Specific Industrial Waste Treatments Specific Industrial Waste Treatments Chemical IndustryChemical Industry

IntroductionIntroduction

The chemical industry comprises the companies that produce industrial The chemical industry comprises the companies that produce industrial chemicals. It is central to modern world economy, converting raw materials chemicals. It is central to modern world economy, converting raw materials (oil, natural gas, air, water, metals, minerals) into more than 70,000 different (oil, natural gas, air, water, metals, minerals) into more than 70,000 different products.products.

The chemical industry involves the use of chemical processes such as The chemical industry involves the use of chemical processes such as chemical reaction and refining methods to produce a wide variety of solid, chemical reaction and refining methods to produce a wide variety of solid, liquid, and gaseous materials. liquid, and gaseous materials.

Most of these products are used in manufacture of other items, although a Most of these products are used in manufacture of other items, although a smaller number are used directly by consumers. Solvents, pesticides, dye, smaller number are used directly by consumers. Solvents, pesticides, dye, washing soda, and portland cement are a few examples of product used by washing soda, and portland cement are a few examples of product used by consumers. consumers.

The industry includes manufacturers of inorganic and organic industrial The industry includes manufacturers of inorganic and organic industrial chemicals, ceramic products, petrochemicals, agrochemicals, polymers and chemicals, ceramic products, petrochemicals, agrochemicals, polymers and rubber, oleochemicals (oils, fats, and waxes), explosives, fragrances rubber, oleochemicals (oils, fats, and waxes), explosives, fragrances flavors and pharmaceutical industry.Other closely related industries include flavors and pharmaceutical industry.Other closely related industries include petroleum, glass, paint, ink, sealant, adhesive, and food processing petroleum, glass, paint, ink, sealant, adhesive, and food processing manufacturers.manufacturers.

The chemical industry comprises the companies that produce industrial The chemical industry comprises the companies that produce industrial chemicals. It is central to modern world economy, converting raw materials chemicals. It is central to modern world economy, converting raw materials (oil, natural gas, air, water, metals, minerals) into more than 70,000 different (oil, natural gas, air, water, metals, minerals) into more than 70,000 different products.products.

The chemical industry involves the use of chemical processes such as The chemical industry involves the use of chemical processes such as chemical reaction and refining methods to produce a wide variety of solid, chemical reaction and refining methods to produce a wide variety of solid, liquid, and gaseous materials. liquid, and gaseous materials.

Most of these products are used in manufacture of other items, although a Most of these products are used in manufacture of other items, although a smaller number are used directly by consumers. Solvents, pesticides, dye, smaller number are used directly by consumers. Solvents, pesticides, dye, washing soda, and portland cement are a few examples of product used by washing soda, and portland cement are a few examples of product used by consumers. consumers.

The industry includes manufacturers of inorganic and organic industrial The industry includes manufacturers of inorganic and organic industrial chemicals, ceramic products, petrochemicals, agrochemicals, polymers and chemicals, ceramic products, petrochemicals, agrochemicals, polymers and rubber, oleochemicals (oils, fats, and waxes), explosives, fragrances rubber, oleochemicals (oils, fats, and waxes), explosives, fragrances flavors and pharmaceutical industry.Other closely related industries include flavors and pharmaceutical industry.Other closely related industries include petroleum, glass, paint, ink, sealant, adhesive, and food processing petroleum, glass, paint, ink, sealant, adhesive, and food processing manufacturers.manufacturers.

In the chemical industry, waste is either considered inert or contaminated.  Inert In the chemical industry, waste is either considered inert or contaminated.  Inert waste can be recycled or released into the environment.  Malformed polymer or waste can be recycled or released into the environment.  Malformed polymer or leaking steam may be considered inert wastes, although they are not chemically leaking steam may be considered inert wastes, although they are not chemically inert.inert.

Treating wastewater is certainly an expensive endeavor.  In short, there is much Treating wastewater is certainly an expensive endeavor.  In short, there is much motivation to minimize waste in the chemical and other industries.motivation to minimize waste in the chemical and other industries.

