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Long-Running Comparison of Feed-Water Scaling in Membrane Distillation - PubMed

  • ️Wed Jan 01 2020

Long-Running Comparison of Feed-Water Scaling in Membrane Distillation

Mohammad Rezaei et al. Membranes (Basel). 2020.

Abstract

Membrane distillation (MD) has shown promise for concentrating a wide variety of brines, but the knowledge is limited on how different brines impact salt scaling, flux decline, and subsequent wetting. Furthermore, past studies have lacked critical details and analysis to enable a physical understanding, including the length of experiments, the inclusion of salt kinetics, impact of antiscalants, and variability between feed-water types. To address this gap, we examined the system performance, water recovery, scale formation, and saturation index of a lab-scale vacuum membrane distillation (VMD) in long-running test runs approaching 200 h. The tests provided a comparison of a variety of relevant feed solutions, including a synthetic seawater reverse osmosis brine with a salinity of 8.0 g/L, tap water, and NaCl, and included an antiscalant. Saturation modeling indicated that calcite and aragonite were the main foulants contributing to permeate flux reduction. The longer operation times than typical studies revealed several insights. First, scaling could reduce permeate flux dramatically, seen here as 49% for the synthetic brine, when reaching a high recovery ratio of 91%. Second, salt crystallization on the membrane surface could have a long-delayed but subsequently significant impact, as the permeate flux experienced a precipitous decline only after 72 h of continuous operation. Several scaling-resistant impacts were observed as well. Although use of an antiscalant did not reduce the decrease in flux, it extended membrane operational time before surface foulants caused membrane wetting. Additionally, numerous calcium, magnesium, and carbonate salts, as well as silica, reached very high saturation indices (>1). Despite this, scaling without wetting was often observed, and scaling was consistently reversible and easily washed. Under heavy scaling conditions, many areas lacked deposits, which enabled continued operation; existing MD performance models lack this effect by assuming uniform layers. This work implies that longer times are needed for MD fouling experiments, and provides further scaling-resistant evidence for MD.

Keywords: antiscalant; long-term performance tests; reverse osmosis brine; scaling; vacuum membrane distillation; wetting.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure A1
Figure A1

Water desalination by an RO + VMD integrated process. The schematic diagram shows the hybrid reverse-osmosis membrane-distillation process for maximizing the system recovery rate and minimizing the environmental impact of brine waste when a VMD unit is added downstream of the RO unit for further recovery.

Figure A2
Figure A2

Brine conductivity and water recovery as functions of time for the system with tap water as feed.

Figure A3
Figure A3

Membrane surface after the NaCl experiment.

Figure A4
Figure A4

Permeate flux and water recovery as functions of time when using an aqueous NaCl solution as feed.

Figure A5
Figure A5

Brine conductivity and water recovery as functions of time for the synthetic brine feed system without antiscalant.

Figure A6
Figure A6

Brine conductivity and water recovery as functions of time for the synthetic brine feed system with antiscalant.

Figure A7
Figure A7

Effect of forced (i.e., high-pressure) wetting on system performance.

Figure A8
Figure A8

Effect of scaling on system performance.

Figure A9
Figure A9

Aqueous speciation equilibrium of synthetic RO feed and retentate. Solid lines indicate the saturation indices of secondary precipitated minerals in the synthetic RO feed, while dashed lines indicate the corresponding saturation indices for the retentate. Modeled using PHREEQC software. At low temperatures, silica minerals typically form colloids rather than precipitates. Dolomite requires temperatures above 100 °C for precipitation activation.

Figure 1
Figure 1

Vacuum membrane distillation (VMD) process schematic. The setup consisted of a feed container, a stirred membrane cell, a distillate collection vessel, a feed container, a magnetic stirrer, an external condenser, a balance, and a vacuum pump.

Figure 2
Figure 2

Linz tap water VMD test showing permeate flux and permeate electrical conductivity against time. The system exhibited no reduction in permeate flux during the experiment.

Figure 3
Figure 3

Permeate flux and permeate electrical conductivity as functions of time for the synthetic feed system without antiscalant. The permeate flux decreased, which indicates fouling, but permeate electrical conductivity remained at low levels, which indicates the absence of wetting. Feed temperature, permeate temperature, and gauge pressure were kept constant at 75 °C, 21 °C, and −30 mbar during the experiments, respectively.

Figure 4
Figure 4

Aqueous speciation equilibrium of synthetic RO brine feed without antiscalant and its corresponding retentate. Solid lines indicate the saturation indices of the main mineral precipitates in synthetic RO brine feed without antiscalant, while dashed lines indicate the corresponding saturation indices for the retentate. The shaded area indicates the drop in saturation indices caused by precipitation on the membrane surface. Modeled using PHREEQC software.

Figure 5
Figure 5

Permeate flux and permeate electrical conductivity as functions of time for the synthetic brine feed system with antiscalant. The antiscalant led to higher initial permeate flux compared to the system without antiscalant. Permeate electrical conductivity remained relatively low.

Figure 6
Figure 6

After the experiment with synthetic RO brine: (a) Membrane module; (b) Membrane surface. Salt crystals deposits are visible, which can easily be removed by washing with water.

Figure 7
Figure 7

Effect of forced (i.e., high-pressure) wetting on system performance. Operation of the process above liquid entry pressure (LEP) led to rapid passage of liquid feed to the permeate side of the membrane, which drastically increased permeate electrical conductivity.

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