PAGE one OF FOUR Waste to Waste Disposal Vessel 3 Vessel Backwash Brine Vessel 2 Vessel Raw Water Raw is a process that also uses Source: U.S. Environmental Protection Agency, 1989 Agency, Protection U.S. Environmental Source: Alarm and Shutdown Nitrate Analysis Vessel 1 Vessel Monitor systems where there is a high degree of sea- theresystems where is a high degree sonal fluctuation in water demand. Electrodialysis in electrodialysis, direct membranes. However, is used to attract ions to one electrical current Electrodialysis chamber. side of the treatment water, systems include a source of pressurized currentsupply, and a pair of power a direct selective membranes. Conductivity

○○○○○○ ○○○○○○○○○ A NATIONAL DRINKING WATER CLEARINGHOUSE FACT SHEET FACT CLEARINGHOUSE WATER DRINKING A NATIONAL Treated Water Treated Blending Valves Well Supply Well Tank Brine Salt Loading Figure 1. Ion exchange treatment system Figure 1. Ion exchange treatment Pressure Tank System Distribution Potable Water to Brine System Overflow units can be used to remove any soda ash, without soda ash, distillation, ization, the above. 1995) systems are mineralization

• filtration, • coagulation and filtration, • cold lime with or without • hot lime with or • evaporation or • electrodialysis, • RO, • continuous deion- • ultrafiltration, • degasification, or • combinations of (Source: Owens, n Exchange and n Exchange Ion exchange but are charged water, (ionic) substance from usually used to remove and nitrate hardness from groundwater. is pretreated to reduceWater the suspended load to solids and total dissolved solids (TDS) pretreatmentthe ion-exchange unit. Methods of include: RO compact, simple to operate, and require mak- minimal labor, ing them suitable for small systems. They are also suitable for NATIONAL DRINKING WATER CLEARINGHOUSE DRINKING WATER NATIONAL Summary are membrane processes Ion exchange and in water and wastewater becoming used extensively used for the removal Ion exchange is primarily of hardnesstreatment. ions, such as magnesium and (RO) and electrodialysis,water demineralization. Reverse osmosis calcium, and for both membrane processes, remove dissolved solids from water using membranes.

Ion Exchange and Demineralization Are Becoming Widely Used Ion Exchange and Demineralization

De Io two

PAGE OF FOUR Ion Exchange/DemineralizationSystems: knowledge of theresinexchange capacity, theinfluent breakthrough timefor anionexchange unitrequires weight perunitvolume oftheresin. The calculationofthe The resinexchange of capacityisexpressed interms per day. cycles areonce economicalregeneration the shortest ions.appropriate Becauseoftherequired “down time,” suitable themediumwith solution,whichresaturates the contaminantions, themediumisregenerated witha exchange medium. When therearenoionsleftto replace regenerant ionsbecausethey arepreferred by the During ion exchange, thecontaminantionsreplace with theexchangeable ionbefore treatmentoperations. (negatively charged). The exchange mediumissaturated resin surface isdesignedaseithercationic oranionic medium, whichisusuallyasyntheticplasticresin. This substituted for anotheronthechargedsurface ofthe As thenameimplies, oneionis ions ontoaresinexchange medium. ofcontaminant through adsorption Inorganics removal isaccomplished that needtoremove radionuclides. Ion exchange smallsystems isusuallythebestchoicefor selenium,arsenic(V),chromium(VI),andnitrate.nitrites, of barium,cadmium,chromium(III),silver, radium, Ion exchange effectively removes morethan90percent Ion exchange Ionexchange unitsalsoare • Pretreatmentrequiredformost • Usuallynotfeasible withhighlevels • Potential for unacceptable levels • Ionexchange waste ishighly • Largevariety ofspecificresinsareavailable from • impossible. Effluentcontaminationisvirtually • Ionexchange process,like reverse osmosis, can • regenerated morefrequently.regenerated mediummustexchange be medium,andthe exchange ions)for(positive spaceonthe will competewithothercations influent withhighlevels ofhardness competing ions. For example, sensitive tothepresenceof surface waters. of TDS. (peaks) ofcontaminationineffluent. disposal. andrequirescareful concentrated specific contaminants. suppliers. Eachresiniseffective inremoving be usedwithfluctuatingflow rates. PROCESS ADVANTAGES LIMITATIONS Product toDegasifier PO Feed Figure 2.Schematicofareverseosmosissystem and Reservoir 4 Feed

