Improving the efficiency of rapid sandfilters by adding small amounts of ferric salts (βFe ≤ 0.1 mg·L-1)

Abstract

Improved removal of suspended matter during the treatment of water from Lake Constance has been achieved by pre-ozonation (βO3 = 1 mg·L-1 corresponding to 0.8 mg O3/mg DOC) and the addition of small quantities of iron salts (βFe≤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.