Importance of the co-presence of silica and organic matter in membrane fouling for RO filtering MBR effluent

• Real wastewater treated by an MBR was fed for RO.
• Contribution of cake/biofilm to permeability decline in RO was limited.
• Silica was dominant inorganic component in the foulants.
• Silica did not cause severe fouling without organic matter.
• Organic matter also needed silica to exhibit large filtration resistance.

Reverse osmosis (RO) is an important technique for potable water reuse. However, the problem of membrane fouling in RO must be addressed for its widespread use. In this study, fouling in RO was investigated with 2540 membrane elements fed with effluents from a membrane bioreactor (MBR) treating municipal wastewater. The results of chemical cleaning, elemental analysis for a cross section of the fouled membrane and microwave digestion of the fouled membrane suggested that silica was a major player in the membrane fouling. Also, organic matter present in the MBR effluent (mainly composed of soluble microbial products) was found to play an important role in the evolution of membrane fouling. A unique experimental setup enabled investigation of the interaction between silica and organic matter in the membrane fouling in RO: concentrations of silica and organic matter in the feedwater were manipulated. Remarkable membrane fouling occurred only when feedwater containing both silica and organic matter was filtered. It was found that silica or organic matter alone did not cause severe membrane fouling in the RO. The co-presence of silica and organic matter seems to have synergetic effects that cause severe membrane fouling in RO.