Alaa H. Aly Utah State University, Logan, Utah

E. J. Anthony CANMET, Ottawa, Canada

Milovan S. Beljin University of Cincinnati, Cincinnati, Ohio

Douglas M. Brown The Logistics Management Institute, Bethesda, Maryland

M. S. Chandrasekharaiah Houston Advanced Research Center, The Woodlands, Texas

Allen P. Davis University of Maryland, College Park, Maryland

Steven K. Dentel University of Delaware, Newark, Delaware

Merrit P. Drucker Army Management Staff College, Fort Belvoir, Virginia

Jeffrey C. Evans Bucknell University, Lewisburg, Pennsylvania

A. Samer Ezeldin Stevens Institute of Technology, Hoboken, New Jersey

Hsai-Yang Fang Lehigh University, Bethlehem, Pennsylvania

Ronald G. Fender Environmental Resources Management Group, Exton, Pennsylvania

Alan C. Funk Environmental Resources Management Group, Exton, Pennsylvania

John Hanna The University of Alabama, Tuscaloosa, Alabama

Christopher A. Hazen Miles Inc., New Martinsville, West Virginia

Marilyn Hewitt Environmental Resources Management Group, Exton, Pennsylvania

Richard F. Hoff Chester Environmental, Monroeville, Pennsylvania

Ahmad I. Jamrah University of Delaware, Newark, Delaware

Robert J. Jupin Chester Environmental, Monroeville, Pennsylvania

Thomas Kear OP&L, Inc., San Diego, California

Thomas R. Klos Envirovest Management, Houston, Texas

Prasad S. Kodukula Woodward-Clyde Consultants, Overland Park, Kansas

George P. Korfiatis Stevens Institute of Technology, Hoboken, New Jersey

Ronald J. Kotun Chester Environmental, Monroeville, Pennsylvania

Paul D. Kuhlmeier Consulting Environmental Engineer, Boise, Idaho Agnes Y. Lee U.S. Bureau of Mines, Rolla, Missouri

George Losonsky Eastman Christensen Environmental Systems, Houston, Texas James Lounsbury National Roundtable of State Pollution Prevention Programs, Silver Spring, Maryland

David M. Manis EET, Austin, Texas

J. L. Margrave Houston Advanced Research Center, The Woodlands, Texas

Diane McCausland Chester Environmental, Monroeville, Pennsylvania

Tom McDowell Siallon Corporation, Laguna Niguel, California

Namunu J. Meegoda New Jersey Institute of Technology, Newark, New Jersey

Alvin F. Meyer A. F. Meyer and Associates, Inc., McLean, Virginia

Patrick B. Moroney Chester Environmental, Monroeville, Pennsylvania

Vernon R. Miller U.S. Bureau of Mines, Rolla, Missouri

James I. Myers Miles Inc., New Martinsville, West Virginia

Dan Nickens Earth Resources Corporation, Ocoee, Florida

S. C. Niranjan Rice University, Houston, Texas

O. A. Ogunseitan University of California, Irvine, California

Karnig Ohannessian OP&L, Inc., San Diego, California

Osawaru A. Orumwense The University of Alabama, Tuscaloosa, Alabama

Patrick D. Owens Tosco Refining Company, Martinez, California

D. L. Patton Applied Environmental Services, Inc., San Juan Capistrano, California

Richard C. Peralta Utah State University, Logan, Utah

Anna Peteranecz OP&L, Inc., San Diego, California

Dan A. Philips Pensacola Junior College, Pensacola, Florida

F. Preto CAN MET, Ottawa, Canada

Ray Ruemekorf NIRO, Inc., Columbia, Maryland

Stella S. Schramm University of Tennessee, Knoxville, Tennessee

William E. Schramm Oak Ridge National Laboratory, Oak Ridge, Tennessee

Klaus Schwitzgebel EET, Austin, Texas

Kanti L. Shah Ohio Northern University, Ada, Ohio

S. S. Shukla* Houston Advanced Research Center, The Woodlands, Texas

Michael G. Stapleton University of Delaware, Newark, Delaware

Herminio H. Suguino Utah State University, Logan, Utah

S. M. Testa Applied Environmental Services, Inc., San Juan Capistrano, California George J. Trezek Greenfield Environmental, Carlsbad, California and University of California at Berkeley, Berkeley, California T. Viraraghavan University of Regina, Regina, Canada Donald K. Walter U.S. Department of Energy, Washington, D.C. Robert L. Ward Ohio Northern University, Ada, Ohio Ann M. Wethington U.S. Bureau of Mines, Rolla, Missouri

