In this article we will discuss about how to treat by: 1. Physical Methods 2. Thermal Methods 3. Solidification and Stabilisation 4. Chemical Treatment of Wastes.
Introduction:
The effects of chemical waste on the environment are reflected by the effects on organisms and effects on the overall ecosystem. And, it is also worth remembering that virtually all chemical wastes are poisonous to a degree, some extremely so.
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As with all environmental pollutants, such chemical wastes eventually reach a state of physical and chemical stability and equilibrium with the environment, although it may take many centuries to occur. In many cases, the behaviour (or reactivity) and ultimate fate of a chemical waste is a function of its physical properties and surroundings. On the basis that it is not possible to wait for the chemical to reach an equilibrium with its surroundings, methods of remediation are essential.
The first step in considering the appropriate technology to employ to treat a specific waste is to determine whether the waste is hazardous or non-hazardous. And, the definition of a hazardous/non-hazardous waste is determined by the relevant legislation. There is the general tendency to think of a chemical waste as some obscure and ill-defined sludge discarded from a process. However, chemical waste can also be in the form of volatile organic compounds, semi-volatile organic compounds, metals, radioactively contaminated materials, or a mixture of any or all of these types.
The ideal treatment process reduces the quantity of chemical waste to a small fraction of the original amount and converts it to a non-hazardous form if such a conversion is possible. Another consideration in selecting a treatment technology is the location where the wastes are to be treated. For example, wastes may be treated in place (in-situ), within the confines of the site, or at an off-site facility (ex-situ).
There are various alternative waste treatment technologies, such as chemical treatment and biological treatment. Physical treatment processes (which also include thermal methods such as incineration) as well as solidification or stabilisation are the subject. These processes are used to- (i) recycle the waste and reuse waste materials, (ii) reduce the volume and toxicity of the waste stream, and (iii) produce a final residual material that is suitable for disposal.
The effectiveness of the application of each of these technology groups to a specific waste varies depending on- (i) the type of waste, (ii) the concentration of the individual components in the waste, (iii) the physical phase of the material, (iv) the desired level of treatment, and (v) the final method or disposal of any remaining residue.
The waste characteristics can include such properties as volatility (gases, volatile solutes in water, gases or volatile liquids held by solids, such as catalysts), liquid phase materials (waste-water, organic solvents), dissolved or soluble materials (water soluble inorganic species, water-soluble organic species, compounds soluble in organic solvents), semisolid materials (sludge, grease), and solid materials (dry solids, including granular solids with a significant water content, such as de-watered sludge, as well as solids suspended in liquids). Waste treatment may occur at three major levels – primary, secondary, and tertiary (polishing).
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Primary waste treatment is generally regarded as preparation for further treatment, although it can result in the removal of by-products and reduction of the quantity and hazard of the waste. Secondary waste treatment detoxifies, destroys, and removes hazardous constituents. Polishing usually refers to treatment of a waste product for safe discharge. An example is the treatment of recycled water that is removed from wastes so that it may be safely discharged. In addition, especially in the case of water treatment, the processes employed can be generally categorised as external treatment or internal treatment.
External treatment uses processes such as flotation and clarification to remove material, including suspended or dissolved solids, hardness, and dissolved gases. Following this basic treatment, the water may be divided into different streams, some to be used without further treatment and the rest to be treated for specific applications.
Physical Methods of Waste Treatment:
In contrast to the chemical methods of waste treatment, physical processes for the treatment of chemical waste include processes that separate components of a waste stream or change the physical form of the waste without altering the chemical structure of the constituent materials.
These processes are very useful for separating hazardous materials from an otherwise non-hazardous waste stream so that they may be treated in a more concentrated form, separating various chemical components for different treatment processes, and preparing a waste stream for ultimate destruction in a biological or thermal treatment process.
