1、Designation: D4993 08 (Reapproved 2014)Standard Practice forCalculation and Adjustment of Silica (SiO2) Scaling forReverse Osmosis1This standard is issued under the fixed designation D4993; the number immediately following the designation indicates the year oforiginal adoption or, in the case of rev
2、ision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () 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
3、 reverse osmosissystem. 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
4、brackish waters 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 th
5、is standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:D859 Test Method for Silica in WaterD1067 Test Methods for Acidity or Alkalinity of WaterD1129 Terminology Relating to W
6、aterD1293 Test Methods for pH of WaterD3739 Practice for Calculation and Adjustment of theLangelier Saturation Index for Reverse OsmosisD4194 Test Methods for Operating Characteristics of Re-verse Osmosis and Nanofiltration DevicesD6161 Terminology Used for Microfiltration, Ultrafiltration,Nanofiltr
7、ation and Reverse Osmosis Membrane Processes3. Terminology3.1 DefinitionsFor definitions of terms relating to waterused in this practice, refer to Terminology D1129 and D6161.4. Summary of Practice4.1 This practice consists of calculating the potential forscaling by SiO2in a reverse osmosis concentr
8、ate stream fromthe 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 t
9、he temperature of the feedwater.5. Significance and Use5.1 In the design and operation of reverse osmosisinstallations, it is important to predict the SiO2scaling prop-erties of the concentrate stream. Because of the increase in theconcentration of SiO2and the change in pH, the scalingproperty of th
10、e concentrate stream will be quite different fromthat of the feed solution. This practice permits the calculationof the scaling potential for the concentrate stream from thefeedwater analysis and the reverse osmosis operating param-eters.5.2 Scaling by SiO2will adversely affect the reverse osmo-sis
11、performance. 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 c
12、oncentration of SiO2in the feed streamin accordance with Test Method D859.6.2 Measure the temperature of the feed solution.6.3 Measure the pH of the feed solution using Test MethodsD1293.NOTE 1If acid is used for control of CaCO3scale, measure the pHafter acid addition.6.4 Determine the total alkali
13、nity of the feed solution usingTest Methods D1067 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 Su
14、bcommittee D19.08 on Membranes and IonExchange Materials.Current edition approved Jan. 1, 2014. Published March 2014. Originallyapproved in 1989. Last previous edition approved in 2008 as D4993 08. DOI:10.1520/D4993-08R14.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshoh
15、ocken, PA 19428-2959. United States17. 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
16、concentrate as SiO2, mg/L,SiO2f= silica concentration in feed as SiO2, mg/L,Y = recovery of the reverse osmosis system, expressedas 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 conc
17、entrate stream from the pHof the feed stream using the procedure given in PracticeD3739.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 andth
18、e concentrate pH as a function 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.).NOT
19、E 5Temperature of the concentrate 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
20、 of SiO2corrected for pH(SiO2corr.) 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., s
21、ilica scalingwill occur and 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
22、 SiO2cis less than SiO2corr.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
23、.or the recommendedfraction 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 emplo
24、ying either mag-nesium oxide 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 formati
25、on of insolublemetal silicates 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 with2Alexander, G. B., Hester, W. M., and Iler, R. K., “The Solubility of AmorphousSilica in Water,” Journal of Physical Chemist
26、ry, Vol 58, 1954, p. 453.FIG. 1 Solubility of SiO2Versus TemperatureFIG. 2 SiO2pH Correction FactorD4993 08 (2014)2either acid or base can permit a higher recovery with respect tosilica scaling. However, the reverse osmosis membrane mustbe able to operate at the adjusted pH and for the high pH,CaCO3
27、scaling must be prevented. Check with supplier 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 mus
28、t be able to operate in the adjustedtemperature 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
29、concentrate stream and compared with the projectedscaling 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 Inter
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33、dards, at the address shown below.This standard 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). Permission rights to photocopy the standard may also be secured from the ASTM website (www.astm.org/COPYRIGHT/).D4993 08 (2014)3