David S. Wilson Environmental Resources Management Group, Exton, Pennsylvania

*Current affiliation: Lamar University, Beaumont, Texas

Prevention Pollution Policy

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Engineering Issues in Pollution Control and Waste Minimization

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Process Engineering for Pollution Control and Waste Minimization

John Hanna and Osawaru A. Orumwense

The University of Alabama Tuscaloosa, Alabama


As a part of the material cycle, ores and fossil fuels are extracted from the earth, processed, and converted into metals, chemicals, and other processed (high value added) materials. Hence, any expansion in the world economy increases the demand for minerals and metals with subsequent increases in the amount of waste generated.

Wastes are generated by the mining, mineral processing, metallurgical, and chemical industries at an estimated annual rate of over 2.3 billion tons. The accumulated solid wastes at both active and inactive mining sites approach a whopping 30 billion tons [1]. These wastes include gases, dusts, sludges or solutions, ashes, and a variety of massive solid materials such as overburden, waste rocks, tailings, and slags that must be disposed of at low cost with a minimum of environmental degradation.

A large volume of the wastes is normally disposed of at locations close to either the mining sites or processing plants. Evidence of these can be found in Minnesota, Utah, Alabama, California, Tennessee, Idaho, Montana, and other states having high mining and industrial activities.

A significant amount of the tailings is disposed of in impoundments, which range in size from a few acres to large ponds covering thousands of acres. Wastes from processing plants pose the most difficult disposal and environmental problems in view of the physical and chemical properties of the wastes as well as the enormous volumes involved, and consequently a large expanse of land must be used for the disposal [2]. A typical example is the Florida phosphate slimes. The overburden and waste rocks with characteristic high contents of pyrite, heavy metals, and radioactive materials also present potential environmental and health problems.

Unfortunately, most of the mineral processing wastes have been excluded from the Resource Conservation Recovery Act of 1976 (RCRA). However, this situation is changing with the stringent environmental regulations introduced in recent years, which have necessitated that precautions be taken to both minimize and control waste disposal. Hence, the mineral or metal constituents in wastes, whether of little or no economic value, must be amended for their environmental impact or as a source of resource supply.


A. Mining and Processing Solid Wastes

Most operations in the extraction and processing cycle generate wastes (refer to Figure 1), but the extent to which a material can be classified as a waste depends on a number of factors. These include

1. Sources and volumes of waste produced

2. Potential dangers to health and the environment

3. Long-term reactivity with air and/or water and mobilization to the environment

4. Present disposal practices and alternative disposal methods

5. Cost of disposal and potential use of the waste

RAW MATERIALS EXTRACTION mining, quairying. dredging, exploration

Ores, crude oil. coal. elc.

RAW MATERIALS BENEFICIATION milling, washing, concenlrallon, upgrading

Concentrales, refined oll. gas. minerals

PROCESSING BULK RAW MATERIALS smelting, refining, processing, utilities

Ingots, pigs, chemicals, energy

MANUFACTURING AND SERVICES assembly, packing, transportation, energy dlstrib.

Goods and services

Overburden Sub-grade minerals — Slurries — — — Fines Spoils

Tailings Sands —Slurries — Dusts Solutions

Slags Smoke Fumes Muds — Drosses Solutions Residues Ash


Chemicals Dust Smoke




Clippings c i



SECONDARY MATERIALS PROCESSING smelting, refining, processing, segregation

Municipal Waste —

Fumes Dusts _ -Slags Drosses Solutions Pulp Smokes

Figure 1 Mineral waste materials supply, utilization, and disposal system.

The following are typical examples of the waste generated by mining and related processing industries.

It has been reported that in the production of about 1.6 million tons of copper in 1976, 1 billion tons of materials were processed. This breaks down to 684 million tons of overburden, 264 million tons of tailings, 5 million tons of slag, 3.3 million tons of sulfur dioxide, and about 100 billion gallons of process water [3],

The iron industry is one source of enormous amounts of waste, since most of the iron concentrates used in the manufacture of iron and steel are derived from relatively low grade ores. Typically, raw ores assaying 25-33% Fe are mined and beneficiated to produce high quality pellets assaying 60-65% Fe and 5% Si for the manufacture of iron and steel. About 330 million long tons of iron ore was mined in the United States in 1976, and the amount of wastes generated was about 200 million tons [4], excluding the slag and dust wastes from the steel-making step.