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Knowledge of the physical behaviour of wastes has been used to develop various unit operations for waste treatment that are based upon physical properties. These operations include – (i) phase separation (filtration), (ii) phase transfer (extraction, sorption), (iii) phase transition (distillation, evaporation, precipitation), and (iv) membrane separations (reverse osmosis, hyper-filtration and ultrafiltration).
The important aspect of physical methods of remediation is to isolate the contaminant so that it can be recovered/destroyed while being contained within a specific area. In terms of containment, there are several technologies available which usually involve construction of a containment wall (usually concrete) around the contaminated area on the side where leakage into nearby water systems can occur.
One aspect of remediation that is evolving is the application of soil-mix wall technology. This technology consists of mixing soils in situ with cement grout using multiple shaft augers to construct overlapped cement columns. The use of augers makes it possible to define the treatment zone clearly and to confine the contaminant.
Thermal Methods of Waste Treatment:
Outside of the traditional containment methods (land filling and capping), application of some type of thermal process has, until recently, been the most common form of treatment for chemical waste.
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Thermal treatments have lost some popularity recently due to the threat of emissions from incomplete combustion. Except for vitrification, thermal technologies are ex situ processes requiring the wastes to be transported to the processing unit. However, the use of a thermal destruction technology may be (depending upon the nature of the emissions) superior to wet scrubbing technologies for emissions cleaning.
These are designed depending on the quality and quantity of the waste feed, the required process time in consideration with the kind of thermal processing that suits the wastes. Hearth furnaces constitute a common variety used in this process. Wastes are fed through a feed port into the incinerator, which is built with arms to rotate inside to keep the material dynamic.
The retention period for the wastes inside the incinerator is selected carefully and maintained, as a result of the design of such arms only. The control over the overall temperature of the incinerator needs to be overemphasised. The resultant gas from the incineration is led through accessories like pre-cooler, particle separator, filter, sub-cooler, fan etc.
In an alternative type, viz. fluidised bed incinerator, the material is retained in a fluidised state with the help of gases blown in. Here, pre-heating of the bed is mandatory. Moreover, excess air is limited to not more than 35-40 per cent above the stoichiometric requirement.
The liquid waste incinerators are considered as labour-friendly, as most of the cumbersome requirements of the other types of incinerators are done away with this. The general feed requirement is that the feed wastes act as a liquid, the solids are molten, so that they are pumpable and attract atomisation in the liquid waste incinerator.
The waste gas incinerators act in either of the following three principles:
a. Direct flame method.
b. Multiple hearths.
c. Catalytic combustion.
While the multiple hearths have been discussed in the solid waste treatment section, the direct flame method is applicable only when the wastes or their combustion produce do not pose any threat to the environment and demands a very high temperature levels that generally runs in the range of 500°C. The catalytic combustion process uses the most common property that the general gaseous wastes do not possess the fuel value enough to initiate or support or sustain self-ignition. This method is applicable to the waste gases that have hydrocarbon levels lower than lower explosive limits. The process is considered advantageous in the sense that this produces clean hot gas.
Multiple chamber incinerators are most commonly employed to treat wastes of plastic nature. The arrangement of chambers classifies them as either retort or inline type. Molten salt incinerators are composed of a salt mixture, generally containing sodium carbonate and sodium sulphate in 9:1 ratio. Changes in the salt types and mix proportions can be envisaged to result in different temperature ranges. One attractive feature of this process is that eutectic mixtures of reactive salts can be developed which render astonishing additional benefits like containment of toxicity.
As a general rule of nature, every coin has two sides. While this process is attractive in the above said aspect, it also brings in with it the challenge of treating of the salt wastes. They have to be either regenerated or incinerated or used as say, road fills. This also calls for requirement of fuels for start-up and support.
Rotary kilns are designed to keep the material in continuous movement to ensure proper mixing while process is on. Wet air oxidation method is solely applicable for treatment of oxidisables in their aqueous state. The underlying principle here is that organic compounds increase their rate of oxidation with pressure. This will aid in arriving at an incomplete oxidation in liquid phase that will help destroy the organics.
