Flocculation kinetics: a comparative study on the use of polyaluminium chloride with high and low basicity and alum

Abstract

Aluminum sulfate (AS) is widely used as a coagulant in conventional drinking water treatment facilities. Alternative coagulants are commercially available, including polyaluminium chloride (PAC). However, comparative studies between AS and PAC with high (PAC-AB) and low (PAC-BB) basicity regarding flocculation kinetics are still scarce. In this sense, the aim of this work is to compare the use of PAC-BB, PAC-AB, and AS via coagulation, flocculation, and sedimentation as well as to evaluate the effect on electrical conductivity, sludge formation, alkalinity, pH, and turbidity removal based on flocculation kinetic and floc morphology. The results indicated that the increase in electrical conductivity was 8.1% and the sludge formation was 30.6 mg.L-1, and that there was no significant difference between coagulants. PAC-AB presented the lowest alkalinity consumption and, consequently, the lowest pH reduction. Turbidity removal was statistically equal between PAC-BB (91.8 +/- 3.7%) and PAC-AB (91.5 +/- 1.1%), but higher than in AS (82.2 +/- 6.4%). This difference is accentuated with the increase in critical settling velocity. The floc aggregation constant (K-A) is statistically equal among the coagulants, and the better turbidity removal performance of the PACs in relation to the AS is due to the lower floc breaking constant (K-B) and may, therefore, infer that the flocs formed with PAC-AB and PAC-BB are stronger than those formed with SA.