What is Reverse Osmosis Used for

An RO system is designed to remove ions from solution. Rejection of most ions ranges from about 96% to 99+%, depending of the nature of the ion and the type of membrane used (see Chapter 4.2). 

Although RO membranes also act as barriers to suspended solids, it is not recommended that they be used for this purpose. The membranes will foul with suspended solids, resulting in higher operating pressure, frequent membrane cleaning, and shorter membrane life. To avoid fouling, pretreatment is required to remove suspended solids from the RO feed water (see Chapter 8). 

The most common uses of RO are for desalination of seawater and brackish water for potable and industrial applications. However, as demand for fresh water grows, RO is being pressed into service for wastewater and reuse applications. These will require extensive pretreatment, sometimes involving other membrane technologies such as micro- or ultrafiltration, to minimize fouling of the RO membranes (see Chapter 16). 

Nanofiltration (NF) and RO are closely related in that both share the same composite membrane structure and are generally used to remove ions from solution. However, NF membranes use both size and charge of the ion to remove it from solution whereas RO membranes rely only on solution-diffusion transport to affect a separation (see Chapters 16.2 and 4.1, respectively). Nanofiltration membranes have pore sizes ranging from about 0.001 to 0.01 microns, and therefore, 

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Now that we have determined what processes the facility will be used for, we can finalize utility requirements. The following utilities are required for our solid-dose facility heating, ventilation, and air conditioning (HVAC), hot and cold water, steam, electrical service, compressed air, vacuum systems, dust collection, chillers, effluent stream, and purified water. For the more specialized processes or special material handling, we may need specialized gases and breathing air. Purified water is one of the more difficult utilities to maintain the quality of. From a source of potable water, a series of treatments must be performed to control microbiological quality. Typical treatment options include carbon filters, reverse osmosis, and UV radiation. 

What do we think of when we hear fiber Clothing, certainly, and other textiles such as sheets and blankets, curtains and upholstery. Some fiber goes into carpeting—for offices, homes, automobiles, and recreational vehicles. Automobile, truck, and bus tires are strengthened with the use of fibers called cords. Some fiber is used for industrial purposes such as insulation and filtration. Hollow fibers that act as membranes are used in the desalination of seawater by reverse osmosis and in kidney dialysis. As we have already seen, extremely high-strength fibers are used to make bulletproof safety equipment and to reinforce polymers for high performance ap-... 

Blatt et al.(29) developed what has become known as the "gel polarization" theory for ultrafiltration, in which the amount of macromolecular material in the fouling layer is controlled by its back-diffusion rate into the feed stream. The gradual decline in flux observed in some practical systems was explained in terms of an irreversible consolidation of the gel layer with time, leading to a reduction in the layer s permeability. Kimura and Nakao (,1) used Blatt s approach to model the fouling of reverse osmosis... 

A hollow-fiber separator used for reverse osmosis is suspected of having flaws in the 0.1-pm dense layer, since the salt rejection is only 97 percent when tested with seawater at 1000 Iby /in. abs, and the predicted rejection is 99.5 percent The measured product flux is 6.5 gal/day-ft. (a) If the flaws are 0.01-pm pinholes in the dense layer, how many holes per square centimeter would be needed to account for the lower salt ngection (fe) What is the corresponding fraction hole area in the dense layer ... 

What is important to recognize from the discussion is that the boundary condition for mass transfer through a semipermeable membrane is directly analogous to that for a mixed heterogeneous reaction. A consequence of this is that what is said about the one problem can be translated to the other, despite the somewhat different physics and chemistry. The example of reverse osmosis is therefore used as an illustration of a mixed heterogeneous reaction. The major part of the discussion will, however, be confined to the developing layer, where... 

Suppose that one wishes to use reverse osmosis to reduce the salt content of brackish water containing 0.22 M total salt concentration to a value of 0.01 M, thus rendering it usable for human consumption. What is the minimum pressure that needs to be applied in the permeators (Figure 18.19) to achieve this goal, assuming that the operation occurs at 298 K Hint Refer to Section 13.5.)... 

A membrane filtration plant suitable for process water from reactive dyeing and printing of cotton is a two step plant Pre-filtration by ultrafiltration and a final treatment by reverse osmosis. Pre-treatment technologies for RO spiral wound membrane filtration have focused on flat sheet polymer UF membranes in a high cross-flow filter. The quality of water produced by this plant will go beyond what most dyehouses use today and will be well suited for all processing, including reactive dyeing of cotton." ... 

A process unit for purifying salt water is based on the reverse osmosis principle. In such a device, hollow fibers are used that retain the salt but permit water to diffuse out. For a unit using 900,000 hollow fibers (diameters of 85 and 42 x 10 m, respectively) that are 0.9 m long, the volumetric output (feed pressure of 2.86 x 10 pascals) is 8.76 x 10 m /sec. What is the pressure drop in an individual fiber from inlet to outlet ... 

Let us now turn to a more detailed examination of these processes. Both nanofUtration (NF) and reverse osmosis (RO) draw on principles of osmosis for their implementation. The principal features of this phenomenon and its manifestation in reverse-osmosis operations are illustrated in Figure 8.13a. Consider a selective membrane (i.e., one that is freely permeable to water but much less so to salt) separating a salt solution from water, as shown in Part 1. In such an arrangement, water will flow from the pure-water side into the side less concentrated in water (i.e., the saltwater side). This process is referred to as normal osmosis. If, now, a hydrostatic pressure is applied to the salt side, the flow of water will be retarded, and if that pressure is sufficiently high, the flow will ultimately cease completely. At this point, we will have reached what is termed osmotic equilibrium (Part 2), and the hydrostatic pressure associated with this state is referred to as the osmotic pressure. A further increase in applied pressure will act to reverse the flow from the salt solution to the pure-water side (Part 3), and that flow will rise as the pressure is increased. This phenomenon, termed reverse osmosis, is exploited in all RO operations. The membranes used in these processes are so dense that no discrete pores can be discerned. Permeation proceeds instead by a solution-diffusion mechanism that makes use of a statistical distribution of... 

It is known that the theoretical maximum electrical resistivity for water is approximately 182 kQ-m at 25 °C. This figure agrees well with what is typically seen on reverse osmosis, ultra-filtered and deionized ultra-pure water systems used, for instance, in semiconductor manufacturing plants. A salt or acid contaminant level exceeding even 100 parts per trillion (ppt) in ultra-pure water begins to noticeably lower its resistivity level by up to several kOhm-m (or hundreds of nanosiemens per meter). 

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