Incineration of solid and liquid chemical wastes, such as polychlorobiphenyl materials, is a common and efficient method of destruction. The destruction of polychlorobiphenyls commences at ca. 800°C (1470°F) and commercial incinerators operate at temperatures in excess of 1000°C (1800°F). Major concerns in this type of incineration involve potential generation and emission of by-products of incomplete combustion. Such by-products, if produced, can often be more harmful to the environment.
Suitable for the purpose are stationary incinerators, cement kilns, incinerator ships, and smaller mobile thermal destruction units. Efforts have also been spent in developing efficient catalysts that allow the use of lower temperature reactions with higher efficiency.
This, of course, is a convenient point at which to note only one of the many ways in which environmental events on the land, in the air, and in the water are connected. And the connection between the air, land, and water systems is not guaranteed to be sequential –
Land systems ↔ water systems ↔ atmosphere
Briefly, emissions from the incinerators may be gaseous (SOx, NOx, particulate matter), which can pollute the atmosphere or, through the formation of the constituents of acid deposition, the water systems and the land systems, and therefore control is necessary.
Alternatively, these emissions might have a direct effect on the vegetation by deposition of the gases or particulate matter directly on to the land at a point downwind of the incinerator. In addition, solid waste (ash) from an incinerator can pollute the land and, as a result of leaching by rain (acidic or otherwise) pollute the water systems. Therefore, attention must be focused on the disposal of the ash.
The air emission systems from these thermal reactors, as well as from the furnaces that burn solids emanating from waste-water plants, must be designed to prevent chemical and particulate air pollution. These air emission systems include electrostatic precipitators (ESP) and afterburners.
In addition, sulphur-containing fuels emit sulphur dioxide and other sulphur containing gases as a result of combustion and these sulphur-containing gases have the potential for conversion to sulphates (sulphuric acid in the atmosphere), which can be deposited on the land as sulphate.
Nitrogen oxides (also produced during fuel combustion) are converted to nitrates in the atmosphere and the nitrates eventually are deposited on soil. Soil also adsorbs nitric oxide (NO) and nitrogen dioxide (NO2) readily and these gases are oxidised to nitrate in the soil. Carbon monoxide is converted to carbon dioxide and possibly to biomass by soil bacteria and fungi. Elevated levels of heavy metals (such as lead) from mines and smelters are also found in the soil near such facilities.
Thermal treatment of chemical waste can be used to accomplish most of the common objectives of waste treatment – (i) volume reduction, and (ii) removal of volatile, combustible, mobile organic matter and destruction of toxic and pathogenic materials. Incineration utilises high temperatures, an oxidising atmosphere, and often turbulent combustion conditions to destroy wastes.
The effective incineration of wastes depends upon the combustion conditions, which are – (i) a sufficient supply of oxygen in the combustion zone, (ii) thorough mixing of the waste, the oxidant, and any supplemental fuel, (iii) combustion temperatures above 900°C (↔ 1650°F), and (iv) sufficient residence time to allow reactions to occur.
Usually, the heat required for incineration comes from the oxidation of organically bound carbon and hydrogen contained in the waste material or in supplemental fuel –
C(organic) + O2 = CO2 + heat
4H(organic)+ O2 = 2H2O
These reactions destroy organic matter and generate heat required for endothermic reactions, such as the breaking of C-CT bonds in organic chlorine compounds.
Incineration detoxifies chemical waste by destroying organic compounds, reduces the volume of the waste, and converts liquid waste to a solid product by reacting and vaporising any fluids present in the wastes. The primary use of incineration is for the destruction of volatile organic compounds and semi- volatile organic compounds. Incinerators have been extremely capable of destroying organic compounds in waste. Removal efficiencies as high as 99.9999 per cent have routinely been achieved (often referred to as the six-9s treatment level).