Conventional magnetic and gravity separation processing of magnetic and nonmagnetic taconites of the Lake Superior Region resulted in substantial iron losses of about 20-30% in the tailing products [5]. On the other hand, the more advanced beneficiation of the tailings from techniques such as flocculation and flotation processes reduced the iron lost in the rejects to as low as 10%. The loss is partly due to the mineralogical composition of the ores and the grain size of both iron and gangue. High iron losses are observed for Birmingham red hematite ore, for instance, because it produces more slimes than taconite ores. This is one of the factors responsible for the relatively poor recovery of iron from run-of-mine material and the generation of large tonnages of wastes, particularly in large-scale beneficiation operations.

The Florida phosphate industry is another source of a tremendous volume of wastes. In the production of phosphate, the soft minerals in the matrix, particularly clays and the very fine phosphate aggregates, are dispersed readily in water, forming slimes during the hydraulic mining, transportation, and separation steps. These slimes are difficult to recover, and in addition they impair the beneficiation operation. About one-third of the phosphate content of the matrix is lost in the slimes, which are generally discarded as wastes. The Florida phosphate slimes are characterized by very slow settling and trap a high volume of water. Currently, impounded slimes are stored behind earth dams and pose a serious threat to the environment. The reclamation of the land and enormous volumes of water are important for resource conservation and in order to comply with stringent environmental regulations. The recovery of the phosphate values discarded in the slime and tailing fractions containing about 30%-40% of the phosphate present in the mined matrix would enhance the economy of the phosphate industry and expand the available resources. The impact of this on reducing potential environmental hazards is enormous. The phosphate losses at the current rate of rock production of about 40 million tons per year includes over 11 million tons of high grade phosphate that is lost in the slimes annually. Vasan [6] has estimated that about 1.5 billion tons of phosphate slimes is accumulated over the years in dams together with about 4.5 billion tons of water.

The coal mining industry is another source of a large volume of solid wastes. The methods used in the past for cleaning coal were highly inefficient and resulted in high coal losses in waste streams during the mining and washing operations [2]. The washer waste fines are normally stored in above-ground impoundments. Quite a number of processing plants still indulge in the practice of discarding coal fines. As a result, about 25% of the coal mined is disposed of as waste. Based on the current rate of coal production of about 1 billion tons, about 250 million tons of wastes is produced annually, and out of this, about 200 million tons is coarse particles and 50 million tons is fines. The amount of coal in coarse waste particles is more than 30 million tons of carbon per year, while the corresponding amount in the fine fraction is about 30 million tons on an annual basis. The disposal of coarse waste particles is not a serious problem in most coal preparation plants, as they are used as landfill. The fine-size wastes, on the other hand, are a problem because of the difficulties experienced in dewatering and the characteristic relatively low structural strength of fine particles, which prevent fines from being used as landfill [7],

B. Mining and Processing Liquid Wastes

Effluents from coal preparation plants and drainage from waste disposal sites have a characteristic dark color and have high concentrations of suspended particles that cause not only silt-ation due to the settling of coarse particles, but also water pollution, both of which have negative effects on aquatic lives [2,7]. Effluents from coal cleaning plants and mines are also reputed to have a great impact on the environment through the phenomenon known as acid mine drainage (AMD). This is one of the causes of the destruction of forests and vegetation today. Acid drainage is reported to be caused by the reaction between oxygen, water, and iron sulfides such as pyrite and marcasite. Microorganisms are known to enhance the rate of this reaction. The most common techniques for mitigating acid drainage are neutralization using either lime, limestone, soda ash, or caustic soda; reverse osmosis; and treatment involving silicates [7], These techniques are discussed in detail later. The highly acidic solutions produced dissolve several heavy metals in the waste piles or impounded material and become loaded with a host of environmentally undesirable heavy metal species, sulfates, and other anions.

