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: