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.