Membrane based separation processes have become an integral part of water treatment and reuse industry. Even though the mode of operation and driving force for different membrane based separation processes may be different, all are affected by membrane scaling at high water recoveries that increases mass transfer resistance and greatly affects performance. Previous studies evaluated the fouling behavior in pressure-driven processes and thermally-driven processes (i.e., membrane distillation) separately and the comparison was mostly qualitative. Pressure-driven processes experience a severe flux decline due to scaling while the fouling layer in membrane distillation may be quite porous and have a much lower impact on system performance. We quantitatively studied the difference in the fouling process in pressure-driven (nanofiltration, NF) and thermal membrane processes (direct contact membrane distillation, DCMD). Preliminary results indicated a severe decline in NF permeate flux when treating real abandoned mine drainage while DCMD exhibited negligible flux decline with the same wastewater. To further investigate this phenomenon, fouling in DCMD and NF was compared using calcium sulfate and calcium carbonate as foulants under identical experimental conditions (i.e., ionic strength, temperature, hydrodynamic conditions, water recovery, and scale mass). CaSO4 resulted in instantaneous permeate flux decline (i.e., 89%) followed by a further decrease of 52% over 12 hours with NF. CaSO4 scaling in the case of DCMD occurred after 2 hours and no initial flux drop was observed. However, the flux decreased by 67% over the next 12 hours accompanied by membrane wetting. Imaging of the scales indicated uniform needle-like CaSO4 crystals on DCMD membranes and patchy flat needles on NF membranes. The differences can be explained by the 40-fold difference in the operating feed pressures in the two processes. On the other hand, CaCO3 fouling occurred instantaneously in both DCMD and NF processes. However, permeate flux decline in DCMD was only 17% and remained constant over the next 12 hours while continuous flux decline with time was observed in NF. Results of the ongoing tests to explain the impact of membrane fouling on the flux and rejection efficiency with two different membrane processes will be presented at the conference.

Impact of operating conditions in membrane-based separation processes on the characteristics of inorganic scales on membrane surface

256th National Meeting (2018)

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