What Reverse Osmosis Is

When pure water and a salt solution are introduced on opposite sides of a semipermeable membrane in a vented container, the pure water diffuses through the membrane and dilutes the salt solution, At equilibrium, the liquid level on the saline water side of the membrane will be above that on the freshwater side; this process is known as osmosis and is depicted in Figure 9. The view on the left illustrates the commencement of osmosis and the center view presents conditions at equilibrium. The effective driving force responsible for the flow is osmotic pressure. This pressure has a magnitude dependent on membrane characteristics, water temperature, and salt solution properties and concentration. By applying pressure to the saline water, the flow process through the membrane can be reversed. When the applied pressure on the salt solution is greater than the osmotic pressure, fresh water diffuses in the opposite direction through the membrane and pure solvent is extracted from the mixed solution; this process is termed reverse osmosis (RO). The fundamental difference between reverse osmosis and electrodialysis is that in reverse osmosis the solvent permeates the membrane, while in electrodialysis the solute moves through the membrane.

Reverse osmosis is a means for separating dissolved solids from water molecules in aqueous solutions as a result of the membranes being composed of special polymers which allow water molecules to pass through while holding back most other types of molecules; since true "pores" do not exist in the membrane, suspended solids are also retained by superfiltration. In an actual reverse osmosis system, operating in a continuous-flow process, feed water to be treated or desalinated is circulated through an input passage of the cell, separated from the output product water passageway by the membrane.

Nornul Ojmotic

Oimoui Equilibrium

Nornul Ojmotic

Oimoui Equilibrium

Figure 9. Principle of Reverse Osmosis

The feed stream is split into two fractions - a purified portion called the product water (or permeate) and a smaller portion called the concentrate' containing most of the impurities in the feed stream. At the far end of the feed-water passage, the concentration (dewatered) reject stream exits from the cell. After permeating the membrane, the product (fresh-water) flow is collected. The percentage of product water obtained from the feed stream is termed the recovery, typically around 75 percent.

The ratio (F-P)/F, or the concentration of a solute species in the feed (F) minus that in the product (P) over the concentration in the feed, is called the rejection of that species. Rejections may be stated for particular ions, molecules, or conglomerates such as TDS or hardness. Solids rejection depends on factors such as types and forms of solids, membrane types, recovery, pressure, and pH. Suspended solids (typically defined as particles larger than 0.5 micron mean diameter, and including colloids, bacteria, and algae) are rejected 100 percent; that is, none can pass through the membrane. Weakly ionized dissolved solids (usually organics, but may include other materials such as silicates) undergo about a 90 percent rejection at normal recoveries for certain membranes. Although pH can strongly influence the rejection, when the molecular weight of these solids is less than 100, rejection decreases appreciably. Ionized solids, or salts, are rejected independent of molecular weight and at molecular weights considerably below 100. At 75 percent recovery and pressures greater than 250 pounds per square inch, overall rejection of total dissolved solids (TDS) is about 90 percent. Rejections vary with pressure because the actual salt flow through the membrane remains fairly constant, but the water permeation depends nearly linearly on pressure, affecting the ratio of concentrations. For example, rejection of sodium chloride can fall from 90 percent at 300 pounds per square inch to 20 percent at 50 pounds per square inch, indicating the need to operate at the highest pressures possible.

Cellulose acetate is a common membrane material, but others include nylon and aromatic polyamides. The mechanism at the membrane surface involves the influent water and impurities attempting to pass through the pressurized side, but only pure water and certain impurities soluble in the membrane emerge from the opposite side.

Various configurations of membranes with different surface-to-volume ratios and different flux capabilities (gallons per day per square foot, or gpd/ ft') have been developed. Each type of membrane is a flexible plastic filmno more than 4 to 6 mils thick, firmly supported. Basic designs include the plate and frame, the spiral-wound module (jellyroll configuration), the tubular, and the newest of the process designs, the hollow-fine fiber. Fibers range from 25 to 250 microns (0.001 to 0.01 in.) in diameter, can withstand enormous pressure, are self-supporting, and can be bundled very compactly within a containment pipe. While product flow per square foot of fiber surface is less than that for an equivalent area of flat membrane, the difference in surface area more than compensates for the reduced unit flux.

Major problems inherent in general applications of RO systems have to do with (1) the presence of particulate and colloidal matter in feed water, (2) precipitation of soluble salts, and (3) physical and chemical makeup of the feed water. All RO membranes can become clogged, some more readily than others. This problem is most severe for spiral-wound and hollow-fiber modules, especially when submicron and colloidal particles enter the unit (larger particulate matter can be easily removed by standard filtration methods). A similar problem is the occurrence of concentration-polarization, previously discussed for ED processes. Concentration-polarization is caused by an accumulation of solute on or near the membrane surface and results in lower flux and reduced salt rejection.

