Water Supply Vol 2 No 1 pp 191–204 © IWA Publishing 2002
Improving the efficiency of rapid sandfilters by adding small amounts of ferric salts ( Fe ≤ 0.1 mg.L-1)
R. Schick*, R. Winzenbacher**, H.-H. Stabel*** and M. Jekel****
*Zweckverband Bodensee-Wasserversorgung,Water Control and Research Laboratory, Süflenmühle, D-78354 Sipplingen, Germany. (E-mail: roland.schick@zvbwv.de)
**Zweckverband Landeswasserversorgung, Water Control and Research Laboratory, Postfach 1257, D-89123 Langenau, Germany. (E-mail: winzenbacher.r@lw-online.de)
***Zweckverband Bodensee-Wasserversorgung,Water Control and Research Laboratory, Süflenmühle, D-78354 Sipplingen, Germany
****Technical University of Berlin, Department of Water Quality Control, Institute of Environmental Technology, Strafle des 17. Juni 137, D-10623 Berlin, Germany. (E-mail: wrh@itu201.ut.TU-Berlin.de)
ABSTRACT
Improved removal of suspended matter during the treatment of water from Lake Constance has been achieved by pre-ozonation (O3 = 1 mg.L-1 corresponding to 0.8 mg O3/mg DOC) and the addition of small quantities of iron salts (Fe≤0.1 mg.L-1; "Fe(III)-assisted filtration") followed by rapid sand filtration. As shown by investigations on a large-scale installation (bypass mode) over several years, this procedure reliably reduces particulate matter in the water by about 3 orders of magnitude in long-term use. However, the high efficacy of Fe(III)-assisted filtration cannot be explained on the basis of known coagulation mechanisms (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 only occurs in the presence of alkaline earth cations (especially Ca2+). According to further experiments, the enhanced solid-liquid separation was ultimately traced to chemical interactions such as the formation of calcium-NOM bridges between the iron hydroxides and other solids.
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