Pictures of the Future      Spring 2005

Cannibals, Porous Fibers and Purity

Although pollutants in wastewater are becoming increasingly complex, higher quality is also being demanded of purified water. Specialists at Water Technologies, a Siemens business unit, and at Siemens Corporate Tech­nol­o­gy have come up with new methods for purifying water more effectively while cutting costs.

Practically every household sends an unappetizing mixture of organic materials into the sewer system. The food industry contributes grease, flour, whey and cleansing agents; industrial wastewater often contains chemicals; and rain­water washes dirt into gutters—all of which adds up to an enormous challenge for wastewater treatment plants. Furthermore, stricter environmental standards and increased awareness of health issues are driving demand for better treat­ment solutions. Water treatment facilities are expected to remove complex pollutants, be inexpensive to run, take up little space and generate minimal hazardous waste.

The most advanced facilities use developments from Siemens Water Technol­o­gies such as MemJet—an innovative membrane bioreactor—and the "Canni­bal" process—a purely biological process that substantially reduces the quantity of sludge produced by the treatment process.

Sludge mainly consists of the remains of the bacteria used for the treatment process. Depending on the level of pollutants and pathogens in the dry sludge, it can either be used as agricultural fertilizer or sent to a landfill for disposal. "The cost of sludge handling, transport and disposal has risen sharply in recent years," says Betty-Ann Curtis, Director of Biological Processes at Water Technologies Envirex Products in Waukesha, Wisconsin.

Ravenous Bacteria. The Cannibal process introduced by Water Technologies, however, allows the total amount of sludge to be drastically reduced—by 90 to 95 percent compared to conventional methods. The process utilizes the ravenous hunger of various types of bacteria to thoroughly break down the organic components of wastewater. Its end products are nothing more than water and carbon dioxide (see graphic). The sludge that accumulates in a clarifier is first pumped into a special facility where non-biodegradable or not readily biodegradable constituents such as hair, trash, grit and inert material are separated out using fine screens and a hydrocyclone. This—for the bacteria indigestible—residue is easily disposed of at local landfills.

The amount of bacteria in the plant thus remains constant and there is no accumulation of sludge. Nevertheless, the entire facility is cleaned out occasionally since some of the particulates cannot be removed by the solids separation module. In a typical facility, which treats 5.6 million liters of waste­water per day (for a population equivalent to about 15,000 U.S. residents) the new system could save up to $300,000 a year. The Cannibal process, which is now installed at some 25 facilities in the U.S., can be easily added to existing wastewater treatment facilities, thus increasing their capacitiy.

In recent years, the trend has been to separate solids from the water with the help of membranes, rather than relying on gravity in a clarifier. These mem­branes are generally located in the aeration basin itself, thus obviating the final clarifier. Whereas in a conventional plant only those bacteria that quickly settle on the bottom of the clarifier are allowed to multiply in the aeration basins, membrane bioreactors can use the bacteria that are most effective at purifying wastewater. "The biological treatment process can be optimized without worrying about settleability," says Edward Jordan, Vice President of Water Technologies Memcor Products.

Siemens’ MemJet process employs an elegant solution to deal with the problem of fouling. Since the membranes consist of bundles of hollow fibers, the wastewater is led along the fibers’ exterior together with a jet of air bubbles. As in other membrane systems, the interiors of the fibers hold a vacuum that sucks the water molecules into the fibers through tiny pores. Dirt particles, viruses and bacteria are left outside. The air bubbles take the solids along and prevent them from accumulating on the membrane. The bubbles also create a uniform flow along the membrane’s surface, preventing "dead spots" that are particularly susceptible to fouling. Thanks to this solution, MemJet tanks need to be chemically cleaned only once or twice a year. Ten such facilities are now in operation in the U.S., with others on the way.

Mini Treatment Plant. Since membrane technology has recently become far more affordable and water in the U.S. is increasingly being reused right after treatment, the trend is toward small, decentralized facilities. "These plants are being built where the water is needed—near irrigated fields or golf courses, for example," says Jordan. The satellite plants are connected to a municipal sewer line and consist only of an aeration tank, a membrane tank and a disinfecting facility. The extracted solids are fed back into the drain, where they are washed back to the central wastewater treatment plant. Plant operation is largely automatic, and such facilities need to be inspected only once or twice a week. According to Jordan, the satellite plants can be set up even in densely populated areas since they produce no unpleasant odors. What’s more, the small tanks are inconspicuous and are often not even recognizable.

Membranes are also suitable for treating industrial wastewater, which often contains substances that conventional sewage systems can’t handle, such as mineral oils, salts or heavy metals. Industrial wastewater is thus often pre­treated at company plants before it reaches the sewage system. As part of the EU-sponsored Mewaprev (Metal Waste Prevention) project, Frank Walachowicz and his team of engineers at Siemens Corporate Technology in Berlin have developed a solution for wastewater from the electroplating industry. Electro­plating involves the application of a thin layer of metal to a plastic or metal surface. It is used to enhance vehicle parts, cell phone housings and jewelry. "These companies’ wastewater contains all kinds of metallic salts, particularly salts of copper, nickel, chromium and precious metals," says Walachowicz. Although the concentration of metals in such wastewater is sometimes as high as that found in ore deposits, the metal could, until recently, not be eco­nom­i­cally recycled.

Precious Metals from Wastewater. With the new Siemens method, however, metal ions can be inexpensively extracted from wastewater. The system’s core component is a hollow fiber module. The wastewater flows past the exterior of these fibers, while a second liquid known as a "strip" and consisting largely of sulfuric acid flows in the fibers’ interior. The sides of the fibers are studded with pores filled with a kerosene-like liquid, whose composition varies depending on the metal extracted. This liquid acts like a membrane, allowing the metal ions to migrate through the pores while the water remains outside. As a result, the metal concentration in the strip is successively increased. "Even though this concentration is eventually a thousand times higher than in the waste­water, the metals remain in solution due to low pH values," says Walachowicz. The extracted metals can be reused for electroplating.

A pilot plant has already been set up in Berlin that can purify 50 liters of waste­water per hour – which is not enough for full-scale commercial electroplating facilities. "But it wouldn’t be a problem in terms of technology to enlarge the plant," says Walachowicz. The CT pilot plant can treat one cubic meter of waste­waster for about 80 €. In contrast, it costs electroplating plants about 100 € to do the same with conventional—but not very environmentally compatible—treatment methods. And that doesn’t include the cost of disposing of the electroplating slurry.

Walachowicz and his colleagues are still developing a fully automatic control system for the plant. It will use a real-time analysis chip developed by a Siemens Corporate Technology team headed Dr. Frank Arndt. The chip features microscopically small channels. The metal content of tiny wastewater droplets can be analyzed directly on the chip on the basis of electrical conductivity. "From dosing and identification to the separation of metal ions—everything will take place on the chip itself," says Arndt. "What’s more, it will all be done electrically, without requiring any pumps or other mechanical aids." Arndt and Walachowicz plan to integrate the chip into the pilot plant later this year to allow automatic control of the system.
                                                                                                                         Ute Kehse