TBT-contaminated Sediments: Treatment in a Pilot Scale (9 pp)

-  Dedicated to Prof. Dr. Ulrich Förstner on his 65th birthdayBackground, Aims and Scope   Sediments in harbours and nearby shipyards demonstrate widespread contamination with tributyltin (TBT). Therefore, reuse and relocation of dredged material from these locations are prohibited. Even if the International Marine Organization (IMO) convention concerning TBT-based paints is ratified (Champ, 2003) the TBT problem in sediments will continue to remain for many years due to the persistence of TBT. Methods   An electrochemical process has been developed to treat polluted sediments. Dredged materials with high and low TBT-contents were studied on a technical and a pilot scale. The treatment process was assessed by chemical analysis and a biotest battery. Additionally, an economic analysis was performed to check the economic feasibility of the process to treat dredged material from two different locations at different operating conditions. Furthermore an up-scaling estimation was performed to evaluate treatment costs at a larger scale, i.e. for a plant having a capacity of 720 000 t/a.Results and Discussion   Butyltin species and polycyclic aromatic hydrocarbons (PAH) were decomposed due to electrochemically-induced oxidation, while the treatment did not alter heavy metal and PCB concentrations. The bacteria luminescence test indicated a reduced toxicity after the electrochemical treatment, while the algae growth inhibition test and bacteria contact test did not confirm these results. Based on a small consumer price of €0.12/kWh, treating the high-contaminated sediment in the pilot plant would cost €21/m³ and €31/m³ for the low contaminated sediment, respectively. Assuming an industrial consumer price of €0.06/kWh for electricity in an up-scaled process with a capacity of 720 000t/a, the total treatment costs for the low contaminated sediment would be €13/m³. Conclusion   The results of treating dredged material from Bremerhaven and the fine-grained fraction from the METHA plant show that the effectiveness of the process performance is more related to the binding form and sediment composition than to the initial concentration of TBT. The electrochemically treated material complies with chemical criteria for relocation of dredged material, but post-treatment, e.g. washing and/or reduction of remaining oxidants with Fe-II-salts, is needed to fulfil ecotoxicological criteria for relocation. Economic investigations have shown that the electrochemical treatment might be a technical option to treat TBT contaminated, dredged material. However, the technique is not fully developed and cannot remove all chemicals of concern, e.g. heavy metals. The decision as to whether this technique can be applied is site-specific and should be taken based on the case-by-case approach.Recommendation and Outlook   Since biotests integrate the effects of all contaminants present in a sample, process-accompanying evaluation procedures need additional assessment methods such as TIE (toxicity identification evaluation) as a second tier following ecotoxicological tests, in which the reasons for the effects are identified. For reasons of sustainability, a much more effect-oriented and long-term cost effective approach should be applied in future to avoid the release of harmful substances into the environment. Life cycle assessment should be carried out to identify and quantify impacts of sediment treatment processes in order to take into account both the distant effects of local actions and local effects of distant actions.