| Abstract: | The treatment of water from eutrophic lakes is disturbed by extensive development of plankton, which is caused by particulate algogenic organic substances and frequently also by dissolved ones. They are released by excretion, diffusion, cell destruction, separation of cell wall substances and metabolic processes in the course of the cellular reproduction in dependence on the physiological state of the algae. At a sufficient dosage of metallic salts as a flocculant, the extracellular organic substance (EOS) may act as an anionic flocculation aid in the concentration range of 1 … 2 mg/1 C. There is decisive the mass ratio: Me (metal as flocculant)/EOS (as TOC). Below a mass ratio Fe : TOC =2 … 3 the EOS disturbs the process of flocculation and filtration. Flocculation and filtration tests are carried out with the centrifugate of the culture liquid of eight algae species in flocculation test apparatus in order to clarify the disturbance of flocculation and filtration. The following essential reaction mechanisms influencing flocculation have been found: complexation of the metal-ions added as flocculation aids, precipitation reactions, neutralization of the positive charge of the metal hydroxo complexes of the flocculant with formation of surface complexes, increasing of the negative charge concentration and formation of protective colloids. The following counter-measures may be technically used with success: increasing of the addition of flocculant (Fe : TOC =2 … 3), shifting of the pH-value below the iso-electric point, addition of calcium-ions (1 … 2 mmol/l) and limited application of ozone prior to flocculation. |
