To categorise, appropriately label, decide on treatment, disposal techniques and to avoid storage of incompatible wastes together, it is very essential to characterize these wastes. Wastes are considered to be hazardous if they exhibit any of the four criteria (EPA):
1. Ignitability
2. Corrosivity
3. Reactivity
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4. Toxicity.
1. Ignitability
Ignitability is the characteristic used to define as hazardous those wastes that could cause a fire during transport, storage, or disposal.
A waste exhibits the characteristics of ignitability if a representative sample of the waste has any of the following properties:
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It is a liquid other than an aqueous solution containing less than 24% alcohol by volume with flash point less than 60°C.
It is a not a liquid and is capable, under standard temperature and pressure, of causing fire through friction, absorption of moisture, or spontaneous chemical changes. When ignited, the waste burns vigorously and persistently thereby creating fire hazard.
For example, waste oil and used solvents.
2. Corrosivity
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Corrosivity, as indicated by pH, was chosen as an identifying characteristic of a hazardous waste because highly acidic or alkaline waste can react dangerously with other wastes or cause toxic contaminants to migrate from certain wastes.
Corrosive wastes exhibit the following characteristics:
An aqueous solution having pH less than 2 or greater than 12.5.
It is a liquid and corrodes steel at a rate greater than 6.35 mm per year at a temperature of 55°C.
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For example, acidic bath waste.
3. Reactivity
Reactivity was chosen as an identifying characteristic of a hazardous waste because unstable wastes can pose an explosive problem at any stage of the waste management cycle.
A reactive waste exhibits the following properties:
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1. It is normally unstable and readily undergoes violent change without detonation.
2. It reacts violently with water.
3. It forms potentially explosive mixtures with water.
4. Generates toxic gases and vapours when mixed with water (for example, FeS).
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5. It is capable of detonation or explosive decomposition at standard temperature and pressure.
Hazardous waste minimization
The most effective technique for waste minimization utilizes a combination of common sense and good engineering. Implementation of a comprehensive waste minimization plan requires commitment from top-level decision makers, appropriate financial and technical resources, and involvement at each level of the production process.
Waste minimization techniques can be categorized into four overlapping types of approaches:
1. Source reduction
2. Waste exchange
3. Recycling
4. Treatment
Each of these approaches is briefly summarized below:
1. Source Reduction
Source control investigations should focus on changes to input raw materials, process technology, and the human aspect of production. Input material changes can be classified into three elements:
1. Purification
2. Substitution
3. Dilution.
Purification of input raw materials prevents inert or impure materials from entering the production process. Substitution involves replacing a toxic material with a less toxic or more environmentally desirable material. Dilution is a minor component of input material changes. An example of dilution is the use of more dilute solution to minimize drag outs in metal part cleaning.
Production processes can be modified to make better use of raw materials and to minimize waste generation. The efficiency of the production process can be improved through simple operational and maintenance procedures. In other cases, more involved material changes and process equipment modifications are needed.
2. Waste Exchange
Waste exchange is a reuse function involving more than one facility. An exchange matches one industry’s output to the input requirement of another. Waste exchange organizations act as brokers of hazardous materials by purchasing and transporting them as resources to another client. Waste exchanges commonly deal in solvent, oil, concentrated acids and alkalies, and catalysts. Limitations include transport distance, purity of the exchange product and reliability of supply and demand.
3. Recycling and Reuse
Recycling techniques allow reuse of waste materials for beneficial purposes. A recycled material is used, reused, or reclaimed. Recycling through reuse involves returning waste material to the original process as a substitute for an input material or to another process as an input material. Recycling through reclamation involves processing a waste for recovery of valuable materials. Recycling can help eliminate waste disposal costs, reduce raw material costs, and generate revenue from saleable wastes.
Recycling and reuse processes can occur on-site or at a centralized off-site location. Advantages of on-site operations include reduced waste handling, transport, and reporting requirements. Off-site operations also offer several advantages, as listed below:
1. Potential economics of sale associated with accepting wastes from many generators
2. Economically viable material transfers between industries.
3. Flexibility in quantity and timing.
Recycling is the second option in the pollution prevention hierarchy and should be considered only when all source reduction options have been investigated and implemented.
