Science: 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...