Advantages. Advantages. Disadvantages. Disadvantages. Membranes (reverse osmosis & electrodialysis) Advantages. Activated alumina Advantages

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2 arsenate, etc.) can be removed when polyvalent hydrolyzable metal ions are added to water. Coagulants include aluminum sulfate, lime, and iron salts; anionic and cationic polyelectrolytes are added as coagulant ions hydrolyze to form hydroxymetallic complexes, which polymerize to form the insoluble metal hydroxides. Although some anions form complexes with the polyvalent cations and coprecipitate with the metal hydroxide, removal of anions by enmeshment in and sorption by the Generally, ferric ions are less soluble than aluminum ions, and they can be used over a wider ph range. The best range for alum coagulation is ph 5.5 to 8.0, whereas precipitation of ferric ions is effective over the range of 4.0 to Actual removal efficiency depends on alkalinity, ph, and competing ion and chelant concentrations. Laboratory jar tests and pilot studies are required to determine the optimum ph and the most effective coagulant, coagulant aid, and chemical dosage. Alum and iron coagulants can move more than 90 percent of phosphate at dosages in the range of a 1-to-3 metal ion/phosphorus molar ratio. Lime reacts with bicarbonate to precipitate CaC03, and the dosage on the phosphate concentration. Enhanced removal of phosphorus is obtained using a combination of lime and iron coagulants. Enough lime is added to raise ph to 9.5 to 10.0, and an optimal FeCI3 dose (4.2mg/L as Fe) is added. Conventional coagulation using 3Omg/L doses of ferric sulfate or alum results in greater than 90 percent removal of arsenate, and filtration achieves an additional 5 percent removal. The optimal ratio of precipitant ion to arsenic ion varies from 1.5 to 4.0. Because arsenite removal is ineffective with these coagulants, precoagulation oxidation with chlorine or permanganate to arsenate is recommended. Hardness and some anionic contaminants in water can be reduced by adding lime (lime softening) or lime and soda ash (lime-soda softening). Calcium carbonate and magnesium hydroxide precipitates, which are capable of co-precipitating anions, are produced. The optimum lime and soda ash dosage can be calculated from the reaction stoichiometry and equilibrium equations or from laboratory studies that simulate plant conditions. Calcium carbonate requires a ph of 9.5 and magnesium hydroxide requires a ph of 10.8 for precipitation; softening is usually carried out above ph 11. Softening can be carried out at ambient conditions (cold process, 5 to 32 C) or at elevated temperatures (hot process, 95 to 100 C). The advantages of temperature above 50 C include lower soiubility of calcium and magnesium, faster reaction kinetics, improved setting characteristics, and better adsorption of silica and other contaminants on the Mg(OH)2 floc. Fluoride can be removed by coprecipitation with magnesium hydroxide in the lime softening process. The amount of magnesium added during softening depends on the desired amount of fluoride removal. Silica, as the silicate ion, forms magnesium silicate complex and is adsorbed by the magnesium hydroxide floc during lime softening. The removal of silica increases with temperature, and normally the warm (50 C) or hot (95 C) lime softening process is used. The optimal ph is 10.5, and a 15-minute retention time is sufficient. The removal efficiency is enhanced with iron or aluminum salts because of their adsorption properties. Hydroxide, Sulfide, and Carbonate Precipitation The most common method of reducing heavy-metal concentrations in wastewater is by hydroxide precipitation. Excess lime or caustic soda is used to precipitate metal hydroxides, which are relatively insoluble in alkaline solutions. Polymers are added to aid coagulation. Hydroxide precipitation is inexpensive, reliable, and well suited to automatic control as long as influent flow and concentration variations are not excessive. A staged precipitation process can be used for mixed-metal waste because of the ph variation of their minimum hydroxide solubilities. One disadvantage of hydroxide precipitation is that large quantities of gelatinous sludge, which is difficult to dewater, are generated. In addition, the technique is ineffective in the presence of complexing and chelating agents, and the startup and shutdown times are longer than those for packed-bed and membrane processes. Sulfide precipitation using soluble Na2S or sparingly soluble FeS can remove heavy metals effectively even in the presence of complexing and chelating agents. Because metal 0 Low cost for high volume 0 Often improved by high ionic strength 0 Reliable process well suited to osmotic control 0 Stoichiometric chemical additions required 0 High-water-content sludge must be disposed of 0 Part-per-billion effluent contaminant levels may require two-stage precipitation 0 Not readily applied to small, intermittent flows 0 Coprecipitation efficiency depends on initial contaminant concentration and surface area of primary floc Activated alumina 0 Operates on rjmaw! 