The first priority is to reduceThe first priority is to reduce the amount of waste generated. Again the amount of waste generated. Again manufacturing processes can be modified to yield substantial waste reduction. manufacturing processes can be modified to yield substantial waste reduction. For example changes in the chemical reaction conditions can minimize For example changes in the chemical reaction conditions can minimize production of by-product hazardous substances. In some instances potentially production of by-product hazardous substances. In some instances potentially hazardous catalysts, can be replaced by catalysts that are non hazardous or hazardous catalysts, can be replaced by catalysts that are non hazardous or that can be recycled rather than discarded.that can be recycled rather than discarded.

Many industrial processes that use solvents are equipped for solvent recycle for Many industrial processes that use solvents are equipped for solvent recycle for reasons of both economics and pollution control. A number of operations are reasons of both economics and pollution control. A number of operations are used in solvent purification. Entrained solids are removed by settling, filtration, used in solvent purification. Entrained solids are removed by settling, filtration, or centrifugation.or centrifugation.

Even with after vigorous hazardous waste reduction program, there will still be Even with after vigorous hazardous waste reduction program, there will still be large quantities of hazardous wastes that will require treatment and disposal. large quantities of hazardous wastes that will require treatment and disposal. The treatment technologies have been categorized as physical, chemical, The treatment technologies have been categorized as physical, chemical, biological, thermal, or stabilization.biological, thermal, or stabilization.

Physical treatment Physical treatment processes include gravity separation, phase change processes include gravity separation, phase change systems, such as air and steam stripping of volatiles from liquid wastes, and systems, such as air and steam stripping of volatiles from liquid wastes, and various filtering operations, including carbon adsorption. various filtering operations, including carbon adsorption.

Chemical treatment Chemical treatment transforms waste into less hazardous substances transforms waste into less hazardous substances using such techniques as pH neutralization, oxidation or reduction, and using such techniques as pH neutralization, oxidation or reduction, and precipitation. Biological treatment uses microorganisms to degrade organic precipitation. Biological treatment uses microorganisms to degrade organic compounds in the waste stream.compounds in the waste stream.

Thermal destruction Thermal destruction processes include incineration, which is increasingly processes include incineration, which is increasingly becoming a preferred option for the treatment of hazardous wastes, and becoming a preferred option for the treatment of hazardous wastes, and pyrolysis, which is the chemical decomposition of waste is brought about by pyrolysis, which is the chemical decomposition of waste is brought about by heating the material in the absence of oxygen heating the material in the absence of oxygen

Stabilization Stabilization techniques involve removal of excess of water from a waste techniques involve removal of excess of water from a waste and solidifying the remainder either by mixing it with a stabilizing agent such and solidifying the remainder either by mixing it with a stabilizing agent such as Portland cement, or vitrifying it to a glassy substance as Portland cement, or vitrifying it to a glassy substance

Physical methods of treatment of hazardous wastesPhysical methods of treatment of hazardous wastes

ElectrodialysisElectrodialysis Reverse osmosisReverse osmosis AdsorptionAdsorption SedimentationSedimentation Solvent extractionSolvent extraction DistillationDistillation EvaporationEvaporation FiltrationFiltration Flocculation Flocculation

Electrodialysis (ED)Electrodialysis (ED)

What is Electrodialysis (ED)?What is Electrodialysis (ED)?

Electrodialysis is an electro membrane process in which ions are Electrodialysis is an electro membrane process in which ions are

transported through ion permeable membranes from one solution to another transported through ion permeable membranes from one solution to another under the influence of a potential gradient. The electrical charges on the under the influence of a potential gradient. The electrical charges on the ions allow them to be driven through the membranes fabricated from ion ions allow them to be driven through the membranes fabricated from ion exchange polymers. Applying a voltage between two end electrodes exchange polymers. Applying a voltage between two end electrodes generates the potential field required for this. Since the membranes used in generates the potential field required for this. Since the membranes used in electro dialysis have the ability to selectively transport ions having positive electro dialysis have the ability to selectively transport ions having positive or negative charge and reject ions of the opposite charge, useful or negative charge and reject ions of the opposite charge, useful concentration, removal, or separation of electrolytes can be achieved by concentration, removal, or separation of electrolytes can be achieved by electro dialysis. electro dialysis.