On the other hand, the water discharged from some mines contains valuable metals such as copper and uranium that could be recovered economically. Copper is usually extracted from such discharges by either cementation or liquid ion exchange. Mine drainage containing 9-12 ppm U208 is stripped by ion exchange as exemplified by the operation in the Ambrosia Lake district [8]. The acid mine drainage containing 300-600 ppm A1203 and 10-20 ppm U2Og, on the other hand, is stripped by a combination of ion and liquid ion exchanges [9],

C. Coal Utilization Wastes

As a result of burning coal in boilers and electric power plants, a large quantity of ash is produced. The amount of ash generated by power plants in 1977 is estimated to have been about 67.8 million tons, of which 48.5 million tons was fly ash, 14.1 million tons bottom ash, and the remainder boiler slag. During that year, about 6.3 million tons of the fly ash, 4.6 million tons of the bottom ash, and 3.1 million tons of the boiler slag, or approximately 21% of the total ash generated, was recycled in such products as concrete blocks, asphalt, and roofing materials [10-12],

D. Metallurgical Wastes

The production of alumina by the Bayer process each year is accompanied by simultaneous formation of about 7 million tons of red mud that consists of a substantial amount of valuable minerals and dissolved salts. These wastes are estimated to contain a large amount of caustic soda, 1.2 million tons of alumina, 1.7 million tons of iron, and about 450,000 tons of titania [13], These pose severe environmental and health hazards.

In steelmaking, over 2 million tons of dust and gases is generated by electric and basic oxygen furnaces annually. The dust contains a substantial quantity of lead (0.4-2.6%), zinc (6.3-24.8%), manganese (0.5-5.3%), and copper (0.03-0.27%) in addition to iron [14],

Similarly, in manufacturing stainless steel, a large amount of metals is lost as wastes. Powell et al. [15] estimated that approximately 5 million pounds of molybdenum is lost in stainless steel furnace dust each year.

Process Engineering for Pollution Control Table 1 Characterization of Foundry Dust

Toxicity (mg Pb/L)

Sample location




New Hampshire



West Virginia

530 440 764 188 460

A large amount of dust is produced by brass and bronze foundries and secondary smelters annually in the United States. The baghouse dusts vary in composition, but the main constituents are zinc (40-78%), copper (10-15%), and small amounts of lead and tin. Most of the zinc is present at ZnO, while the remainder is in the form of brass or bronze alloys. A typical characterization of dust from some foundries is presented in Table 1. These wastes are considered to be hazardous because of the high lead contents.

During the production of elemental phosphorus using an electric furnace, a large amount of toxic wastes such as sludge, slag, gases, and phossy water are also generated. It is known that between 5 and 10% of the elemental phosphorus that is produced is left behind in the sludge. The composition of the other solid constituents of the sludge is 40-60% Si02, 5-15% CaO, 2-4% Fe203, and 2-5% P205 [16]. In general, the ratio of phossy water and sludge that are formed to the amount of elemental phosphorus produced is about 5:1. Phosphorus wastes pose both environmental and fire hazards, and these wastes are produced at a rate of 1.5 million tons annually.


A number of measures are taken to minimize or render bulk solid wastes safe for disposal. These include the extraction of heavy metals or toxic constituents from the waste materials using either physical, chemical, or bioremediation techniques. On the other hand, some wastes are either recycled or used directly, but more often a combination of these techniques is applied to achieve maximum process efficiency. The following methods are classified according to the source of the solid wastes.

A. Copper Mine Wastes

Copper mine wastes are increasingly important because of the very low grade of most available copper ores. Rule and Siemens [17] have shown that the bulk flotation method is effective in extracting such metal values as copper, cobalt, and nickel from copper mine wastes with recoveries in the range of 54-95%. The primary problem in using the flotation method for this purpose is the intimate association of the valuable minerals or metals (minor) with the predominant gangue materials. Consequently, a high degree of fineness is necessary in order to ensure liberation and subsequent separation of the metal values. However, reagent consumption is also expected to be high. In most instances, the residues still contain fairly high levels of valuable minerals or heavy metals and as a result must be subjected to further treatment. Pressure leaching or bacterial leaching (bioleaching) is often used for this purpose.

B. Iron Ore Wastes

In the past, many iron tailing ponds were subjected to gravity concentration [18] to recover the iron contents. Jones and Laughlin Steel Corporation in Calmet, Minnesota, is an example of a company that at one time combined flotation and gravity concentration for treating iron wastes to recover the metal values. The presence of a large amount of slimes and the high impurity contents of either the initial ores or the wastes impaired the recovery of iron from the wastes.

