Membrane filtration process

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. 

Intensive technologies are derived from the processes used for the treatment of potable water. Chemical methods include chlorination, peracetic acid, ozonation. Ultra-violet irradiation is becoming a popular photo-biochemical process. Membrane filtration processes, particularly the combination microfiltration/ultrafiltra-tion are rapidly developing (Fig. 3). Membrane bioreactors, a relatively new technology, look very promising as they combine the oxidation of the organic matter with microbial decontamination. Each intensive technique is used alone or in combination with another intensive technique or an extensive one. Extensive... 

Membrane filtration processes, such as reverse osmosis, and micro and ultra filtration, are used to filter out dissolved solids in certain applications see Table 10.9. These specialised processes will not be discussed in this book. A comprehensive description of the techniques used and their applications is given in Volume 2, Chapter 8 see also Scott and Hughes (1995), Cheryan (1986), McGregor (1986) and Porter (1997). 

Membrane filtration processes have been successfully applied to the field of environmental engineering for air pollution control,34 potable water purification,22-24 groundwater decontamination,35,36 industrial effluent treatment,37 hazardous leachate treatment,35,36 and site remediation,36 mainly because membrane filtration can remove heavy metals and organics. 

Microfiltration. Microfiltration is a pressure-driven membrane filtration process and has already been discussed in Chapter 8 for the separation of heterogeneous mixtures. Microfiltration retains particles down to a size of around 0.05 xm. Salts and large molecules pass through the membrane but particles of the size of bacteria and fat globules are rejected. A pressure difference of 0.5 to 4 bar is used across the membrane. Typical applications include ... 

Prodnces higher qnahty treated water than traditional membrane filtration processes. 

Mass-transport limitations are common to all processes involving mass transfer at interfaces, and membranes are not an exception. This problem can be extremely important both for situations where the transport of solvent through the membrane is faster and preferential when compared with the transport of solute(s) - which happens with membrane filtration processes such as microfiltration and ultrafiltration - as well as with processes where the flux of solute(s) is preferential, as happens in organophilic pervaporation. In the first case, the concentration of solute builds up near the membrane interface, while in the second case a depletion of solute occurs. In both situations the performance of the system is affected negatively (1) solute accumulation leads, ultimately, to a loss of selectivity for solute rejection, promotes conditions for membrane fouling and local increase of osmotic pressure difference, which impacts on solvent flux (2) solute depletion at the membrane surface diminishes the driving force for solute transport, which impacts on solute flux and, ultimately, on the overall process selectivity towards the transport of that specific solute. 

List of Applied Pressure for Typical Membrane Filtration Processes... 

C. Blocher, J. Dorda, V. Mavrov, H. Chmiel, N. K. Lazaridis, and K. A. Matis, Hybrid flotation— membrane filtration process for the removal of heavy metal ions from wastewater. Water Research 37, 4018-1026 (2003). 

A simple centrifugation or MF step in the primary clarification may directly be succeeded by a secondary clarification or an enrichment step, such as extraction, precipitation, or adsorption. The concentrated product may then be subjected to membrane filtration processes. Ultrafiltration might be done earlier and then followed by extraction or precipitation with salts. Diafiltration units can subsequently be used to remove the... 

Figure 2a. Flow Diagram of Two-Stage Pilot Scale Membrane Filtration Process for Recovery of an Extracellular Protease...

These NMR images prove that oil polarization layers form after feedstock flow ha,s been turned on. The layers dissipate rapidly when the flow of feedstock is turned off. This particular investigation illustrates the importance of NMR-imaging techniques for analy.sis of technologically relevant membrane filtration processes. 

The development of the membrane production to real large-scale production will increase the realiability of membrane-filtration plants and increase the competitive power of the membrane-filtration process. 

Problem 5-12. Flow Through a Porous Tube. Let us consider flow through a cylindrical porous tube, which occurs in many membrane filtration processes. The tube is very long with radius R. At the inlet of the tube, the pressure is/ /. Fluid permeates or leaks out through the wall of the tube with a velocity k(P — Ps) ///, where P is the local pressure in the fluid, Ps is the pressure on the other side of the membrane, A is a permeation coefficient, and // is the viscosity of the fluid. We wish to determine how much fluid is filtered as a function of the length of the tube. 

The variation of throughput with filtration rate has far-reaching implications on the most economical way to run conventional membrane filtration processes. For example, if a fixed volumetric flow rate (gal/min) must be filtered, the total volume (in gallons) which may be processed before plugging (run to a set AP) is... 

MICRO FILTRATION - A membrane filtration process, which forces water through a porous barrier. Pores are usually between 0.1 to 20 m, when used for water purification. For filtering purposes, pore sizes are. 045 m. 

Likewise, nanofiltration can be integrated into waste water treatment. Combined reverse osmosis/nanofiltration processes can offer higher water recovery than either process alone [122]. Moreover, nanofiltration can be combined with other membrane filtration processes [123], electrodialysis [124], or other waste water treatment processes such as ozonation [125]. 

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