The Silent Treatment

No one can hear the high-frequency tones, but their echoes can be tracked far beyond the research labs where they were first detected. Today, ultrasonics, otherwise known as the science of noiseless sound, has become one of the more versatile tools of technology. Vibrating at roughly 18,000 or more cycles per second, quiet bursts of ultrasonic energy can probe the ocean depths, clean teeth, cut through steel, even look for brain damage. In their latest incarnation, ultrasonic waves are being put to work in a novel sewage-treatment system.

Most American cities still treat raw sewage with relatively primitive methods. Larger particles of waste are usually allowed to settle out of the polluted water, which is filtered through gravel and sand and then chlorinated before being piped to a nearby river or lake. Trouble is, such "purified" sewage is almost always contaminated with a rich brew of bacteria and viruses, to say nothing of stubborn industrial wastes that may serve as fertilizers and cause such rapid plant growth in the waters where they are dumped that all other forms of life are eventually choked off.

The new purifying process, called Sonozone, practically eliminates that leftover pollution with a swift one-two punch. A small, vibrating metal disk at the bottom of a tank through which the sewage water flows sends out a steady stream of ultrasonic energy. At the same time, ozone, a highly active form of oxygen that readily combines with other materials, is bubbled into the tank from a nearby generator, which produces the gas by shooting electric arcs through ordinary air.

Larger Test. Scientists are not sure exactly what takes place when the sewage is subjected to the combination of buzzes and bubbles. But whatever happens is highly effective. Clumps of bacteria and viruses disintegrate; longer chemical molecules break apart. In a pilot project at the University of Notre Dame that processes 20,000 gal. of sewage daily, less than 60 seconds of Sonozone treatment has proved itself capable of destroying 100% of the fecal bacteria and viruses it attacks, 93% of the phosphates and 72% of the nitrogen compounds.

Some European cities already use ozone in place of chlorine to purify water (TIME, Sept. 3). Engineers at Long Island's Telecommunications Industries, Inc., which designed Sonozone, say that the addition of ultrasonic waves produces a second whammy that makes the ozone much more effective than the chemical alone. One reason is that the high-frequency vibrations keep the ozone bubbles small and separated, thus providing a maximum surface area on which the cleansing reactions can take place. Sonozone will soon be put to a larger test. A full-scale plant is about to start up in the Indiantown, Fla., area, where 5,000 people produce some 600,000 gal. of sewage per day.

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