The ion permeable membranes used in electro dialysis are essentially sheets The ion permeable membranes used in electro dialysis are essentially sheets of ion-exchange resins. They usually also contain other polymers to improve of ion-exchange resins. They usually also contain other polymers to improve mechanical strength and flexibility. mechanical strength and flexibility. The resin component of a cation-exchange The resin component of a cation-exchange membrane would have negatively charged groups (e.g., -SO3-) chemically membrane would have negatively charged groups (e.g., -SO3-) chemically attached to the polymer chains (e.g. styrene/divinylbenzene copolymers).attached to the polymer chains (e.g. styrene/divinylbenzene copolymers). Ions Ions with a charge opposite to the fixed charge (counter ions) are freely exchanged with a charge opposite to the fixed charge (counter ions) are freely exchanged at these sites. The concentration of counter ions (e.g., Na+) is relatively high; at these sites. The concentration of counter ions (e.g., Na+) is relatively high; therefore, counter ions carry most of the electric current through the therefore, counter ions carry most of the electric current through the membrane. The fixed charges attached to the polymer chains repel ions of the membrane. The fixed charges attached to the polymer chains repel ions of the same charge (co-ions), in this case the anions. Since their concentration in the same charge (co-ions), in this case the anions. Since their concentration in the membrane is relatively low, anions carry only a small fraction of the electric membrane is relatively low, anions carry only a small fraction of the electric current through a cation permeable membrane. current through a cation permeable membrane. Attachment of positive fixed Attachment of positive fixed charges (e.g., -NR3+ or C5H5N+R where commonly R = CH3) to the polymer charges (e.g., -NR3+ or C5H5N+R where commonly R = CH3) to the polymer chains forms anion permeable membraneschains forms anion permeable membranes, which are selective to transport of , which are selective to transport of negative ions, because the fixed -NR3+ groups repel positive ions. This negative ions, because the fixed -NR3+ groups repel positive ions. This exclusion, as a result of electrostatic repulsion, is called Donnan exclusionexclusion, as a result of electrostatic repulsion, is called Donnan exclusion. .

Ion Permeable Membranes

The process depends on special synthetic membranes, usually based on The process depends on special synthetic membranes, usually based on ion exchange resins which are permeable only to a single type of ion.ion exchange resins which are permeable only to a single type of ion.

Cation exchange membranes permit passage only of positive ions, under Cation exchange membranes permit passage only of positive ions, under the influence of electric field, while anion exchange membranes permit the influence of electric field, while anion exchange membranes permit passage only of negatively charged ions.passage only of negatively charged ions.

A dc potential applied across the stack causes the positive and negative A dc potential applied across the stack causes the positive and negative ions to migrate in opposite directions.ions to migrate in opposite directions.

This technique has been used for desalination to produce potable water This technique has been used for desalination to produce potable water from brackish well water.from brackish well water.

In food industry electrodialysis is used for desalting whey and deashing In food industry electrodialysis is used for desalting whey and deashing

sugars. The chemical industry uses this technique for enriching or depleting sugars. The chemical industry uses this technique for enriching or depleting solutions, and for removing mineral constituents from product streams. solutions, and for removing mineral constituents from product streams.

Reverse osmosis

Reverse osmosis process

Reverse osmosis is a filtration process that is often used for water. It works by using pressure to force a solution through a membrane , retaining the solute on one side and allowing the pure solvent to pass to the other side. This is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied

This technique which is most widely used consists of a membrane This technique which is most widely used consists of a membrane permeable to solvent but impermeable to most dissolved species, permeable to solvent but impermeable to most dissolved species, both organic and inorganic.both organic and inorganic.

