Assessment of particle size distribution for polyaluminum chloride-based coagulation–flocculation processes

A particle counter is a very useful tool to analyse the hydrolysis and particles agglomeration processes after coagulant dosing and hence provides important data to state the optimum flocculation time and the type and dose of commercial PACl (polyaluminum chlorides) which differ in basic parameters (basicity, degree of Al polymerization of aluminum hydroxocomplexes). The aim of the research was to determine the influence of coagulant properties and process conditions on the size of particles formed during coagulation/flocculation and their distribution, which determines the subsequent course of separation of flocs. The paper presents the results of the particle size distribution changes during flocculation at different pH values. The most effective agglomeration of hydrolysis products was noted for PACl3 which is characterized by the highest share of monomeric Al species. The share of Al monomers in coagulant was decisive for floc size distribution. At the pH 7.9 resulting flocs were larger and more eager to agglomerate in relation to the pH 6.8 although there was no effective reduction in the number of fine particles. At the higher pH coagulant overdosing was observed, manifesting as the increase of particles smaller than 5 μm along with the flocculation time. It was the most noticeable for the high-polymerized PACl2. Such particles are susceptible to removal only in sweep coagulation under optimal technological conditions.