The degree of concentration that can be achieved by RO may be limited by the precipitation of soluble salts and the resultant scaling of membranes. The most troublesome precipitate is calcium sulfate. The addition of polyphosphates to the influent will inhibit calcium sulfate scale formation, however, and precipitation of many of the other salts, such as calcium carbonate, can be prevented by pretreating the feed either with acid or zeolite softeners, depending on the membrane material.

Hydrolysis of cellulose acetate membranes is another operational problem and occurs whenever the feed is too acid or alkaline; that is, the pH deviates beyond designed range limits. As may readily happen, whenever C02 passes through the membrane, the resultant permeate has a low pH. The operational solution is to remove the gas from the permeate by deaerators, by strong-base anion resins or a complementary system-for example, RO and ion exchange, in series. Aromatic polyamide or nylon membranes are much less sensitive to pH than cellulose acetate. Compounds such as phenols and free chlorine that are either soluble in the membrane or vice versa will be poorly rejected and may damage the membrane. Procedures to improve feed-water makeup and thus reduce such membrane damage include acid pretreatment of the feed water, dechlorination, periodic cleaning or replacement of the membrane, sequestration of cations, coagulation and filtration of organics, and use of alternative, more durable membrane materials.

Reverse osmosis process is applied-or undergoing evaluation for imminent application-to a number of water-upgrading needs including high-purity rinse water production for the electronics industry (semiconductor manufacturing), potable municipal water supplied for newly-developed communities (for example, large coastal plants to upgrade brackish well water contaminated by seawater intrusion), boiler feed-water supplies, spent liquor processing for pulp and paper mills, and treatment of acid mine drainage.

In desalting operations, distillation plants have provided the major portion of the world's capacity. As the world's requirements for treated water increase, however, and water quality standards become more stringent, the membrane treatment processes in general and commercial RO processes in particular have been undergoing appreciable development. Important factors in the expansion of commercial RO applications are their favorably low power requirements and the realization of continuous technical improvements in membranes which are used in RO systems. A general guideline in water benefication is that RO is most frequently considered for cases in which the TDS is greater than 2,000 to 3,000 ppm; ED generally applies when the TDS is less than 2,000 to 3,000 ppm. However, many exceptions exist, based on feed-water species and product requirements.

One of the most important applications of RO is in the reclamation of large volumes of municipal and industrial wastewaters and the concentration of the solids for

Glossary of Salt Water

The general term for all water over 1,000 ppm (mg/L) total dissolved solids.

Fresh Water : <1,000 TDS

Brackish : 1,000-5,000

TDS

Highly Brackish : 5,000-

15,000 TDS

Saline : 15,000-30,000 TDS

Sea Water: 30,000-40,000

TDS

—-i-B-lh-'^ll

simplified disposal. The value of the reclaimed water offsets the cost of RO, and dilute wastewater concentration leads to economies in any further required liquid waste treatment.

Unrestricted use of reclaimed wastewater for drinking water, however, requires careful examination. While practically a complete barrier to viruses, bacteria, and other toxic entities that must be kept out of a potable supply, RO membranes could pose serious problems should any defect develop in their separation mechanism. Given the purity and clarity of RO-treated wastewaters, however, it might be advantageous to use RO and then subject the product to well-established disinfection procedures.

You should remember that RO uses a semi-permeable membrane. As such, the membrane is permeable to only very light molecules like water. Under atmospheric condirtions the fresh water flows into the solution which is called osmotic flow. But for purification purposes, this is no use, and hence we employ the reverse of osmotic flow. For this to happen, we need to apply external pressure in excess of osmotic pressure. The osmotic pressure is given by:

Of course, you should be familiar with this equation (the Ideal Gas Law), where 'n' is the molar concentration of solute, R is the universal gas law constant, and T is absolute temperature in °K. The permeate flow can be calculated from:

In this expression, Am is the membrane permeability coefficient.

It is useful to compare the merits of the various processes for seawater desalination. Although the comparison will be primarily qualitative, it should be helpful in providing a deeper insight into the strengths and weaknesses of process. Foremost among the aspects of comparison is the energy consumption of each process you consider. With the known process specification, it is theoretically possible to calculate the minimum work or energy needed for separation of pure water from brine. For the real process, however, the actual work required is likely to be many times the theoretically possible minimum. This is because the bulk of the work is required to keep the process going at a finite rate rather than to achieve the separation.

The minimum work needed is equal to the difference in free energy between the incoming feed (i.e. seawater or brackish water) and outgoing streams (i.e. product water and discharge brine). For the normal seawater (3.45 per cent salt) at a temperature of 25° C, for usual recoveries the minimum work has been calculated as equal to about 0.86 kWh/m-3. Table 5 makes the desired comparison.

Table 5. Energy requirements of four industrial desalination processes. (Source. International Atomic Energy Agency 1992.)

