This study verified the effectiveness and the potential toxic impact of PERACLEAN Ocean ballast water treatment for very cold freshwater (0.1-0.5 degrees C) in real ballast tank (750 m(3)) conditions aboard a ship and in large-volume (4.5 m(3)) polyethylene tanks. Concentrations of peracetic acid (PAA) and hydrogen peroxide (H2O2) gradually dropped by 41-59% over 5 days. The treatment altered the quality of the treated waters by causing a pH drop of 0.9-1.3 units and a fourfold to sevenfold increase in dissolved organic carbon and organophosphates concentrations. More than 90% of the biomass of free-floating micro-organisms and viable phytoplankton were eliminated within 48 h after treatment. The treatment resulted in 100% mortality in caged fish exposed to treated waters but was totally ineffective against adult zebra mussels and some nematods living in tank sediments. Toxic response from ecotoxicological assays indicated that treated waters after 5 days should be diluted by a factor of 1:2 to 1:200 to reduce toxicity below selected endpoints of acute lethality tests. On the basis of PAA degradation rate, fresh waters treated with 100-ppm PERACLEAN Ocean should be kept in ballast tanks for 15-20 days after treatment to reduce toxicity. It is concluded that the treatment can be an effective biocide to rapidly eliminate organisms of the water column inside the ballast tanks over a wide range of environmental conditions, but that the discharge of the toxic treated waters should be properly managed to minimize potential environmental impact.