Hybrid coagulation–NF membrane processes for brackish water treatment: Effect of pH and salt/calcium concentration

• Hybrid coagulation–NF membrane process for brackish water desalination was studied.
• Influence of water chemistry on the performance of hybrid process was investigated.
• Properties of the foulants in the solution were altered by coagulation process.
• Distinct membrane fouling phenomena compared to single fouling study were observed.
• Membrane foulant cake layer was inspected and linked to explain fouling mechanisms.

In order to reduce membrane fouling propensity, membrane technologies in water treatment and desalination processes have been coupled with coagulation process. However, the fouling issues still persisted. It is believed that coagulation might affect the fouling mechanisms involved. Hence, this study was intended to investigate the influence of water chemistry (solution pH, calcium concentration, and total dissolved solids) on hybrid coagulation–nanofiltration membrane processes. Assessment will be done based on permeate flux, salt rejection trend, and foulant layer autopsy. It was discovered that coagulation process altered the properties of the foulants in the solution and resulted in distinct membrane fouling phenomena compared to most of the standalone membrane fouling studies. This situation prevailed in the presence of calcium ions in the solution. It was postulated that coagulation process weakened the interaction between the oppositely charged calcium ions and foulants. Therefore, instead of forming complex with the foulants, calcium ions had higher affinity towards more negatively charged membrane. Consequently, calcium salts precipitated and formed scales on the membrane surface, as supported by membrane performance and SEM-EDX analysis. This indicated that membrane fouling study should consider the impact of pretreatment process for better understanding about the fouling mechanisms involved.