Spatial variation of fouling behavior in high recovery nanofiltration for industrial reverse osmosis brine treatment towards zero liquid discharge

Abstract Nanofiltration (NF), as a cost-efficient pre-concentrating process, has been incorporated into Zero Liquid Discharge (ZLD) treatment system to improve economic feasibility. However, NF is required to operate with an extremely high recovery rate to achieve ZLD, which could cause severe and complex membrane fouling. Understanding the variation of fouling behavior with increase in recovery rate is crucial for the development of effective fouling control strategy. Spatial variation of fouling behavior in a 3-stage NF used for industrial RO brine treatment towards ZLD was investigated in present study. Distinctive fouling characteristics were observed in each operation stage. Membrane fouling was dominated by organics at lead stage, which could be completely removed via simple base cleaning. Scaling by deposition of bulk crystallization occurred on fouling layer at middle stage, which required a combination of acid and base cleaning for its removal. At the tail stage, more refractory scaling by surface crystallization accompanied by irreversible silica and ferric fouling led to the formation of fouling layer that was resistant to conventional chemical cleaning. Significant humics in preformed fouling layer could have facilitated the occurrence of surface crystallization. Minor elements present in the feed such as silica, humics and iron could cause significant fouling and alter preformed fouling layer that induced more complex and refractory fouling/scaling as recovery rate increased.