Membrane Processes: Pervaporation, Vapor Permeation and Membrane Distillation for Industrial Scale Separations

By S. Sridhar

Separation processes are challenging steps in any process industry for isolation of products and recycling of reactants. Membrane technology has shown immense potential in separation of liquid and gaseous mixtures, effluent treatment, drinking water purification and solvent recovery. It has found endless popularity and wide acceptance for its small footprint, higher selectivity, scalability, energy saving capability and inherent ease of integration into other unit operations. There are many situations where the target component cannot be separated by distillation, liquid extraction, and evaporation. The different membrane processes such as pervaporation, vapor permeation and membrane distillation could be used for solving such industrial bottlenecks.

This book covers the entire array of fundamental aspects, membrane synthesis and applications in the chemical process industries (CPI). It also includes various applications of pervaporation, vapor permeation and membrane distillation in industrially and socially relevant problems including separation of azeotropic mixtures, close-boiling compounds, organic–organic mixtures, effluent treatment along with brackish and seawater desalination, and many others. These processes can also be applied for extraction of small quantities of value-added compounds such as flavors and fragrances and selective removal of hazardous impurities, viz., volatile organic compounds (VOCs) such as vinyl chloride, benzene, ethyl benzene and toluene from industrial effluents.

Including case studies, this is a must-have for any process or chemical engineer working in the industry today.  Also valuable as a learning tool, students and professors in chemical engineering, chemistry, and process engineering will benefit greatly from the groundbreaking new processes and technologies described in the volume. 

• Language: English
• Publisher: Wiley
• Release date: Nov 26, 2018
• ISBN: 9781119418351

Book preview

Preface

Advances in Pervaporation, Vapor Permeation and Membrane Distillation for Industrial-Scale Separations

Increasing human concern and general awareness of the rapidly deteriorating environment, substantiated by stringent rules and regulations imposed by governments for protecting the limited resources left in our ecosystem, has already prompted researchers to propose thousands of new research ideas and opportunities for nurturing their knowledge and making a worthwhile contribution. Despite the fact that the domain of process industry is already saturated with countless publications defending the successful history of existing and emerging production and separation technologies, there have always been some open-ended questions which call for further roadmaps to the remedial solution. Because separation processes are essential unit operations involved in any process industry for isolation of products, recycling of reactants and recovery of value-added by-products from process solutions, economical separation of liquid mixtures besides desalination of sea and brackish water assumes utmost significance. Rapid industrial growth, unacceptable industrial discharge, depletion of fossil fuel reserves and water scarcity are major setbacks in developing sustainable, cost-effective and energy-efficient treatment procedures. Membrane technologies have shown immense potential over the last two decades in effluent treatment, desalination and solvent recovery mainly due to their smaller footprint, higher selectivity, scalability, energy-saving capability and inherent ease of integration into other unit operations. Besides the diverse applications, this technology has also shown tremendous potential to deal with some challenging separation processes unable to be addressed by conventional processes and their level of maturity in terms of development. Membrane processes such as pervaporation, vapor permeation and membrane distillation could be used for solving such industrial bottlenecks. As the title indicates, advancement of these second-generation membrane processes using various polymeric membranes and their applications have been emphasized in this book.

The present compilation is an effort to provide fundamental concepts of these three processes along with a detailed understanding of transport phenomena to target a wide audience from undergraduates to research scholars. The book covers the complete arena of fundamental theoretical aspects of membrane-based unit operations and membrane synthesis. It also includes various applications of pervaporation, vapor permeation and membrane distillation processes for solving problems relevant to both industry and society, such as separation of azeotropes and close-boiling mixtures, organic–organic systems, effluent treatment and desalination of sea and brackish water. These processes can also be applied for extraction of small quantities of value-added compounds such as flavors and fragrances, and selective removal of hazardous impurities, viz., volatile organic compounds (VOCs) such as vinyl chloride, benzene, and toluene from industrial effluents besides thermally sensitive liquids like hydrazine. Enhancement of reaction yields in esterification and fermentation processes through continuous dewatering or by-product removal can also be realized through these processes. Starting from a brief introduction on membranes, this book aims to present a clear view on the typical applications of membranes, their synthesis, characterization and in-depth analysis of these separation techniques with a few case studies that highlight their utility value. The book also provides fundamental applications of computational fluid dynamics and molecular dynamics simulation to scale up laboratory process developments to industrial levels, in order to boost its overall appeal as research material or even a classroom teaching aid.