In contrast, selective flocculation and high-intensity and high-gradient magnetic separations [19] are some of the other techniques that can be used effectively to treat such materials. Waste materials can also be subjected to reductive roasting and magnetic separation to reduce the energy required for processing.

C. Phosphate Rock Wastes

Phosphate slimes are known to be not only difficult to recover but also economically unsound. However, the associated adverse environmental impact necessitates treatment. Laboratory tests on waste pond materials, low-grade washer products, and some raw Tennessee phosphate ores have shown that some of the phosphate can be recovered. Market grade phosphate concentrates assaying 60-82% P2Os can be obtained in substantial amounts using the anionic flotation method [20].

Direct digestion of the phosphate matrix with sulfuric acid is an alternative approach for the minimization of slime disposal problems. This process produces a simple waste consisting of gelatinous slime, sand, and gypsum. The composite is a compact sandy cake that could be used as a filling material in mined-out areas while about 95% of the P205 is recovered as useful material [21,22].

D. Fine Coal Wastes

Two major techniques have been proposed for treating coal wastes. These are gravity separation and flotation [23,24]. The use of Humphreys spirals to treat coal wastes has been established. Although such treatments are capable of yielding high-grade coal concentrates, the recovery is relatively low. Also these techniques are only applicable to feeds with particle size coarser than 200 mesh. Besides, a substantial amount of the coal is lost in the tailings—about 10-71% [24]. Therefore, techniques that are suitable for fine particles processing are required to supplement the spirals in order to improve coal recovery. This has led to the development of a process that is based on a combination of gravity separation and froth flotation.

In this process, Humphreys spirals are used to recover the coarse coal particles while column flotation is employed for the minus 200 mesh size fractions [24], Mechanical flotation can also be used in place of spirals to separate the coarse particles. In this manner, both the quality and the recovery of coal are improved significantly.

Similarly, the pyrite present in the wastes can be removed, and by doing so, acid drainage problems can also be mitigated. It is also possible to employ a bioleaching technique to eliminate the pyrite constituents from coal wastes. This can be achieved by allowing bacteria to oxidize the pyrite in coal wastes as feed.

E. Phosphorus Wastes

The methods of treating phosphorus waste include physical, chemical, and bioleaching techniques. The physical methods include sizing, sedimentation, centrifugation, cycloning, and flotation [25-27, 31], while air oxidation, chlorine oxidation, electrolytic oxidation, catalytic oxidation, distillation, CS2 extraction [28-33], and ion exchange constitute the chemical methods. Most of these processes either partially separate or oxidize phosphorus from the impurities. Therefore, a combination of two treatment techniques is necessary for complete remediation of phosphorus wastes. Another factor necessitating this methodology is the associated low operating costs for such schemes.

A combination of clarification and chlorination techniques has been developed for extracting elemental phosphorus from phossy water [26]. However, the associated residual chlorine has an adverse environmental impact, and this renders the technique impractical. The ERCO process is based on the use of nascent oxygen to oxidize elemental phosphorus prior to subsequent separation [33], Another method uses distillation as the basis for the remediation of phosphorus from sludges [29]. The high operating costs associated with these methods have limited their application.

In many cases a major part of the phosphorus wastes are present in the coarse particles. Anazia et al. [31] have shown that between 26 and 29% by weight of the particles in the tested sludge samples (obtained from FMC Corporation of Pocatello, Idaho, and the TVA at Muscle Shoals, Alabama) are coarse phosphorus particles containing 82-91% P4. It was also demonstrated in the same study that about 61-88% of the coarse phosphorus particles can be recovered by screening. The fine fractions represent 71-74% by weight of the sludge and assay 5-21% P4. The as-received unsized sludge can also be subjected to flotation to separate phosphorus concentrates assaying between 61 and 78%. P4 with a recovery in the range of 71-79% depending on the characteristics of the wastes. The tailings assayed between 9 and 18% P4 and constitute about 59-68% of the sludge [31]. It is obvious, therefore, that the fine fractions or tailings must likewise be subjected to further treatment using other methods.

The phosphorus remaining in the physical separation rejects can be extracted after air oxidation treatment at ambient temperatures. These form the basis for the proposed two-step method comprising either flotation or screening and conventional air oxidation for the treatment of phosphorus sludges [31].

