Reverse osmosis silt density index

There are two other types of technologies—reverse osmosis (RO) and electrodeionization (EDI)—that remove dissolved solids from a liquid stream. Both are widely used in the water purification industry and have potential for use in the treatment of CMP wastewater. RO is a method by which water is forced through a semipermeable membrane that does not allow ions to cross. EDI removes ions from a liquid stream by means of an applied voltage. Both the EDI or RO are very effective in removing anions and cations, but the tradeoff is that the feed to the EDI or RO must be preconditioned to prevent damage to the equipment. In particular, the feedwater to an RO should not have a silt density index (SDI) greater than 3.0, which may require additional filtration to ensure all the solids are removed from the liquid stream. Some EDI manufacturers recommend that the feedwater to the EDI be RO permeate or of better quality. 

Binovi R. (1983), A replacement for the silt density index permanganate demand to predict reverse osmosis membrane fouling, Dissertadon, University of Cinciimati. 

SILT DENSITY INDEX - A measure of the tendency of a water to foul a reverse osmosis membrane, based on time flow through a membrane filter at constant pressure. 

Silt density index (SDl) is a sensitive method for determining the ability of a filter to remove colloidal particles 14). SDl (in min ) is used extensively as a criterion in minimizing fouling of reverse osmosis membranes. The lower the SDl value, the cleaner is the stream. Manufacturers of reverse osmosis membranes recommend that the stream be prefiltered so that it has an SDl factor less than 3.0. Typically 1 pm absolnte 6) filters have an SDl of about 4-5. Manufacturers of hallow fiber membrane filters claim SDI s in the range of 1.75 to 2.25. SDl measurements of effluents from the nano alumina media range from 0.5-1.0. The SDl of 32% PAC media was measured and found to be about 1.0. Turbidity as well as SDl tests have confirmed that the extent of shedding of PAC particles into effluent streams is minimal. 

Furthermore, water scarcity has led to the widespread use of desalination to supplement freshwater resources. While reverse osmosis (RO) membranes are used to desalinate seawater and brackish water, MF/UF can be applied as a pretreatment to RO, in an integrated membrane system (IMS). The level of pretreatment, in terms of turbidity and silt density index (SDI), provided by MF/UF was found to be significantly better and more consistent than conventional pretreatment to RO (e.g., coagulation/sedimentation/filtration) for RO feed water. In recent years, the demand for MF/UF as a pretreatment to RO has grown as the need to augment our... 

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