Sundergopal Sridhar

Siddhartha Moulik

Chapter 1

Tackling Challenging Industrial Separation Problems through Membrane Technology

Siddhartha Moulik1,2, Sowmya Parakala1 and S. Sridhar1,2,*

1Membrane Separations Group, Chemical Engineering Division

2Academy of Scientific and Innovative Research (AcSIR), CSIR-Indian Institute of Chemical Technology (CSIR-IICT), Hyderabad, India – 500007

*Corresponding author: [e-mail]

Abstract

Membrane processes such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) for water and wastewater treatment have been developed, and widely used over the past few decades with proven efficiency. Currently emerging membrane techniques such as i.e., pervaporation (PV), vapor permeation (VP) and membrane distillation (MD) are being developed since they portray high potential with additional advantages and minor limitations over the conventional processes, and are thus regarded as next generation technologies. These processes can easily be retrofitted with existing conventional treatment methods for drinking water purifications, domestic, municipal and industrial wastewater treatment. The fundamentals of water resources available and their contamination, water and wastewater and their conventional treatment techniques, and the limitations of first-generation membrane technologies will be discussed in this chapter. The main focus is on technologies which are low cost and energy efficient, and provide high yield and desired purity with minimal sludge. The potential of second-generation membrane processes, i.e., PV, VP and MD has been explored and showed promising results by fulfilling all the ideal characteristics of next-generation membrane separation processes.

Keywords: Membrane technology, water/wastewater treatment pervaporation, vapor permeation, membrane distillation

1.1 Water: The Source of Life

Water has become the ‘mother’ and ‘matrix’ of life, essentially interlinked with the existence of biological lifecycles. Water is certainly the most precious natural resource that exists in the universe for human consumption. Water on the earth is available mostly as salt water (97.5%) with the remaining 2.5% being fresh water [1], which can be further distributed into 0.3% in lakes and rivers, 30.8% groundwater including soil water, swamp water and permafrost, and 68.9% as glaciers and permanent snow covers. Only 30% of the total fresh water resources are easily accessible to humans (as lakes and streams), which is shown in Figure 1.1. This raises a question on whether there will be enough fresh water available for future generations. Hence, there arises the need to make the fresh water resources sustainable; through recycle and treatment of wastewater, desalination of sea and brackish water, purification of contaminated groundwater and surface water for making them safe for drinking or utilization as process water. This chapter discusses the technologies required for drinking water purification and effluent treatment using first and second generation membrane separations.

Figure 1.1 Distribution of water on the earth.

The total mass of water on the earth is constant in its different phases such as ice, atmospheric water, clean water and ground water. The overall hydrologic cycle is a conceptual model that undergoes storage and movement of water between the biosphere, atmosphere, lithosphere and the hydrosphere. There is no starting point for the water cycle. Since three-fourth of the earth’s surface is occupied by water, it will be wise to initiate its cycle from the oceans. Water gets warmed up in daylight and evaporates. Some water gets directly sublimated from the glaciers and icebergs and rises upward. At higher altitude, the gradual decrease in ambient temperature and pressure make the vapors supersaturated and they get condensed, forming clouds. The clouds roam miles across the earth and finally at a supersaturated state, fall in the form of rain, snow, hail, dew, frost or sleet over the Earth. Water is distributed in nature in different forms such as rain, river, spring, mineral water and seepage water, which get stored in different natural reservoirs such as atmosphere, oceans, lakes, rivers, soils, glaciers, snowfields and ground water table [2]. Water is not only vital for sustenance of life, but also essential for socio-economic developments such as agriculture, industry, energy production, transportation, etc. The rapid industrialization and urbanization would have been scarcely possible without adequate supply of water. Renewable surface and ground water are the natural resources recognized to meet the increasing demands of society. With increasing population and exploitation of natural resources for one’s own benefit, mankind has behaved in a wild manner by creating problems of pollution which are hazardous to life as well as aquatic flora and fauna. To combat water pollution, we must understand the sources and problems from the grassroot stages to be a part of the solution.