However, the P4 concentrations in the refuse from the oxidation step can be as much as 4%, which is still high in terms of toxicity. A long air oxidation period of several days or weeks may be necessary to achieve 90-95% conversion of P4 to H3P04 at an ambient temperature of 30°C. Under these conditions the oxidation rate of P4 in water is slow and is influenced by many factors such as pH, oxygen content, temperature, presence of metal ions, and degree of dispersion of colloidal material [34], Therefore, an incomplete conversion of P4 to oxyphos-phorus compounds occurs during the conventional oxidation process because the reaction kinetics appear to be influenced by other factors such as agitation, particle size, and surface coating [34],

This process has been further developed at the University of Alabama such that the oxidation and conversion of P4 to soluble oxyphosphorus compounds are enhanced significantly [32, 35]. In the new process, the insoluble P4 is converted to highly soluble and nontoxic compounds that are easy to extract from the rest of the sludge. This improvement has been achieved by employing a novel reactor design known as HSAD to expedite the remediation operation. Thus, depending on the P4 content of a sludge, an almost complete oxidation of phosphorus is achieved in about 1-3 hr, and the resultant acid solution can be employed in the manufacture of either phosphoric acid or fertilizer by-product by neutralization. The chemically inactive solid waste can be dried and safely disposed of as nonhazardous landfill product. Some of the results obtained employing this process are given in Table 2.

The advantages of the HSAD technique include short processing duration, high efficiency, simple configuration, low cost, and applicability to various phosphorus wastes. The process requires no catalysts, chemical oxidants, or high temperatures [35].

Table 2 Typical Results of HSAD West Oxidation of Phosphorus Sludge [32]

Test No.


Weight (g)

Weight (%)

P4 Analysis (%)

P4 Removal (%)

































"Equivalent P4 analysis of oxyphosphorus compounds.

"Equivalent P4 analysis of oxyphosphorus compounds.

F. Brass and Bronze Foundries Dust

Baghouse zinc dust is processed by using sulfuric acid leaching and electrolysis or crystallization to recover zinc and other metals [36,37]. The zinc extraction attainable with this method is in the range of 89-99%. Basically, the method involves the use of strong sulfuric acid and intense aeration to dissolve the zinc oxide and metallic copper from the dust. The lead, tin, and zinc metal alloys present in the dust are not dissolved by sulfuric acid and remain with the solid residues. The leach residues, which account for 20-50% by weight of the dust and are rich in a number of metals, can be further treated to extract the metallic components [36], The pregnant leach liquor is subjected to electrowinning to produce metallic zinc. However, the zinc electrowinning operation is adversely affected by the presence of chloride ions or some metals. Hence, additional measures are required to eliminate chloride ions and other impurities in order to produce high-grade metallic zinc. This can render the whole process expensive. Alternatively, the crystallization technique is employed to recover the zinc from the leach liquor as zinc sulfate salt.


The most common method of recovering the metal values from low-grade sources such as waste dumps or heaps is leaching. Leaching is a process in which a solid material is contacted with a solvent in order to selectively dissolve some of the components. The objectives of leaching metals from sludge include the dissolution of the metal values for recycling or subsequent separation by other methods, to render wastes nonhazardous, or to render wastes amenable to further treatment. Leaching is known to account for about 10% of the yearly copper production.

The commonly used leaching agents are sulfuric acid, hydrochloric acid, ferric chloride, nitric acid, ferric sulfate, ammonia or ammonium carbonate, hydroxide, and microorganisms such as bacteria, yeast, and fungi.

Unfortunately, many factors concerning leaching such as the size and height of dumps and factors affecting solution percolation and the kinetics and recovery of the valuable metals from the leach of pregnant liquors in general still require detailed studies and information dissemination. The fact that many of the minerals in wastes can be recovered inexpensively by leaching implies that some of the problems associated with the disposal of fine wastes can be alleviated.