C

RO

Possible unit size

60,000

60,000

24,000

24,000

Energy consumption (kWh/m3) - Electrical/mechanical

4-6

2-2.5

7-9

5-7

Energy consumption (kWh/m3) - Thermal

55-120

30-120

None

None

Electrical equivalent for thermal energy (kWh/m 3)

8-18

2.5-10

None

None

Total equivalent energy (kWh/m3)

12-24

4.5-12.5

7-9

5-7

There are no major technical obstacles to desalination as a means of providing an unlimited supply of fresh water, but the high energy requirements of this process pose a major challenge. Theoretically, about 0.86 kWh of energy is needed to desalinate 1 m3 of salt water (34,500 ppm). This is equivalent to 3 kJ kg"1. The present day desalination plants use 5 to 26 times as much as this theoretical minimum depending on the type of process used. Clearly, it is necessary to make desalination processes as energy-efficient as possible through improvements in technology and economies of scale.

Desalination as currently practiced is driven almost entirely by the combustion of fossil fuels. These fuels are in finite supply; they also pollute the air and contribute to global climate change. The whole character of human society in the 20th century in terms of its history, economics and politics has been shaped by energy obtained mostly from oil. Almost all oil produced to date is what is called conventional oil, which can be made to flow freely from wells (i.e. excluding oil from tar sands and shale). Of this vast resource, about 1600 billion barrels have so far been discovered, and just over 800 billion barrels had been used by the end of 1997. It is estimated that there may be a further 400 billion barrels of conventional oil yet to be found. With current annual global consumption of oil being approximately 25 billion barrels, and rising at 2 per cent per annum, the "business as usual" scenario would suggest that the remaining oil will be exhausted by 2050. The supply of oil will undoubtedly be boosted by an increase of supplies from unconventional sources, notably the tar sands and shale of Canada and the "Orinoco sludge" of Venezuela. This oil can only be extracted using high energy inputs, and at very high environmental costs. There will be strong political and international pressure against development of these resources, but, when world oil prices are high enough, production will inevitably increase. In theory, unconventional oil could stretch the world's oil supply by another 30 years. In practice, of course, the rate of consumption of oil will be heavily influenced by economic and many other factors, so that prediction in this area is very difficult. The political situation of two of the world's largest potential producers, Iran and Iraq, could be highly relevant to supplies as well as to the global political economy. It is clear, however, that one of the most important of the influencing factors will be the relative cost of renewable energy and how quickly the world can switch to sustainable technologies. There is nothing to gain by deferring investment in this area, and everything to lose by postponing it any longer.

While salinity or salty water, is generally used to describe and measure seawater or certain industrial wastes, we use the term total dissolved solids ("TDS") to describe water high in various salt compounds and dissolved minerals. While one could have very high total dissolved solids, and very low salinity from a chemistry standpoint, here we are talking about high TDS. Total Dissolved Solids (TDS) refers to the amount of dissolved solids (typically various compounds of salts, minerals and metals) in a given volume of water. It is expressed in parts per million (also known as milligrams per liter) and is determined by evaporating a small amount of amount of water in the lab, and weighing the remaining solids. Another way to approximately determine TDS is by measuring the conductivity of a water sample and converting the resistance in micromhos to TDS. TDS in municipally-treated waters in our area range from 90 ppm to over 1000 ppm. The most common range on city water is 200 - 400 ppm. The maximum contaminant level set by USEPA is 500 ppm. California sets its standard as 1000 ppm, probably due to the high number of ground water sources in the state. The MCL is known as a Secondary Standard and in one sense, refers to the aesthetic quality of a a given water. The higher the TDS, the less palatable the water is thought to be. Sea water ranges from 30,000 to 40,000 ppm. Many brackish ground water supplies are used around California and we have many clients whose private well water has a TDS of 1500 - 2000 ppm. In some cases the levels exceed 7000 ppm. Generally, one wants a TDS of less than 500 for household use. In our experience, it appears that folks can tolerate for general household use, soft clean water with a TDS of up to 1500 ppm. When the levels start to exceed 1500 ppm, most people start to complain of dry skin, stiff laundry, and corrosion of fixtures. White spotting and films on surfaces and fixtures is also common at these levels and can be very difficult or impossible to remove.

TDS affects taste also, and waters over 500 - 600 ppm can taste poor. When the levels top 1500 ppm, most people will report the water tastes very similar to weak alka-seltzer. TDS is removed by distillation, reverse-osmosis or electrodialysis. In our area, most desalination projects, both large and small are accomplished with reverse-osmosis. Depending on the water chemistry, reverse osmosis systems are the most popular, given their low cost and ease of use. Distillers work very well also, and produce very high quality water, but require electricity and higher maintenance than reverse osmosis systems. For whole house treatment, commercial-sized reverse osmosis systems are usually the best approach. You will find a compilation of research and review articles at the end of this chapter that will provide you more in-depth information on each of the technologies covered.

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