Effects of Al species on coagulation efficiency, residual Al and floc properties in surface water treatment

Effects of coagulant dosage and solution pH on the coagulation behaviors using three different Al-based coagulants were studied in terms of dissolved organic carbon (DOC) removal efficiency and residual Al. The coagulants used in this study were aluminum chloride (AlCl3), Al13O4(OH)247+ polymer (Al13) and (AlO4)2Al28(OH)5618+ polymer (Al30). Surface water from a typical water treatment plant in the north China was used to conduct the experiments. The results showed that DOC removal efficiency correlated well to the content of medium polymerized species (Alb), while the flocs formed by Alb were not as large as those formed by colloidal or gel species (Alc). Monomeric species (Ala) showed a high ability to react with some unsatisfied coordinate bonds of organic matter to facilitate DOC removal, and it was the most unstable species, which could be easily transformed into in-situ Alb species. Compared to preformed Alb, in-situ Alb could remove DOC more efficiently and the flocs were much larger via the mechanism of further hydrolysis and transfer into Alc during coagulation. Results showed that pH had significant effects on residual Al, the total residual dissolved Al achieved the minimal concentrations in the neutral pH region. The results of molecular weight (MW) distribution of residual Al showed large differences at the given coagulation pH, and monomeric species were the predominant Al species at pH 6.0 and pH 8.0 for all coagulants.