1、Designation: D 4993 89 (Reapproved 2003)Standard 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 r
2、evision, the year 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 o
3、f a 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 bo
4、th brackish waters andseawaters.1.3 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 establish appro-priate safety and health practices and determine the applica-bility of regulatory limitati
5、ons prior to use.2. Referenced Documents2.1 ASTM Standards:D 859 Test Method for Silica in Water2D 1067 Test Methods for Acidity or Alkalinity of Water2D 1129 Terminology Relating to Water2D 1293 Test Methods for pH of Water2D 3739 Practice for Calculation and Adjustment of Lange-lier Saturation Ind
6、ex for Reverse Osmosis3D 4194 Test Methods for Operating Characteristics of Re-verse Osmosis and Nanofiltration Devices33. Terminology3.1 For definitions of terms relating to water used in thispractice, refer to Terminology D 1129. For terms relating toreverse osmosis, refer to Test Methods D 4194.4
7、. Summary of Practice4.1 This practice consists of calculating the potential forscaling by SiO2in a reverse osmosis concentrate 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 dec
8、reasing the recovery, decreasing the SiO2con-centration in the feedwater, adjusting the pH of the feedwater,and increasing the temperature 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
9、ofthe concentrate stream. Because of the increase in the concen-tration of SiO2and the change in pH, the scaling property ofthe concentrate stream will be quite different from that of thefeed solution. This practice permits the calculation of thescaling potential for the concentrate stream from the
10、feedwateranalysis and the reverse osmosis operating parameters.5.2 Scaling by SiO2will adversely affect the reverse osmo-sis 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 o
11、f SiO2via formation ofinsoluble metal silicates. This practice does not address thisphenomena.6. Procedure6.1 Determine the concentration 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
12、MethodsD 1293.NOTE 1If acid is used for control of CaCO3scale, measure the pHafter acid addition.6.4 Determine the total alkalinity of the 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 a
13、fter acid addition.1This practice is under the jurisdiction of ASTM Committee D19 on Water andis the direct responsibility of Subcommittee D19.08 on Membranes and IonExchange Materials.Current edition approved June 10, 2003. Published August 2003. Originallyapproved in 1989. Last previous edition ap
14、proved in 1998 as D 4993 89 (1998)e1.2Annual Book of ASTM Standards, Vol 11.01.3Annual Book of ASTM Standards, Vol 11.02.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.7. Calculation7.1 Calculate the SiO2concentration in the concent
15、ratestream 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,SiO2f= silica concentration in feed as SiO2, mg/L,Y = recovery of the rever
16、se 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 the feed stream using the procedure given in PracticeD 3739.NOTE 4For
17、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 function of conversion can be used to moreaccurately calculate the concentra
18、te 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 concentrate is assumed equal to tempera-ture of feed solution. If the temper
19、ature of the water is known to vary, usethe minimum temperature for the calculations.7.4 From Fig. 2,4obtain the pH correction factor for theconcentrate pH calculated in 7.2.7.5 Calculate the solubility of SiO2corrected for pH(SiO2corr.) by multiplying the solubility of SiO2obtained in 7.3by the pH
20、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 adjustment is required.NOTE 6Some suppliers may use a safety factor. C
21、heck 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.or the recommendedfraction of SiO2corr., a higher recovery can be used
22、 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 of SiO2corr., a lower recovery must be used to preventscaling. Reitera
23、tion 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 oxide or sodium aluminate can be used in the pretreat-ment system to decrea
24、se 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 silicates in the reverse osmosis system.8.4 Since the solubility of silica in
25、creases 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 must4Alexander, G. B., Hester, W. M., and Iler, R. K., “The Solubility of AmorphousSilica in Water,” Jou
26、rnal of Physical Chemistry, 58, 1954, p. 453.FIG. 1 Solubility of SiO2vs. TemperatureFIG. 2 SiO2pH Correction FactorD 4993 89 (2003)2be able to operate at the adjusted pH and for the high pH,CaCO3scaling must be prevented. Check with supplier ofreverse osmosis device for permitted operating pH range
27、.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 adjustedtemperature range. Check with supplier of reverse osmosisdevice for
28、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 projectedscaling potential calculated above.10. Use of Computer
29、s 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 validity of any patent rights asserted in connection with any
30、item mentionedin this standard. Users of this standard are expressly advised that determination of the validity of any such patent rights, and the riskof infringement of such rights, are entirely their own responsibility.This standard is subject to revision at any time by the responsible technical c
31、ommittee and must be reviewed every five years andif not revised, either reapproved or withdrawn. Your comments are invited either for revision of this standard or for additional standardsand should be addressed to ASTM International Headquarters. Your comments will receive careful consideration at
32、a meeting of theresponsible technical committee, which you may attend. If you feel that your comments have not received a fair hearing you shouldmake your views known to the ASTM Committee on Standards, at the address shown below.This standard is copyrighted by ASTM International, 100 Barr Harbor Dr
33、ive, 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).D 4993 89 (2003)3
