creating shear forces. However, the recirculation of the flows imposes a higher number of members and an additional cost due to recirculation energy. Tangential filtration is used for applications with loaded water or when adding PAC. It ensures that the membrane is scavenged and that deposits on the membrane surface are, to a certain extent, avoided.

In frontal filtration, the feed flow is perpendicular to the membrane and the particles are directly filtered by the membrane. There are no recirculation pumps and the feed flow is equivalent to the product flow. As filtration proceeds, the particles accumulate and form a deposit that clogs the membrane. Frontal filtration is the most common type of filtration for hollow fiber membranes in relatively clear water applications (drinking water, process water, tertiary wastewater). It consumes less energy than tangential filtration, which requires pumping much larger volumes of water.

17.5.1. Batch operation: filtration-backwash

In order to remove the particles that accumulate on the membrane (especially in the frontal mode), backwashes are regularly performed. Backwashes can consist of different phases usually including a washing sequence in the opposite direction of the filtration using permeate. However, they also include phases of sweeping the fiber with water or air. The sequences depend on the filtration direction (internal– external or external–internal). These backwashes are done very frequently, but last for a very short time.

Frequencies can, for example, be from one backwash every 15 min to one backwash every 90 min, and their duration can be from 30 s to 3 min.

Figure 17.17.Filtration-backwash diagram. For a color version of this figure, see www.iste.co.uk/gaid/watertreatment4.zip

17.5.2. Filtration direction

The filtration direction can be internal–external; in other words, the feed water enters the interior of the fibers and exits through the external surface. The filtration direction can also be external–internal; in other words, the feed water enters through the outer surface and exits through the inside of the fibers.

17.5.2.1. Internal–external filtration direction

Internal–external filtration has hydraulic advantages. The backwashing is done from the outside to the inside at a flux often two to three times the filtration flux.

The velocities are high and allow very efficient permeate backwashes.

Figure 17.18.Internal–external filtration. For a color version of this figure, see www.iste.co.uk/gaid/watertreatment4.zip

17.5.2.2. External–internal filtration direction