The effect of membrane characteristics on the performance of membrane distillation system for the treatment of hypersaline brine

Abstract

South Africa is a water-scarce country that continues to experience significant strain around the availability of water resources that are environmentally and economically sustainable. Importantly, this scarcity in the water supply is expected to increase in the future owing to the ramifications of climate change leading to unpredictable rainfall, which the increased evaporation rates will further exacerbate due to elevated average temperatures. Membrane Distillation (MD) is a membrane-based, thermally driven separation process. Only vapour molecules pass through a microporous hydrophobic membrane that acts as a physical barrier separating a hot aqueous feed solution from a cold permeate. The driving force for membrane distillation is the transmembrane vapour pressure differential. Of the various membrane characteristics investigated, membrane pore size was identified as the critical variable in terms of membrane selection. As a result, the research focussed mainly on two polyvinylidene fluoride (PVDF) membranes with pore sizes of 0.22 μm and 0.45 μm. This study examined the use of two membranes with varying pore sizes to treat brine emanating from industrial wastewater. The key aim of this study was to determine the effects of pore size for the treatment of mining and or industrial wastewaters using MD (focussing on flux rate and scaling limitations). The effect of feed temperature and feed concentration on MD system performance using varying pore sizes was also investigated. Two types of brines were investigated: Type 1 Brine, a monovalent ion dominant, non-scaling/fouling brine and Type 2 Brine, which was a divalent and trivalent ion dominant brine with scaling and or fouling potential. Both brines provided the necessary coverage regarding the variability in brine wastewater characteristics from industrial and mining sectors. Synthetically prepared feed solutions were used to establish baseline performance characteristics for the membranes. The study also included testing industrial brine emanating from an RO process, emphasising the most suitable membrane properties for this specific type of brine. The water purity from all investigations yielded acceptable results and very high rejection (>99.94%) with the product water exhibiting low conductivity (<15 μS/cm), which only increased once scaling on the membrane surface and pore wetting caused a decrease in flux rate.

For the results obtained using the Type 1 brine, it was found that for both membrane pore sizes investigated, 0.22 μm and 0.45 μm, an increase in temperature from 40˚C to 80˚C increased the flux rate by up to 6.27 times. Increasing the feed TDS concentration from 35 g/L to 65 g/L did not have any notable effect on the flux rate even when the 0.22 μm and 0.45 μm pore size membranes were subjected to feed concentrations close but below solubility level. The 0.22 μm and 0.45 μm pore size membranes performed similarly, indicating that the vapour pressure driving force was not limited by pore size. However, as the Type 1 brine approached solubility level, the smaller, 0.22 μm pore size membrane performed better in terms of flux and salt rejection. This result may be explained by the crystallising solute in the solution having a small enough particle size to enter the pores of the 0.45 μm pore size membrane but being too large to enter the 0.22 μm pore size membrane’s pores. For the results obtained using the prepared Type 2 brine, it was found that for both membrane pore sizes investigated, 0.22 μm and 0.45 μm, an increase in temperature from 40˚C to 80˚C increased the flux rate by up to 4.22 times. An increase in initial feed TDS concentration 11870 mg/L to 27025 mg/L for the 0.22 μm, and 0.45 μm pore size membranes caused a decrease in the flux of up to 40.3% and 35%, respectively. The smaller 0.22 μm pore size membrane generally performed better in terms of flux for these experiments. The investigation into the effect of scaling/fouling on the performance of MD 0.22 μm and 0.45 μm pore size membranes for the treatment of Type 2 brine with initial feed TDS concentration of 27025 mg/L showed a significant difference between the varying pore sizes. The larger, 0.45 μm pore size membrane yielded a flux two times higher than the 0.22 μm pore size membrane. The maximum suspended solids in the solution before a significant decline in MD performance for the 0.22 μm and 0.45 μm pore size membranes were found to be approximately 3050 mg/L and 3550 mg/L, respectively. Thus the 0.45 μm pore size membrane was more resistant to scaling when compared to the 0.22 μm pore size membrane. The use of actual brine investigated showed the 0.22 μm pore size membrane to perform better in terms of flux and permeate quality, when compared to the 0.45 μm pore size membrane, which was due to the maximum TSS not being reached. Hence, 0.22 μm pore size membrane generally performs better until scaling/fouling occurs.

More research into membrane characteristics, particularly porosity and membrane thickness, needs to be investigated to develop cheaper, better-performing membranes that would result in greater interest from industrial sectors to use MD. In conclusion, this study aimed to add to the body of knowledge and explore MD as a more sustainable industrial wastewater treatment process that reduces wastewater production. There is a significant technological gap in providing a cost-effective brine treatment solution towards achieving zero liquid discharge.