Water, the "universal solvent," the indispensable ingredient to maintain life and health, is a precious commodity that is largely misunderstood and abused.
For all intents and purposes, we have the same quantity of water on this planet as existed when the earth first was formed. Only 3 percent of the water is fresh, and two-thirds of that supply is tied up in glaciers and icecaps. The remaining supply (1 percent) is all that is left for human consumption, agricultural activities and manufacturing requirements. In addition, the acceptability of that water is becoming more and more of a concern.
Of course, acceptable means different things to different people. For example, the highest overall water quality is required for the semiconductor industry for rinsing operations.
On the other end of the spectrum is water required for cooling or rinsing applications. Here, the water simply must not cause corrosion and/or leave objectionable deposits behind.
As scientists gain knowledge about various waterborne contaminants, drinking water quality requirements are becoming more stringent. Similarly, as manufacturing processes become more sophisticated and product quality requirements get stricter, the water quality used in these operations becomes more important. However, we all must work with a fixed quantity of available water that is subjected to increasingly greater usage pressures and contaminants.
Water contaminants can be divided into five general classes. See Table 1.
No water volume is completely free from contaminants. The ability to measure small concentrations of water contaminants has increased, and water suppliers and users now are able to identify more types of contaminants. In the past, the part-per-million (ppm) measurement ability was considered the ultimate in analytical sensitivity. Now part-per-billion (ppb) levels ," and even part-per-trillion (ppt) levels ," are measured routinely.
Some people claim our water supplies are becoming more and more contaminated; in reality, however, this could be a misperception based on improved measurement capabilities. In fact, strong evidence exists that our water is facing less contamination today than it was a few decades ago ," partly as a result of U.S. Environmental Protection Agency rulings and partly as a result of responsible stewardship.
When dealing with contaminated water within chemical and other manufacturing operations, plant personnel must ask:
How should we clean up water that is too contaminated for our particular use requirements (water purification)?
How should we treat water we wish to discharge, but cannot for either legal or ethical reasons without treatment (wastewater treatment)?
In the past, water purification and wastewater treatment were considered two separate animals. In reality, however, they have more similarities than differences.
Many of the technologies used to purify water, including membranes, also are very effective in treating wastewater. On the other hand, most wastewater streams have higher concentrations of contaminants than incoming water, and some of the pollutants might require very specific treatment technologies for effective removal.
Table 2 lists many of the treatment technologies in common use today, and provides approximate indications of their effectiveness in reduction of the classes of contaminants.
To select the optimum treatment technology, whether for water purification or wastewater treatment, plant personnel must ask:
1. What are the kinds and concentrations of contaminants that must be reduced?
2. What is the "purity" level goal?
3. What is the flow rate to be treated?
Selection of the optimum treatment technology based on the above data requires knowledge, experience and objectivity.
For water purification applications, where the contaminants are usually low in concentration and not particularly unusual, a number of well-proven technologies exist, and little or no testing is required to define their performance in a particular application. The skilled professional can peruse a water analysis and, with the water quality requirement and flow rate data, identify the technology candidates that would produce the optimum treatment system. In some cases, such as with reverse osmosis, computer programs are available to help the experienced professional design a complete membrane processing system.
Based on the client's requirements, the design engineer might be asked to develop performance specifications, purchasing documents and even a list of potential suppliers of each technology component. To ensure the optimum technology is selected and the most cost-effective hardware is purchased, the design engineer must have no vested interest in the sale.
Wastewater applications, however, present different challenges. Usually, the goal is to remove as much waste material as possible from the water stream and to make it "go away." The concentration of the contaminants in the portion to be discharged often is very high, potentially creating problems with certain technologies. In addition, the contaminants found in industrial waste streams often are manmade and might resist separation from the stream when traditional treatment technologies are used.
It is usually necessary that wastewater streams be tested thoroughly before the engineer selects the best technology and optimum treatment design. In general, this process requires a professional with even greater knowledge and skills than those required for water purification applications, along with the equipment to perform the testing.
Although widely used for water purification, membrane technologies are relatively unfamiliar in wastewater treatment, despite the fact that they offer a number of advantages over other solids/liquid separation technologies. These advantages include:
A continuous process, resulting in automatic and uninterrupted operation.
Low energy use involving neither phase nor temperature changes.
A modular design ," no significant size limitations.
A minimum number of moving parts and low maintenance requirements.
No effect on the form or the chemistry of contaminants.
A discrete membrane barrier to ensure physical separation of contaminants.
No chemical addition requirements.
Before installing a membrane technology, the plant must conduct testing to determine:
Whether or not the particular polymer provides the desired separation.
What membrane device configuration is optimum in the application.
What processing conditions produce the best results (pressure, flow rates, temperature, etc.).
In general, every stream must be tested to determine design factors such as the specific membrane polymer, membrane element design, total membrane area, applied pressure, system recovery, flow conditions, membrane element array and pretreatment requirements.
Specific stream properties that influence these design factors include those listed in Table 3.
In an ideal system, all contaminants to be removed are separated by the membrane and exit in the concentrate stream. In reality, no membrane is perfect in that it rejects 100 percent of the solute on the feed side. This solute leakage is known as "passage." Expressed as "percent passage," the actual quantity of solute that passes through the membrane is a function of the concentration of solute on the feed side.
Under high-recovery conditions using reverse osmosis and nanofiltration technologies, the concentration of salts on the feed side is increased; therefore, the actual quantity of salts passing through the membrane also increases. Many applications demand that, in addition to a minimum concentrate volume, the permeate quality be high enough for reuse. This "Catch 22" predicament ," where permeate quality decreases as recovery increases ," can impose design limitations. Additionally, the osmotic pressure increases resulting from recovery increases also impose a design limit. Generally, pumping pressures in excess of 1,000 pounds per square inch (psi) (68 bar) are impractical for most applications.
Membrane testing options
Recovery, osmotic pressure, permeate quality, recycle and other factors serve to underscore the value of testing the specific waste stream as thoroughly as possible. Because analysis results on effluent streams often vary as a function of time, it is important that either a composite or a "worst case" sample is obtained for test purposes.
A cell test, an applications test and/or a pilot test should be used to evaluate membrane technology with a particular effluent stream.
On the plus side, the cell test is fast, involves inexpensive equipment and requires only small quantities of test solution. However, the test cannot determine the long-term chemical effect of a solution on the polymer and does not provide engineering scale-up data. In addition, it gives no indication of optimum membrane element configuration and does not provide data on the fouling effects of the test solution.
The test is fast and provides scale-up data such as flow, element efficiency, osmotic pressure as a function of recovery and pressure requirements. It also can provide an indication of membrane stability. On the down side, however, it provides neither the long-term chemical effects nor sufficient data on the fouling effects of the test solution. The figure illustrates a typical applications test schematic.
Typical Applications Test Schematic
Membrane technologies can be a viable option for a chemical plant's wastewater treatment. Their selection and use, however, involves some testing work upfront. When the proper membrane system is selected through testing, chemical plants will be able to reap the benefits associated with the technology.
Cartwright is president of Minneapolis-based Cartwright Consulting Co., which specializes in the application of innovative treatment technologies to water purification, wastewater treatment and chemical/food processing. Contact him at email@example.com, or visit the company's Web site at www.cartwright-consulting.com.