Hazardous waste treatment and disposal
Waste treatment and disposal technologies may be divided into five major types:
1. Physico-chemical processes
2. Biological processes
3. Thermal processes
4. Underground (deep well) injection
5. Land-based systems
1. Physico-chemical Processes
The important physicochemical processes used in the treatment of hazardous wastes are as follows:
1. Air stripping
2. Ion exchange
3. Adsorption
4. Neutralization
5. Precipitation
6. Coagulation and flocculation
7. Oxidation and reduction
These treatment technologies can be utilized for the removal of a host of hazardous wastes from aqueous streams and also from air. Descriptions of these treatment processes are as follows:
Air stripping
The physical process of transferring volatile organic compounds (VOCs) from liquid into air is known as air stripping. In this process, air is passed through containment liquid. Any volatile constituent in the liquid will be preferentially removed into the gaseous phase and will leave the system as vapour.
In air stripping a gas-liquid mass transfer occurs via inter phase diffusion. The driving forces for the transfer in the gaseous phase are a partial pressure gradient and in the liquid phase a concentration gradient.
Ion exchange
Ion exchange is a chemical treatment process used to remove dissolved ionic species from contaminated aqueous streams. Ion exchange processes can achieve treatment of both anionic and cationic contaminants.
Soluble hazardous constituents that are amenable to treatment by ion exchange include arsenic, barium, cadmium, chromium, cyanide, lead, mercury and silver. As ion exchange process costs are generally exorbitant for treatment of highly concentrated waste streams, ion exchange is typically used as a polishing step after chemical precipitation. Stringent discharge limitations that cannot be met by other conventional technologies can be met using ion exchange.
Adsorption
Many hazardous wastes contain organics which are refractory and which are difficult to remove by conventional biological treatment processes. These materials can be frequently removed by adsorption on an active-solid surface. The most widely used adsorbent in environmental applications is carbon that has been processed to significantly increase internal surface area (activated carbon). Activated carbon is available in both granular and powdered form. Granular activated carbon in used widely for removal of a wide range of toxic organic compounds and heavy metals from groundwater and industrial waste streams.
Neutralization
Acid-base reactions are among the most common chemical processes used in wastewater treatment. Neutralization of a waste involves addition of chemical substance to change the pH to a more neutral level (6 to 8). The neutralization reactions are exothermic and require systems similar to that in to avoid excessively high temperatures, which could produce unsafe operating conditions and damage the process equipment.
Precipitation
The undesirable heavy metals present in liquid waste stream can be removed by chemical precipitation. Metals are precipitated at varying pH levels, depending on the metal ion, resulting in the formation of an insoluble salt. Hence, neutralization of an acidic waste stream can cause precipitation of heavy metals and allow them to be removed as a sludge residue by sedimentation followed by filtration. The hydroxides of heavy metals are usually insoluble, so lime or caustic soda is commonly used to precipitate them.
Oxidation and Reduction
The chemical processes of oxidation and reduction can be used to convert toxic pollutants to harmless or less toxic substances. Oxidation is a chemical reaction in which valency increases from the loss of electrons. Chemical reactions that involve both oxidation and reduction are known as redox reactions.
Hexavalent chromium is highly toxic and its presence in a waste requires careful management to avoid harm to human health and environment. Once hexavalent chromium is reduced to trivalent chromium, it can be precipitated as chromic hydroxide, as shown in the following reaction that utilizes sulphur dioxide and lime:
2Cr03 + 3H2S03 _______ > Cr2(S04)3 + 3H2O
Cr2(S04)3 + 3Ca(OH)2 _______ > 2Cr(OH)3 + 3CaS04
The reduction of hexavalent chromium to the trivalent state through techniques similar to the above produces a chromium-containing compound that is less toxic and more acceptable for subsequent recovery or final disposal.
2. Biological Processes
Biological degradation of hazardous organic substances is a viable approach to waste management. Common processes are those originally utilized in treating municipal wastewater, based on aerobic or anaerobic bacteria. In-situ treatment of contaminated soils can be performed biologically. Cultures used in biological degradation processes can be native (indigenous) microbes, selectively adopted microbes, or genetically modified microorganisms.
Biological treatment processes are applied to gaseous, aqueous and solid wastes containing biodegradable organics and inorganic ions such as nitrate, ammonia, sulphate and phosphate. Microorganisms involved in biological hazardous waste treatment processes include:
1. Bacteria
2. Fungi
3. Protozoa
4. Algae.
The most active and diverse group are the bacteria. For microorganisms to degrade hazardous contaminants, conditions that promote their growth and reproduction must be maintained. Required for this are sources of energy, carbon, nitrogen, phosphorus and trace micronutrients, along with proper environmental conditions such as temperature, pH, and moisture. In addition, the success of biological treatment relies on the biodegradability of the contaminants of interest. Major factors that affect the biodegradability of a specific contaminant are:
1. Presence of an appropriate microbial culture
2. Chemical structure of the contaminant
3. Physical characteristics of the contaminant.
Each of these factors must be considered in the choice of the biological reactor configuration to promote the desired degradation reactions.
