Zooplankton sensitivity and phytoplankton regrowth for ballast water treatment with advanced oxidation processes

The ballasting and de-ballasting of ships are two necessary operations with ballast water that provide stability for safe navigation. Empty ships must ballast tanks with water, which contains living organisms and subsequently carries them away from their original distribution. De-ballasting represents an input of still viable zooplankton, phytoplankton, and microorganisms in the destination port, leading to the introduction of alien species, and consequently, the introduction of organisms will alter the local biodiversity. Ballast water treatment is necessary to comply with the International Maritime Organization (IMO) for the maximum viable organisms permitted. It is known that UVC eliminates microorganisms, but there are few studies on the other taxonomical groups, such as phytoplankton and zooplankton. The advance oxidation processes (AOPs) with UV-C can be a good alternative to manage the problem of ballast water, primarily for microorganisms. However, for larger organisms, there is more resistance, and, a stage with filtration (by physical filtration or hydrocyclone) is usually required. The filter can fail, or certain zooplankton organisms can escape across the filter and go to the AOPs or UVC reactor. According to the taxonomic group, there can be a different sensitivity to the treatment, and one could survive and generate a risk. The AOPs tested were natural solar radiation (RAD), UV/H₂O₂, UV/TiO₂, UV/TiO₂/H₂O₂, and UV/TiO₂/H₂O₂/RAD. Natural sea water was pumped and treated with the AOPs. The vital zooplankton organisms counted were polychaetes, cladocerans, ostracods, nauplii and calanoid, cyclopoid, and harpacticoid copepods. For the phytoplankton, the abundance was estimated, and the photosystem II efficiency was determined. To evaluate the phytoplankton regrowth after the treatments, the treated water was stored and populations counted for 20 days. The most effective treatment for the zooplankton groups was UVC/H₂O₂. Regarding the sensitivity, the cyclopoid copepods were the most resistant. The nauplii and polychaetes were more likely to be killed by the AOPs, greatly decreasing the risk from nauplii due to their abundance and ease of passing through the filters with their smaller size. Phytoplankton regrowth was observed in all treatments, and it even reached abundance values higher than in the intake water. For instance, additional dark conditions and retreatment on days 3 or 5 are suggested for any treatment.