If the waste is exposed to high temperatures in an oxygen-poor environment, the process is known as pyrolysis. The products of this process are simpler organic compounds, which may be recovered or incinerated, and a char or ash.
Incineration systems are designed to accept specific types of materials; they vary according to feed mechanisms, operating temperatures, equipment design, and other parameters. The main products from complete incineration include water, carbon dioxide, ash, and certain acids and oxides, depending upon the waste in question. Unlike pyrolysis, incineration is carried out with excess oxygen.
Waste incineration and pyrolysis systems include single-chamber liquid systems, rotary kilns, and fluidised-bed incineration systems. In a single chamber liquid system a brick-lined combustion chamber contains liquids that are burned in suspension. In addition to being the primary parts of an incineration system, these units can be used as after burners for rotary kilns.
A rotary kiln is a versatile large refractory-lined cylinder capable of burning virtually any liquid or solid organic waste; the unit is rotated to improve turbulence in the combustion zone. Fluidised-bed incineration uses a stationary vessel within which solid and liquid wastes are injected into a heated, extremely agitated bed of inert granular material; the process promotes rapid heat exchange and can be designed to scrub off the gases.
The ideal wastes for incineration are predominantly organic materials that will burn with a heating value of at least 5,000 Btu/lb and preferably more than 8,000 Btu/lb. Such heating values are readily attained with waste having a high content of organic constituents. In some cases, however, it is desirable to incinerate wastes that will not burn alone and which require supplemental fuel, such as methane and petroleum liquids.
Examples of such wastes are nonflammable organic chlorine wastes, some aqueous wastes, or soil in which the elimination of a particularly troublesome contaminant is worth the expense and trouble of incinerating it. Inorganic matter, water, and organic heteroelement contents of liquid wastes are important in determining their susceptibility to incineration.
Many wastes, including hazardous waste, are burned to produce fuel for energy recovery in furnaces and boilers. This process (coincineration) uses combustible waste material more for energy generation than for waste destruction. In addition to heat recovery from combustible waste, an existing on-site facility, rather than a separate waste incinerator, can be used for waste disposal.
Incinerators can be designed to handle wastes in any physical state and have proven effective in treating solids, liquids, sludge, slurries, and gases. The effectiveness of an incinerator depends on three factors – (i) temperature of the combustion chamber, (ii) residence time in the chamber, and (iii) amount of mixing of the material with air while in the chamber.
Normal combustion temperatures range between 900° and 1500°C (1650°-2280°F) in some instances the temperatures employed are much higher. Many incinerators for hard-to-burn compounds employ two combustion chambers. The first chamber converts the compounds to gas and initiates the combustion process. In the second chamber, combustion of the gases is completed.
There are four major components of chemical waste incineration systems: preparation, combustion, pollutant removal, and ash disposal. Waste preparation for liquid wastes may require filtration settling to remove solid material and water, blending to obtain the most appropriate mixture for incineration, or heating to decrease viscosity. Solids may require shredding and screening, and good examples include tyres. Atomisation is commonly used to feed liquid wastes. Several mechanical devices, such as rams and augers, are used to introduce solids into the incinerator.
The most common kinds of combustion chambers are liquid injection, fixed hearth, rotary kiln, and fluidised bed. Often the most complex part of a waste incineration system is the air pollution control system, which involves several operations. The most common operations in air pollution control from waste incinerators are combustion gas cooling, heat recovery, quenching, particulate matter removal, acid gas removal, and treatment and handling of by-product solids, sludge, and liquids.
The inert portion of the waste remains as ash after incineration. For liquid waste, the amount of ash remaining is generally minimal, whereas for solid waste, the volume of ash can be as much as one-third by weight of the original material. If the ash contains metals or radioactive material, it must be further treated prior to disposal. The most frequently employed method of treating the ash remaining from the incineration process is solidification/stabilisation.