These devices use pressure to force the contaminated water These devices use pressure to force the contaminated water against the semi permeable membrane. The membrane acts as a against the semi permeable membrane. The membrane acts as a filter, allowing the water to be pushed through the pores, but filter, allowing the water to be pushed through the pores, but restricting the passage of larger molecules that are to be removed. restricting the passage of larger molecules that are to be removed.

The reverse osmosis technique has been widely used for The reverse osmosis technique has been widely used for desalination of sea or brackish water It has also been successfully desalination of sea or brackish water It has also been successfully used in the treatment of electroplating rinse waters, not only to meet used in the treatment of electroplating rinse waters, not only to meet effluent discharge standards, but also to recover concentrated metal effluent discharge standards, but also to recover concentrated metal salt solutions for reuse. It has also been used for treatment of waste salt solutions for reuse. It has also been used for treatment of waste stream from paper and food processing industries.stream from paper and food processing industries.

Adsorption

Adsorption is a process that occurs when a gas or liquid solute Adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or a liquid (adsorbent), forming a accumulates on the surface of a solid or a liquid (adsorbent), forming a film of molecules or atoms (the adsorbate ).film of molecules or atoms (the adsorbate ).

The factors that relate to adsorption capacity are:The factors that relate to adsorption capacity are: • • Greater surface area produces greater adsorption capacity Greater surface area produces greater adsorption capacity [e.g.: activated carbon has large surface area (500-1500 m[e.g.: activated carbon has large surface area (500-1500 m22/g)]/g)] • • Adsorptivity increases as the solubility of the solute (in solvent) Adsorptivity increases as the solubility of the solute (in solvent) decreases. thus, for hydrocarbons, adsorption increases with moleculardecreases. thus, for hydrocarbons, adsorption increases with molecular weight.weight. • • For solutes with ionisible groups, maximum adsorption will beFor solutes with ionisible groups, maximum adsorption will be achieved at a pH corresponding to minimum ionization.achieved at a pH corresponding to minimum ionization. • • Adsorption capacity decreases with increasing temperatureAdsorption capacity decreases with increasing temperature

Carbon adsorption is applicable only to single phase fluid Carbon adsorption is applicable only to single phase fluid waste streams, e.g., liquid solutions or gas mixtures. In waste streams, e.g., liquid solutions or gas mixtures. In actual applications, both aqueous and nonaqueous actual applications, both aqueous and nonaqueous liquids are treated with carbon. the nonaqueous streams liquids are treated with carbon. the nonaqueous streams include petroleum fractions, syrups, animal and include petroleum fractions, syrups, animal and vegetable oils, and pharmaceutical preparations, color vegetable oils, and pharmaceutical preparations, color removal is the most common objective in such cases. removal is the most common objective in such cases.

Distillation

Distillation is expensive and energy intensive and can probably be justified only in cases where valuable product recovery is feasible (e.g., solvent recovery). This technique has only limited application in the treatment of dilute aqueous hazardous wastes.

Distillation is a method of separating mixtures based on differences in their volatilities Distillation is a method of separating mixtures based on differences in their volatilities in a boiling liquid mixture. Distillation is a unit operation, or a physical separation in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction. process, and not a chemical reaction.

1: Heat source2: Still pot3: Still head4: Thermometer/Boiling point temperature5: Condenser6: Cooling water in7: Cooling water out8: Distillate/receiving flask9: Vacuum/gas inlet10: Still receier11: Heat control12: Stirrer speed control13: Stirrer/heat plate14: Heating (Oil/sand) bath15: Stirrer bar/anti-bumping granules16: Cooling bath.

Evaporation

Evaporation process is used for the treatment of hazardous waste such as Evaporation process is used for the treatment of hazardous waste such as radioactive liquids and sludge's and concentrating of plating and paint radioactive liquids and sludge's and concentrating of plating and paint solvent waste among many other applications. solvent waste among many other applications.

It is capable of handling liquids, slurries and sometimes sludge's, both It is capable of handling liquids, slurries and sometimes sludge's, both organic and inorganic, containing suspended or dissolved solids or organic and inorganic, containing suspended or dissolved solids or dissolved liquids, where one of the components is essentially non volatile. dissolved liquids, where one of the components is essentially non volatile.

