Identifying key residual aluminum species responsible for aggravation of nanofiltration membrane fouling in drinking water treatment

Residual aluminum has been identified as one of the key foulants responsible for nanofiltration (NF) membrane fouling in drinking water treatment. However, the extent to which each species of residual aluminum contributes to membrane fouling and the underlying mechanisms are not yet clear. In this study, various pretreatments were applied to a lake water, which generated feed waters of different residual aluminum and species distribution. It was found that Ala (monomeric and oligomeric aluminum species) were mainly in the form of complexes (Ala-NOM) with natural organic matter (NOM), with proportions greater than 70%. The membrane fouling experiments showed that NF operation was sensitive to the presence of Ala, in which a small increase in the Ala concentration of the order of tens of μg/L could greatly aggravate membrane fouling. In contrast, Alb and Alc (intermediately and highly polymeric aluminum species) did not contribute substantially to the membrane fouling process. The membrane fouling potential of Ala increased with increasing coexisting NOM concentration. Results showed that Ala and NOM have a synergistic relationship during membrane fouling. More rigorously, Ala-NOM, rather than Ala itself, are therefore the key residual aluminum species that aggravate NF membrane fouling. The Ala produced by polyaluminum chloride (PACl) had a higher membrane fouling potential than that produced by AlCl3, which was due to the lower polymerization degree of Ala and the more stable complexes with NOM formed when PACl was used. Use of AlCl3 as a coagulant would therefore mitigate NF membrane fouling caused by residual aluminum.