Water is the major requirement for a healthy life. Keeping water sources free from pollution is of utmost importance in the drive towards water conservation. Polluted water is the major source of diseases, and the land also becomes unfit to sustain life. At present, the potable water consumed by 80 to 90% of the population is of poorer quality by international standards and 2.1 billion people lack access to safely managed drinking water services [3]. Thus, the source of water available to humans is much lower than the 1% present in lakes, streams and underground. Misuse, pollution of water bodies and uncontrolled growth of human population further strain this limited resources. Surface water can be found on the land in the form of streams, ponds, marshes, lakes or other fresh (not salty) sources. Rivers are the main source of surface water and as many as 13 rivers in India are categorized as major sources of surface water with a catchment area around 252.8 million hectares. Surface water as reported by CPCB [4] from 120 rivers contains toxic metals. Toxic metals like arsenic, copper, chromium, nickel, mercury and lead were found to be present in rivers mostly in permissible limits except for few. CPCB has identified 1145 industries in the country that pollute river bodies. Ground water is found underground in the sweeps and spaces, soil, sand and rocks. A global scenario of ground water usage is represented in Figure 1.2. Current status of ground water in India is shown in Table 1.1 [5]. It is stored from rain and slowly moves through the geological formations of the soil. Ground water is the biggest source of drinking water in India as more than three-quarters of the Indian population depends on it but in more than 10 states, it is contaminated by arsenic.

Figure 1.2 Global scenario of ground water utilization.

Table 1.1 Status of ground water in India (5).

Unfortunately, water that is available as ground water resource is also being contaminated with naturally occurring chemicals besides fluoride, arsenic, lead, salt, aluminum, chromium, copper, pathogens, ammonia, nitrates or nitrites. Permissible and desirable limits of a few essential characteristics that determine the quality of water can be obtained from Table 1.2 [6]. Metal poisoning and bacterial contamination has also affected the ground water apart from contamination by fluoride, arsenic and nitrates. Arsenic levels beyond permissible limits in drinking water is the main cause of arsenic toxicity and skin cancer in Taiwan, China, Chile, Argentina, Mexico, India, Hungary Bangladesh, the United States and Thailand. Eleven countries in the world have more than 50% of their population drinking fluoridated water: Australia (80% population), Brunei (95%), Chile (70%), Guyana (62%), Hong Kong (100%), the Irish Republic (73%), Israel (70%), Malaysia (75%), New Zealand (62%), Singapore (100%), and the United States (64%). The graphical representation of fluoride and arsenic affected areas in India can be observed in Figure 1.3 (a) and (b) respectively (7, 8).

Figure 1.3 Range of (a) fluoride in ground water (mg/L)

Figure 1.3 (b) Range of Arsenic in ground water (µg/L) as reported.

Table 1.2 Drinking water specifications by Ministry of Water Resources, (Amended) (6).

1.2 Significance of Water/Wastewater Treatment

Countries around the globe have been competing against each other in terms of development through rapid industrialization and urbanization. This has sequentially resulted in depletion and pollution of the available water resources. Water purification processes have been developed from the past few centuries for protection of public health from pathogens and chemicals. The processes are mainly targeted to remove contaminants from water bodies and improve water quality all over the world. Conventional surface water treatment includes screening, wherein primary objects like sand, dust and larger objects are removed. Screening is followed by coagulation and flocculation where coagulants are added for settlement of tiny particles and formation of large flocs; followed by filtration post sedimentation to remove the settled solids using sand filtration or microfiltration. To stop the growth of bacteria or to kill the microbes and purify water, a disinfectant is added to the water. Chemical treatment is also carried out to adjust the pH of the water and prevent tooth decay. At the final stage, the organic or inorganic contaminants are removed using ion exchange resin or carbon column treatment. These processes can be integrated and used routinely for water treatment. Moreover, the choice of process depends on the characteristics of the water, the types of water quality problems and the cost of different treatments methods. Therefore, scientists and engineers have been developing ways of treating water rapidly and effectively in a controlled manner, at lower cost. Sustainable operation of these treatment processes is to be considered based on availability of materials and ease of maintenance [9].

For drinking water purification and desalination, many membrane processes have become popular in the last few decades. Conventional membrane processes like microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), pervaporation and electrodialysis have gained wide acceptance in this field [10]. In the current situation, with no sustainable use of fresh water, it is important to deploy low-cost, and energy-efficient processes. Membrane distillation has been reported as an alternative for water purification. It is defined as a thermally driven process in which the separation is done by phase change. MD is mainly used for applications in which water is the major component present in the feed solution and needs to be separated.

Wastewater can be characterized on the basis of its physical, chemical and biological properties. Physical characteristics of wastewater include color, odor, temperature, suspended and dissolved solids and turbidity. Chemical properties include primary pollutants, fats, oils, proteins, VOCs, pesticides, heavy metals, chlorides, nutrients, and few gases such as methane, oxygen and hydrogen sulfide. Biological characteristics include constituents such as bacteria and viruses, natural organic matter (NOM) and BOD that contaminate open water bodies and treatment plants. Wastewater is the major concern in present times and can be classified into two categories: a) domestic or municipal wastewater; b) industrial wastewater.

Domestic or Municipal Wastewater: Municipal wastewater, also called sewage, contains organic wastes, pathogens, heavy metals and detergents which cause water pollution because of negligence regarding discharge requirements. This wastewater comes from household domestic use including black water, grey water and yellow water. Before releasing the sewage into the environment, physical, chemical and biological processes are carried out to treat such water in a wastewater treatment plant. Steps of treatment involve removal of suspended solids, degradation of organic wastes and separation of inorganic salts, which can be categorized into primary, secondary and tertiary treatments.

Industrial wastewater: Wastes generated in industries during the production processes are directly discharged into the nearby water bodies. Water that has been used in chemical process industries cause major pollution as they are discharged directly into surface water such as lakes, rivers and oceans. Industrial waste water is a major concern since its treatment of such water is highly expensive and requires efficient methods for separation of heavy metals, dissolved inorganic wastes, suspended solids, bio-degradable organic components, carcinogenic and toxic components, highly acidic or alkaline components and colored substances [11].

1.3 Wastewater Treatment Techniques

Technologies include physical, chemical and biological processes for treatment of industrial effluents as well as municipal wastewater which can be classified into primary treatment, secondary treatment and advanced treatment by an increasing order of degree of treatment to remove suspended solids, organic matter and nutrients from the wastewater. Preliminary treatment includes removal of large solids and suspended particles which are generally present in the untreated wastewater via screening, grit chambers and clarifiers. Sedimentation and skimming are generally used to remove the particles which can settle and float and come under primary treatment. Secondary treatment is carried out prior to tertiary treatment, which includes removal of organic waste, and microorganisms. For removal of nutrients such as nitrogen or phosphorous, processes involved in secondary treatment are activated sludge, trickling filters, and rotating biological contactors (RBC) or bio-filters. Tertiary treatment, also considered as advanced treatment further improves the quality of the secondary treated water. Chlorination, ozonation, biological nitrification, denitrification, ultraviolet disinfection, suspended growth processes and membrane techniques can be used for removal of pathogens, nutrients like nitrogen and inorganic salts [12]. Emerging issues in wastewater treatment include high energy consumption, cost of construction of the treatment plants, scarcity of water resources, disposal of bio-solids and dried sludge, with low performance index. Conventional primary treatment processes also have disadvantages although they are efficient in removing 97% of suspended solids. They require high capital investment and large footprint to accommodate a huge number of tanks, which are not easy to clean. Oxidation processes are expensive while chlorination cannot remove all the contaminants in a single step. The major disadvantage with stripping is that bulk pollutants and metals are not removed. Chemical process industries manufacture any particular compound/material/chemical using three main stages; synthesis, separation and purification. Among these, separation processes play a crucial role as they account for 40–90% of capital and operating costs. Separation processes are required mostly when there is incomplete conversion of reactants to products. The need for separation processes in any industry is to reduce the cost of a process by reusing the unutilized reactant by separating it from the product stream, and minimize the possibility of side reactions and unexpected hazards. Major conventional treatment processes including physical, chemical and biological treatment involve high maintenance, labor costs, and huge sludge handling costs. A few conventional separation processes which are well developed include distillation, adsorption, absorption, centrifugation, precipitation, sedimentation, stripping, chemical precipitation, ion-exchange etc [13]. Even with numerous advantages, these methods are not preferable because of some drawbacks. Physical methods produce a high quality effluent with no sludge, but they are not economically feasible while simultaneous formation of by-products and their management becomes another issue. Chemical methods are considered efficient since they remove all types of pollutants and deliver high product quality, but the cost involved in these methods is quite high whereas, the disposal of accumulated concentrated sludge is an added problem. Fouling is a disadvantage with ion-exchange processes mainly when extremely hard water is used. Ion exchange resins also aid growth of bacteria which cannot be heat treated as it will damage the resin. Biological methods are economically attractive but very slow. Also, this method needs favorable conditions which include proper maintenance and nutrition supply. Distillation is a tried and tested method for water purification which, however, involves high energy consumption with nominal efficiency. Industrial wastewater is generally obtained from organic chemical industry, paper and pulp industry, power plants and nuclear plants, food industry, iron and steel industry, mines and quarries, distilleries, textile mills, tanneries, bulk drug units etc. For specific applications such as treatment of industrial brine or the concentration of aqueous streams containing high total dissolved solids (TDS), multi stage flash distillation or multiple effect evaporation can be considered, which yield pure water, although there is a disadvantage of higher energy consumption and possibility of corrosion because of the high solute concentration levels. Municipal wastewater treatment using distillation is not very efficient although it can remove viruses and bacteria, heavy metals like lead, arsenic and minerals from the effluent, due to the phase change involved and the transfer of VOC’s, chlorine and its byproducts to the condensate. Thus, the final product obtained is not completely treated wastewater as it would still contain 80% of the contaminants. Multiple effect evaporators can also be used for water and wastewater treatment primarily for removing moisture through vaporization and reducing the amount of waste product that must be processed in industries.

