| dc.description.abstract | Tunnel wash water (TWW) is highly contaminated with a range of different pollutants, including heavy metals in both solid and dissolved state. Untreated TWW released directly to recipients might cause harm to aquatic organisms. Few tunnels have an arrangement for treating TWW in place, and those that do largely use sedimentation. This treatment method has proved efficient for removing particle bound contaminants, but less so for dissolved substances. As a result, it has been suggested to add another step to the treatment process.\\
Sedimentation in combination with chemical coagulation/flocculation with traditional materials, and/or filtration have been studied. However, existing literature is limited.
%It seems that the tested coagulants and filtermedia are traditional, well-known materials, such as Al- or Fe based metal salts for coagulation and filtermedia such as activated carbon or clay based materials.
Materials such as organic polymers for treatment of TWW has barely/not been explored, but has proved efficient for treatment of other types of contaminated water. Some literature exists on the use of clay based filter media especially developed for metal removal, though few studies concerning treatment of TWW have been published.
In this master's thesis sedimentation, the coagulants chitosan and polyaluminum Chloride (PAX), and the filter media Filtralite HMR is evaluated for treatment efficiency of TWW, in terms of particle and heavy metal removal.
The following research question has been posed:
How is the suitability of chemical precipitation and/or filtration as a post-polishing step for sedimentation
of tunnel washing water with regard to degree of purification?
The efficiencies are evaluated for sedimentation for 30 days, sedimentation for 0, 15 or 30 days followed by either coagulation with chitosan or PAX, and sedimentation for 30 days, subsequently undergoing coagulation with chitosan/PAX, followed by filtration through a small scale, constructed filter with HMR. In the coagulation tests, a range of doses were tested under equal conditions (water at room temperature, precipitation pH 6, and equal mixing conditions).
The treated water is analyzed for turbidity, suspended solids and total fraction of heavy metals (As, Pb, Cd, Cu, Cr, Ni, Zn and Hg).
Based on the results of this study, it can be concluded that coagulation/flocculation is an efficient for post-polishing treatment for sedimentation of TWW. This applies for both chitosan and PAX, though PAX gave a slightly higher reduction in both turbidity and heavy metal concentration. Coagulation/flocculation after 15 days of sedimentation gave almost the same results as coagulation/flocculation following 30 days of sedimentation.
Filtration did not give satisfactory results as a post-polishing method in this project, as concentration of several heavy metals increased following filtration. | |