The Perth Seawater Desalination Plant draws water from the sea and depends on desalination membrane processes conceived and developed by chemical engineers. Photo via The Dow Chemical Company. Courtesy Water Corporation of Western Australia.
With explosive population and industry growth, the need for cost-effective water-purification and wastewater-treatment technologies has become more urgent than ever. Chemical-engineering principles are used to remove harmful pollutants from both raw source water and contaminated wastewater.
Specifically, chemical engineers have developed cost-effective methods to
- Purify water from subsurface aquifers and surface sources, such as rivers and lakes, to produce potable drinking water;
- Produce purified water that meets the increasingly strict requirements for industrial use; and
- Treat contaminated industrial and municipal wastewater and sewage to make them suitable either for discharge to public waterways or for reuse.
Treating water
Modern-day treatment of raw water sources or contaminated wastewater employs a wide array of physical, chemical, and biological techniques.
Chemical engineers refer to separating dangerous materials from good water as a treatment train. At various stages in the multistage treatment process, unwanted constituents are separated using
- Vacuum or pressure filtration,
- Centrifugation,
- Membrane-based separation,
- Distillation,
- Carbon-based and zeolite-based adsorption, and
- Advanced oxidation treatments.
Activated carbon is a highly adsorbent form of carbon that is produced when charcoal is heated. Its extremely intricate internal-pore structure provides exceptionally high internal surface area: just 5 grams of activated carbon has the surface area of a football field. Activated carbon removes impurities via adsorption from both aqueous and gaseous waste.
Membranes allow materials of a certain size or smaller to pass through but block the passage of larger materials. Imaginative arrays of membrane materials in innovative physical configurations are used to separate unwanted solids and dissolved chemicals from tainted water. During operation, purified water diffuses through the microporous membranes and collects on one side of the membrane, while impurities are captured and concentrated on the other side. In places like Key West and Saudi Arabia potable water is produced from seawater using membrane processes.
Today, membranes made from cellulose acetate, ceramics, and polymers are widely used. The applications come in a variety of innovative designs, including tubular, hollow-fiber, plate-and-frame, and spiral-wound configurations. The goals of membrane design are to
- Maximize the available surface area,
- Reduce membrane pore size (to allow for the more precise removal of smaller contaminants),
- Minimize the pressure drop the fluid will experience when flowing through the unit, and
- Identify more cost-effective system designs.
Advanced oxidation
Worldwide, about 85% of childhood sickness and 65% of adult diseases are thought to be produced by waterborne viruses, bacteria, and intestinal protozoa that cause diarrhea and other potentially life-threatening diseases. The addition of oxidizing agents—chemical ions that accept electrons—has proven effective against these microorganisms. Today, a variety of advanced oxidation techniques kill such disease agents and disinfect water, thanks to ongoing developments pioneered by the chemical-engineering community.
Historically, chlorine-based oxidation has been the most widely used, and it is very effective. However, the transportation, storage, and use of chlorine (which is highly toxic) present significant potential health and safety risks during water-treatment operations. To address these concerns chemical engineers and others have developed a variety of alternative oxidation treatments that are inherently safer, and in many cases more effective, than chlorination. These include
- Ultraviolet light,
- Hydrogen peroxide, and
- Ozone.
Each of these powerful oxidizing agents destroys unwanted organic contaminants and disinfects the treated water without the risks associated with chlorine use.
Copyright © 2009 American Institute of Chemical Engineers and Chemical Heritage Foundation. All rights reserved.