Membrane separation processes are the newer separation techniques that have wide applications, thus replacing conventional methodologies over the past few decades. As proposed by André B et al., Figure 1.4 shows the technological maturity of various separation processes [14]. The technology asymptote is an assumption that everything is known to the researcher and there cannot be any further improvements possible. The use of asymptote denotes that the process has been fully developed. The processes that are ranked near the bottom are likely to be developed after extensive research. Thus, membrane processes have a high scope for development to achieve maximum separation.

Figure 1.4 Technology maturity index.

Since the 1960s, membrane technologies have proven helpful in reducing the scarcity of water by treatment of wastewater before it is directly discharged into the water bodies. The cost of earlier membranes affected the usage of earlier membranes systems for wide applications. Later, due to the remarkable fall in the cost, membrane systems replaced many conventional treatment processes because of their feasibility, cost efficiency and product quality.

1.4 Membrane Technologies for Water/Wastewater Treatment

The term Membrane was derived from a Latin word membrana which means skin [15]. A membrane acting as an interface between two bulk fluids can be defined as a semi-permeable material which selectively separates components from a mixture when a suitable driving force is applied across it, i.e., pressure difference, concentration difference, electrical potential difference or temperature difference. These driving forces affect the mass transfer through the membranes which either cause convective or diffusive transport of materials. A few characteristic properties required in an ideal membrane are high selectivity, high thermal and mechanical stability, resistance to chemicals, low fouling potential, high permeability, film forming property and a cost effective manufacturing process. Permeability of the membrane affects the flux of the process, whereas the selectivity of a membrane alters the purity of a product, thus it is important to maintain a high selectivity to reduce the number of stages in purification of a product. The use of membrane is a lot more advantageous since they are compact, necessitate low capital cost, involve easier operation and maintenance schedules and energy efficiency, besides delivering a high quality product. Though membranes are undergoing rapid development and commercialization, a few challenges that remain to be met are uniform pore size distribution, stability at high temperatures and high separation factors.

1.5 Membranes: Materials, Classification and Configurations

The choice of the membrane for a process largely depends upon the application. Different applications use diverse membranes made of dissimilar materials. Generally, membranes can be divided into biological membranes and synthetic membranes. Synthetic membranes are used in separation of industrial liquid or gaseous mixtures, whereas biological membranes are cell membranes constituting plant and animal cell wall. Synthetic membranes can further be classified into organic and inorganic membranes. Polymeric membranes fall under the category of organic membranes which have utmost importance in separation industry because of their ease of preparation, low capital cost, and less resistance to high temperature and destructive chemicals. Cellulose acetate is one of the oldest membrane materials, which was explored in the 1950’s. Porous polymeric membranes with excellent mechanical properties such as polyamide, polysulfone, polyacrylonitrile, polyvinyl alcohol, polyether sulfone, polyethylene, vinyl polymers, polyolefins, poly(ether imide), polypropylene, polycarbonate etc. were developed for various processes depending on their pore size.

