Realistic coagulation mechanisms in the use of aluminium and iron(III) salts

Abstract

The processes leading from coagulant feed to the development of flocs suited to solid-liquid phase separation are examined and analyzed in the light of available information and experimental data. It is concluded that at the relatively high buffering capacity (HCO3− concentration above 1.8 meg/L) and close to neutral pH values (7.0<pH<8.4) typically found in the surface waters of Hungary, flocs suited to solid-liquid phase separation develop from the coagulant added within a very short period of time (10-20 seconds). These flocs are not capable of changing the charge of the particles on the colloidal, quasi-colloidal, or destabilizing the dispersion. Between the hydrolysis of the coagulant and the development of large flocs, “short-lived” water soluble aluminium and iron(III) hydroxide complexes, metal hydroxide sols are formed, which also carry a positive charge. These latter two metal hydroxide types (which also exist for a brief period - a few seconds - only) are the ones capable of destabilizing the dispersion. The bond between the suspended solids to be removed and the metal hydroxide sols and water soluble metal hydroxide complexes must be established within this short period. Rapid mixing of the coagulant will ensure rapid hydrolysis of the coagulant, contact between the sols and the suspended solids and will retard the development of large flocs which are inactive in destabilizing the dispersion.