Comparison of calcium scaling in thermally driven (MD) and pressure driven (NF) membrane desalination processes
Abstract
Membrane-based desalination processes have been applied to desalinate seawater, inland brackish water and shale gas produced water. However, the presence of dissolved components in the target water affects the process performance as the water recovery increases. Chemical precipitation is particularly detrimental as the inorganic scales can dramatically reduce permeate flux and require frequent membrane cleaning. Calcium scaling is a common problem encountered in all membrane desalination processes. The impact of calcium scaling on permeate flux, membrane permeability and energy efficiency is crucial to the development, optimization and application of membrane desalination. Direct contact membrane distillation (DCMD) and nanofiltration (NF) were selected to represent two broad categories of membrane-based desalination processes: thermal and pressure driven membrane processes, respectively. It is expected that the difference in the driving force will results in significantly different calcium scaling and hence, impact these processes differently.
The impact of calcium scaling was evaluated in this study using synthetic solutions of calcium sulfate (CaSO4) and calcium carbonate (CaCO3) with different concentrations of sodium chloride (NaCl). Scaling by CaSO4 and CaCO3 was very different for the two membrane processes depending on the feed salinity. At low feed salinities, CaSO4 scaling was more severe in DCMD than in NF while CaCO3 scaling reduced permeate flux in the case of NF and had no impact on DCMD flux. At high feed salinities, neither of these foulants affected the performance of DCMD while the NF experience permeate flux decline for both CaSO4 and CaCO3. Mass balance analysis and scanning electron microscopy (SEM) showed that the total amount of CaSO4 and CaCO3 solids deposited on the NF membrane was higher than on the DCMD membrane at both low and high feed salinity. The average size of the scale crystals in DCMD system was larger and the scale was more porous compared to than observed in the NF system, which can be explained by the compaction due to the hydraulic pressure in the NF system. The results of this study suggest that calcium scaling would have significantly more severe impact on the performance of pressure driven membrane desalination process than thermally driven membrane process.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMH133...07Z
- Keywords:
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- 1807 Climate impacts;
- HYDROLOGY;
- 1876 Water budgets;
- HYDROLOGY;
- 1880 Water management;
- HYDROLOGY;
- 1884 Water supply;
- HYDROLOGY