1.5.1 Types of Membranes

Synthetic membranes are classified into dense, porous and composite membranes. Porous membranes are further divided into symmetric (Isotropic) and asymmetric (Anisotropic), electrically charged thin film composite and inorganic membranes [16]. Dense membranes allow transport of molecules by chemical potential gradient, electrical potential gradient or pressure gradient. Polymeric membranes are classified in Figure 1.5 (17).

Figure 1.5 Structures of polymeric membranes. (Reproduced with permission from Encyclopedia of Separation Technology) [17].

1.5.1.1 Symmetric Membranes

Symmetric membranes comprise of an uniform overall structure which can be either dense or microporous. Dense symmetrical membranes consist of a nonporous film of pore diameter much lower than 1Å and are generally less than 20 µm thick, through which mixtures of molecules are transported by sorption and diffusion. These membranes are characterized to study the properties achieved via solution casting method. Since the flux obtained from these membranes is low, their utilization is restricted to gas separation and pervaporation.

Microporous symmetrical membrane consists of pores that are uniform throughout the membrane, and they contain a pore size ranging from 0.01 to 10 µm. These membranes are used as filters since particles larger than the pore size are retained at the membrane surface, allowing only a few components to pass through. These membranes are characterized for porosity, pore diameter and tortuosity.

1.5.1.2 Asymmetric Membranes

Asymmetric membranes introduced by Loeb and Sourirajan [18] comprise of a very thin skin layer on a porous support containing voids all over to serve as the substrate for the skin layer. Asymmetric membranes have a thickness of less than 50 nanometer (nm). These membranes are characterized for thickness and pore size of skin. Also, the thin skin layer helps in determining the rate of mass transfer through these membranes. Asymmetric membranes are further classified into integrally skinned membrane (formation of skin layer via phase inversion) and non-integrally skinned membrane (formation of lumen side skin layer by the bore fluid during extrusion of hollow fibers). The advantages of the higher fluxes provided by anisotropic membranes are so great that almost all commercial processes use such membranes. The transport rate of a species through a membrane is inversely proportional to the membrane thickness.

1.5.1.3 Electrically Charged Membranes

Microporous membranes consisting of fixed positive and negative charges in the pore walls are defined as electrically charged membranes. Membranes with positively charge ions present in it are called anion exchange membranes and the ones with negatively charge ions are called cation exchange membranes. Separation is carried out by excluding ions of the same charge. These charged membranes are used for processing electrolyte solutions containing ionizable species in electrodialysis process.

1.5.1.4 Inorganic Membranes

Inorganic membranes are mostly based on ceramic and zeolite materials to constitute a special class of microporous membranes, useful in cases where high thermal stability and resistance to solvents are necessary. Compared to metal membranes which have a strong stability even at 500–800 °C, ceramic membranes can show stability up to 1000°C. Supported liquid membranes are also developed for facilitated and selective transport processes. Metal membranes based on palladium are developed for separation of hydrogen gas from its mixture with other gases in refineries, water-gas shift reaction and ammonia purge gas.

1.5.2 Membranes Modules and Their Characteristics

Generally, industrial-scale separations require a large surface area to perform the separation process. To achieve the desired separation economically and effectively, different modules have been developed which consider cost, power consumption and ease of replacement. The four membrane modules and their advantages are discussed briefly (19, 20).

Tubular

Spiral wound

Hollow fiber

Plate and frame

Different types of membrane geometries that are currently used are represented in Figure 1.6.

Figure 1.6 Schematic of various membrane modules (20) (Reproduced with permission from Royal Society of Chemistry).

Tubular Module: Tubular modules are like shell and tube heat exchanger. These modules contain over 30 perforated tubes each with a diameter of 0.5 to 1.0 cm, accommodated in a large single tube. The feed usually passes through the tubes and permeate is collected through the shell side. These modules are highly resistant to membrane fouling due to easy cleaning but exhibit mechanical stability. However, because of the cross flow mode of operation, the concept of concentration polarization is eliminated, and the module is successful in processing feeds containing large concentrations of suspended solids due to their robust structure. Cleaning of tubular membrane modules is done by forcing sponge balls through the tubes, thus eliminating use of chemicals. These modules are generally used for fruit juice clarification, and oily wastewater treatment resulting in a membrane life of up to 10 years depending on the application.

