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Treatment of Food Industry Foods and Wastes by Membrane Filtration 239
2. MEMBRANE PROCESSES, MODULES, AND EQUIPMENT
2.1. Membrane Processes
Membranes are porous filtration mediums, which can be cationic, anionic, or nonionic in
nature, and acts as a barrier to prevent mass movement of selected phases, but allows passage
of other remaining phases. The phases include solid phase (such as suspended solids, organic
solids, inorganic solids, etc.), liquid phase (such as water, ethanol, chloroform, etc.), and gas
phase (such as air, nitrogen, oxygen, etc.). Membrane processes include the following five
main categories for processing water and wastewater (12, 13):
1. Microfiltration (MF): MF is a pressure filtration process for the separation of suspended solids in
the particle size range of about 0.08–10 mm. The primary function affecting solids separation from
water is the size of the solids. The hydraulic pressure applied in MF is about 1–2 bar, or
15–20 psig, for overcoming the resistance of the “cake.”
2. Ultrafiltration (UF): UF is another pressure filtration process for the separation of macromolecular
solids in the particle size range of about 0.001–0.1 mm. The hydraulic pressure required by UF
for overcoming hydraulic resistance of the polarized macromolecular layer on the membrane
surface is about 1–7 bar.
3. Nanofiltration (NF): NF membranes are multiple-layer thin-film composites of polymer consisting
of negatively charged chemical groups, and are used for retaining molecular solids (such as
sugar) and certain multivalent salts (such as magnesium sulfate), but passing substantial amounts
of most monovalent salts (such as sodium chloride), at an operating pressure of about 14 bar or
200 psig. Both molecular diffusivity and ionic charge play important roles in the separation
process. The sizes of molecular solids and multivalent salts to be rejected by NF are normally in
the range of 0.0005–0.007 mm.
4. Reverse osmosis (RO): RO membranes are mainly made of cellulose acetate (CA) with pore sizes
of 5–20 Angstrom units. These membranes are used for rejecting salts (as high as 98%) and
organics (as high as 100%), at an operating pressure of 20–50 bar or 300–750 psig. Hydraulic
pressure (through a pump) is used to provide the driving force for permeation, or for overcoming
the chemical potential difference between the concentrate and the permeate, expressed in terms of
osmotic pressure. Sizes of molecular solids and salts (multivalent as well as monovalent) to be
rejected by RO are normally in the range of 0.00025–0.003 mm.
5. Electrodialysis (ED): ED uses voltage or current as the driving force to separate ionic solutes. The
size of ionic solutes to be rejected or separated by ED is normally in the range of
0.00025–0.08 mm, depending on the pore size of ED membranes. EDR is the electrodialysis
reversal (or reverse electrodialysis) process – which is similar to ED, but its cathodes and anodes
can be reversed for automatic cleaning operation.
Figure 6.1 illustrates the relationships among MF, UF, NF, RO, ED, and conventional
separation processes (such as cloth/fiber filters, screens, gel chromatography, dialysis, ion
exchange, distillation, freeze concentration, solvent extraction, foam and bubble fractionation,
ultracentrifuges centrifuges, liquid cyclones, and gravity sedimentation. Membrane
bioreactor (MBR) adopts either MF or UF. Figure 6.2 shows the capabilities of MF, UF, NF,
and RO and Fig. 6.3 illustrates the capabilities of ED and EDR processes (14).
The applied pressure for typical membrane filtration process is given in Table 6.1:
Membrane processes are characterized by two basic process parameters: