A NATIONAL DRINKING WATER CLEARINGHOUSE FACT SHEET Ion Exchange and DeSummarymineralization Ion exchange and membrane processes are becoming used extensively in water and wastewater treatment. Ion exchange is primarily used for the removal of hardness ions, such as magnesium and calcium, and for water demineralization. Reverse osmosis (RO) and electrodialysis, both membrane processes, remove dissolved solids from water using membranes. Ion Exchange and Demineralization○○○○○○○○○ Are Becoming Widely Used Ion exchange units can be used to remove any systems where there is a high degree of sea- charged (ionic) substance from water, but are sonal fluctuation in water demand. usually used to remove hardness and nitrate from groundwater. Electrodialysis is a process that also uses membranes. However, in electrodialysis, direct Water is pretreated to reduce the suspended electrical current is used to attract ions to one solids and total dissolved solids (TDS) load to ○○○○○○ side of the treatment chamber. Electrodialysis the ion-exchange unit. Methods of pretreatment systems include a source of pressurized water, include: a direct current power supply, and a pair of • filtration, selective membranes. • coagulation and filtration, • cold lime with or without soda ash, Figure 1. Ion exchange treatment system • hot lime with or Salt Loading without soda ash, • evaporation or Brine distillation, Overflow Tank • electrodialysis, Brine • RO, • continuous deion- Raw Water ization, • ultrafiltration, • degasification, or Potable • combinations of Water the above. to Brine Vessel 1 Vessel 2 Vessel 3 (Source: Owens, System Blending Valves 1995) Treated Water RO systems are Pressure Tank compact, simple to Backwash OF FOUR PAGE operate, and require Nitrate Waste to minimal labor, mak- Analysis Disposal ing them suitable for Alarm and one small systems. They Distribution Shutdown are also suitable for System Conductivity Monitor Well Supply Source: U.S. Environmental Protection Agency, 1989 NATIONAL DRINKING WATER CLEARINGHOUSE Ion Exchange/Demineralization Systems: ○○○○○○○○○○○○○○○ Ion exchange contaminant concentration, and the desired effluent Ion exchange effectively removes more than 90 percent quality. of barium, cadmium, chromium (III), silver, radium, nitrites, selenium, arsenic (V), chromium (VI), and nitrate. EQUIPMENT Ion exchange is usually the best choice for small systems Typical ion exchange units consist of prefiltration, ion that need to remove radionuclides. exchange, disinfection, storage, and distribution ele- ments. (See figure 1 on page 1.) ADVANTAGES • Ion exchange process, like reverse osmosis, can CHEMICALS be used with fluctuating flow rates. Sodium chloride is often used to regenerate the exchange • Effluent contamination is virtually impossible. medium in ion exchangers because of the low cost of the ○○○○○○○○ • Large variety of specific resins are available from chemical. However, this can result in a high sodium suppliers. Each resin is effective in removing residual in the finished water, which may be unacceptable specific contaminants. for individuals with salt restricted diets. This problem can be avoided by using other regenerant materials, such as LIMITATIONS potassium chloride. • Ion exchange waste is highly concentrated and requires careful disposal. Figure 2. Schematic of a reverse osmosis system • Potential for unacceptable levels (peaks) of contamination in effluent. Generator Set • Usually not feasible with high levels of TDS. • Pretreatment required for most surface waters. • Ion exchange units also are High Service Pumps sensitive to the presence of Brine to competing ions. For example, Percolation influent with high levels of hardness Pond will compete with other cations Permeators (positive ions) for space on the exchange medium, and the exchange medium must be Product to Degasifier regenerated more frequently. Sodium and Reservoir Control Panel Silicate Feed and Pumps PROCESS PO Feed 4 Acid Feed Inorganics removal is accomplished Filter Mixing through adsorption of contaminant Cleaning Feed Tank ions onto a resin exchange medium. Solution As the name implies, one ion is Source: U.S. Environmental Protection Agency, 1989 ○○○○○○○○ substituted for another on the charged surface of the medium, which is usually a synthetic plastic resin. This Reverse osmosis (RO) resin surface is designed as either cationic or anionic RO can effectively remove nearly all inorganic contami- (negatively charged). The exchange medium is saturated nants from water. It removes more than 70 percent of with the exchangeable ion before treatment operations. arsenic (III), arsenic (IV), barium, cadmium, chromium (III), chromium (VI), fluoride, lead, mercury, nitrite, During ion exchange, the contaminant ions replace