Hot ash is often quenched in water and, prior to disposal, it may require dewatering and chemical stabilisation. A major consideration with waste incinerators and the types of wastes that are incinerated is the disposal problem posed by the ash especially in respect to potential leaching of heavy metals.
Waste incinerators may be divided among the following, based upon type of combustion chamber:
(i) Rotary kiln incinerators in which the primary combustion chamber is a rotating cylinder lined with refractory materials and an afterburner downstream from the kiln to complete destruction of the wastes;
(ii) Liquid injection incinerators that burn liquid waste dispersed as small droplets;
(iii) Fixed-hearth incinerators with single or multiple hearths upon which combustion of liquid or solid wastes occurs; and
(iv) Fluidised bed incinerators that have a bed of granular solid (such as sand) maintained in a suspended state by injection of air to remove pollutant acid gas and ash products.
Advanced design incinerators, including plasma incinerators, make use of an extremely hot plasma of ionised air injected through an electrical arc. Medical wastes are often incinerated by these systems. Examples are electric reactors which use resistance-heated incinerator walls at around 2200°C (3990°F) to decompose the waste by radiative heat transfer. There are also infrared systems which generate intense infrared radiation by passing electricity through silicon carbide resistance heating elements. Molten salt combustors use a bed of molten salt, such as sodium carbonate, at 900°C (1650°F) to destroy the waste and retain gaseous emissions through chemical reaction. Finally, there are also molten glass processes which use a molten glass to transfer heat to the waste and to retain products in a non-leachable form.
Incinerators may be mobile, transportable, or stationary (fixed). Mobile reagent incinerators are normally relatively small units that are mounted on a flat-bed trailer and transported to the hob site. Transportable incinerators are larger units that can be disassembled into manageable components and move from one site to another by a caravan of trucks. Stationary/fixed incinerators are permanently erected at a site, and the wastes are brought to the site for treatment.
Thermal desorption is the process of heating a waste in a controlled environment thereby volatilising any organic constituents. Thermal Desorption works especially well for volatile organic compounds but can also be employed for semi-volatile organic compounds. Removal efficiencies ranging from 65 to 99 per cent have been achieved depending upon the type of waste.
Prior to entering the thermal desorption unit, the wastes are screened to eliminate coarse pieces. If the wastes have a high moisture content, this excess moisture is also removed. The wastes are then passed to a furnace which operates at temperatures in the range 300°-600°C (570°-1110°F). Volatile organic compounds become gaseous in the process and are either collected on an adsorbent, such as activated carbon, for further treatment or passed through an incinerator connected in-line with the thermal desorption unit.
This is a process that is defined, at least theoretically as a zero air indirect heat process. Pyrolysis is an air-starved process, wherein the combustion proceeds with ‘less than stoichiometric level’ air. The aim is to distil or vapourise the compounds to give out combustible gases. Wastes that are considered worthy of treatment via this process are only those that possess a high calorific value, inherent in them. Here the otherwise end product problems that are associated with common incineration, viz. ash and clinker are not derived, as they are also converted to combustibles. The feed to this process can be combined with the otherwise non-treatable wastes of various other treatment methods (a few of which are being discussed), so that they too meet a useful end as getting converted to combustibles.
Pyrolysis is a chemical change brought about by the action of heat. The process differs from incineration, which is the combustive destruction of a material in direct flame in the presence of oxygen. Pyrolysis can also be defined as destructive distillation in the absence of oxygen (or other oxidant) with the simultaneous removal of volatile products.
Pyrolysis converts wastes containing organic material to combustible gas, charcoal, organic liquids, and ash/metal residues. In some instances, the organic liquid fraction produced during pyrolysis has the potential to produce constituents for synthetic crude oil. As a different note, pyrolysis can also be applied to oil shales to produce liquid hydrocarbons for synthetic crude oil.