0 Insensitive to flow and total dissolved solids background 0 Low effluent contaminant level possible 0 Highly selective for fluoride and arsenic Both acid and base are required for regeneration B Media tend to dissolve, producing fine particles B Slow adsorption kinetics B Spent regenerant must be disnoswi nf 0 Operates on demand 0 Relatively insensitive to flow variations 0 Essentially zero level of effluent contamination possible 0 Large variety of specific resins available 0 Beneficial selectivity reversal commonly occurs upon regeneration 0 Potential for chromatographic effluent peaking 0 Spent regenerant must be disposed of 0 Variable effluent quality with respect to background ions 0 Usually not feasible at high levels of total dissolved solids Membranes (reverse osmosis & electrodialysis) 0 All contaminant ions and most dissolved non-ions are removed 0 Relatively insensitive to flow and total dissolved solids level 0 Low effluent concentration possible 0 In reverse osmosis, bacteria and particles are removed as well 0 High capital and operating costs 0 High level of pretreatment required 0 Membranes are prone to fouling 0 Reject stream is 20-90% of feed flow The National Environmental Journal July/August

3 ~ oagulant aids adaed; lime added to raise ph minimum solub~ltty of metal hydroxtde, sulliae, or carbonate proddct ne resid-a. contam oab81dy is determine concentration and floc area for coorecioitatlon and adsorotion onto hvdrous oxides. sulfides are generally less soluble than the corresponding hydroxides, better removal efficiencies are achieved with SUIfide precipitation over a broad ph range. In addition, metal SUIfide are less amphoteric and less likely to resolubilize than metal hydroxides. Finally, sulfide sludges usually have smaller volumes and easier to dewater than hydroxide sludges. An obvious disadvantage of the process, however, is the generation of toxic H2S gas during precipitation. Furthermore, the excess sulfide ions in the treated water must be removed by aeration or oxidation with chlorine. Although technically effective, sulfide precipitation is a relatively complex and expensive process. Occasionally, carbonate precipitation with soda ash is used to remove heavy metals because of the lower operating ph and lower solubility of the metal carbonate compared with the metal hydroxide. Heavy metals, such as barium, cadmium, copper, lead, mercury, silver, nickel, and zinc, can be removed by hydroxide sulfide precipitation or a combination of the two processes. Factors affecting heavy-metal precipitation include choice of the precipitant and coagulant aid, operating temperature and ph, valence state of the metal, and the presence of complexing agents. Bench scale and pilot scale studies should be conducted to determine optimal configurations and the effects of process variables. Oxidation and Precipitation Ferrous ions in solution are readily oxidized to ferric ions by oxidizing agents such as chlorine, permanganate, and ozone. The ferric ions hydrolyze to form ferric hydroxide precipitate, which has minimum solubility at ph 8.0 to 9.5. The rate of oxidation depends on ph, buffer capacity, and organic content (organic substances tend to complex iron and react with chlorine to decrease the oxidation rate). For manganese removal, oxidation above 9.4 ph is required because of the low reactivity of manganous ions with oxygen. Breakpoint chlorination with a free-chlorine residual of 0.5 mg/l or permanganate treatment (1.5 to 2.0 mg/l KMnO, dose) also causes precipitation of manganese dioxide. The recommended method for iron and manganese removal involves ph adjustment, oxidation, and direct filtration on a single medium filter. Sufficient reaction time (5 to 30 min- I utes) is necessary for oxidation to occur and for the oxjdized species to agglomerate to a filterable size. Precipitates of hydrous iron oxide and manganese dioxide, which coat the filter medium, have good sorptive capacities. Reduction and Precipitation Hexavalent chromium can be reduced to trivalent chromium, which can then be removed by precipitation. The ph of aqueous solutions is reduced to about 2.0 with hydrochloric or sulfuric acid. The aqueous ph controls the reaction rate, which is extremely slow above a ph of 3. Reducing agents such as sulfur dioxide and sodium metabisulfite are added and lime or caustic soda is added to raise the ph and precipitate trivalent chromium. Precipitation is carried out at ph 8.5 to 9.5; chromium solubility is minimal in this range. Ferrous sulfate can be used to reduce Cr6+ to Cr3+ and pre- cipitate ferric and chromium hydroxide, as follows: r3+ + 3Fe3+ + 4H20 The process requires no ph adjustment, but it produces more sludge than sulfur dioxide or metabisulfite reduction. It has, however, been found to be more economical than sulfite reduction for the removal of chromium from ion exchange regenerant brines. Cation Exchange Processes Sodium softening is the best known of the ion exchange process. Feed water that contains Ca", Mg2+, Fez+, and other cations is passed through a bed of cation resin in the sodium form to achieve the following exchange: RNa + Ca R2Ca + 2 Na+ R represents the resin, usually a cross-linked polystyrene polymer with negatively charged exchange sites (-SO3-) present at a typical concentration of two equivalents per liter of resin. In water of low ionic strength (<0.05 M) the divalent ions (such as Ca2+) are highly preferred over the monovalent ions (such as Na+). Regeneration is accomplished using 0.5 