Efficacy and residual toxicity of a sodium hydroxide based ballast water treatment system for freshwater bulk freighters
The efficacy and residual toxicity of a sodium hydroxide (NaOH) based ballast water treatment system (BWTS) were tested aboard the Great Lakes carrier M/V American Spirit in 1000 L mesocosms containing water from the ship's ballast tanks. NaOH was added to elevate the pH to 11.5 or 11.7 for 48 h, after which pH was reduced to < 9 before discharge by sparging with carbon dioxide to form sodium bicarbonate. In 4 trials, pH 11.7 NaOH BW was highly effective in reducing the densities of organisms relative to uptake water and met the ballast water discharge standards of the US Coast Guard (USCG), the US Environmental Protection Agency vessel general permit (USEPA VGP) and the International Maritime Organization (IMO) G8 for the classes of regulated organisms: ≥ 50 μm, ≥ 10 μm to < 50 μm and indicator bacteria < 10 μm. In addition, densities of heterotrophic bacteria were reduced > 96% in pH 11.7 treated discharge water relative to uptake densities. Seven day whole effluent toxicity tests indicated pH 11.7 NaOH BW met the USEPA VGP daily maximum criteria for residual toxicity. Organism densities in uptake water did not meet the minimum densities for IMO G8 shipboard test validity in 2 of 4 trials for organisms ≥ 10 μm to < 50 μm or in any trials for the < 10 μm size class. The high efficacy and low residual toxicity observed indicates that a NaOH BWTS has great potential for successfully treating large volumes of ballast water released into freshwater systems.