Investigations on the filtration of natural aquatic suspensions supported by dosing small amounts of Fe(III)-salts (β_Fe ≤ 0.1 mg.L^-1): mode of action and basic design criteria

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 (β_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 β_Ca/DOC ratio above 40 mg.mg^-1 and pre-oxidation with ozone dosages not exceeding 2 mg O₃/mg DOC was recommended.