1、Designation: D 4993 08Standard Practice forCalculation and Adjustment of Silica (SiO2) Scaling forReverse Osmosis1This standard is issued under the fixed designation D 4993; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year
2、of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers the calculation and adjustment ofsilica (SiO2) for the concentrate stream of a reverse osmosi
3、ssystem. The calculations are used to determine the need forscale control in the operation and design of reverse osmosisinstallations. This practice is applicable for all types of reverseosmosis devices (tubular, spiral wound, and hollow fiber).1.2 This practice is applicable to both brackish waters
4、 andseawaters.1.3 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to
5、establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:D 859 Test Method for Silica in WaterD 1067 Test Methods for Acidity or Alkalinity of WaterD 1129 Terminology Relating to WaterD 1293 T
6、est Methods for pH of WaterD 3739 Practice for Calculation and Adjustment of theLangelier Saturation Index for Reverse OsmosisD 4194 Test Methods for Operating Characteristics of Re-verse Osmosis and Nanofiltration DevicesD 6161 Terminology Used for Microfiltration, Ultrafiltra-tion, Nanofiltration
7、and Reverse Osmosis Membrane Pro-cesses3. Terminology3.1 For definitions of terms relating to water used in thispractice, refer to Terminology D 1129 and D 6161.4. Summary of Practice4.1 This practice consists of calculating the potential forscaling by SiO2in a reverse osmosis concentrate stream fro
8、mthe concentration of SiO2in the feed solution and the recoveryof the reverse osmosis system.4.2 This practice also presents techniques to eliminatescaling by decreasing the recovery, decreasing the SiO2con-centration in the feedwater, adjusting the pH of the feedwater,and increasing the temperature
9、 of the feedwater.5. Significance and Use5.1 In the design and operation of reverse osmosis installa-tions, it is important to predict the SiO2scaling properties ofthe concentrate stream. Because of the increase in the concen-tration of SiO2and the change in pH, the scaling property ofthe concentrat
10、e stream will be quite different from that of thefeed solution. This practice permits the calculation of thescaling potential for the concentrate stream from the feedwateranalysis and the reverse osmosis operating parameters.5.2 Scaling by SiO2will adversely affect the reverse osmo-sis performance.
11、This practice gives various procedures for theprevention of scaling.5.3 The presence of certain metals, for example, Al+3, maysignificantly alter the solubility of SiO2via formation ofinsoluble metal silicates. This practice does not address thisphenomena.6. Procedure6.1 Determine the concentration
12、of SiO2in the feed streamin accordance with Test Method D 859.6.2 Measure the temperature of the feed solution.6.3 Measure the pH of the feed solution using Test MethodsD 1293.NOTE 1If acid is used for control of CaCO3scale, measure the pHafter acid addition.6.4 Determine the total alkalinity of the
13、 feed solution usingTest Methods D 1067 and express as CaCO3.NOTE 2If acid is used for control of calcium carbonate (CaCO3)scale, determine the total alkalinity after acid addition.1This practice is under the jurisdiction of ASTM Committee D19 on Water andis the direct responsibility of Subcommittee
14、 D19.08 on Membranes and IonExchange Materials.Current edition approved May 1, 2008. Published May 2008. Originallyapproved in 1989. Last previous edition approved in 2003 as D 4993 89 (2003).1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United
15、States.7. Calculation7.1 Calculate the SiO2concentration in the concentratestream from the SiO2concentration in the feed solution, therecovery of the reverse osmosis system, and the SiO2passageas follows:SiO2c5 SiO2f31 2 YSPSiO2!1 2 Ywhere:SiO2c= silica concentration in concentrate as SiO2,mg/L,SiO2
16、f= silica concentration in feed as SiO2, mg/L,Y = recovery of the reverse osmosis system, ex-pressed as a decimal, andSPSiO2= silica passage, expressed as a decimal.NOTE 3SPSiO2can be obtained from the supplier of the reverseosmosis system.7.2 Calculate the pH of the concentrate stream from the pHof