Plate and Frame: One of the earliest membrane modules developed are plate and frame configuration but owing to their relatively high cost, these modules are preferred only for electrodialysis and forward osmosis applications. The built of these membrane modules mainly depends on the thin support plates which are sealed with flat sheet membranes on either side, preventing leakage. The resulting permeate is collected through the conduits present in these thin plates after passing through the membrane. A few advantages of these modules are that the membranes can be taken out and cleaned easily, thus controlling fouling, but, efficiency is low as the packing density is less.

Spiral wound: Large membrane envelopes are rolled around a central collection pipe with a permeate spacer sandwiched in between two membranes with the support side facing the permeate spacer and membrane side facing feed distributor. The feed flows parallel to the perforated collection tube wherein permeate falls after flowing along the permeate spacer in a direction perpendicular to that of feed. Industrially, tubular pressure vessels containing up to 6 spiral wound modules of 40 inch length and 8 inch diameter are used. These modules are highly commercialized as they are compact, can withstand excess pressure and face minimum concentration polarization.

Hollow fiber: In recent years, polymeric hollow fiber membranes have gained huge potential in various separation technologies [21]. Compared to the flat-sheet, the hollow-fiber configuration has a much larger membrane area per unit volume in the module and hence, higher productivity [22]. Nowadays, hollow fibers are widely used in ultrafiltration, dialysis and supported liquid membrane extraction. Synthetic hollow fibers are spun by forcing a polymeric solution, under pressure, through a plurality of orifice formed in a spinneret from which the polymeric material issues in the form of continuous fibers which undergo phase inversion in an appropriate non-solvent [23, 24]. Three key elements that determine the potential and applications of a hollow fiber membrane are pore diameter and pore size distribution, selective layer thickness, and inherent properties (chemistry and physics) of the membrane material. Pore size and its distribution usually determine membrane applications, separation factor, or selectivity. The selective layer thickness determines the membrane flux or productivity. Material chemistry and physics governs biocompatibility of membranes used in dialysis besides biomedical and tissue engineering.

1.6 Introduction to Membrane Processes

Over the years, membrane processes have been developed for treatment of water and wastewater that are generated from various industries. In the current scenario of water scarcity, these membrane processes have either substituted or complemented conventional treatment methods for effluent treatment and drinking water purification. Membrane processes for water/wastewater treatment include Reverse osmosis (RO), Nanofiltration (NF), Ultrafiltration (UF) and Microfiltration (MF). Prefiltration is carried out using MF and UF as they remove bigger particles as well as bacterial microbial load. RO completely removes all the inorganic pollutants, ions and pathogens including virus, whereas NF can be used for removal of heavy metals or dissolved organics from the wastewater. Membrane bioreactors are a combination of biological treatment methods such as aerobic and anaerobic processes and membrane techniques like MF and UF.

1.6.1 Conventional Membrane Processes

Reverse Osmosis (RO)) is a green technology used for the treatment of boiler feed water, ground water purification and recovery of wastewater for reuse. Even though the technology has been widely used, it is limited by high osmotic pressures exerted by seawater or effluents containing high TDS. The principle of RO is shown in Figure 1.7 where the solvent which is mostly water is forced through the membrane at a much faster rate than dissolved solids like salts. The net effect is that a solute-solvent separation occurs rendering pure water as product. Water molecules naturally move from pure water side to the salty water side exerting osmotic pressure on the membrane. An external hydraulic pressure is applied to the salty side to overcome the osmotic pressure, thus rejecting the salts and allowing pure water to diffuse through the semipermeable membrane. Pore size of the membrane is in the range of 1 Å (0.1 nm) in diameter, which helps to hold back dissolved salts, colloidal particles, sediments and microorganisms. It operates at a pressure range of 10–50 bar which results in a flux rate of 20–50 L/m²h. RO treatment removes particles with a molecular weight cut off of > 50 Daltons, i.e., almost all the dissolved contaminants along with reducing total dissolved solids (TDS), viruses and bacteria, organic pollutants and heavy metals. Also, trihalomethanes (THM’s) like chloroform, bromoform, dibromochloromethane, and bromodichloromethane and volatile organic compounds (VOC’s) are removed. Membranes used in RO are of two types i.e., asymmetric and thin film composite made of materials like cellulose acetate and aromatic polyamides with the most commercially successful modules being spiral wound. RO has wide range of applications such as brackish water desalination, treatment of effluents from chemical, pharmaceutical, textile and other process industries, concentration of fruit juices, recovery of essential components from wastewater and concentration of sludge. Major disadvantage of RO for water/wastewater treatment is that the membrane might get damaged because of the constituents present in the wastewater such as strong acids, bases and free chlorine. Also, there is a high risk of membranes getting fouled due to high concentration polarization by organic and inorganic contaminants, biological matter and metal oxides. N-nitrosodimethylamine (NDMA), a strong carcinogen, which causes cancer, can also be removed using RO. The major advantage of RO membrane is that it removes all impurities in a single step. It offers an ideal method for water purification with low operating cost which is less than seven paisa per liter of purified water and high production capacity. RO is advantageous when the feed wastewater is first pretreated (using microfiltration) to remove colloidal residues and any suspended solids. The disposal of residual wastewater and the rejected concentrate is a major issue. Also, the process is not very cost efficient compared to conventional treatment. Thus, RO can be best suitable for ground water or pretreated wastewater. But owing to the limitations discussed earlier, the demand grows for newer membrane techniques that can offer better results with minimal disadvantages.

