Nanofiltration is routinely applied as an advanced water treatment technology after conventional water treatment. However, the residual coagulant after coagulation process may affect the nanofiltration process and to our best of knowledge, few studies focused on this phenomenon. To address such issues, ferric and aluminum ions were adopted as the model coagulant, and the influences of coagulant concentrations, coagulant conditioning time and fouling strategies on gypsum scaling were systematically investigated. The results indicated that coagulant conditioned on the membrane surface could improve membrane flux, enhance scaling, and increase the conductivity of permeate. The contents of coagulant accumulated on the membrane surface gradually increased with its increasing concentration in feed solution and extending conditioning time, resulting in severer scaling and flux decline. Interestingly, the formation of heterogeneous scaling layer will contribute to membrane fouling alleviation and prevent the further flux decline regardless of the ongoing increase of coagulant concentrations in the feed water as well as on the membrane surface. As a result, a critical value of coagulant concentrations in the feed water was obtained in present conditions. Furthermore, it's found that successive fouling strategy could lead to less gypsum scaling but severer flux decline, compared to simultaneous fouling strategy. Both the scaling quantity and scaling morphologies conferred significant influence on the flux decline. It is suggested that the concentrations of coagulant should be strictly controlled prior to nanofiltration process, especially with practical relevance for the applications of it in treating the water rich in calcium ions and sulfate anions. (C) 2019 Elsevier B.V. All rights reserved.