17、 the feed stream using the procedure given in PracticeD 3739.NOTE 4For seawater systems, the calculated pH of the concentratestream can be 0.1 to 0.2 higher than measured pH values if the feed pH isabove 7.0. In these cases, empirical correlations between the feed pH andthe concentrate pH as a funct
18、ion of conversion can be used to moreaccurately calculate the concentrate pH. Check with the supplier of thereverse osmosis device to determine if empirical correlations should beused.7.3 From Fig. 1, obtain the solubility of SiO2as a functionof temperature (SiO2temp.).NOTE 5Temperature of the conce
19、ntrate is assumed equal to tempera-ture of feed solution. If the temperature of the water is known to vary, usethe minimum temperature for the calculations.7.4 From Fig. 2,2obtain the pH correction factor for theconcentrate pH calculated in 7.2.7.5 Calculate the solubility of SiO2corrected for pH(Si
20、O2corr.) by multiplying the solubility of SiO2obtained in 7.3by the pH correction factor obtained in 7.4.7.6 Compare the silica concentration in the concentrate(SiO2c) obtained in 7.1 with the silica solubility (SiO2corr.)obtained in 7.5.IfSiO2cis greater than SiO2corr., silica scalingwill occur and
21、 adjustment is required.NOTE 6Some suppliers may use a safety factor. Check with thesupplier of the reverse osmosis device to determine if some fraction of theSiO2corr., for example, 0.9 SiO2corr., should be used to compare with SiO2c.8. Adjustments for Scale Control8.1 If SiO2cis less than SiO2corr
22、.or the recommendedfraction of SiO2corr., a higher recovery can be used with respectto scaling by silica. Reiteration of the calculations at higherrecovery can be used to determine the maximum conversionwith respect to scaling by silica.8.2 If SiO2cis greater than SiO2corr.or the recommendedfraction
23、 of SiO2corr., a lower recovery must be used to preventscaling. Reiteration of the calculations can be used to deter-mine the allowable recovery with respect to scaling by silica.8.3 If the maximum allowable recovery is lower thandesired, lime plus soda ash softening employing either mag-nesium oxid
24、e or sodium aluminate can be used in the pretreat-ment system to decrease the SiO2concentration in the feedstream and thus permit higher conversion with respect toscaling by silica. It is important that the softening process beperformed properly in order to prevent formation of insolublemetal silica
25、tes in the reverse osmosis system.8.4 Since the solubility of silica increases below a pH ofabout 7.0 and above a pH of about 7.8, pH adjustment witheither acid or base can permit a higher recovery with respect tosilica scaling. However, the reverse osmosis membrane must2Alexander, G. B., Hester, W.
26、 M., and Iler, R. K., “The Solubility of AmorphousSilica in Water,” Journal of Physical Chemistry, 58, 1954, p. 453.FIG. 1 Solubility of SiO2vs. TemperatureFIG. 2 SiO2pH Correction FactorD4993082be able to operate at the adjusted pH and for the high pH,CaCO3scaling must be prevented. Check with supp
27、lier ofreverse osmosis device for permitted operating pH range.8.5 The maximum allowable recovery with respect to silicascaling can be increased significantly by increasing the watertemperature using a heat exchanger. However, the reverseosmosis membrane must be able to operate in the adjustedtemper
28、ature range. Check with supplier of reverse osmosisdevice for permitted operating temperature range.9. Reverse Osmosis in Operation9.1 Once a reverse osmosis system is operating, the scalingpotential of SiO2can be directly calculated from the analysesof the concentrate stream and compared with the p
29、rojectedscaling potential calculated above.10. Use of Computers for the Determination of ScalingPotential10.1 The preceding calculations are adaptable to simplecomputer analysis.11. Keywords11.1 fouling; reverse osmosis; scaling; silica; solubilityASTM International takes no position respecting the
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33、ndard is copyrighted by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959,United States. Individual reprints (single or multiple copies) of this standard may be obtained by contacting ASTM at the aboveaddress or at 610-832-9585 (phone), 610-832-9555 (fax), or serviceastm.org (e-mail); or through the ASTM website(www.astm.org).D4993083
