Chlorine dioxide as a treatment for ballast water to control invasive species: Shipboard testing

•Chlorine dioxide (5 ppm) added to ballast water eradicates most planktonic organisms.•Disinfection does not persist past a few days for bacteria, especially during summer.•No resurgence of phytoplankton occurs for several days.•Zooplankton are eliminated unless sheltered in sediments or within protective coatings.

The efficacy of chlorine dioxide (ClO2) in eliminating organisms present in estuarine ballast water of a containership was determined under actual operating conditions by comparing the survival of planktonic communities present in waters of treated and control ballast tanks. Sampling was via ballast-tank hatches. The treatment (5 mg L−1 ClO2 without pre-filtration) delivered by a prototype ClO2-generating system was generally effective against planktonic assemblages, although bacterial communities rebounded after a few days. Regardless of temperature, ClO2 was very effective against phytoplankton; the effect was immediate, without resurgence. Some zooplankters in the ⩾50-μm fraction may survive the biocide, especially those able to find refuge within a protective coating (e.g., cysts, resting eggs, and shells) or in sediment. In order to boost efficacy, a pre-filtration step is recommended (now installed as standard equipment) to lower the intake of the ⩾50-μm fraction and lessen the challenge posed by this size class.