the ○○○○○○○○○○○○○○○○○○○○○○ selenium (IV), selenium (VI), and silver. Properly regenerant ions because they are preferred by the operated units will attain 96 percent removal rates. RO exchange medium. When there are no ions left to replace can also effectively remove radium, natural organic the contaminant ions, the medium is regenerated with a substances, pesticides, and microbiological contami- suitable solution, which resaturates the medium with the nants. RO is particularly effective when used in series. appropriate ions. Because of the required “down time,” Water passing through multiple units can achieve near the shortest economical regeneration cycles are once zero effluent contaminant concentrations. per day. ADVANTAGES The resin exchange capacity is expressed in terms of • Removes nearly all contaminant ions and most weight per unit volume of the resin. The calculation of the dissolved non-ions. breakthrough time for an ion exchange unit requires • Relatively insensitive to flow and TDS level, and thus two knowledge of the resin exchange capacity, the influent suitable for small systems with a high degree of PAGE OF FOUR PAGE PAGE OF FOUR PAGE seasonal fluctuation in water demand. FOUR • May 1997 ○○○○○○○○ • RO operates immediately, without any minimum Electrodialysis break-in period. Electodialysis is very effective in removing fluoride and • Low effluent concentration possible. nitrate, and can also remove barium, cadmium, and • Bacteria and particles are also removed. selenium. (See figure 3 below.) • Operational simplicity and automation allow for less ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ operator attention and make RO suitable for small ADVANTAGES system applications. • All contaminant ions and most dissolved non-ions are removed. LIMITATIONS • Relatively insensitive to flow and TDS level. • High capital and operating costs. • Low effluent concentration possible. • Managing the wastewater (brine solution) is a potential problem. LIMITATIONS • High level of pretreatment is required in some cases. • High capital and operating costs. • Membranes are prone to fouling. • High level of pretreatment required. • Reject stream is 20–90 percent of feed flow. PROCESS • Electrodes require replacement. RO removes contaminants from water using a semiper- meable membrane that permits only water, and not PROCESS dissolved ions (such as sodium and chloride), to pass The membranes adjacent to the influent stream are through its pores. Contaminated water is subject to a charged either positively or negatively, and this charge high pressure that forces pure water through the attracts counter-ions toward the membrane. The mem- membrane, leaving contaminants behind in a brine branes are designed to allow either positively or nega- solution. Membranes are available with a variety of pore tively charged ions to pass through the membrane, thus sizes and characteristics. ions move from the product water stream through a membrane to the two reject water streams. (See figure 4 EQUIPMENT on page 4.) Typical RO units include raw water pumps, pretreatment, membranes, disinfection, storage, and distribution EQUIPMENT elements. (See figure 2 on page 2.) These units are able The three essential elements of the system are (1) a to process virtually any desired quantity or quality of source of pressurized water, (2) a direct current power water by configuring units sequentially to reprocess supply, and (3) a pair of selective membranes. The waste brine from the earlier stages of the process. The average ion removal varies from 25 to 60 percent per principal design considerations for reverse osmosis stage. Multistage units can increase the efficiency of units are: removal. Many membrane pairs are “stacked” in the • operating pressure, treatment vessel. • membrane type and pore size, • pretreatment requirements, and • product conversion rate (the ratio of the influent CHEMICALS Fouling of membranes may limit the amount of water recovered as waste brine water tothe finished water). treated. Fouling is caused when membrane pores are clogged by salt precipitation or by physical obstruction of Figure 3. Basic components of an suspended particulates. electrodialysis unit Particulates, suspended in Membrane water, can be removed in Stack Product Water pretreatment but salts that exceed their solubility Raw Electrode product at the membrane Feedwater Cartridge surface must be controlled Filters chemically by pH reduction (to reduce carbonate Holding concentration) or chelation Tank of metal ions (by use of phosphate, for example). A Special Treatment Low- reversal of the charge on the Pressure Electrode (If Required) membranes, a process Circulation Pump called electrodialysis PAGE OF FOUR PAGE reversal (EDR), helps to OF FOUR