3. Thermal Treatment
Thermal treatment applies high temperature to convert hazardous wastes to forms that are significantly less toxic, have lower volume, and are more easily disposed of. In general, two types of thermal technologies are used. They are:
1. Incineration
2. Pyrolysis
Incineration involves the combustion of wastes in the presence of oxygen. Pyrolysis is thermal decomposition of molecules in the absence of oxygen.
Incineration
Hazardous waste streams containing significant quantities of organics and minimal quantities of inorganics are most suitable for incineration. Wastes containing high concentration of halogenated compounds or volatile metals are unsuitable for incineration. Incineration is an oxidation process in which wastes are heated to a high temperature in the presence of oxygen. Organics within the waste are converted to C02 and water, nitrogen and sulphur are oxidized to inorganic gases, halogens are converted to acidic gases and salts and metals are oxidized and precipitated into ash or volatilized. Ash is an inherent byproduct of incineration that must be collected and ultimately disposed by landfill method. The choice of an appropriate configuration depends on the characteristics and volume of the waste, incinerator availability and cost, the details of which are beyond the scope of this text.
Pyrolysis
Pyrolysis is applicable for the treatment of wastes that are not amenable to conventional incineration. The major advantages associated with pyrolytic processes for hazardous waste treatment include more efficient energy recovery compared to incineration. Major limitations of this process include the requirement of auxiliary heating during the endothermic stage, long residence times compared to incineration units, potentially hazardous products in the gaseous emissions, and potentially hazardous leachate residues resulting from treatment of metals and salt-bearing wastes. Consequently, pyrolysis is currently not applied for the treatment of hazardous waste nearly as often as incineration.
4. Underground (Deep Well) Injection
Underground injection involves using specially designed wells to inject liquid hazardous waste into deep earth strata containing non-potable water. In this method, a wide variety of hazardous waste liquids are pumped underground into deep permeable rocks that are separated from freshwater aquifers by impermeable layers of rock above, below and lateral to the waste layer. The depth of injection ranges from 300 to 2500 m and varies according to the geographical factors of the area
The cross-section of a typical injection well is shown in particulate matter present in the liquid should be removed to prevent plugging of the injection equipment. The deep well must be constructed such that potable water zones are isolated and protected. For hazardous liquid waste to be deep-well injected, the following guidelines must apply:
1) Liquid waste must have a volume and a high concentration
2) Must be biologically inactive
3) Must be non-corrosive
4) Must be difficult to be treated by other methods.
Thus the method should be used only for those liquid wastes with no other feasible management options.
5. Land-based Systems
Sanitary landfills were developed for municipal solid waste disposal to replace existing open dumps. New secure landfills are used to bury non-liquid hazardous wastes in artificially lined depressions. Secure landfills for hazardous waste disposal are now equipped with double lines, leak dection, leachate monitoring and collection systems.
The cross-section of a typical injection well is shown the particulate matter present in the liquid should be removed to prevent plugging of the injection equipment. The deep well must be constructed such that potable water zones are isolated and protected. For hazardous liquid waste to be deep-well injected, the following guidelines must apply:
1. Liquid waste must have a volume and a high concentration
2. Must be biologically inactive
3. Must be non-corrosive
4. Must be difficult to be treated by other methods.
Thus the method should be used only for those liquid wastes with no other feasible management options.
6. Land-based Systems
Sanitary landfills were developed for municipal solid waste disposal to replace existing open dumps. New secure landfills are used to bury non-liquid hazardous wastes in artificially lined depressions. Secure landfills for hazardous waste disposal are now equipped with double lines, leak, leachate monitoring and collection systems.
1. Surface run-off should be intercepted and diverted.
2. The integrity of soil covers should be maintained.
3. Surface erosion should be prevented.
4. Artesian pressure should be released.
5. Ground water does not move laterally.
6. Liners should be properly chosen and installed.
7. Leachate collection and removal systems are provided.
8. Post-closure maintenance is provided.
9. Environmental monitoring around the site is conducted.