The introduction of the zebra mussel (Dreissena polymorpha) to North America in 1986 has had a dramatic impact on water users throughout the continent. This non-indigenous bivalve is thought to have been introduced to Lake St. Clair from transoceanic ballast discharge. In ten years it has colonized the entire Great Lakes system, the Mississippi River and associated tributaries, and now threatens inland waterways throughout North America. More recently, a second exotic dreissenid, the quagga mussel (Dreissena bugensis), was identified in the Great Lakes, (May and Marsden, 1992).

As a result of the mussels' ability to block industrial and municipal water intakes and service water piping, significant sums of money have been expended to develop methods of controlling infestation.

Methods have included mechanical cleaning, chlorine, thermal treatments, ultraviolet light, cathodic protection, and molluscicides, among others. Traditional methods of chemical control using oxidants, specifically chlorine, have been very successful in treating once-through systems. Research continues with the goal of identifying viable methods that will reduce or eliminate the use of chemicals as a control option.

Sampling ocean-going vessel ballast water to detect the potential of new exotic species entering our waterways.

Systems that are semi-static in nature, such as closed-loop cooling or fire protection distribution systems, present different challenges in terms of control and monitoring strategies and have recently become a major focus of zebra mussel control. Fire systems in particular are easily disrupted by infestation since only a small amount of shell material is required to obstruct a fire hose nozzle or sprinkler head during emergency use. These systems were initially considered to be immune to mussel colonization because of their stagnant nature, however, recent data has shown otherwise.

Scientists have developed a method to assess fire protection systems using dissolved oxygen as an indicator. Many surveys have shown that, in fact, significant amounts of fresh water are moved through these piping networks as a result of intermittent use (i.e., dust control, cooling water, vehicle washing) and/or leakage. In addition, most systems are flushed or flow tested on an annual, semi-annual or more frequent basis. In this way, zebra mussels colonize intermittent flow systems, become established, and are maintained by these sporadic infusions of fresh water.

Semi-static water systems have been difficult to effectively treat using traditional methods, such as oxidants, because it is necessary to maintain continuous chemical injection and flow through these systems to ensure sufficient residual levels for the duration of treatments. In addition, treatment using oxidants or non-oxidizing biocides is only an economical solution when used under warm water conditions (>15°C). In many cases it is not convenient for industrial water users to undergo treatments during busy summer months.

A new approach to treating semi-static systems has been developed using potassium (K+) in the form of commercially available potash. Initially used to optimize the effects of oxidants (i.e., chlorine), experiments found that potassium alone worked better and faster than oxidants and, at low levels, appeared to be relatively selective as a molluscicide. The low cost, benign nature, and commercial availability of this product have generated widespread enthusiasm among industrial water users. In addition, potash is naturally occurring in the environment at detectable levels (>2 ppm ­ parts per million) and has been used by the farming industry for decades. Each year millions of tons of potash are applied as fertilizer to aid the agricultural process.

This article has been abridged from May 1997 issue.