Experiments with ultraviolet light, ultrasound and ozone technologies for onboard ballast water treatment

The introduction of invasive marine species into new environments through ships' ballast water, attached to ships' hulls and via other vectors has been identified as one of the four greatest threats to the world's oceans. Different treatment methods have been proposed for onboard ballast water treatment options to reduce this effect, among those also ultraviolet light (UV), ultrasound (US) and ozone (O3) treatments. The literature survey that was carried out during the first phase of the project indicated that all of the methods have potential for ballast water treatment and numerous reports were available presenting the research activities carried out around the world. The technology that has been studied most widely appears to be UV, whereas US seems to have very limited application in terms of ballast water treatment. In addition to single technologies, the combinations of US + UV and UV + hydrogen peroxide (H2O2) were also tested as part of the hurdle experiments. During the first phase, the methods were tested under laboratory both in Finland and the UK. After an evaluation of the laboratory test results, onshore trails were carried out in Tvärminne, Finland, in order to confirm the proper operation of the devices and to obtain information about the efficiency of the treatment options against the organisms in the Baltic Sea marine environment. The effects on phytoplankton and bacteria were not studied. The results from the laboratory trials were partly confusing due to the various scale effects related to the test system and thus the results were difficult to explain. The results from Tvärminne onshore trials, with considerable reliability for UV, varied between 78-100%, for US treatment between 80-99% and for ozone treatment 95-100% depending on the organism group, flow rate and ozone dosages. The combination of US and UV achieved mortality rates of between 97-100% and the combination of UV + H2O2 between 94-100%. Even in those cases where 100% mortality was observed, the requirements for the maximum allowable number of viable organisms per water volume set by IMO were not necessarily confirmed due to the relatively small sampling volume. It must also be emphasised that only moderate (200-1,600 l/h) flow rates were used. During the trials in the UK, a possible modification of ballast water properties and contents by the treatments was also identified. Ozone treatment causes a significant increase in Redox potential with possible consequences on metal corrosion, coatings and gaskets. However, these effects can be minimised by careful material selection. Costs evaluations were carried out in order to provide rough estimations of treatment costs for each treatment option on two different case study ships. It appears that the costs for treated ballast water varies between 0.045-0.11 /m3 for UV treatment, for US 0.39-0.43 /m3 and for ozone 0.20-0.24 /m3. The effect on the shipping costs due to the treatment varies between 1-14% per voyage for these case study ships. These values represent the cost evaluation for a full-scale application based on the current level of treatment technology available. It is more likely that treatment costs would lower due to technology development. It must be kept in mind that different source and background information has been available for each study and therefore reasonable comparison between the methods is difficult. In most of the cases, the treatment processes are not predictable due to the different water properties and operational aspects. Therefore, further studies and full-scale trials are required in order to optimise the process conditions for each treatment technology. One option for the testing and evaluation of various treatment methods could be container installations, where treatment processes would be designed for full-scale flow rates and water volumes. This option would also enable different marine environments to be included in the test programme. In addition to the secondary treatment options, primary treatment options, i.e., filters and cyclons, should also be included since many secondary treatment options require primary treatment in order to perform efficiently. In addition to the treatment technologies, the sampling and analysing methods also need to be developed in order to ensure reliable results and easy-to-use samplers for the ship's crew. The long-awaited guidelines for test and performance specifications adopted in the IMO MEPC 53 meeting in July 2005 standardised the testing procedures and provides technology developers and manufacturers with a uniform approach to the challenge.