Spatial evolution of membrane fouling along a multi-stage integrated membrane system: A pilot study for steel industry brine recycling

The establishment of a multi-stage membrane system that integrates different membrane technologies has been regarded as an essential process in industry brine recycling. Although membrane fouling has been intensively studied, in particular, on single membrane unit at the lab-scale, the actual fouling behavior in multi-stage integrated pilot systems has not yet been fully understood. In this study, membrane fouling was systematically investigated in a pilot-scale RO-NF process by monitoring the surface properties of fouling layer and the deposition behavior of foulants. Distinct spatial fouling variation along the flow direction of the multi-stage integrated system was observed. At the first stage of the RO process, severe fouling occurred on the membrane surface under the synergistic interaction of several organic and inorganic foulants, resulting in a rough and thick cake layer. The membrane fouling at the second RO stage became less severe. During the following NF process, the amounts of metal oxides/hydroxides and organic foulants depositing on the membrane surface decreased. The results indicated that the foulants firstly deposited on the front stage of the integrated RO-NF process. Overall, the observations for spatially varied membrane fouling help to advance understanding of membrane technologies to achieve zero liquid discharge in the steel industry.