Particle colloidal filtration

The effectiveness of UF in particle/colloid filtration can be measured with an MF plugging test similar to the "silt density index" (see Chapter 4, Figure 4.10). Water is fed at 30 psig through a 47 mm diameter 0.22 m membrane filter... 

Buesseler KO, Bauer JE, Chen RF, Eglinton TI, Gustafsson O, Landing W, Mopper K, Moran SB, Santschi PH, Vernon Clark R, Wells ML (1996) An intercomparison of cross-flow filtration techniques used for sampling marine colloids overview and organic carbon results. Marine Chem 55 1-31 Buffle J, Perret D, Newman M (1992) The use of filtration and ultrafiltration for size fractionation of aquatic particles, colloids, and macromolecules. In Enviroiunental particles. Buffle J, van Leeuwen HP (eds) Lewis Publishers, Boca Raton FL, pl71-230... 

Suspended particles in natural and wastewaters vary in diameter from 0.001 to about 100 pm (1 10 9 to 10 4 m). For particles smaller than 10 pm, terminal gravitational settling will be less than about 10 2 cm sec 1. The smaller particles (colloids) can become separated either by settling if they aggregate or by filtration if they attach to filter grains. 

Buffle, J., D. Perret and M. Newman (1992), The use of Filtration and Ultrafiltration for Size Fractionation of Aquatic Particles, Colloids and Macromolecules", in J. Buffle and H.P. v. Leeuwen, Eds., IUPAC Series in Environmental Physical and Analytical Chemistry, Lewis Publ., Chelsea, Ml. 

Electrostatic and electrical double-layer forces play a very important role in a number of contexts in science and engineering. As we see in Chapter 13, the stability of a wide variety of colloids, ranging from food colloids, pharmaceutical dispersions, and paints, to colloidal contaminants in wastewater, is affected by surface charges on the particles. The filtration efficiency of submicron particles can be diminished considerably by electrical double-layer forces. As we point out in Chapter 13, coagulants are added to neutralize the electrostatic effects, to promote aggregation, and to enhance the ease of separation. 

Water for use in homes, agriculture, and industry is generally obtained from freshwater lakes, rivers, or underground sources. The water you drink must be purified to remove solid particles, colloidal material, bacteria, and other harmful impurities. Important steps in a typical purification process include preliminary filtration, sedimentation, sand filtration, aeration, and sterilization (Figure 14.11). 

Filtration (water treatment) Refers to the physical separation of particles, colloids, or other contaminants from water by passing the liquid through permeable or semipermeable materials (compare with microfiltration, nanofiltration, reverse osmosis, and ultrafiltration). 

At bench-scale purification, a two-step operation of depth filtration followed by sterile filtration (or centrifugation followed by sterile filtration) can efficiently remove large particles, colloidal particles, and bacteria. At industrial scale, however, the clarification step is usually completed by three stages in series as shown in Figure 32.6. 

The separation and characterization of submicron-sized particles in water is difficult, in particular because of artifacts from sampling and concentration techniques. Lead et al. (1997) have presented a critical review of the different techniques for separation and analysis of colloids (filtration, dialysis, centrifugation, but also volta-metry, gels (DET/DGT), field-flow fractionation, SPLITT). Ultrafiltration membranes have been developed with nominal cutoff sizes ranging from the thousands of daltons (Da) to hundreds of thousands of daltons, which have been used to separate the colloidal pool into several fractions. 

Capillary colloidal filtration occurs when the dry substrate comes into contact with a dispersion and the pore surface is wetted by the dispersion liquid. The capillary suction of the substrate which occurs, in effect drives particles to the interface. If the surface is not permeable for the particles, the particles concentrate at the substrate-dispersion boundary and a compact layer is formed. The thickness of the compact layer grows with time according to the well-known square root time law (see Section 6.3.1), until the substrate is saturated with dispersion liquid or, in case of a dispersion of small colloidal particles, a stationary state due to back-diffusion occurs. Figure 6.8 summcirises the capillary filtration mode of dip-coating. 

A particle packing process is a process which causes an increase in the volume fraction of a particulate material, i.e. the dispersion. The densification takes place stepwise in the colloidal filtration process and becomes more gradual... 

Colloidal filtration is selected as the dip-coating mechanism for the first trials in the development path. This means that cake filtration should occur when the suspension comes into contact with the substrate. So the particle size in the suspension should not be much smaller than 1 pm (approximately 4 times less than the mean pore size in the substrate) otherwise too much penetration and clogging of the substrate occurs prior to cake build-up. This would give rise to an extra high interfacial flow resistance during application of the MF membrane. 

The size ranges of some filter types used to remove particulate materials from natural waters are also shown in Fig. 10.1. The standard 0.45 /zm-membrane filter clearly has pores too large to filter out many colloidal-sized materials, which can include metal oxyhydroxides, clays, and viruses. Removal of colloidal-sized particles by filtration often takes place, however, when soil water or... 

Two phenomena are responsible for flux decline during membrane operation with real fluids [27]. One is related to polarization phenomena (concentration and temperature polarization), which normally are reversible processes. Thus, at a finite time, when steady-state conditions have been attained, the flux stabilizes at a value always less than the original one. Membrane fouling is the second phenomenon responsible for flux decline. It consists of the deposition of retained particles, colloids, anulsions, suspensions, macromolecules, salts, etc., on/in the membrane. Fouling always results in a continuous (ir)reversible decrease of membrane flux with time and constitutes one of the major problem to be managed during filtration plant operation. 

Buffle J, Ferret D, Newman M (1992) The use of filtration and ultrafiltration for size fractionation of aquatic particles, colloids, and macromolecules. In Buffle J, van Leeuwen HP (eds.) Environmental Particles, vol. 1, pp. 171-230. Chelsea, MI Lewis Publishers. 

A. UltrafiUration. — By the use of filters that allow electrolytes to pass freely through, but retain the colloidal particles, colloidal stannic acid must have, after filtration, not only its ultramicrons, with their attendant anions, but also an equivalent amount of alkali ion molecules. The excess of the electrolytes, KOH, KaSnOa, etc., that were dissolved in the disperse medium, have passed through. The adsorbed portion of the alkali, regardless of whether it is dissociated or not, is an essential part of the hydrosol for if it is removed the colloid will coagulate. Duclaux, who has studied the behavior of colloidal iron oxide and cupric ferrocyanide in this connection, has proposed the name Micells for the ultramicrons together with their adsorbed molecules... 

Nowadays, ultrafiltration (UF) or microfiltration (MF) membrane processes are widely used because of their ability to remove particles, colloidal species and microorganisms from different liquids feeds. However a limitation inherent in the process is membrane fouling due to the deposition of suspended matter during filtration. Therefore the understanding of formation and transport properties of particle deposits responsible for membrane fouling is a necessary step to optimize membrane processes. Thus it is necessary to obtain local information in order to analyze and model the basic mechanisms involved in deposit formation and then to further predict the process operation. Besides, it is also useful to control the deposit formation and to plan preventive or curative actions with a controlled efficiency. Nonetheless, local parameters such as cake thickness and porosity are hardly reachable with conventional techniques. 

The importance of colloids on the lanthanide composition of rivers waters was developed in Elderfield et al. (1990) who suggested that lanthanides are contained in three pools particles, colloids and solution. The combination of the latter two pools determined the lanthanide composition reported as dissolved concentrations (i.e., 0.22 and 0.45 [xm filtrates). In fig. 11 we have up-dated their plot of Nd versus the Er/Nd (mole) ratio of river waters using data collated in table Al. This... 

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