Acoustics

The use of sound energy (acoustics) is developing as a zebra mussel control strategy, though investigations of its effectiveness have been inconsistent and more research is needed to adequately develop this strategy. Acoustics has several potential advantages over other methods; it is less likely to kill nontargeted organisms, has no obvious residue effects, and equipment can be installed relatively easily (Kowalewski, Patrick, and Christie 1993). There are three major approaches to using acoustic energy:

1. Cavitation - the formation and collapse of microbubbles. Such bubble formation occurs at the rarefaction phase of pressure in a highly intensive ultrasonic wave or in high velocity turbulent water flow.

2. Sound treatment - the use of water-borne acoustic energy (acoustic waves) having an intensity below the cavitation threshold. These include sound (20-Hz to 20-kHz) and ultrasound (above 20-kHz) waves. The sound waves having frequency below 1 kHz are called low frequency sound.

3. Vibration - the use of solid-borne acoustic energy (vibration) in mechanical structures (pipes, walls, etc.).

Kowalewski, Patrick, and Christie (1993) conducted experiments on the effectiveness of using acoustic energy (3- to 18 kHz) as a potential control measure for zebra mussels. Experiments using solid-borne sound at sonic frequencies were effective in preventing attachment of juvenile mussels in a pipe section. In the 8- to 10 kHz range, with acceleration of vibration to about 150 m/sec²., nearly 100 percent control (i.e., detachment) and 75- to 95 percent mortality was achieved. In the of 10- to 12 kHz range, almost 100 percent unattachment and mortality occurred at vibration accelerations exceeding 200 m/sec². Although, vibration amplitude needed for effectiveness appeared to increase with frequency, these were well within the permissible limits for normally operating equipment such as piping.

Donskoy and Ludyanskiy (1995) studied the effectiveness of low-frequency sound techniques to control zebra mussel fouling. Sound treatments were found to stress and, immobilize the veligers, causing them to drop out of the water column. Treatments using a combination of sound energy and vibration exposure caused a higher rate of mortality than sound treatment alone. Veligers responded to sound energy by the loss of their free swimming ability and subsequent sinking to the bottom. The vibration energy traveling in the pipe mechanically dissipated the immobilized veligers. This control strategy was found to be most effective in low-frequency range (below 200 Hz). Low-frequency sound was also effective in limiting the settlement of translocators into the study volume.

From their research, the following results of acoustic techniques, applied frequencies and mussel life stages can be inferred (Donskoy and Ludyanskiy 1995):

1. Ultrasonic cavitation at frequencies between 10 to 380 kHz have been shown to kill veliger, juvenile, and adult zebra mussels.

2. Sound treatments of low frequency (<500 Hz), have been shown to be effective against zebra mussel veligers.

3. Vibration treatments have been shown to be effective below 200 Hz and between 4 to 100 kHz against zebra mussel juveniles, and below 200 Hz and between 10 to 100 kHz against zebra mussel veligers.

These results indicate that, with further development, acoustic energy may be a practical mitigation strategy against mussel attachment in water handling facilities. There is a concern, however, about the destructive effect of vibration on structures, especially in the vicinity of the vibrator attachment. Further studies are necessary.

Prospective Control Methods

Contents