Water Supply Vol 2 No 2 pp 91–98 © IWA Publishing 2002
Investigations on the filtration of natural aquatic suspensions supported by dosing small amounts of Fe(III)-salts ( Fe ≤ 0.1 mg.L-1): mode of action and basic design criteria
R. Winzenbacher*, R. Schick**, H.-H. Stabel*** and M. Jekel****
*GELSENWASSER AG, Department of Planning, Maintenance, Plant Construction, Willy-Brandt-Allee 26, D-45891 Gelsenkirchen, Germany. (at present: Zweckverband Landeswasserversorgung, Water Control and Research Laboratory, D-89123 Langenau, Germany. (E-mail: winzenbacher.r@lw-online.de))
**Zweckverband Bodensee-Wasserversorgung, Water Control and Research Laboratory, Süflenmühle, D-78354 Sipplingen, Germany. (E-mail: roland.schick@zvbwv.de)
***Zweckverband Bodensee-Wasserversorgung, Water Control and Research Laboratory, Süflenmühle, D-78354 Sipplingen, Germany. (E-mail: hans.stabel@zvbwv.de)
****Technical University of Berlin, Institute of Environmental Technology, Department of Water Quality Control, Strafle des 17. Juni 137, D-10623 Berlin, Germany. (E-mail: wrh@tu-berlin.de)
ABSTRACT
Improved removal of particles during the treatment of natural aquatic suspensions has been achieved by pre-ozonation and the addition of small quantities of iron salts (Fe ≤ 0.1 mg.L-1; "Fe(III)-assisted filtration") followed by rapid filtration. As shown by investigations on a large-scale installation at Lake Constance Water Supply, this procedure reliably reduces suspended solids by at least 2-3 powers of ten in long-term use. However, the high efficacy of Fe(III)-assisted filtration cannot be explained on the basis of known coagulation mechanisms (like adsorption-charge neutralization, co-precipitation). Instead, the essential step was found to be the conditioning of the filter medium by coating it with colloids containing Fe(OH)3, and this "Fe coating" process occurs only in the presence of alkaline earths (especially Ca2+). According to further experiments, the enhanced solid-liquid separation was ultimately traced to chemical interactions such as the formation of calcium-organic association structures between the iron hydroxides and other solids. For design of Fe(III)-assisted filtration steps, finally, a Ca/DOC ratio above 40 mg.mg-1 and pre-oxidation with ozone dosages not exceeding 2 mg O3/mg DOC was recommended.
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