This section was developed using information and original text from the Zebra Mussel (Dreissena polymorpha) Control Handbook for Facility Operators.
A number of chemicals have been tested with varying degrees of success and acceptability. Claudi and Mackie (1994) pointed out that the major advantage offered by most chemical treatments is that they can be engineered to protect almost the entire facility. The disadvantage rests in limiting the discharge of toxic materials back into the environment and meeting environmental regulations. Chemical treatment technologies are subjected to continued scrutiny, and environmental concerns further limit their use. However, until suitable substitutes are found, facility managers have no real choice but to rely on chemicals as a component of their overall control strategy for problem infestations of zebra mussels. Although many chemical treatments have been tested, chlorination is most widely used. In particular, sodium hypochlorite (NaOCl) is used more often than any other chlorination treatment method. The Zebra Mussel Chemical Control Guide and the Zebra Mussel Control Handbook for Facility Operators contain case studies dealing with successes and failures of various chemical applications to control the zebra mussel, as well as providing further information on mitigation techniques. New control strategies are being proposed and tested in anticipation of more stringent environmental limitations.
The applied chemical treatment strategy is as important as the type of chemical used. Five different chemical treatment strategies have been proposed by Claudi and Mackie (1994) for zebra mussel infestations: end-of-season, periodic, intermittent, continuous, and semicontinuous. A chemical zebra mussel control strategy may consist of a single treatment scenario or a combination of treatments. The treatments most applicable to a particular facility depend on: the extent of zebra mussel infestation, the degree of permissible infestation, water quality, existing facility systems, economics, permit requirements, and environmental regulations. An effective chemical treatment design, allows for flexibility in treatment applications in accordance with the facilities’ entire zebra mussel control program.
End-of-season treatment is generally a reactive strategy, acceptable in systems that can tolerate limited macrofouling. Limited macrofouling can be anticipated if chemical treatments are applied once during the year, usually after the spawning season or at the end of the growing season. Treatments after the spawning season increase chemical effectiveness and reduce required concentrations as adult mussels are weakened. Also shells and soft tissue debris of young-of-the-year mussels more easily pass through facility systems. Mitigation of established mussels by end-of-season treatments requires higher dosages of chemicals over an extended period of time (2 to 3 weeks) (Claudi and Mackie 1994). Chemical concentrations and exposure times are dependent on the chemical used, water quality, and health of the mussels. Defining absolute levels applicable to all locations at all times is very difficult. Byssal threads remaining after end-of-season treatment can promote the settlement of veligers, cause corrosion, and add surface friction.
Periodic chemical treatment, like end-of-season treatment, is usually a reactive treatment (usually conducted on a regular basis, such as every 2 months) designed to eliminate adults that have accumulated since the previous application. Again, limited infestations must be tolerable, but because treatments are more frequent, infestations will be proportionally smaller. The chemical concentration and exposure time should be comparable to end-of-season values, though the total removed biomass would be smaller.
Intermittent chemical use is designed to prevent initial zebra mussel infestation at facilities that cannot tolerate macrofouling. Dosing at frequent intervals (i.e., 6, 12, 24 hr) destroys post-veligers that have settled since the previous treatment. Post-veligers are more susceptible to oxidizing chemicals than are adults; thus, the concentration of the chemical and exposure times will be considerably less than if adults were the target. Because post-veligers with shells about 250 µm long can easily pass through the system, disposal and under-deposit corrosion are eliminated.
Semicontinuous treatment is a preventative control method developed by Ontario Hydro. Because zebra mussels will stop filtering and close their shell when exposed to a toxic substance, frequent on-off cycling of chlorine was more effective than continuous chemical treatments. Treatment schedules can be adjusted to 15 minutes on and 15 to 45 minutes off. Chlorination treatments consisting of 15 minutes on and 15 or 30 minutes off at the 0.5-mg/L level have been as effective as continuous treatment (Claudi and Mackie 1994). Semicontinuous treatment is ideal for facilities where several discrete systems need to be treated, and it results in less chemical usage than continuous chlorination.
Continuous chemical treatment is designed for facilities that cannot tolerate any level of macrofouling. Low chemical concentrations applied continuously prevent any post-veliger settlement, and are stressful enough to either kill adult mussels or cause them to detach and move out of the system. Continuous treatment should be carried out for the entire zebra mussel breeding season.
For further information on the use of chemical control and its methods of application, see the
Zebra Mussel Chemical Control Guide
Management and Control Contents
Management and Control Options
Reactive Control Methods
Zebra Mussel Chemical Control Guide
Effectiveness of Chemical Control Methods: An Overview