to 2.0 M NaCl in a cocurrent or countercurrent fashion. At the higher ionic strengths used in regeneration, selectivity is reversed and monovalent ions are preferred, making it easy to drive off the calcium. If the regenerant flow through the bed is in the same direction as the exhaustion flow (cocurrent regeneration), undesirable leakage of the trace calcium remaining on the resin can take place during the next exhaustion run. When necessary, this can be minimized using countercurrent regeneration, which also is stoichiometrically more efficient than cocurrent regeneration. Most ion exchange resins are completely regenerable and can be operated for numerous cycles before being replaced. However, usehi iives can be ShGrtei-ied drastically by fcluling (accumulation of deposits on the beds). The most common foulants for cation exchanges are clays and iron; humic materials and silica typically foul anion resins. Anion Exchange Processes Using strong-base resins at ph 3 to 10 or weak-base resins at ph 4, the anion exchange reaction can be used to remove nitrate from water: RCI + NO3- --t RN03 + CI- Fortunately, all commercially available anion resins prefer nitrate to chloride, and many bed volumes of water can be treated before the resin is exhausted. Again, regeneration is accomplished by using 0.5 to 2.0 M NaCI. 54 The National Environmental Journal July/August 1993

4 Unlike softening (a divalent-monovalent exchange reaction),.no reversal of selectivity between chloride and nitrate takes place because both are monovalent ions. Thus, excessive amounts of NaCl are required for complete regeneration of nitrate-spent resins. This disadvantage has largely been overcome by resorting to partial regeneration, in which only half of the sorbed nitrate is removed using a stoichiometric amount of chloride during a cocurrent regeneration. The subsequent high-nitrate leakage is not a problem because the allowable limit in drinking water is 10 mg/l NOa as nitrogen. Activated Alumina Packed beds of activated alumina can be used to remove fluoride, arsenic, selenium, silica, and humic materials from water. The mechanism, which is one of exchange of contaminant anions for surface hydroxides of alumina, is generally called adsorption, although ligand exchange or chemisorption are more appropriate terms for the highly specific surface reactions involved. The typical activated aluminas used in water treatment are 28 to 48 mesh (0.3 to 0.6 mm diameter) mixtures of amorphous and A1203 prepared by low-temperature (300 to 600 C) dehydration of AI(OH)3. They have surface areas of 50 to 300 m2/g. Assuming that the alumina surface is hydroxylated and subject to protonation or deprotonation, the following ligand exchange reaction can be written for fluoride adsorption in acid solution (alumina exhaustion) in which = AI is the alumina surface: AI-F + HOH The equation for fluoride desorption by hydroxide (alumina ation) is -F + OH F- Activated alumina processes are sensitive to ph, and anions are best adsorbed below ph 8.2, the zero point of charge, where the alumina surface has a net charge and excess protons are available to fuel the reaction. Above the zero point of charge alumina is predominantly a cation exchanger. Activated alumina differs from anion exchange resins in that some of the ions least preferred by resins are most preferred by alumina. Activated Carbon Adsorption Granular activated carbon (GAC) adsorption has often been found useful for adsorption of traces of some inorganic contaminants, particularly radionuclides and toxic metals. GAC does not enjoy the same acceptance as an inorganic adsorbent that it has as an adsorbent for neutral organic molecules. The best-adsorbed inorganics are neutral molecules such as radon, radioactive inert gas for which GAC adsorption is the process of choice. GAC adsorption also is the technology of choice for removing neutral forms of cobalt-60 and ruthenium-106, which are passed through treatment consisting of alum coagulation, upflow clarification, filtration, cation exchange, and anion exchange. Despite their solubility and ionic character, traces of toxic metals have been removed from water in laboratory tests using GAC adsorption. Designs of activated alumina exhaustion and regeneration cycles and processes are similar to those for ion exchange resins, with some exceptions. Contaminant leakage is inherently greater with alumina, and breakthrough curves are more gradual because alumina adsorption kinetics are much slower than strong-resin kinetics. Effluent chromatographic peaking of the contaminant (fluoride, arsenic, selenium, or silica) is not exhibited during aluminum adsorption because these contaminants are the most preferred ions in the feed water. Finally, complex, two-step base-acid regenerations are required to rinse out the excess base and return the alumina to a useful form. 7HE AavnvrGROUC INC. Groundwater Treatment Complete Environmental Management Senrice CS Industrial Wastewater Treatment c3 Stormwater Pollution Prevention 0 Air Quality Management CS Solid and Hazardous Waste Management CS RCRA and CERCLA Investigations Innovative Solutions to Environmental Challenges Offices in Nashville, TN; Philadelphia, PA; Louisville, KY; Washington, D.C., St. Louis, MO; and Charleston, SC Phone (615) FAX (615) Circle 148 on card.