It can be used to reduce waste volume prior to land fill disposal or It can be used to reduce waste volume prior to land fill disposal or incineration. incineration.

The major disadvantages of evaporation are high capital and operating The major disadvantages of evaporation are high capital and operating costs and high energy requirements. This process is more adaptable to costs and high energy requirements. This process is more adaptable to waste waters with high concentrations of pollutants waste waters with high concentrations of pollutants

Chemical methods of treatment of hazardous wastesChemical methods of treatment of hazardous wastes

Chemical Oxidation and reduction Chemical Oxidation and reduction Acid-base neutralization Acid-base neutralization Chemical precipitation Chemical precipitation Hydrolysis Hydrolysis Ion exchange Ion exchange Thermal treatment methods Thermal treatment methods Photolysis Photolysis Biological treatment of hazardous wastes Biological treatment of hazardous wastes

Chemical Oxidation and reductionChemical Oxidation and reduction Oxidation reduction methods provide another important chemical treatment Oxidation reduction methods provide another important chemical treatment

alternative for hazardous wastes. One important chemical redox treatment alternative for hazardous wastes. One important chemical redox treatment involves the oxidation of cyanide wastes from metal finishing industry, using involves the oxidation of cyanide wastes from metal finishing industry, using chlorine in alkali solution. In this reaction CNchlorine in alkali solution. In this reaction CN-- is first converted to a less toxic is first converted to a less toxic cyanate. Further chlorination oxidizes the cyanate to simple carbon dioxide cyanate. Further chlorination oxidizes the cyanate to simple carbon dioxide and nitrogen gas.and nitrogen gas.

NaCN + ClNaCN + Cl22 + 2NaOH →NaCNO + 2NaCl +H2O + 2NaOH →NaCNO + 2NaCl +H2O 2NaCNO + 3Cl2NaCNO + 3Cl22 + 4NaOH → 2CO + 4NaOH → 2CO22 + N + N22+ 6NaCl +2H+ 6NaCl +2H22OO

Another important redox treatment process is the reduction of hexavalent Another important redox treatment process is the reduction of hexavalent chromium Cr (VI) to trivalent chromium Cr (III) in large electroplating chromium Cr (VI) to trivalent chromium Cr (III) in large electroplating operations Sulphur is used as the reducing agent and the reactions are as operations Sulphur is used as the reducing agent and the reactions are as follows. follows.

3 SO3 SO22 + 3 H + 3 H22O → 3 HO → 3 H22SOSO44

2 CrO2 CrO3 3 + 3 H+ 3 H22SOSO44 → Cr → Cr22SOSO44 + 3 H + 3 H22OO

Iron (II) in solution can be precipitated as ferric hydroxide by oxidation. Iron (II) in solution can be precipitated as ferric hydroxide by oxidation.

4 Fe 4 Fe 2+2+ - O - O22 + 10 H + 10 H22O → 4 Fe (OH)O → 4 Fe (OH)33 + 8 H + 8 H++

In a similar way sulphurdioxide is oxidised to sulphuric acidIn a similar way sulphurdioxide is oxidised to sulphuric acid

2SO2SO2 2 +O +O22 + 2H + 2H22O → 2HO → 2H22SOSO44

A large variety of oxidisable contaminants in waste water and sludges are A large variety of oxidisable contaminants in waste water and sludges are oxidised by ozone which can be generated on site by an electrical discharge oxidised by ozone which can be generated on site by an electrical discharge through dry air or oxygen.through dry air or oxygen.

