This section was developed using information and original text from the Zebra Mussel (Dreissena polymorpha) Control Handbook for Facility Operators.
Of all the chemical methods tested thus far, chlorination has gained the greatest acceptance. It has generally satisfied the economic feasibility of zebra mussel control, and it is reasonably easy to apply at most facilities. Change is on the horizon, however, and new control strategies are being proposed and tested in anticipation of more stringent environmental limitations. Chlorination forms trihalogenated methanes and other hydrocarbons that are carcinogenic. In 1993, Ontario Hydro gave its researchers 5 years to develop alternative control strategies that might replace chlorination. There is a trend in the United States toward tighter environmental restrictions regarding the use of chlorination. Like their counterparts in Canada, U.S. facility managers will need to have chlorination available to them for zebra mussel control, especially when operation is in jeopardy or efficiency is greatly reduced. Thus, guidelines are needed for the reasonable and prudent use of chlorination as a means of zebra mussel control at power-generating facilities.
Chlorination systems have gained wider acceptance than other treatment technologies, mainly because they are highly effective in controlling zebra mussel infestations. McMahon, Ussery, and Clarke (1994) reviewed European and North American control methods and indicated that there was a large and varied body of literature on both chemical and nonchemical control technologies. Their tables are useful for comparing the merits of individual control methods. The treatment methods listed with the oxidizing molluscicides include chlorination, chlorine dioxide (ClO2), and chloramines. The literature clearly shows a variation in not only the methods of chlorine treatment, but also the concentrations and duration of application and the lethality of these treatments. McMahon, Ussery, and Clarke (1994) report the following application and effect ranges for chlorine treatments:
Table 1.
Application and Effect Ranges for Chlorine Treatments (McMahon, Ussery, and Clarke 1994)
Toxicity studies have shown that adult zebra mussels are slightly more resistant to chlorine than veliger stages (Claudi and Mackie 1994). Chlorine treatments are more effective at the end of a growing season due to the physiologically exhausted state of the mussel following the reproductive effort. There is an inverse relation between the population biomass and the treatment effectiveness. Dense thick mats of mussels, particularly individuals at the bottom of the mat, are less vulnerable than are single layer colonies (Claudi and Mackie 1994). Thus, multiple applications or multiple treatment methods may be necessary in problem infestations.
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