Many substances occurring in wastewater are colloidal (diameter 10^-9 - 10^-5 m). Such substances are hardly settling and cannot be separated in conventional filters.
Based on the Stoke’s law, a settling velocity of only 10^-2 cm / s is calculated for a particle with a size of 10 μm and a density of 3 g / cm³. Crucial here is the diameter of the particle, which indicates the square of the settling velocity.
By coagulation, larger particles are generated and thereby achieve a higher settling rate. By adding inorganic coagulum, the negatively charged surfaces of the sludge floc are being neutralized. Small sludge particles then attach and combine to larger and heavier floc. With these larger particles, a higher settling rate is achieved. The effect of filtration is also improved, as it depends largely on the diameter of the particles.
Due to hydrolysis, inorganic coagulants, mostly aluminum or ferric compounds, also change the physicochemical properties of the water to be treated (pH, conductivity, charge potential ...).



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