(CH(CH22O) + 2[O]→COO) + 2[O]→CO22 +H +H22OO

CHCH33CHO+ [O] →CHCHO+ [O] →CH33COOHCOOH

Acid-base neutralizationAcid-base neutralization Hazardous wastes are categorized as corrosive when their solution pH is less than 2 Hazardous wastes are categorized as corrosive when their solution pH is less than 2

or more than 12.5. Such wastes can be chemically neutralised.or more than 12.5. Such wastes can be chemically neutralised. Generally acidic wastes are neutralised with slaked lime [Ca(OH)2] in a continuously Generally acidic wastes are neutralised with slaked lime [Ca(OH)2] in a continuously

stirred chemical reactor. The rate of addition of lime is controlled by feed back control stirred chemical reactor. The rate of addition of lime is controlled by feed back control system which monitors pH during addition. Lime is least expensive and is widely used system which monitors pH during addition. Lime is least expensive and is widely used for treating acidic wastes. Since the solubility of lime in water is limited, solution of for treating acidic wastes. Since the solubility of lime in water is limited, solution of excess lime do not reach extremely high pH values.excess lime do not reach extremely high pH values.

Alkaline wastes may be neutralised by adding sulphuric acid. It is a relatively Alkaline wastes may be neutralised by adding sulphuric acid. It is a relatively inexpensive acid. For some applications acetic acid is preferable since it is non toxic inexpensive acid. For some applications acetic acid is preferable since it is non toxic and biodegradable. Alkaline wastes can also be neutralised by bubbling gaseous and biodegradable. Alkaline wastes can also be neutralised by bubbling gaseous carbondioxide forming carbonic acid. The advantage of COcarbondioxide forming carbonic acid. The advantage of CO22 is that it is often readily is that it is often readily available in the exhaust gas from any combustion process at the treatment site. available in the exhaust gas from any combustion process at the treatment site.

Chemical precipitationChemical precipitation This technique can be applied to almost any liquid waste stream containing a This technique can be applied to almost any liquid waste stream containing a

perceptible hazardous constituent perceptible hazardous constituent By properly adjusting pH, the solubility of toxic metals can be decreased, By properly adjusting pH, the solubility of toxic metals can be decreased,

leading to the formation of a precipitate that can be removed by settling and leading to the formation of a precipitate that can be removed by settling and filtration.filtration.

Lime [Ca (OH)Lime [Ca (OH)22] or caustic soda is used for precipitation of the metal ions as ] or caustic soda is used for precipitation of the metal ions as metal hydroxidesmetal hydroxides

MM+2 +2 + Ca (OH) + Ca (OH) 22 M (OH) M (OH) 22 + Ca + Ca+2+2

Chromium is precipitated as hydroxide.Chromium is precipitated as hydroxide. CrCr3+3+ + (OH) + (OH) - - → Cr (OH) → Cr (OH) 3 3.. Sodium carbonate also has been used to precipitate metals as hydroxidesSodium carbonate also has been used to precipitate metals as hydroxides (Fe(OH)(Fe(OH)33.XH.XH22O), carbonates (CdCOO), carbonates (CdCO33), basic carbonate salts ), basic carbonate salts

(2PbCO(2PbCO33.Pb(OH).Pb(OH)22).). Carbonate ion hydrolyses in water to give hydroxide ion.Carbonate ion hydrolyses in water to give hydroxide ion. CO3CO32-2- + H + H22O → HCOO → HCO3-3- + OH + OH−− Reducing agents such as sodium borohydride can be used to precipitate the Reducing agents such as sodium borohydride can be used to precipitate the

metal ions from solution in the elemental form.metal ions from solution in the elemental form. 4Cu4Cu2+2+ + NaBH + NaBH44 + 2H + 2H22O → 4Cu + NaBOO → 4Cu + NaBO22 + 8H+ + 8H+

HydrolysisHydrolysis Hydrolysis treatment can be given to those hazardous waste Hydrolysis treatment can be given to those hazardous waste

constituents which are very reactive with water. Examples of those constituents which are very reactive with water. Examples of those substances are halides, carbide, hydride, alkoxide, and active metal.substances are halides, carbide, hydride, alkoxide, and active metal.

