End-of-season treatment. This treatment is given to flush out relatively small amounts of accumulated adults. Debris can be a problem; end-of-season (November) chlorination is not optimal if this is the only treatment or chemical being used.
Periodic treatment. This consists of at least three treatments of several weeks length; if given over the breeding season (May to November), adult mussels will be eliminated. Less debris is generated, as mussels are smaller.
Intermittent treatment. This treatment is useful for prevention of new primary veliger settlement, especially where adult mussels cannot be tolerated. However, it is not effective against established adults. It can be combined with an out-of-season continuous chlorination treatment to eliminate adults.
Semicontinuous. Frequent on/off cycling of treatment can have effects similar to continuous chlorination in keeping zebra mussels in a stressed status of shell closure. This treatment has a lower cost of material due to reduced exposure time.
Continuous. Constant presence of chlorine at low levels can prevent veliger settling and survival.
Chlorinating compounds can be added to water as gas (Cl2), liquid (e.g., NaOCl), or solid (e.g., calcium hypochlorite, Ca(OCl)2). Due to difficulty and hazard of handling pressurized chlorine gas, the majority of applications are of liquid hypochlorite. Various concentrations of sodium hypochlorite are available, with 12 percent active chlorine by weight being most commonly used by industry (Claudi and Mackie 1994). Chlorine gas requires potable water supply for maintenance and operation of a gaseous chlorine system, and air scrubbers to filter exhaust.