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

Improved removal of suspended matter during the treatment of water from Lake Constance has been achieved by pre-ozonation (β_O3 = 1 mg·L^-1 corresponding to 0.8 mg O₃/mg DOC) and the addition of small quantities of iron salts (β_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.