Figure 1.7 Principle of reverse osmosis.

Nanofiltration (NF): Nanofiltration is an advanced pressure process that falls between RO and UF which removes dissolved contaminants and particulate matter using a nanoporous semi-permeable membrane whose pore size ranges from 1–10 nm which corresponds to the molecular weight cut-off of 300–500 g/mole. A flux rate of 20–200 L/m²h is obtained when the process is operated at pressures 5–21 bar. The principle of NF, as shown in Figure 1.8 [38], is similar to RO where a pressure gradient transports solvent molecules through the membrane which removes solutes (TDS), color, hardness, dissolved organics and turbidity. NF membranes are comprised of aromatic polyamides and cellulose acetate membranes along with polysulfone. Spiral wound modules are currently used for NF applications, but hollow fiber geometry could be preferred in future for reducing the costs. NF can be applied for purification of moderate TDS ground water (up to TDS of 800 ppm), and reduction in hardness by removing multivalent ions as a better option than lime softening process. A few other applications of NF include pretreatment in desalination of sea water, processing of dairy and textile industry effluents, concentration of sugar solutions, clarification, concentration and deacidification of fruit juices, treatment of landfill leachate and separation of heavy metals from aqueous solutions. NF could play an important part as a low pressure substitute to RO by reducing the energy requirement with production of water of desired quality. Here, the separation occurs by molecular sieving or charge based exclusion wherein monovalents salts such as NaCl pass through the membrane while bivalent ones like CaCO3 undergo maximum retention. NF finds wide application for brackish water treatment [25], water softening [26], industrial waste water treatment [27], food processing [28] etc.

Figure 1.8 Principle of nanofiltration [38] (Reproduced with permission from Elsevier).

Ultrafiltration (UF): Ultrafiltration is also a pressure driven process where membrane pore size varies from 0.005 to 0.02 µm which enables separation of colloidal substances and macromolecules, retention of suspended solids and high molecular weight solutes UF is carried out at ambient temperatures, and operates at pressures ranging from 1 to 10 bar producing a flux rate of 50–1000 L/m²h. As shown in Figure 1.9, when a process feed solution is pressurized against a semi-permeable membrane, the solutes that are smaller than the molecular weight cut-off (MWCO) of the membrane pass through it whereas atleast 85% of larger molecules are retained. Membranes used in UF are also composite in nature and made of materials like cellulose acetate, polysulfone, polyethersulfone or polyvinylidene fluoride. UF removes bacteria and viruses but not dissolved solids like hardness, nitrates or heavy metals from the feed. Major drawbacks of ultrafiltration include membrane fouling leading to a fall in flux and rejection. Fouling can be prevented by pre-treating the feed water to extend the life of UF membrane to 3 years. Major applications of UF are concentration of proteins, enzymes and antibiotics besides clarification of fruit juices and sugar solutions. UF is also increasingly being used to replace sand and activated carbon columns for the pretreatment step in RO or NF systems.

Figure 1.9 Process Flow Diagram of skid mounted UF system for surface water purification.

Microfiltration (MF): MF process operates over a pressure range of 0 to 2 bar for removal of suspended particles, bacteria and large colloids from feed solutions. MF mostly uses porous membranes