Typical systems for applying chemicals to systems and sites usually require specific chemical feed equipment, piping for chemical transport, diffusors to introduce chemical to the water, and areas for chemical storage or generation (Lawrence 1997). A variety of treatment timings can be used. Claudi and Evans (1993) summarize them as reactive strategies, used after zebra mussels have become established in a raw water system or have fouled external structures, and proactive strategies designed to prevent settlement:
- End-of-season. Targets adult mussels. Chemical is applied for a period sufficient to kill all adults established in the system at the end of the breeding season; thus, the system must be able to tolerate one season of fouling. Oxidizing or nonoxidizing chemicals may be used. Oxidizing chemicals are expected to require dosing at high levels for at least 2 weeks to overcome mussel closure. Ten to twenty percent of individuals are expected to survive, and prolonging treatment to achieve 100 percent mortality may be impractical.
- Periodic. Targets adult mussels on a regular basis; usually carried out when densities and size of adults remain low, so that debris removal is lessened. System must be able to accept some macrofouling. It is not necessary to achieve complete (100 percent) mortality. Oxidizing or nonoxidizing chemicals can be used.
- Ongoing intermittent/(continuous pulse). Low levels of chemicals target postveligers to prevent infestation. Since postveligers are more susceptible than adult mussels, lower concentrations can be used; however, these will then not control established adults. For use in clean systems where no plugging can be tolerated (e.g., thin piping). Little debris duced.
- Continuous. To discourage all postveliger settling. For use where there is no tolerance of obstruction or fouling; lower concentrations can be used but they must be constant. Established adults affected only if chemical applied all season. Carried out only with oxidizing chemicals.
Treatment strategies also involve the type of application and the extent of the system treated at any one time. There are several options, depending on the system configuration and location of current or potential problems:
- Entire raw water treatment: Addition of chemical to the forebay or injected into suction or discharge of system pump piping.
- Entire system treatments: Addition of chemical so that it is present and circulating in all water within the operating system.
- Forebay treatments: Treatment of water in a holding area before it is brought into the main operating system.
- Targeted treatments: Addition or application of chemical to only certain portions of the complete operating system or parts of multiple systems.
- Recirculation treatments: Treat forebay, then isolate forebay from incoming water and recirculate through system.
Where applicable, closed-loop systems may be set up to reduce the amount of chemical needed per application.
Zebra Mussel Chemical Control Guide
The Use Guide Outline