SiClSiCl44 + 2H + 2H22O → SiOO → SiO2 2 + 4HCl+ 4HCl CaCCaC22 + 2H + 2H22O → Ca(OH)O → Ca(OH)22 + C + C22HH22

NaAlHNaAlH44 + 4H + 4H22O → 4HO → 4H22 +NaOH+ Al(OH) +NaOH+ Al(OH)33

NaOCNaOC22HH55 +H +H22O → NaOH+ CO → NaOH+ C22HH55OHOH Ca + 2HCa + 2H22O → Ca(OH)O → Ca(OH)22 + H + H22

Ion exchangeIon exchange Ion exchange is judged to have some potential for the application of Ion exchange is judged to have some potential for the application of

interest in situations where it is necessary to remove dissolved interest in situations where it is necessary to remove dissolved inorganic species.inorganic species.

This is the use of anion exchanges for the removal of anionic nickel This is the use of anion exchanges for the removal of anionic nickel cyanide complex and chromate ions from waste solutions.cyanide complex and chromate ions from waste solutions.

2Res2Res++ OH OH-- + [Ni (CN) 4] + [Ni (CN) 4]2-2- → (Res → (Res++) 2 [Ni (CN) 4]) 2 [Ni (CN) 4]2-2- + 2OH + 2OH--

2Res2Res++ OH OH-- + CrO4 + CrO42-2- → (Res → (Res++) 2 (CrO) 2 (CrO442-2-) + 2OH) + 2OH--

Ion -exchange resins have also been used in the removal of Ion -exchange resins have also been used in the removal of radionuclide from radioactive wastes. radionuclide from radioactive wastes.

Thermal treatment methodsThermal treatment methods Thermal incineration is a process that uses high-temperature Thermal incineration is a process that uses high-temperature

thermal oxidation to convert a waste to a less bulky, less toxic or thermal oxidation to convert a waste to a less bulky, less toxic or less noxious materialless noxious material

It can be considered as a volume-reduction process in that many of It can be considered as a volume-reduction process in that many of the component elements of organic materials, including the most the component elements of organic materials, including the most common ones (carbon, hydrogen and oxygen) are converted wholly common ones (carbon, hydrogen and oxygen) are converted wholly or partially to gaseous form, leaving only the non combustible or partially to gaseous form, leaving only the non combustible inorganic volume. inorganic volume.

The principal products of incineration are carbon dioxide, water The principal products of incineration are carbon dioxide, water vapour and ash vapour and ash

C (organic) + OC (organic) + O22 → CO → CO22 + heat + heat 4H (organic) + O4H (organic) + O22 → 2 H → 2 H22O + heatO + heat

The solid and liquid effluents may require treatment prior to The solid and liquid effluents may require treatment prior to ultimate disposal or discharge.ultimate disposal or discharge.

The most critical factors that determine complete combustion of The most critical factors that determine complete combustion of hazardous wastes arehazardous wastes are

(i)(i) High combustion temperatures above about 900High combustion temperatures above about 900ooC to ensure that C to ensure that

the thermally resistant compounds react.the thermally resistant compounds react. (ii) Availability of sufficient amount of oxygen for combustion.(ii) Availability of sufficient amount of oxygen for combustion. (iii) Sufficient residence time to allow the reactions to occur.(iii) Sufficient residence time to allow the reactions to occur.

PhotolysisPhotolysis In photolysis, chemical bonds are broken under the influence of In photolysis, chemical bonds are broken under the influence of

light.light. In primary photochemical process, the target species is converted to In primary photochemical process, the target species is converted to

an electronically excited state, usually a diradical, which is an electronically excited state, usually a diradical, which is sufficiently energetic to undergo chemical reactionsufficiently energetic to undergo chemical reaction

The fate of the excited molecule and therefore the effectiveness of a The fate of the excited molecule and therefore the effectiveness of a photolysis treatment process, depends on its chemical structure and photolysis treatment process, depends on its chemical structure and on the medium in which it is carried outon the medium in which it is carried out

For the photolysis process to be effective in the treatment of For the photolysis process to be effective in the treatment of hazardous wastes stream, the radiation source must be sufficiently hazardous wastes stream, the radiation source must be sufficiently energetic, must be absorbed by the target species and the final energetic, must be absorbed by the target species and the final photochemical products must be less toxicphotochemical products must be less toxic

To date much of the research work on the treatment of highly toxic wastes To date much of the research work on the treatment of highly toxic wastes has centered on two types of constituentshas centered on two types of constituents

Polychlorinated biphenyls (PCBs) and chlorinateddibenzo-p-dioxins (CDDs) Polychlorinated biphenyls (PCBs) and chlorinateddibenzo-p-dioxins (CDDs) eg: tetrachloro dibenzo-p-dioxin (TCDD). eg: tetrachloro dibenzo-p-dioxin (TCDD).

