Pressure-driven nanofiltration membranes

Membrane bio/processes have also been used to remove inorganic anions within drinking water supplies Apart from the typical F removal by pressure-driven nanofiltration, dialytic membrane bio/processes by using ion exchange membranes have been employed to remove F (Velizarov et al., 2004). 

The majority of PMRs described in literature combines photocatalysis with a pressure-driven membrane technique, such as nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF), in which the catalyst is contained in the pressurized side of the membrane. 

As the quality of drinking water sources gets worse, the methods of water treatment or the traditional water treatment systems need to be modernized. Pressure-driven membrane systems such as reverse osmosis (RO), nanofiltration (NF) and ultrafiltration (UF) and electric-driven membrane system such as... 

The work described in this chapter is especially concerned with three of the most widely used pressure driven membrane processes microfiltration, ultrafiltration and nanofiltration. These are usually classified in terms of the size of materials which they separate, with ranges typically given as 10.0-0.1 xm for microfiltration, 0.1 p.m-5 nm for ultrafiltration, and 1 nm for nanofiltration. The membranes used have pore sizes in these ranges. Such pores are best visualised by means of atomic force microscopy (AFM) [3]. Figure 14.1 shows an example of a single pore in each of these three types of membrane. An industrial membrane process may use several hundred square meters of membrane area containing billions of such pores. 

A practically useful predictive method must provide quantitative process prediction from accessible physical property data. Such a method should be physically realistic and require a minimum number of assumptions. A method which is firmly based on the physics of the separation is likely to have the widest applicability. It is also an advantage if such a method does not involve mathematics which is tedious, complicated or difficult to follow. For the pressure driven processes of microfiltration, ultrafiltration and nanofiltration, such methods must be based on the microhydrodynamics and interfacial events occurring at the membrane surface and inside the membrane. This immediately points to the requirement for understanding the colloid science of such processes. Any such method must account properly for the electrostatic, dispersion, hydration and entropic interactions occurring between the solutes being separated and between such solutes and the membrane. 

Nanofiltration (NF) is a pressure-driven membrane separation technology used to separate ions from solution. Nanofiltration membranes were widely available beginning in the 1980 s. This technology uses microporous membranes with pore sizes ranging from about 0.001 to 0.01 microns. Nanofiltration is closely related to RO in that both technologies are used to separate ions from solution. Both NF and RO primarily use thin-film composite, polyamide membranes with a thin polyamide skin atop a polysulfone support (see Chapter 4.2.2). 

Liquid separation. Separation can take place between solvents and solutes, macromolecules or particles or between species in liquid media by the effect of size exclusion. That is, those molecules or colloids larger than the size of the membrane pores will be retained or rejected while those smaller ones can pass through the membrane. The size exclusion mechanism predominates in pressure driven membrane processes such as microfiltration, ultrafiltration and even nanofiltration which has a molecular selectivity on the order of one nanometer. 

Some areas of application are the nuclear industry and the treatment of radioactive liquid wastes, with two main purposes reduction in the waste volume for further disposal, and reuse of decontaminated water. Pressure-driven membrane processes (microfiltration, ultrafiltration, nanofiltration, and reverse osmosis [RO]) are widely used for the treatment of radioactive waste. 

During the last two decades, pressure-driven membrane processes namely reverse osmosis (RO), nanofiltration (NF), and ultrafiltration (UF) have found increased applications in water utilities and chemical industries. Unlike RO, NF, and UF, the Donnan membrane process (DMP) or Donnan dialysis is driven by an electrochemical potential gradient across an ion-exchange membrane. Theoretically, the DMP is not susceptible to fouling because particulate matter or large organic molecules do not concentrate on the membrane surface, as commonly observed with pressure-driven membrane processes. DMP has been used in the past in hydrometallurgical operations [19,20], for concentration of ionic contaminants [21,22] and for separation of... 

Separation processes as a whole have grown in importance because of increasingly stringent requirements for product purity [1]. Among the different membrane techniques, pressure-driven processes such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) were the first to undergo rapid commercialization [2-A], These processes basically differ in pore size distribution of membranes used and the types of compounds recovered. A typical schematic of the exclusion of various compounds through different membrane processes is illustrated in Figure 42.1. 

Nanofiltration is a rapidly advancing membrane separation technique for concentration/separation of important fine chemicals as well as treatment of effluents in pharmaceutical industry due to its unique charge-based repulsion property [5]. Nanofiltration, also termed as loose reverse osmosis, is capable of solving a wide variety of separation problems associated with bulk drug industry. It is a pressure-driven membrane process and indicates a specific domain of membrane technology that hes between ultrafiltration and reverse osmosis [6]. The process uses a membrane that selectively restricts flow of solutes while permitting flow of the solvent. It is closely related to reverse osmosis and is called loose RO as the pores in NF are more open than those in RO and compounds with molecular weight 150-300 Da are rejected. NF is a kinetic process and not equilibrium driven [7]. 

