Nanofiltration water softening

Micro-, ultra-, and nano-filtration can separate smaller particles using media with defined porous sizes (i.e., 10 1—1 pm in microfiltration, 10 2—10 pm in ultrafiltration, and 10 3— 10 2 pm in nanofiltration). Residual colloidal and suspended solids can be removed by microfiltration. Selected salts, most organic compounds, bacteria, protozoan cysts, oocysts and viruses are removed by nanofiltration, so that the treated water will be disinfected. This advanced filtration is used for the treatment of effluents for indirect potable reuse applications such as groundwater injection, water softening, decoloriza-tion, or removal of micropollution. 

Nanofiltration <2 nm Sieving + specific interactions 10-40 bars Purification, water softening, ... 

Design NF or softening of nanofiltration or softening Industrial or municipal Surface water at... 

Ground and surface water applications include water softening [115], water disinfection by-product removal [116], natural organic matter removal [116-117], pesticide removal [118], and removal of a wide range of other pollutants [119]. Nanofiltration also can be used to remove microorganisms and viruses [120-121]. Such applications do not require the low molecular weight selectivity of reverse osmosis and are well suited for low-pressure nanofiltration. 

While reverse osmosis and ultrafiltration were being established in several applications, there was a lack of available membranes with cutoffs between 400 and 4000 g/mol. Increasing interest in NF membranes developed in the last decade. An extensive review on principles and applications of nanofiltration has been published recently [38]. Nanofiltration is important for water softening [39] and removal of organic contaminants. In the food industry, nanofiltration can be applied for concentration and demineralization of whey, concentration of sugar and juice. Nanofiltration also finds application in the pulp and paper industry, in the concentration of textile dye effluents and in landfill leachate treatment. The improvement of solvent stabihty of available NF membranes opens a wide range of potential applications in the chemical and pharmaceutical industry as weU as in metal and acid recovery. 

RO membrane separation has been traditionally used for seawater and brackish water desalination, and production of high-purity water for food, pharmaceutical processing and industrial waste treatment, as discussed in Chapter 1. The development of nanofiltration (NF) membranes has opened up many areas of apphcation including water softening, removal of disinfection by-product precurson (trihalomethanes), removal of total organic carbon (TOC), food processing and industrial water treatment [5]. 

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... 

Water softening (nanofiltration—see Chapter 16.2) 50 psig up to 150 psig... 

Fig. Z8 Schematic representation of water softening using nanofiltration membrane...

Groundwater generally has a stable composition, but it is often too hard for use as process water or drinking water. Softening is necessary in many cases. Nanofiltration is here a competitive alternative for lime softening a comparison between both for application in Florida by Bergman (1995) showed that for lime softening operation and maintenance... 

Rahimpour, A., M. Jahanshahi, N. Mortazavian, S. S. Madaeni, and Y. Mansourpanah. 2010. Preparation and characterization of asymmetric polyethersulfone and thin-film composite polyamide nanofiltration membranes for water softening./4pp/. Surf. Sci. 256 1657-1663. 

A.R. Anim-Mensah, W.B. Krantz, R. Govind, Studies on polymeric nanofiltration-based water softening and the effect of anion properties on the softening process, European Polymer Journal 44 (2008) 2244-2252. 

The individual membrane filtration processes are defined chiefly by pore size although there is some overlap. The smallest membrane pore size is used in reverse osmosis (0.0005—0.002 microns), followed by nanofiltration (0.001—0.01 microns), ultrafHtration (0.002—0.1 microns), and microfiltration (0.1—1.0 microns). Electro dialysis uses electric current to transport ionic species across a membrane. Micro- and ultrafHtration rely on pore size for material separation, reverse osmosis on pore size and diffusion, and electro dialysis on diffusion. Separation efficiency does not reach 100% for any of these membrane processes. For example, when used to desalinate—soften water for industrial processes, the concentrated salt stream (reject) from reverse osmosis can be 20% of the total flow. These concentrated, yet stiH dilute streams, may require additional treatment or special disposal methods. 

Whey concentration, both of whole whey and ultrafiltration permeate, is practiced successfully, but the solubility of lactose hmits the practical concentration of whey to about 20 percent total sohds, about a 4x concentration fac tor. (Membranes do not tolerate sohds forming on their surface.) Nanofiltration is used to soften water and clean up streams where complete removal of monovalent ions is either unnecessary or undesirable. Because of the ionic character of most NF membranes, they reject polyvalent ions much more readily than monovalent ions. NF is used to treat salt whey, the whey expressed after NaCl is added to curd. Nanofiltration permits the NaCl to permeate while retaining the other whey components, which may then be blended with ordinaiy whey. NF is also used to deacidify whey produced by the addition of HCl to milk in the production of casein. 

For RO pretreatment, NF is typically used to pre-soften and reduce color from RO feed water (when appropriate NF membranes are employed). Nanofiltration replaces sodium softening (for hardness removal) and augments clarification (for color removal). 

Nanofiltration (NF), also referred to as ultra-low-pressure reverse osmosis, was developed to fill the gap between reverse osmosis and ultrafiltration. In water purification, nanofiltration has been found to effectively remove selected salts to reduce total hardness at lower pressures than RO systems. The nanofiltration membrane employed for this apphcation has been referred to as a softening membrane. The softening membrane also effectively removes color and trihalomethane (THM) precursors. 

Nanofiltration softening membranes may be an economic alternative to conventional softening. Possible advantages of membrane filtration for softening include smaller space requirements, no lime requirement, superior quality water, and less operator attendance. As for all membrane separation processes, suitability of softening membranes for a given apphcation must be made on a case-by-case analysis. 

Nanofiltration (NF) and RO are closely related in that both share the same composite membrane structure and are generally used to remove ions from solution. However, NF membranes use both size and charge of the ion to remove it from solution whereas RO membranes rely only on solution-diffusion transport to affect a separation (see Chapters 16.2 and 4.1, respectively). Nanofiltration membranes have pore sizes ranging from about 0.001 to 0.01 microns, and therefore, the rejection of ions in solution by an NF membrane is not a good as that by an RO membrane (see Figure 1.1). Because NF is similar to RO, but with lower rejection, NF is sometimes called loose or leaky RO. Nanofiltration is commonly used to soften potable water or to remove color and organics from RO feed water. 

A comparison between nanofiltration and pellet softening [combined with granular activated carbon (GAC) adsorption for organics removal] was made by Sombekke et al. (1997), based on a hfe-cycle analysis (LCA). Both treatment schemes were found to have a comparable impact, except that NF was advantageous for quality and health aspects. Other positive aspects for NF were the investments and costs, and the impact on the landscape. The main environmental impact for both alternatives is caused by the use of energy. The Water Supply Company of Overijssel (WMO), The Netherlands, decided to extend current treatment capacity with nanofiltration on the basis of this LCA. 

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