Impacts of water hardness on coagulation-UF-NF process using aluminum salts

Abstract

Surface water supplemented with reclaimed water can effectively alleviate water shortage. In this study, an integrated coagulation ultrafiltration-nanofiltration (CUF-NF) process was explored to treat reclaimed water. The effects of water hardness and different aluminum salts on the CUF-NF process were specifically investigated focusing the flocs size, membrane fouling, removal of organic matter, and disinfection by-product formation potential (DBPs-FP). Moreover, the treatment performance of two coagulants (i.e., AlCl3 and PACl25) were also compared. Results showed that the increased water hardness remarkably improved the UF membrane flux in CUF process (i.e., 51.4% to 188.3%), however, a slightly reduced flux (up to 29.5%) was noticed for NF membrane. In addition, parallel factor (PARAFAC) analysis showed that higher water hardness contributed to higher removal efficiencies of humic- (up to 40.6%) and protein-like substances (up to 60.6%), together with the reduction of total DBPs-FP (i.e., total trihalomethanes; 52.6% and total haloacetic acids; 20.4%). Furthermore, 2D-COS-FTIR also revealed the correlation between water hardness and the functional groups in UF/NF fouling layers. Overall, this work not only provides insight into the impacts of water hardness and different aluminum salts on CUF-NF processes treating surface water, but also provides new insights into the formulation of hardness standards for reclaimed water.