The effectiveness of waste destruction by pyrolysis depends upon – (i) the residence time within the retort, (ii) the rate of temperature increase, (iii) the final temperature, and (iv) the composition of the feed material.
Pyrolysis units, which operate at temperatures from 500° to 800°C (930°-1470°F) have achieved up to 99.9999 percent destruction/removal efficiencies.
Plasma torch processes apply the principles of pyrolysis at temperatures in the range 5,000° to 15,000°C (9,000°-27,000°F). The wastes are fed into the thermal plasma, where they are dissociated into their basic atomic components which recombine in the reaction chamber to form carbon monoxide, nitrogen, and hydrogen, as well as methane and ethane.
Acid gases which are removed from the emissions by scrubbers and any solid products are either incorporated into the molten bath at the bottom of the chamber or removed from exhaust gases by particulate scrubbers or filtres.
Solidification and Stabilisation:
Solidification and stabilisation are treatment systems which move beyond the older concept of ‘dig and move’ and are designed to – (i) improve the handling and the physical characteristic of the waste, (ii) decrease the surface area across which transfer or loss of contained pollutants can occur, and (iii) limit the solubility of, or detoxify, any chemical constituents in the waste.
Stabilisation/solidification processes are being used to minimise the potential for groundwater pollution from land disposal of hazardous wastes. Many variations are used, but most rely on pozzolanic reactions to chemically stabilise and physically solidify the waste. Portland cement alone or in combination with fly ash, cement kiln dust lime or other ingredients is the principal solidifying agent used.
Stabilisation and solidification processes are very effective at immobilising most heavy metals present in sludge, contaminated soils and other wastes. They are not as effective at immobilising toxic organic materials. Organically modified clays are now being evaluated as an additive to stabilisation and/or solidification processes in order to adsorb and retain these organic pollutants in the solidified waste form.
The environmental acceptability of stabilisation and solidification processes depends on the long term ability of the waste form to retain contaminants. This will be governed by the chemical binding mechanisms involved and by the durability of the waste for widespread acceptance of stabilisation and solidification processes will be hampered until the long-term durability of the waste form can be demonstrated.
Stabilisation usually involves the addition of materials that ensure that the chemical constituents are maintained in their least soluble or least toxic form. In the solidification process, the results are obtained primarily, but not exclusively, via the production of a monolithic block of treated waste with high structural integrity. Stabilisation techniques limit the solubility of, or detoxify), the waste contaminants; even though the physical characteristics of the waste may not be changed.
Both of these techniques might also have been classified under phase transition or phase conversion. Encapsulation, another perhaps more specific form of waste stabilisation, is a method by which mixed wastes, i.e. waste containing radioactive material and higher level radioactive wastes can be rendered less harmful to the environment. Cross-linked polyethylene, chosen for its durability, has been suggested as a suitable encapsulating reactive polymers such as maleic anhydride grafted polypropylene (pp-g-MA) are also being considered for this purpose.
Chemical Treatment of Wastes:
In any industrial organisation, the one way that is important and beneficial both economically and environmentally is to consider excess materials as an additional resource that can be utilised elsewhere either after they get discarded or retreated. It is in the better side of the humanity welfare that this aspect must be given the highest support and importance even without any regulations by any governmental organisations or so.
Environmental protection must be given the top most priority while meeting commercial targets. This is because of the undeniable fact that the costs of protecting the environment must have to be paid, if not today, tomorrow and if not by us, by our off-springs. Incorporation of available techniques that will render only environment-friendly outputs from any set-up has to be thought of and envisaged by industries, humanity and every individual.
It may seem costly today in some stray cases but, with inflation, it is never going to be cheaper than this. Is it justified to put our future generations in the altar for saving us a few dimes? Is not justice delayed justice denied? Long-term effects must not be neglected. In order to prevent degradation of the quality of the environment, we should no longer allow the treatment of the wastes remain to be a burning issue in today’s world.