Multistage membrane-integrated zero liquid discharge system for ultra-efficient resource recovery from steel industrial brine: Pilot-scale investigation and spatial membrane fouling

Zero liquid discharge (ZLD) strategy provides significant potential for industrial brine treatment. Still, the application has been constrained by the inadequate water recovery and membrane fouling of its membrane-based brine concentration system. Herein, a pilot-scale system was established that integrated pretreatment, reverse osmosis (RO), nanofiltration (NF), and disk tube reverse osmosis (DTRO), and demonstrated over a 6-month operation for the first application in treating steel industry brine. Meanwhile, novel insights were offered into the spatial evolution of DTRO membrane fouling. The overall ZLD system reached ultra-efficient water recovery at 91% with relatively low energy consumption (7.27 kWh/m 3 ). The RO and NF units concentrated the brine and selectively separated ions of different valences, simultaneously eliminating various contaminants. The DTRO unit enriched the highly saline NF permeate with a remarkable 7.5-fold concentration effect, producing an extremely hypersaline brine salinity (138,472 ± 34,804 mg/L, Cl − /SO 4 2− mass ratio of 56) that ultimately yielded ultra-pure NaCl salt. Additionally, DTRO special fouling patterns revealed a distinctive "W"-shaped fluctuation in humic acid along the device water flow, with concurrent reduction in iron/aluminum oxides leading to their deposition at the outlet. This synergistic fouling, coupled with silicon, coincides with the transformation of silica colloids into silicate compounds. The multistage membrane-based strategy and the spatial membrane fouling evolution pattern proposed and investigated in this work provide a highly efficient and cost-effective solution for industrial brine treatment with ZLD.