Biological remediation of wastes is accomplished by using naturally occurring microorganisms such as bacteria, yeast, and fungi to treat contaminants. Its use is rapidly increasing. However, the microorganisms require a wide range of macro and micro nutrients for their met abolic activities and growth. The environment is generally poor in the nutrients such as nitrogen, phosphorus, and carbon required by the microorganisms for sustenance, and some contaminants exhibit a certain degree of resistance to different microorganisms. These are the primary cause of the slow rate of breakdown of contaminants. Therefore, a successful bioleach-ing operation requires the growth of appropriate microorganisms that can be induced by manipulating conditions such as the availability of nutrients, temperature, electron acceptors, and aeration.

B. Precipitation

Precipitation is one of the common means of remediating wastewater. In this method, chemicals are used to alter the physical state of dissolved or suspended metals and to enhance subsequent separation using sedimentation techniques. Chemicals such as caustic soda, lime, soda ash, sodium borohydroxide, sodium phosphate, ferrous sulfide, and sodium sulfides are used to induce precipitation. It is sometimes necessary to subject wastewater to some form of pre-treatment such as filtration, destruction or organic matter and cyanides, metal reduction, neutralization, and/or oil separation prior to precipitation.

Some metals as typified by hexavalent chromium are difficult to precipitate in the form in which they occur and must be reduced if the operation is to be successful. Reducing agents commonly used include sulfur dioxide, sodium bisulfite, sodium metabisulfite, and ferrous sulfate. Similarly, to effect sedimentation and subsequent separation of precipitates, flocculants are sometimes required. Lime, alum, and polyelectrolytes are used for this purpose. The major characteristics of wastes that have an impact on precipitation operations are the type and concentration of metals, amount of total dissolved solids, concentration of residual complexing agents, and amount of oil and grease present in the wastes.

Metal-laden wastewater resulting from electroplating, pigment manufacture, the photographic industry, battery manufacture, and nonferrous metal industries are usually subjected to precipitation treatment.

C. Ion Exchange

In the ion-exchange process, metal ions in a dilute solution are substituted for identically charged ions electrostatically bonded to the surface of an immobile solid medium. The solid medium can be either a naturally occurring inorganic zeolite or a synthetic organic resin. Ion exchange is a reversible chemical reaction. Therefore, the loaded resin or exchange medium is placed in a pure solution of appropriate pH and the trapped metal ions are released. This method is applicable only to liquid wastes or pregnant solutions.

The performance of the process depends on (1) the concentration and valence of the metal constituents, (2) the presence of competing ion species, and (3) the presence of dissolved or suspended solids and organic compounds. Therefore, the feed to an ion-exchange system must be subjected to pretreatment. This method results in about 95% metal recovery and high purity products. This method is fully developed and is used commercially to remove chromium, copper, nickel, cadmium, silver, and zinc from wastewater.

D. Electrolytic Remediation

Electrolytic cell is the primary device used in electrolytic remediation. It consists of an anode and a cathode immersed in an electrolyte. When an electric current is applied to an electrolyte solution, the dissolved metals are reduced and subsequently deposited at the cathode. The electrolytic remediation technique is also known as electrowinning because the metals recovered are of high purity. This is one of the most effective methods for remediating chelated metals, which are difficult to retrieve by other techniques. This method has the advantage of producing metal-laden free sludge, but it is limited to solutions containing a few types of elements.

Electrolysis can be used to remediate cadmium, chromium, copper, lead, tin, and zinc. However, such treatments involve a high energy expenditure. Wastes containing copper and certain other elements must be leached with hydroxides before being subjected to electrolytic treatment.

A variation of the electrolytic technique known as electrodialysis is obtained when a membrane is placed between the anode and cathode such that the mobility of some ions through the membrane is obstructed. Electrodialysis can be used for remediating wastes from such sources as gold-, chromium-, silver-, zinc-, nickel-, and tin-plating operations where the ion concentration is low and would not be economical for electrowinning. Most feeds for electrodialysis treatment must be filtered to remove suspended solids. Besides, pH control is a compulsory pretreatment measure because of the effect on metal separation. When electrodes having a high surface area are employed, metals removal of about 98% can be achieved.

E. Membrane Separation

The membrane separation method encompasses such techniques as filtration (microfiltration, ultrafiltration, etc.), reverse osmosis, and electrodialysis. The filtration technique is used after the sludge has been pretreated for the removal of metals. The technique is also used to pretreat feeds destined for subsequent treatment by both reverse osmosis and electrodialysis.

