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Figure 6. Range of common particle sizes (diameter) over range of UF pore size.
The retention efficiency of membranes is dependent on particle size and concentration, pore size and length, porosity, and flow rate. Large particles that are smaller than the pore size have sufficient inertial mass to be captured by inertial impaction. In liquids the same mechanisms are at work. Increased velocity, however, diminishes the effects of inertial impaction and diffusion. With interception being the primary retention mechanism, conditions are more favorable for fractionating particles in liquid suspension.
In contrast to reverse osmosis, where cellulose acetate has occupied a dominant position, a variety of synthetic polymers has been employed for ultrafiltration membranes. Many of these membranes can be handled dry, have superior organic solvent resistance, and are less sensitive to temperature and pH than cellulose acetate. Polycarbonate resins, substituted olefins, and polyelectrolyte complexes have been employed among other polymers to form ultrafiltration membranes.
Preparation details for most of the membranes are proprietary. As noted earlier, molecular weight cutoff is used as a measure of rejection, however, shape, size, and flexibility are also important parameters. For a given molecular weight, more rigid molecules are better rejected than flexible ones. Ionic strength and pH often help determine the shape and rigidness of large molecules. Membrane lifetimes are usually two years or more for treating clean streams (water processing), but are drastically reduced when treating comparatively dirty streams (e.g., oily emulsions). Membrane guarantees by manufacturers are determined only after pilot work is done on the particular stream in question. In some cases as little as a 90-day guarantee may be given for oil/water waste applications. There are in fact both current and many emerging uses of ultrafilters in the areas of biological research, processing sterile fluids, air and water pollution analysis, and recovery of corrosive or noncorrosive chemicals. The technology is applicable to dewatering some sludges, but this use is highly dependent on the particular sludge itself. There are no commercial uses of UF for sludge dewatering at this time, but several sources have been found which claim that this represents a possible near-future application. Pollution of water supplies within the food industry is a significant problem, since many food wastes possess
extremely high biochemical oxygen demand (BOD) requirement. In the potato starch industry, for example, waste effluent containing valuable proteins, free amino acids, organic acids, and sugars can be processed by ultrafiltration. Reclamation of these materials, which are highly resistant to biodégradation, are providing an economic solution to this waste removal problem. For the concentration of juices and beverages, RO is preferable to evaporation due to lower operating costs and no degradation of the product. Processing by RO retains more flavor components than does heated, vacuum pan concentration. Since ultrafiltration does not retain the low molecular weight flavor components and some sugar, UF is employed as a complement to RO. A two-stage process may be used in which the first stage, UF, allows the passage of sugars and other low molecular weight compounds. This permeate is then dewatered by RO and recycled back to the main stream. High juice concentrations are possible in this manner because the UF removes the colloidal and suspended solids which would foul the RO, and helps relieve some of the high hydraulic pressure due to high osmotic pressure of the juice. In a process as shown in Figure 7, a citrus press liquor, or multiphase suspension, is ultrafiltered following a coarse prefiltration. The resultant clear permeate is processed through ion exchange and granular activated carbon adsorption units to remove low molecular weight contaminants and inorganic salts. The product is a natural citrus sugar solution suitable for reuse. The concentrated suspended solids are used in making animal feed. Pectins are a family of complex carbohydrates which are used to form gels with sugar and acid in the production of jellies, preserves, and other confections from fruit juices. The recovery of starch and other high molecular weight compounds from waste effluents is an important application for UF. The output from a 30-ton-per-day starch plant would be about 432,000 gpd with a solids content of 0.5 percent to 1.0 percent, and 9,000 to 14,000 mg/Liter COD.
Press Liquor Prefiltration Ultrafiltration
Press Liquor Prefiltration Ultrafiltration
Treatment systems which can both reduce the strength of this waste and recover valuable by-products, such as proteins, are an obvious advantage to this industry. Heat and acid coagulation, distillation, and freezing techniques are more costly and less efficient than UF in protein recovery. Reverse osmosis is also a competing process, but protein recovery by UF would lead to a somewhat higher purity. In the production of single-cell proteins as a food source, UF has several applications. For harvesting cells, UF can replace centrifugation in some applications since the efficiency of centrifugation decreases rapidly with particle size. Ultrafiltration is also well suited for recovering and concentrating the metabolic products of fermentation (enzymes, for example). In a related application, UF is also able to concentrate and desalt protein products, being more efficient than dialysis for this purpose. Moreover, a UF membrane module may be coupled with a fermenter so that toxic metabolites can be continuously removed from the system as fresh substrate is introduced. This permits the growth limitations of a batch fermenter to be relieved and permits a substantial increase in productivity. A membrane
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