Generator Set

○○○○○○○○○○○○○○○○○○○○○○○ ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ potassium chloride. be avoided usingotherregenerant by materials, suchas for diets. individualswithsaltrestricted This problem can residual inthefinishedwater, whichmay beunacceptable chemical. However, thiscanresultinahighsodium medium inionexchangers becauseofthelow costofthe isoftenusedtoregenerate theexchangeSodium chloride ments. (Seefigure1onpage1.) exchange, disinfection, storage, ele- anddistribution Typical ionexchange ion unitsconsist ofprefiltration, zero effluentcontaminantconcentrations.zero Water passingthroughmultiple unitscanachieve near nants. effective RO isparticularly whenusedinseries. substances, pesticides,andmicrobiologicalcontami- can alsoeffectively remove radium, organic natural operated unitswillattain96percentremoval rates. RO selenium (IV),(VI),andsilver. Properly (III), chromium(VI),fluoride, lead, mercury, nitrite, arsenic (III),(IV),barium,cadmium,chromium nants fromwater. Itremoves morethan70percentof RO caneffectively remove allinorganiccontami- nearly Reverse osmosis(RO) quality. contaminant concentration, andthedesiredeffluent Relatively insensitive and toflow TDS level, andthus • Removes allcontaminantionsandmost nearly • Acid Feed seasonal fluctuation inwater demand. suitable for smallsystemswithahigh degree of dissolved non-ions. Permeators Tank Mixing Filter Source: U.S.Environmental ProtectionAgency, 1989 ADVANTAGES CHEMICALS EQUIPMENT FOUR•May 1997 High Service Pumps High Service Solution Cleaning Control Panel and Pumps Percolation Brine to Pond Silicate Feed Sodium PAGE OF FOUR

PAGE three OF FOUR suspended particulates. Particulates, suspended in in can be removed water, salts that pretreatment but their solubility exceed product at the membrane be controlled must surface pH reduction chemically by (to reduce carbonate or chelation concentration) use of of metal ions (by A example). for phosphate, of the charge on the reversal a process membranes, called electrodialysis (EDR), helps to reversal flush the attached ions from thus surface, the membrane the time between extending cleanings. PROCESS EQUIPMENT CHEMICALS LIMITATIONS ADVANTAGES Water Product Discharge Concentrate Stack Membrane are removed. • non-ions most dissolved All contaminant ions and •TDS level. to flow and insensitive Relatively • possible. effluent concentration Low • costs. High capital and operating • required. of pretreatment High level • flow. of feed Reject stream is 20–90 percent • Electrodes require replacement. The three essential elements of the system are (1) a The three essential elements of the system (2) a direct current power source of pressurized water, The membranes. of selective and (3) a pair supply, to 60 percent per from 25 varies ion removal average of Multistage units can increase the efficiency stage. in the “stacked” pairs are membrane Many removal. treatment vessel. limit the amount of water may of membranes Fouling pores are is caused when membrane Fouling treated. obstruction of salt precipitation or by physical clogged by Electrodialysis fluoride in removing and is veryElectodialysis effective barium, and cadmium, nitrate, also remove and can figure 3 below.) (See selenium. stream are The membranes adjacent to the influent and this charge or negatively, charged either positively The mem- the membrane. counter-ions toward attracts or nega- either positively branes are designed to allow to pass through the membrane, charged ions thus tively stream through a from the product water ions move (See figure 4 streams. reject water membrane to the two on page 4.)

○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ ○○○○○○○○ Electrode Electrode Filters Cartridge Supply (Rectifier) DC Power Low- Pump Pressure Circulation PROCESS EQUIPMENT LIMITATIONS Tank Holding AC Power Source potential problem. operator attention and make RO suitable for small for suitable RO attention and make operator system applications. break-in period. recovered as waste brine water tothe as waste recovered water). finished • cases. of pretreatment is required in some High level • Membranesare prone to fouling. • costs. and operating High capital • (brine the wastewater Managing a solution) is • possible. Low concentration effluent • Bacteria and particles are also removed. • less allow and automation simplicity Operational for • minimum any without immediately, ROoperates • pressure, operating • membrane type and pore size, • and pretreatment requirements, • of the influent rate (the ratio product conversion (If Required) NATIONAL DRINKING WATER CLEARINGHOUSE DRINKING WATER NATIONAL Typical RO units include raw water pumps, pretreatment, water RO units include raw Typical and distribution disinfection, storage, membranes, ableThese units are (See figure 2 on page 2.) elements. desired quantity or quality of to process virtually any to reprocess units sequentially by configuring water The brine from the earlierwaste stages of the process. osmosis reverse principal design considerations for units are: RO removes contaminants from water using a semiper- contaminants from water removes RO and not only water, that permits meable membrane to pass ions (such as sodium and chloride), dissolved to a Contaminated water is subject through its pores. through the pure water high pressure that forces behind in a brine contaminants leaving membrane, of pore with a variety Membranes are available solution. and characteristics. sizes Source: U.S. Agency for International Development, 1980 Source: Special Treatment Figure 3. Basic components of an electrodialysis unit Raw Feedwater f o ur PAGE OF FOUR Figure 4.Movementofionsintheelectrodialysisprocess (NDWC); however, copyingcostsapply.Thefirst the NationalDrinkingWater Clearinghouse fromThese publicationsalsomaybeordered Development at(513)569-7562. mental Protection AgencyOffice and ofResearch Both maybeordered from free theU.S.Environ- Water Treatment Facilities Upgrading ExistingorDesigningNewDrinking ties Drinking Water forSmallCommuni- Treatment PollutionControlEnvironmental Alternatives: was primarilyobtainedfrom twosources: onionexchange/demineralization Information moreWhere canIfind information? , EPA/625/5-90/025; and Note 1 Electrode Positive Membranes Anionic Concentrate Cells Battery Membranes Cationic Note 2 , EPA/625/4-89/023. Electrode Negative Technologies for Technologies Product Water Feedwater Switch

Brine

○○○○○○○○○○○○○○○○○○○○○○○ shipping andhandlingcharges. availableatnocost,exceptfor sheets are (800) 624-8301or(304)293-4191.Thesefact Control,” item#DWBLPE52calltheNDWCat Brief:Corrosion item #DWBLPE47,or“Tech Brief:Disinfection,” item #DWBLPE50,“Tech Brief:Filtration,” tion,” item#DWBLPE56,“Tech “Tech Brief:IonExchangeandDemineraliza- copiesof ortoorder For furtherinformation alsoapply. Shipping andhandlingcharges #DWBKDM04, a209-pagebook,costs$30.05. tion, costs$11.82;andthesecond,item book, item#DWBKGN09,an82-pagepublica- TDS (productwater). andtheotherwithamuch brine concentrated lower concentration of the concentratecells. Two streamsemergefromthedevice: Oneof andaretrappedabout halfoftheionsinproduct cellsmigrate in Feedwater entersboththeconcentrate andproductcells. Upto remove ionsfromincomingsalinewater onacontinuous basis. The phenomenonillustrated above isusedinelectrodialysisto a highresistancetocurrentflows. havewould level alower ofdissolved have constituentsandwould cells(concentrate cells).alternate The othercells, ions, whichlack almostalltheionswould inthe If thiscontinued, becometrapped fixed Anegatively willallow charge(cationic) membrane Note 2: Apositively charge(anionic) membrane fixed willallow Note 1: cells formed. ions,of themigrating theionswillbetrappedbetween thealternate toionsoftheoppositecharge)areplacedinpath permeable fixed chargedmembranes (whichareselectivelyIf alternately the electrodeofoppositecharge. andtheionstendtomigrate the solutionby thechargedparticles orotherdirectcurrentsource, through thecurrentiscarried battery electrodesareplacedinasolutionofionsandenergizedIf two by a evenly throughoutasolution. moleculesandtobediffusedintimes,attract thedipolarwater fairly Cl- charge. anelectric carry Typical oftheionsinbrackish water are they ionize;water thatis, thecompoundsdissociateintoionswhich (TDS) inbrackish water arestrongelectrolytes. When dissolved in upthe totaldissolvedMany ofthesubstanceswhichmake solids Source: U.S. Agency forInternationalDevelopment, 1980 1 , Na +1 positive but ionstopass, willrepelnegativeions. ionstopass,negative but ions. willrepelpositive , HCO 3 -1 , Mg +2 FOUR•May 1997 , SO 4 -2 , andCa +2 . These ionstendto