Effectiveness and potential environmental impact of a yeast-based deoxygenation process for treating ship ballast waters

We assessed the effectiveness and potential environmental impact of a yeast-based deoxygenation process considered for treating ship ballast waters to reduce the risk of aquatic species introduction. Laboratory experiments were conducted to test three treatment concentrations (0.33%, 0.67% and 1.0% v v−1) at five temperatures (4, 10, 15, 20 and 25 °C) in both fresh- and saltwater, with and without mixing. Complete anoxia (<0.3 mg L−1) was achieved in all experiments, and there were no significant differences in effectiveness between fresh- and saltwater or between mixing levels. Time to hypoxia was inversely related to temperature, ranging from half a day at 25 °C to nearly 7 days at 4–5 °C. The process can quickly generate and maintain anoxic conditions over a long enough period of time to effectively eliminate a wide variety of aquatic organisms. Results of six bioassays indicated that treated waters were not toxic at the end of experiments and would not pose a toxic risk to natural receiving waters. Increased concentrations of ammonia, organic carbon and particulate matter resulting from yeast production in treated waters may cause some potential adverse environmental effects. The practicality of implementing this process for treating ballast water in ships is discussed.