By Rinita Banerjee
Zebra mussels were first discovered in Canada in 1988 and have become a well-established nuisance in the Great Lakes. Their rapid colonization rate and environmental resilience make them difficult to control. Zebra mussels can adhere to hard surfaces such as PVC, plastics and metal surfaces to form multilayer colonies (Griffiths et al, 1991). This results in blockage or reduced flow in pipes and water intake systems, leading to adverse long-term economic impacts.
Various design techniques have been employed to control infestations of zebra mussels. The most common include a combination of chemical treatment and mechanical removal. However, concerns have been raised about the harmful effects on non-target aquatic species.
Traditional methods for dealing with zebra mussels
Traditional control methods include:
Chemical. Oxidizing chemicals such as chlorine, bromine, potassium permanganate and ozone are used extensively, with the help of injectors in pipe systems. This requires continuous application.
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Thermal. 35ºC for two hours results in ~100% removal efficiency. Needs to be repeated two or three times a year.
Mechanical. Pipe pigging, scraping, high pressure hose and sand blasting.
Non-traditional methods for dealing with zebra mussels
Non-traditional methods include:
Foul release coating. Silicone coated grating and pipes have been found to be ~80% successful in minimizing zebra mussel attachment in both static and dynamic systems. Various coatings were investigated in a study by the U.S. Department of the Interior, Bureau of Reclamation. Recommended coating materials include silicone FR #5 (Bioclean White) and bronze. Although silicone is highly effective in controlling attachment, the soft coating makes it vulnerable to abrasion.
The effectiveness of this method depends on site-specific conditions, such as flow rate, amount of floating debris, etc.
Low frequency magnetism. Research by Ener-Tec Inc. has proven the success of extremely low frequency (ELF) magnetism in preventing shell formation in zebra mussels, resulting in effective control of colonization (Ener-Tec Inc., 2015). The linear kinetic cell (LKC) system designed by Ener-Tec introduces a weak electric current into the water.
This causes rapid calcium loss in mussels, leading to death by exposure. This system may be used in 0.0127 m to 6 m diameter pipes. It is installed directly into the water line and requires electrical connections. The advantage of the LKC system is that it does not produce any chemicals and does not require continuous monitoring.
Another similar control device, Patent US 5476595 A, uses a combination of current and air bubbles to achieve the same result (Baddour et al., 1995). Current is supplied between electrodes that extend radially at equiangular positions around a cylindrical water inlet. Bubbles are formed by an annular air chamber of the water intake below the cylindrical inlet. The bubbles allow free-swimming microscopic zebra mussel larvae (veligers) to remain suspended in the electric field for a longer period of time. This results in higher removal efficiency.
This control device works well with vertical bellmouth intake pipes, such as those used by Great Lakes water plants
Pulse acoustics. The operation principle behind acoustic technology is that the vibrations cause stress and result in immobilization of veligers (Legg et al. 2015). Solid-borne sound at sonic frequencies has been found to be effective in preventing attachment of juvenile mussels in a pipe section. In the 8 – 10 kHz range, with acceleration of vibration to about 150 m/sec2, nearly 100% control may be achieved.