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Calcium Scaling in Direct Contact Membrane Distillation (DCMD) at High Salinities

Zhang, Zhewei (2022) Calcium Scaling in Direct Contact Membrane Distillation (DCMD) at High Salinities. Doctoral Dissertation, University of Pittsburgh. (Unpublished)

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Direct contact membrane distillation (DCMD) is a thermally driven membrane desalination process with the capability of treating hypersaline solutions. However, calcium scaling is commonly encountered in DCMD with a profound impact on permeate flux and energy efficiency. The goal of this study was to investigate the crystallization of calcium in hypersaline solutions during the desalination process, analyze the severity of calcium scaling in DCMD in comparison with other membrane separation process and provide potential pretreatment strategy to alleviate this problem.
Among the calcium salts most often associated with scaling issues in water treatment systems, gypsum (CaSO4∙2H2O) is one of the major foulants that adversely impact performance (i.e., fouling, wetting). The solubility indices (SI) of gypsum in hypersaline solutions (0.5M~5M) were first predicted with multiple models and then compared with the results obtained using in-situ electrochemical impedance spectroscopy (EIS) to monitor the crystallization process. EIS results showed reasonable agreement with both equilibrium and kinetic predictions.
Calcium scaling (i.e., gypsum and calcite) was also studied in thermal (DCMD) and pressure driven (nanofiltration, NF) membrane processes operated at identical feed temperature (i.e., 40 °C) and shear conditions (i.e., Re = 771±28) at feed salinities from 3,000 to 30,000 mg/L. This study demonstrated that the impact of calcium scaling is affected by both feed salinity and separation driving force, and is much less severe in thermally than pressure driven membrane processes.
Calcium scaling is a significant challenge when treating inland brackish water reverse osmosis (BWRO) reject that contains high concentration of both calcium and sulfate and a proper mitigation strategy is needed. Barite precipitation was utilized to remove about 4,000 mg/L of sulfate from actual BWRO concentrate to prevent scaling. The pre-treated BWRO concentrate was used as feed in DCMD system and 85% water recovery was achieved without any indication of membrane scaling.
Based on the comprehensive understanding of calcium crystallization under hypersaline conditions, this study offers fundamental insights into the impact of this adverse phenomena in both thermal and pressure driven membrane processes and options for calcium scaling control in membrane desalination processes.


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Item Type: University of Pittsburgh ETD
Status: Unpublished
CreatorsEmailPitt UsernameORCID
Zhang, Zheweizhz114@pitt.eduzhz1140000-0001-5814-7302
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Committee ChairVidic, Radisavvidic@pitt.eduVIDIC0000-0001-7969-6845
Committee MemberKhanna, Vikaskhannav@pitt.eduKHANNAV0000-0002-7211-5195
Committee MemberNg,
Committee MemberLi, Leilel55@pitt.edulel550000-0002-8679-9575
Date: 6 September 2022
Date Type: Publication
Defense Date: 13 July 2022
Approval Date: 6 September 2022
Submission Date: 30 July 2022
Access Restriction: 1 year -- Restrict access to University of Pittsburgh for a period of 1 year.
Number of Pages: 155
Institution: University of Pittsburgh
Schools and Programs: Swanson School of Engineering > Civil and Environmental Engineering
Degree: PhD - Doctor of Philosophy
Thesis Type: Doctoral Dissertation
Refereed: Yes
Uncontrolled Keywords: Desalination, Membrane Distillation, Scaling Control
Date Deposited: 06 Sep 2022 16:38
Last Modified: 06 Sep 2023 05:15


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