The cost-effective oxidizing activity of chlorine has made it the most commonly used compound for all types of water disinfection and biofouling control in North America, and it has been used in potable water treatment since the beginning of the 1900's (Van Benschoten et al. 1993). Chlorination, primarily via sodium hypochlorite, has dominated the chemical control of zebra mussel in both Europe and North America, and remains the least expensive and most popular method of removal. Chlorination effects can be provided by a range of compounds—the hypochlorites of sodium, potassium, or calcium; chlorine and chlorine dioxide gases; and sodium chlorite—and their toxic properties can be used to control zebra mussels and related nuisance mollusk species. Chlorine is able to kill or prevent settling of planktonic veliger larvae in raw water piping systems. In general, chlorine treatment for zebra mussel control should be applied at the most suitable time, for the shortest period, and at the lowest concentration to be efficacious (Jenner and Janssen-Mommen 1993). However, adult mussels will close at concentrations of from 1 to 2 mg L-1 and remain closed for up to 2 weeks.
Claudi and Mackie (1994) discuss chlorination processes for zebra mussel control in industrial systems in detail, along with its pros and cons, and Netherland (1997) gives a brief summary of chlorine usage. Chlorine has a number of important advantages: it is relatively inexpensive, it works in most raw water systems, it is toxic at low concentrations and quickly loses toxicity without bioaccumulating, and it can be applied with simple mechanisms. However, there are several drawbacks to the chlorination process. The transport and storage of gaseous or liquefied chlorination products involve hazards, and their corrosive properties can harm system components, so that they all require special handling. Discharge also presents problems because carcinogenic compounds known as trihalomethanes may be formed where organic compounds are present in water. AOX (adsorbable organic halides) may also be formed, but do not present risks. Differences in water quality may incur extra costs where discharge concentrations may be unpredictable due to varying chlorine demand.
There has been concern that the cumulative effects of extensive chlorine use for zebra mussel control in large rivers in North America could be problematic due to toxicity to nontarget organisms and formation of trihalomethanes. Although chlorine discharge into natural water bodies is already regulated, it is possible that it may be prohibited or severely restricted in the future if usage increases significantly (Claudi and Mackie 1994). However, to date, these products provide the most effective and low-cost control in the majority of situations.
The next section reviews the general properties of chlorination via hypochlorite and chlorine gas; additional chlorine oxidizers (chlorine dioxide, sodium chlorite) are discussed separately.