Reverse osmosis and electrodialysis are used to retrieve metals or plating compounds from wastewater. The electrodialysis method is described in the preceding subsection. Reverse osmosis (RO) systems are characterized by having a number of modular units connected either in parallel or series or a combination of the two. The application of this method to the remediation of metal-laden sludge is limited by the pH range in which the membrane can be used. Cellulose acetate membranes are not suitable for use at pH above 7, while amide and polysulfone membranes can be used in the pH range between 1 and 12. The performance of RO systems is impaired by the presence of colloidal matter, dissolved organics, and insoluble constituents. It is recommended that the feeds to RO systems be subjected to such pretreatments as pH adjustment, carbon adsorption, and filtration. The method is used commercially to remove brass, chromium, copper, nickel, and zinc from metal-finishing wastes.

These techniques can be used to produce effluents with very low metal constituents, provided, of course, that adequate pretreatments have been carried out. Metal removal on the order of 99% can be achieved by making use of a combination of precipitation and filtration.

F. Evaporation

Evaporation is a simple method for remediation of mixed materials based on the difference in volatility. Hence, the concentration of metals is brought about by the reduction in the volume of the waste. The primary instrumentation used for this purpose includes rising film, flash, and submerged tube evaporators. Cadmium, chromium, nickel, zinc, copper, and silver from plating baths are retrieved in the electroplating industry by using this method. However, this method of remediating wastes is cost-effective only when a very small volume of waste is involved.

G. Encapsulation

Soluble silicates and their derivatives are very effective for the stabilization and fixation of hazardous wastes. Silicates are used in waste treatment because of their inherent characteristics such as alkaline nature (pH 20-14), ability to form gels, and reactivity with multivalent cations, and because their disposal poses no potential danger to the environment.

Soluble silicates are polymeric and condense on aging to form anions having a silicon-oxygen-silicon linkage that are complex and exist in various chain lengths and cyclic structures. Silicates react with metal ions to form insoluble amorphous metal silicates. These metal silicate complexes are insoluble over a large pH range compared with simple metal hydroxides. This is responsible for the increased resistance to leaching of metals in solidified wastes and is perhaps the main feature of silicates in waste treatment. Soluble silicates are made by fusing sodium carbonate or potassium carbonate and sand in a furnace at 1450°F. The resultant nSi02Na20 compound has silica (Si02) to alkalinity (Na20) ratio in the range of 1.6:3.9. The Si02Na20 ratio has great significance in subsequent use of silica because only compounds having high ratios are employed in the manufacture of products such as gels, precipitated silica, and zeolites and in the treatment of wastes.

Setting agents commonly used in waste treatment include Portland cements, pozzolanic fly ashes, and cement or lime kiln dust. The active components in setting agents are such derivatives as the mono-, di-, and tricalcium silicates formed when the agent is mixed with water. The physical properties and behavior of setting agents are strongly influenced by the calcium silicate content, as this is directly related to the number and strength of the resultant bonds formed. Silicates also reduce the permeability by reducing calcium hydroxide inclusion formation or the presence of voids in the structure of the material. The treatment of hazardous waste with setting agents can be subdivided into two categories, stabilization and fixation.

Stabilization is a chemical process of transforming a liquid waste into a solid. The setting agents are mixed with the waste, and when they "set up" or harden, the waste material is entrapped in the structure. The procedure used in the stabilization operation involves premixing the waste and setting agents before introducing soluble silicate. The role of silicates in the stabilization process includes the reduction of setting time, decreasing of the permeability, increasing of the compressive strength, and reduction of both the amount of setting agents employed and the volume of the treated waste.

Fixation, on the other hand, is similar to stabilization in many respects, but rather than merely entrapping the wastes as inclusions, the wastes are modified and bonded into a cementlike matrix. Hence the solubility or leachability of hazardous components is reduced dramatically. In this manner, the toxicity and mobility of heavy metal wastes are changed by the treatment. The treatment steps involve mixing the waste with cement or kiln dust as a setting agent and water. Thereafter, a soluble silicate is introduced and mixed thoroughly. The procedure is recommended if a good result must be obtained, and cement must be used when a waste is to be fixed. Portland cement is the most effective setting agent that can be used with silicates for this purpose. The reason is that during hydration cement produces gels that help to encapsulate waste. Lime-based materials do not produce a large amount of gels during hydration, so the amount of bonded wastes is reduced. Therefore, lime-based setting agents should not be used for waste fixation.


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