Water desalination using membrane distillation with acidic stabilization of scaling layer thickness

Membrane scaling during water desalination by membrane distillation has been studied. A tap water was applied as a feed and diluted HCl solutions were used for periodical cleaning of module. The membrane morphology and the composition of deposits were determined using scanning electron microscopy coupled with energy dispersion spectrometry. The CaCO3 deposit covered not only the membrane surfaces but was also formed in the pores' interior. During the membrane rinsing an HCl solution dissolved the deposit inside the pores, which is synonymous with flooding of these pores by liquid. Due to the membrane wetting, a significant flux decline was observed. It was confirmed that the rate of membrane wettability might be limited by shorting a period between the cleaning operations. In this work the membranes were cleaned every 1–5 h, and as a result, the penetration of deposit into the pores was restricted up to the depth of 30 μm during 200 h of MD process. A periodical rinsing of membranes by HCl solutions did not have a negative influence on the membrane performance, and the MD module efficiency was stabilized during this period. © 2015 Elsevier B.V. All rights reserved.

The intensive CaCO3 scaling was observed during tap water desalination by MD process. Diluted HCl solutions successfully removed the deposit and the membranes were cleaned after rinsing operation for 3–5 min. The SEM observation revealed that the images of membrane samples after 200 h of MD process using tap water (with cyclic HCl rinsing) were similar to those observed for new membranes. However, the precipitation of CaCO 3 crystallites may initiate the membrane wettability. The application of cyclic rinsing of MD module by HCl solutions restricted the size of formed crystalline deposits. The small CaCO3 crystallites (diameter below 20 μm) accelerate the heterogeneous crystallization, and they exhibit a tendency to facilitate a selective adsorption of several ionic species, which enable the formation of other compounds in the deposit being formed, e.g., silicates and sulfates, leading to membrane scaling. The efficiency of heterogeneous crystallization decreased when the deposit layer covered the entire surface of the membrane. Therefore, the formed deposit should be systematically removed from the membrane, e.g., by rinsing with acid solutions every 1–5 h. An increase of frequency of membrane rinsing by HCl solution limited the size of crystallites formed inside the pores, and as a result, allowed to eliminate the cracking of the membrane. The studies confirmed that the surface wettability of used polypropylene Accurel PP S6/2 occurred during the initial 50–60 h of MD process operation. The wettability was accelerated when precipitates were deposited on the membrane surface. In this case, the dissolution of deposit facilitates the penetration of liquid into the pores of hydrophobic membranes, which promotes their wettability and the internal scaling was observed. Due to these phenomena it can be expected that membrane wall will be wetted up to 50–100 μm. However, the wall thickness of used Accurel PP membranes amounted to 400 μm. Therefore, the wettability of the surface pores even on a significant depth still allows to maintain a gaseous gap between feed water and obtained distillate. This excludes a leakage of the feed and the distillate of high purity was obtained, having the electrical conductivity below 3 μS/cm.