Coagulation behavior of Al species in polyaluminum chloride on the destabilization of particulates and dissolved organic matter

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The Al speciation of PACl coagulants and the characteristics of organic matter (OM) in raw water critically affect the removal of OM by coagulation. In general, Taiwan’s reservoirs frequently contain high amount of natural organic matter (NOM) and turbid particles. Additionly, when algal blooming occurs, it may cause the rapid increase of algogenic organic matter (AOM) contributing to the NOM water pool. The eutrophication of algae not only deteriorates water quality but also causes numerous operational problems toward water treatment plants. This study aimed to investgate the effects of Al speciation on the destabilization of OM and turbid particles by coagulation of water simultaneously containing NOM and AOM. Humic acid extracted from a synthetic humic substance powder was used to represent for NOM. Turbid particles were kaolinite samples. Two AOM origins were isolated as extracellular organic matter (EOM) and cellular organic matter (COM) from the stationary phase of Chlorella sp.. Two kinds of PACl coagulants, which were PACl-H (Ala/Alb/Alc=22/53/25) and PACl-C (Ala/Alb/Alc=46/11/43), were used to evaluate the effect of Al speciation on destabilization of organic matter samples with turbid at neutral pH (pH=7±0.1) by jar tester. PACl-H has strong ability of adsorption and charge neutralization. Meanwhile, PACl-C has strong ability of adsorption and sweep flocculation.The formation of flocs was analyzed via Mastersizer 2000 and FlowCAM. Finally, the composition and molecular weight distribution of organic matter determined by EEM and HPSEC methods. The results showed that AOM had insignificant negative effects on the removal (>90%) of turbid particles (10 and 100 NTU) for both of PACl coagulants. However, its removal rates by two PACl coagulations were relatively low (>33%) for both NOM and AOM. PACl coagulants had a great removal performance for high molecular weigh matter (> 1k Da), and had a poor removal in low molecular weigh matter (< 1k Da). In formation of flocs, the flocs aggregating by PACl-C had higher fractal dimension (Df >2) than the flocs aggregating by PACl-H. It means that the flocs aggregating by PACl-C were denser than the flocs aggregating by PACl-H. In addition, the flocs fromed by PACl-C (300~900 μm) was bigger than those fromed by PACl-H (200~850 μm) as well. The experimental results found that for the raw water including AOM, PACl containing high amout of Alc species has the high removal of OM than PACl containing Alb species. Moreover, PACl coagulants have great removal rates (>90%) in turbid particles.