Prevention and management of silica scaling in membrane distillation using pH adjustment

• Published in: Journal of membrane science 2018-05, Vol.554 (C), p.366-377
• Main Authors: Bush, John A., Vanneste, Johan, Gustafson, Emily M., Waechter, Christopher A., Jassby, David, Turchi, Craig S., Cath, Tzahi Y.
• Format: Article
• Language: English
• Subjects: GEOTHERMAL ENERGY; Membrane cleaning; Membrane distillation; Membrane fouling; Membrane scaling; Silica
• ISSN: 0376-7388; 1873-3123
• DOI: 10.1016/j.memsci.2018.02.059

Membrane scaling by silica is a major challenge in desalination, particularly for inland desalination of brackish groundwater or geothermal water, which often contain high concentrations of silica and dissolved solids. Adjustment of feed pH may reduce silica scaling risk, which is important for inland facilities that operate at high water recoveries to reduce brine disposal costs. However, water recovery of reverse osmosis is also limited due to increased osmotic pressure with feed water concentration. Membrane distillation (MD) is a thermally driven membrane desalination technique that is not limited by increased osmotic pressure of the feed. In this investigation, pH adjustment was tested as a strategy to reduce silica scaling risk in the MD process. With feed water pH less than 5 or higher than 10, scaling impacts were negligible at silica concentrations up to 600 mg/L. Scaling rates were highest at neutral pH between 6 and 8. Cleaning strategies were also explored to remove silica scale from membranes. Cleaning using NaOH solutions at pH higher than 11 to induce dissolution of silica scale was effective at temporarily restoring performance; however, some silica remained on membrane surfaces and scaling upon re-exposure to supersaturated silica concentrations occurred faster than with new membranes. [Display omitted] •Silica scaling in membrane distillation can be reduced with feed pH adjustment.•At similar pH and concentration, scaling rates correlate to polymerization rates.•Acidification is less costly than alkalization for comparable scale reduction.•Silica scale may be partially removed by rinsing with NaOH solution.•Silica scale has negligible impact on MD performance with temperatures reversed.