Search

Ballast water tanks in ships are today often corrosion-protected by sacrificial zinc anodes in addition to protective coating. Dissolved zinc has been found to be toxic to early life stages of many freshwater and marine organisms. We have therefore examined if the zinc released into ballast water could reach concentrations previously known to affect aquatic biota. When assuming a simplistic model for the release of zinc from the anodes, the zinc concentrations in ballast water could reach from 1.16 mg/l at pH 8.2 to 29 mg/l at pH 7.5. The former corresponds roughly to one, and the latter corresponds to two orders of magnitude times the levels found to kill or injure early life stages of various aquatic organisms. The use of zinc anodes in corrosion protection may thus have possible negative impacts on the biota in enclosed harbour waters, but can reduce the numbers of non-indigenous aquatic organisms surviving in ballast water.

Ozone has been investigated for its potential to remove marine dinoflagellate algae from ships’ ballast water. Dinoflagellate algae, Amphidinium sp. isolated from the Great Barrier Reef, Townsville, Australia were used as indicators since these produce a type of cyst that is difficult to inactivate, but are relatively easy to culture. The ozonation experiments have demonstrated a high ozone demand for inactivation of the algal cultures, which increases as the culture ages. The main ozone demand in seawater is due to its reaction with bromide to form bromine compounds. The non-bromide ozone demand has been estimated by measuring the residuals produced after various doses of ozone. The Amphidinium sp. show an unexpected response to both ozonation and bromination, with an instantaneous inactivation of the organisms for all doses that produced an oxidant residual in the seawater, followed by an effect of the disinfection residual. The standard design procedure of comparing Ct will not be effective for predicting the response of the organism to varying dose, C , and contact time, t , and a plot of ozone produced oxidant residual against organism inactivation for various contact times is proposed for design purposes. High doses of ozone (5–11 mg/L) and up to 6 h of residual contact were required for a 4-log inactivation of the Amphidinium sp. Ozonation is likely to be a difficult technology to implement for organisms with this ozone requirement in combination with characteristics of ballast tanks, which contain areas of sediments high in detritus and areas of corrosion.