Challenge

This Northern electrical utility operates a fossil fuel plant on the Mississippi River and uses 250-300 million gallons per day of water for once through cooling. Over time, this facility became progressively infested with zebra mussels. Although no restrictions in the water flow had been measured, plant personnel took a proactive approach to prevent further infestation.

Solution

A comprehensive chemical and microbiological survey of the water system was performed to establish the most efficient treatment program. Infestation had occurred in the following areas:

• The inlet bays
• The fire water bay
• The service water system

A recommendation was made to treat the system with chlorine dioxide at low concentrations. Chlorine dioxide is an effective molluscicide for both batch and continuous treatment. An 8,000-pounds per day capacity, three-chemical chlorine dioxide generator was hard piped into the inlet bays. A solution of chlorine dioxide was generated and injected into the inlet water stream. An average treatment level sufficient to remove the zebra mussels was maintained. Precursors for the generator were supplied from bermed portable tanks. An extensive header and valve system was installed to insure thorough treatment of the inlet bays. Chlorine dioxide was fed at low concentrations for 72 hours.

Results

A 100% mortality of adult zebra mussels was achieved in the inlet system. Mortality was monitored using mussel bioboxes placed downstream of the treatment as well as visual inspection by divers. The facility was fully operational during the treatment and no downtime or reduced wattage was necessary. An additional benefit observed was an increase in heat exchanger efficiency. This was a result of the removal of biological growth, which had coated the inside of the piping and exchangers. The customer was extremely satisfied with the results.