Dependence of floc properties on coagulant type, dosing mode and nature of particles

•AlCl3 and PACl show great differences as coagulants.•Adding particles after AlCl3 gives different floc structure. No effect with PACl.•Particle nature and concentration greatly affect optical monitor response.•Properties of 'sweep' flocs dominated by nature of hydroxide precipitate.

Kaolin suspensions were coagulated with AlCl3 and a high-basicity PACl at pH 7, at dosages that gave zeta potentials close to zero. The actions of the two coagulants were completely different. With AlCl3, the formation of an amorphous hydroxide precipitate played a dominant role. When the coagulant was added to the suspension, flocs grew rapidly and incorporated most of the kaolin particles within the hydroxide precipitate. When the suspension was added some time after the coagulant, the clay particles were found to be mainly on the outer floc surfaces, although the floc size was about the same. The light scattering properties of the flocs were very dependent on the number and location of particles in the precipitate. With PACl, delaying the addition of kaolin had no influence on the final floc properties.In further tests, different suspensions over a range of concentrations were coagulated with alum at pH 7. Monitoring by a 'turbidity fluctuation' technique showed an apparent increase in floc size with increasing particle concentration. However, floc sizes determined from microscope images were very nearly constant, independent of particle nature and concentration. With different particle types, the monitoring results were greatly dependent on the light scattering properties of the particles.Particles incorporated within hydroxide flocs appeared to have no influence on floc properties, such as size and strength.