5 Membrane Processes Of the many membrane processes available for the separation of ions from solutions, only two, reverse osmosis (RO) and electrodialysis (ED), have reached the practical application stage for the removal of inorganic contaminants from drinking water and wastewater. Both processes remove salt from seawater and thus are commonly classified as desalination processes. Both membrane processes use semipermeable membranes to separate salts from solutions. ED was developed and became commercially available in the 1950s, about a decade before RO was practically applied for desalting. Although RO and ED are used extensively worldwide for desalting brackish water (2,000 to 5,000 mg/l total dissolved solids), RO is used almost exclusively for seawater desalting and has far surpassed ED brackish-water desalting in the United States. RO is a pressure-driven reversal of the natural process of osmosis. Water is forced from a concentrated salt solution through a semipermeable membrane into a solution of low salt concentration by the application of hydraulic pressure. System performance can be predicted by considering the significant factors that influence the capacity and salt passage of an RO membrane, including membrane composition and configuration; pressure, temperature, and concentration of the feed water; and recovery, product pressure, and time in operation. RO membranes were made almost exclusively of cellulose acetate (CA) up to a few years ago. Other polymeric materials have been developed, and two kinds of membranes are presently marketed in addition to CA membranes: aromatic polyamide, and thin-film composites of various polymers. Each have special properties for hydraulic resistance, operating ph, temperature range, chlorine tolerance, and resistance to biodegradation. Membranes are produced in two basic configurations: spiral wound and hollow fiber. Each configuration should be considered on the basis of maximum recovery, production rate per unit volume of module, and fouling properties. Membranes are commonly rated by manufacturers in terms of percentages of salt passage or rejection as determined by standard tests under specific conditions. The rejection of specific ions will vary depending on ionic charge, size, and other physicochemical factors. In general, the larger the ion, the better the rejection, and the higher the valence of an ion, the greater will be its rejection. Thus, divalent ions are always more highly rejected than monovalent ions. For some ions, notably fluoride and bicarbonate, rejection is ph-dependent because ph determines both the ionic speciation and the charge on the membrane surface. Rejection of these ions increases with increasing ph. Efforts have been made to develop membranes that have high salt rejection at low operating pressure. Originally, RO systems treating brackish waters containing 1,500 to 3,000 mg/l of total dissolved solids were designed to operate near feed pressures of 2,800 kpa (400 psi). Lower pressure membranes have appeared on the market but the higher wa?er flux a?!ower pressure sometimes results in a higher salt flux and lower quality product water. Even this situation has changed with the availability of low pressure, high-rejection membranes. Reverse osmosis has been used to treat industrial and domestic wastewater for many years. Many applications reported have been on a small or pilot scale; large-scale plants are in operation, and more are being planned. The advantage of using RO for treating wastewater is its ability to remove nearly all contaminants efficiently, but costs are relatively high and extensive pretreatment to prevent membrane deterioration and fouling is required. Continuing improvements in productivity, fouling resistance, and chemical stability of membranes, particularly the thin-film composites, should serve to enhance the future of RO for wastewater treatment. 0 Circle 149 on card. 56 July/August 1993