Hybrid coagulation – NF membrane process for brackish water treatment: Effect of antiscalant on water characteristics and membrane fouling

Antiscalants have been used to inhibit the formation of scales and reduce the risk of membrane scaling. However, issues such as the increased of biofouling tendency and contradictory observations of organic fouling propensity under the influence of antiscalants have been reported by other researchers. Such observations indicate the need for thorough understanding about the interaction between antiscalants and membrane/foulants in the solution. This study investigated the influence of antiscalant on the water characteristics and the performance of hybrid coagulation–nanofiltration membrane process for brackish water treatment. It was observed that scaling was mitigated with the addition of antiscalants but membrane fouling was exacerbated at higher dosage, probably due to the changes in foulant properties induced by antiscalants. Analysis carried out on the water showed that antiscalants altered the characteristics of the foulants and formed precipitates with calcium/aluminum and iron (from coagulant) ions, which eventually deposited as foulant layer on the membrane surface. This was proven with the existence of phosphorous element on the membrane surface using SEM–EDX. The findings demonstrated how the antiscalants altered the water characteristics and provided insight into the reactivity of antiscalants present in different concentrations, which could be used to interpret the contradicting findings from other researchers. © 2016 Elsevier B.V. All rights reserved.

Throughout the water analysis, we can predict the possible interactions between antiscalant and the foulants/ions in the solution based on the changes in water characteristics; turbidity and zeta potential. Antiscalant managed to prohibit the formation of calcium scales, regardless of its dosage. However, excessive dosage might lead to the formation of agglomerates or water soluble complex between antiscalant with calcium ions and foulants in the solution. These compounds were filtered out and deposited on the membrane surface. Furthermore, the excessive antiscalant may alter the membrane surface properties. Consequently, membrane performance was severely affected with the drastic decline in flux. This study showed that the reactivity of antiscalant might change with its concentration, which could affect its interaction with the substances in the solution and membrane surface. This study showed that the incompatibility between the antiscalant and foulants in the solution could lead to ineffective membrane performance, even though the scaling issue was resolved. Membrane performance is highly dependent on how compatible it is with the different chemicals applied in the pretreatment process. Hence, the selection and dosage of antiscalants and coagulant are very crucial for this hybrid coagulation– membrane process in order to avoid unnecessary loss. Acknowledgment This paper was made possible by NPRP grant #[5-1425-2-607] from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the author[s]. The authors also wish to acknowledge the Ministry of Education Malaysia for sponsoring W.L. Ang's postgraduate study via MyBrain.