The three types of acoustic measures investigated are:
- Cavitation. Formation and collapse of microbubbles at the rarefaction phase of pressure in a highly intensive ultrasonic wave or in high-velocity turbulent water flow. Frequency range from 10 – 380 kHz killed veliger, juvenile, and adult zebra mussels.
- Sound Treatment. Acoustic waves below the cavitation threshold frequency (<500 Hz) was effective against zebra mussel veligers.
- Vibrations. Vibration treatments were effective below 200 Hz and between 4 – 100 kHz against zebra mussel juveniles and below 200 Hz and between 10 – 100 kHz against zebra mussel veligers.
According to an experimental study by Schaefer, Claudi and Grapperhaus, sparker pressure pulses may be used to achieve acoustic shock waves which can inhibit mussel growth. This technology finds application primarily in intake pipes since the pipe structure helps to propagate shock waves. The electrodes, usually copper-tungsten, erode over time and will need to be replaced periodically (Schaefer et al, 2010).
Light technology. The effectiveness of UV light technology has been investigated and is recommended for medium- to small-scale water systems. UV light (wavelengths between 40 and 4,000) is effective in killing juvenile zebra mussels, as well as adult mussels if exposed for a longer period of time (Boelman et al, 1996). Average exposure time is two hours to achieve 85% veliger mortality. Large units can deal with flow rates of up to 2.5 m3/sec.
Lamps can be installed in pipes perpendicular to the flow or along side walls. Despite its effectiveness, UV light technology can mean high cost and maintenance requirements. Also, in the event of a breakdown, the bulb may release mercury into the water system, which is a health hazard. Additionally, high suspended solids or debris in water will reduce the effectiveness of this technology.
Zebra mussels are found in many of the lakes and water systems in Canada. As they are able to attach themselves to most hard surfaces and form multilayer colonies, costly blockage of intake pipes and screens occurs. Various techniques have been employed and research is still being conducted to find environmentally sound alternatives, which can effectively remove zebra mussels, while preserving lake ecosystems.
Rinita Banerjee is with The Municipal Infrastructure Group Ltd. This article was featured in ES&E Magazine’s April 2016 issue.
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