Reverse osmosis configurations

The major advantages of the tubular reverse osmosis configuration are the ability to tolerate high suspended solids concentrations in the feed and the possibility of mechanical membrane cleaning. The disadvantages are the excessive number of tube end fittings in proportion to the active membrane area in each pressure vessel, the bulkiness of the reverse osmosis plant and the high cost. 

For most hydrardic pressure-driven processes (eg, reverse osmosis), dense membranes in hoUow-fiber configuration can be employed only if the internal diameters of the fibers are kept within the order of magnitude of the fiber-wall thickness. The asymmetric hoUow fiber has to have a high elastic modulus to prevent catastrophic coUapse of the filament. The yield-stress CJy of the fiber material, operating under hydrardic pressure, can be related to the fiber coUapse pressure to yield a more reaUstic estimate of plastic coUapse ... 

In general there are three main types of hoUow-fiber flow configurations. In the most common, for reverse osmosis and ultrafiltration, the feed... 

The earhest reverse osmosis and ultrafiltration units were based on flat membrane sheets ia arrangements similar to that of a plate and frame filter press. Siace then, mote efficient membrane configurations, ie, tubular, spiral wound, and hoUow fiber, have emerged (96—98). 

Configurations used include tubes, plate-and-frame arrangements and spiral wound modules. Spiral wound modules should be treated to remove particles down to 20 to 50. im, while hollow fiber modules require particles down to 5 im to be removed. If necessary, pH should be adjusted to avoid extremes of pH. Also, oxidizing agents such as free chlorine must be removed. Because of these restrictions, reverse osmosis is only useful if the wastewater to be treated is free of heavy contamination. The concentrated waste material produced by membrane processes should be recycled if possible but might require further treatment or disposal. 

First multi-leaf spiral wound membrane module developed by Don Bray and others at Gulf General Atomic, under US Patent no. 3,417,870, "Reverse Osmosis Purification Apparatus," December, 1968. A multi-leaf spiral configuration improves the flow characteristics of the RO module by minimizing the pressure drop encountered by permeate as it spirals into the central collection tube. 

Reverse osmosis membranes for industrial applications are typically modularized using configurations that pack a large amount of membrane area into a relatively small volume. This makes the RO system more economical to use in that the system requires a smaller footprint, and membranes can be replaced in smaller modules rather than system wide. 

Process Descriptions Selectively permeable membranes have an increasingly wide range of uses and configurations as the need for more advanced pollution control systems are required. There are four major types of membrane systems (1) pervaporation (2) reverse osmosis (RO) (3) gas absorption and (4) gas adsorption. Only membrane pervaporation is currently commercialized. 

The treatment of ADUF by reverse osmosis [13] was found to be useful in concentrating activity in small volume while making a larger volume of the decontaminated effluent for direct disposal after required dilution. Porous cellulose acetate membranes were used in plate module configurations. The concentration of ammonium nitrate in the permeate stream is not very different from that of the contaminated retentate. With the addition of flocculating aids, the decontamination factors in the range of 1000 with VRFs in the range of 100 were achieved. 

Ultraflltration process is basically a physical filtration process on a very fine scale. It can retain large molecules, suspended, and colloidal particles, but can allow dissolved salts to pass through the bulk of the water. To keep the concentrate mostly in a uniform suspension requires high flows of liquid along the membrane surface. The technique uses similar configurations of equipment layouts as are used in reverse osmosis processes. 

Stack, and a potential difference sufficient to force current through the stack is applied between the two electrodes placed at each end of the stack. For current to pass between the electrodes, ions must be transported through each of the membranes. By arranging the feeds to the various intermembrane compartments, it is possible to force ionic salts to pass from the dilute stream to the concentrated stream. In this way, a salt can also be split into its acid and base components. By combination of several cell pairs that comprise an anion- and a cation-selective membrane sheets in parallel, a stream concentrated in the original salts may be prepared. This configuration is the common method for industrial use, in which electrodialysis gives broadly the same result as reverse osmosis and has found very similar applications to general water treatment. 

In this chapter, we will introduce fundamental concepts of the membrane and membrane-separation processes, such as membrane definition, membrane classification, membrane formation, module configuration, transport mechanism, system design, and cost evaluation. Four widely used membrane separation processes in water and wastewater treatment, namely, microfiltration (MF), ultrafiltration (UF), nanofiltrafion (NF), and reverse osmosis (RO), will be discussed in detail. The issue of membrane foufing together with its solutions will be addressed. Several examples will be given to illustrate the processes. 

TABLE 5.3 A Comparison of the Performance Characteristics of Three Commercial Reverse Osmosis Membrane Configurations"... 

During the 1960 s, the DuPont Company screened numerous polymers to determine the suitability of materials other than cellulose acetate for use in reverse osmosis desalination. The results of this work indicated that aromatic polyamides were the "choice as the best polymer type for use in the DuPont commercial permeators".7 The company was most successful in developing an asymmetric aromatic polyamide reverse osmosis membrane in a hollow fine fiber configuration which successfully competed with cellulose acetate in the market place. 

The Office of Saline Water, U.S. Department of Interior, sponsored much of the development of membrane packaging configurations for reverse osmosis membrane. The configurations which have been developed and evaluated are as follows ... 

In the case of reverse osmosis, the relative flow configuration does not affect the performance to any large extent. As already indicated, the situation is quite different for gas permeation. While countercurrent flow improves the separation efficiency of hollow fiber modules in reverse osmosis only slightly, as far... 

Modern distillation plants have unit capacities of about 25,000 t/d. The energy consumption is approximately 95 KWh/to distillate in the form of saturated steam of a pressure of about 2.2 bar and 4 to 5 KWh/to distillate in form of power for the pump drives. All distillation processes produce a distillate containing only 20 to 30 ppm salts (total dissolved solids). Reverse osmosis (RO) will be the alternative to distillation. The modules which are successfully employed are of the "spiral wound" or the "hollow fiber" configuration. Under favorable conditions, the economic service lifetime of the modules resp. the membranes is about 5 years and warranties for such a figure are given by manufacturers. 

The compartments indicated in Figure 5.57a are a schematic representation of reverse osmosis process. In practice, the reverse osmosis process is conducted in a tubular configuration system (Figure 5.57b). Raw waste water flows under high pressure (greater than osmotic pressure) through an inner tube made... 

FIGURE 5.57 (a) Schematic representation of reverse osmosis process, (b) Tubular configuration system for... 

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