Fouling characteristics and their implications on cleaning of a FO-RO pilot process for treating brackish surface water

• Fouling characteristics of a pilot FO-RO plant using natural surface water was studied.
• Severe flux decline was caused by high scaling/fouling potential feed solutions.
• Fouled membrane autopsy found inorganic scaling was the main cause of fouling.
• Physical and chemical cleaning was ineffective to restore declined flux.
• A sufficient cleaning strategy is important to remove inorganic scalants.

In this study, a laboratory pilot scale forward osmosis-reverse osmosis (FO-RO) hybrid system was used to desalinate both actual and spiked brackish surface water (BSW). An overall performance evaluation was conducted and the membrane foulings were characterised by comprehensive techniques. Severe flux decline was observed during the treatment of high scaling/fouling potential feed solutions. It was found that when the raw BSW was used as feed, the flux can be completely recovered by hydraulic cleaning. When the raw BSW was spiked with nutrients and/or scaling ions for accelerated scaling and biofouling, more severe flux decline was observed. Inorganic scaling caused by calcium and phosphate, and their interactions with organic constituents in the feed solutions were the dominant cause of the declined system performance. For the spiked feed water, the combined physical and chemical cleaning using two chemical agents was not able to restore reduced flux to its initial value. This study identified the need for implementing a sufficient cleaning strategy targeting different membrane foulants, particularly for inorganic scalants; as well as confirms the need of fouling-resistant membrane.