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Wastewater Reclamation and Reuse Treatment Technology
Published in Donald R. Rowe, Isam Mohammed Abdel-Magid, Handbook of Wastewater Reclamation and Reuse, 2020
Donald R. Rowe, Isam Mohammed Abdel-Magid
Membrane scaling is due to deposition of salts that are dissolved in the feed water. The salts may include calcium carbonate, calcium sulfate, silica, strontium sulfate, barium sulfate, and calcium fluoride. Methods used for the control of scaling include: Conversion control to avoid exceeding the solubility of the salts.Elimination of one of the ions responsible for a scale-forming material (methods used to accomplish this include softening, adjusting pH by addition of sulfuric acid or hydrochloric acid to precipitate calcium carbonate and reduce scaling).Inhibiting the crystal growth of a scale-forming substance.
Physical Processes
Published in Ralph L. Stephenson, James B. Blackburn, The Industrial Wastewater Systems Handbook, 2018
Ralph L. Stephenson, James B. Blackburn
Membrane scaling occurs when some of the salts dissolved in the feed water precipitate on the membranes. The feed water salts are concentrated by a factor of two to five times, and their solubility limits can be exceeded. When this happens precipitation occurs, and it occurs at the membrane surface plugging the membrane. Maldistribution within the system can cause even higher concentrations in some parts of the system. The most common scales encountered are Calcium Carbonate and Calcium Sulfate. Other compounds that can cause problems are silica, Strontium Sulfate, Barium Sulfate, and Calcium Fluoride. Since one of the main uses of Reverse Osmosis Systems is to remove dissolved solids in wastewater treatment, it is advisable to check solubility limits carefully. If the limits will be exceeded, then pretreatment may be required, such as softening to remove Calcium salts. Adjusting the pH can also work, however, membranes must operate within a relatively narrow pH range. Feeding precipitation inhibiting chemicals such as Sodium Hexam- etaphosphate can also be of benefit.
Membrane Filtration
Published in Paul N. Cheremisinoff, Handbook of Water and Wastewater Treatment Technology, 2019
Membrane scaling is caused by the precipitation of some of the salts dissolved in the feed water. The salts in the feed water are concentrated in the RO process; two times at 50% conversion and four times at 75% conversion. This alone can cause their solubility limits to be exceeded and precipitation to occur. Concentration polarization and unequal distribution within the module can cause greater concentrations to occur in some places. Concentrate flowrates must be maintained above minimum values to minimize those effects and conversion rates kept at reasonable levels.
Effect of combined fouling on desalination performance of membrane capacitive deionization (MCDI) during long-term operation
Published in Journal of Dispersion Science and Technology, 2020
Chengyi Wang, Lin Chen, Liang Zhu
Additionally, the surface crystal growth rate could be described by the widely adopted kinetic expression[11]: where ms is mass of surface crystal which is crystallized on membrane surface, ks is the rate constant of the surface crystallization, Sm is the surface area of active sites on the membrane surface, Cw is the wall concentration of calcium near the membrane surface, Cs is the saturation concentration and n is the order of the reaction rate. Sm represents the active sites on the membrane surface and is proportional to the number of available growth sites present on the membrane surface, which was generally assumed constant in the surface crystallization. With calcium concentration increasing, calcium ions had more chance to accumulate membrane surface and result in an increment in wall concentration. The rate of surface crystallization was directly proportional to the wall concentration of calcium, indicating that higher calcium concentration would accelerate membrane scaling.