The three requirements of photolysis of TCDD are The three requirements of photolysis of TCDD are 1) Dissolution in a light transmitting film1) Dissolution in a light transmitting film 2) Presence of organic hydrogen odour 2) Presence of organic hydrogen odour 3) Ultraviolet light.3) Ultraviolet light. In photolysis Reactions initially a reactive intermediate such as HO• is In photolysis Reactions initially a reactive intermediate such as HO• is

formed which participitate in chain reactions that lead to the destruction of formed which participitate in chain reactions that lead to the destruction of the compound the compound

Biological treatment of hazardous wastesBiological treatment of hazardous wastes Biological processes are in general, the most cost effective techniques for Biological processes are in general, the most cost effective techniques for

treating aqueous waste streams containing organic constituents. The treating aqueous waste streams containing organic constituents. The physical and chemical properties of the compound influence its physical and chemical properties of the compound influence its biodegradability. With appropriate organisms and under right conditions, biodegradability. With appropriate organisms and under right conditions, even phenol which is considered to be biocidal can be degradedeven phenol which is considered to be biocidal can be degraded

The microorganism must be allowed to acclimate to the waste to be treated The microorganism must be allowed to acclimate to the waste to be treated prior to routine operation of the processprior to routine operation of the process

Even some compounds which were considered as biorefractory may be Even some compounds which were considered as biorefractory may be degraded by microorganisms adapted to their biodegradation. DDT for degraded by microorganisms adapted to their biodegradation. DDT for example is degraded by properly acclimated pseudomonasexample is degraded by properly acclimated pseudomonas

The relatively highly Chlorinated PCBs are degraded by anaerobic bacteria The relatively highly Chlorinated PCBs are degraded by anaerobic bacteria under less anaerobic condition. These products can be further decomposedunder less anaerobic condition. These products can be further decomposed

To increase the biodegradability of the hazardous wastes, the pH of To increase the biodegradability of the hazardous wastes, the pH of the medium should be adjusted to an optimum value of 6-9 and the the medium should be adjusted to an optimum value of 6-9 and the oxygen level should be highoxygen level should be high

Concentrations of soluble inorganic in the hazardous wastes should Concentrations of soluble inorganic in the hazardous wastes should be kept low so that enzymatic activity is not inhibited. Trace be kept low so that enzymatic activity is not inhibited. Trace concentrations of inorganic may be partially removed from the liquid concentrations of inorganic may be partially removed from the liquid waste stream during the biological treatment, because of adsorption waste stream during the biological treatment, because of adsorption onto the microbial cell coatingonto the microbial cell coating

In aerobic waste treatment, hazardous wastes such as chemical In aerobic waste treatment, hazardous wastes such as chemical processes wastes and land fill leachates can be degraded by processes wastes and land fill leachates can be degraded by aerobic microorganisms such as bacteria and fungi in the presence aerobic microorganisms such as bacteria and fungi in the presence of oxygen. In anaerobic waste treatment, microorganisms degrade of oxygen. In anaerobic waste treatment, microorganisms degrade different organic wastes in the absence of oxygen. During the different organic wastes in the absence of oxygen. During the process H2S is generated which precipitates toxic heavy metal ions process H2S is generated which precipitates toxic heavy metal ions as their sulphides. The overall degradation of the hypothetical as their sulphides. The overall degradation of the hypothetical organic compound (CHorganic compound (CH22O) can be written as follows.O) can be written as follows.

2(CH2(CH22O) →COO) →CO22 +H +H22OO

ReferencesReferences

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