ASTM_D_4993_-_08_2014.pdf

Designation:D499308(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 revision,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 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 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 this 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 WaterD1293 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,Nanofiltration 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 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 decreasing 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 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 the 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 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 concentration 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.NOTE1If acid is used for control of CaCO3scale,measure the pHafter acid addition.6.4 Determine the total alkalinity of the feed solution usingTest Methods D1067 and express as CaCO3.NOTE2If 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 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 Conshohocken,PA 19428-2959.United States1 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,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.NOTE3SPSiO2can 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 PracticeD3739.NOTE4For 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