Nanofiltration is a pressure-driven process where the solvent is forced through the membrane by pressure, and other feed constituents randomly pass through the membrane by diffusion. The relative rates of solvent and solute passage determine the quality of the product. Nanofiltration membranes are adept at the separation of small, neutral, and charged solutes in aqueous solutions because they allow the passage of monovalent ions and retain multivalent ions due to their charge. Nanofiltration membranes exhibit two important features in their actual apphcations. They provide... 

Nano filtration According to the International Union of Pure and Applied Chemistry (lUPAC) recommendations [16] nanofiltration is a pressure-driven membrane-based separation process in which particles and dissolved molecules smaller than about 2 nm are retained. ... 

Membrane processes are widely used in oil water separation. In general, a membrane is classified into two groups pressure-driven membrane and electrical membrane, known as electrodialysis. The most applicable process for oily wastewater removal is the former type. The pressure-driven membrane applications include microfiltration (MF), ultrafil-tration (UF), nanofiltration (NF), and reverse osmosis (RO). All of them are categorized by the molecular weight or particle size cut-off of the membrane as shown in Table 5. 

A range of membrane processes are used to separate fine particles and colloids, macromolecules such as proteins, low-molecular-weight organics, and dissolved salts. These processes include the pressure-driven liquid-phase processes, microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), and the thermal processes, pervaporation (PV) and membrane distillation (MD), all of which operate with solvent (usually water) transmission. Processes that are solute transport are electrodialysis (ED) and dialysis (D), as well as applications of PV where the trace species is transmitted. In all of these applications, the conditions in the liquid boundary layer have a strong influence on membrane performance. For example, for the pressure-driven processes, the separation of solutes takes place at the membrane surface where the solvent passes through the membrane and the retained solutes cause the local concentration to increase. Membrane performance is usually compromised by concentration polarization and fouling. This section discusses the process limitations caused by the concentration polarization and the strategies available to limit their impact. 

For many years, polymeric membranes have been widely utilized in practical appHca-tions without having precise information on their pore size and pore size distribution, despite the fact that most commercial membranes are prepared by the phase inversion technique, and the performance of those membranes is known to be governed by their pore characteristics in a complicated manner [1]. These pore characteristics are influenced both by the molecular characteristics of the polymer and by the preparative method [2]. Crudely, membranes applied for pressure-driven separation processes can be distinguished on the basis of pore diameter as reverse osmosis (RO, < 1 nm), dialysis (2-5 nm), ultrafiltration (UF, 2-100 nm), and microfiltration (MF, 100 nm to 2 J,m). Nanofiltration (NF) membranes are a relatively new class and have applications in a wide range of fields [3]. The pore sizes of NF lie between those of RO and UF membranes. 

Nanohybrid materials have been furthermore used for ultra-/nanofiltration applications. Nanofiltration is a pressure-driven membrane separation process and can be used for the production of drinking water as well as for the treatment of process and waste waters. Some apphcations are desalination of brackish water, water softening, removal of micropollutants, and retention of dyes. Ultrafiltration membranes based on polysulfones filled with zirconia nanoparticles are usually prepared via a phase-inversion technique and have been used since 1990 [328]. Various studies were done in order to assess the effect of the addition of Zr02 to polysulfone-based ultrafiltration membranes [329] and the influence of filler loading on the compaction and filtration properties of membranes. The results indicate that the elastic strain of the nanohybrid membranes decreases and the time-dependent strain... 

Various pressure-driven membrane processes can be used to concentrate or purify a dilute (aqueous or non-aqueous) solution. The characteristic of these processes is that the solvent is the cominueous phase and that the concentration of the solute is relatively low. The particle or molecular size and chemical properties of the solute determine the structure, i.e. pore size and pore size distribution, necessary for the membrane employed. Various processes can be distinguished related to the panicle size of the solute and consequently to membrane structure. These processes are microfiJtration, ultrafiltration, nanofiltration and reverse osmosis. The principle of the four processes is illustrated in figure VI - 2. 

Membrane separation processes have been applied to many industrial production systems for the purpose of clarification, concentration, desalting, waste treatment, or product recovery. Broadly speaking, membrane filtration can be classified as microfiltration, ultrafiltration, nanofiltration, reverse osmosis, and dialysis or electrodialysis. In this section, the discussion will only cover microfiltration and ultrafiltration, both of which are pressure-driven membrane processes. 

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