Influence of coagulation mechanisms on the residual aluminum--the roles of coagulant species and MW of organic matter.

Aluminum (Al) based coagulants are widely used in coagulation process to enhance the removal of turbidity and dissolved substances in the drinking water treatment. However, it raised more concerns due to the increase of residual aluminum in treated water, which can cause even more issues. In this study, the effects of organic matter molecular weight and coagulants species on the concentration and aluminum distribution in residual aluminum were investigated. The residual aluminum concentration decreases as the organic matter (OM) molecular weight (MW) rises. Charge neutralization mechanism was found to be the most important factor that determines the residual aluminum concentration directly. Basically, higher Ala percentage leads to lower residual Al concentration at acidic conditions, and Alb/Alc plays an important role in controlling the residual Al concentration at neutral and alkaline condition. The flocs structures formed by charge neutralization mechanism will be more compact for the mid and high MW OM, and fractal dimension (Df) was important to reflect the dissolved residual aluminum rather than the flocs size. The total dissolved residual aluminum concentration of Al13 and Al30 was mainly contributed by the fractions with low and/or high MW, especially by the fraction with MW range of 0-1 kDa.