With the ever growing waste generation and accumulation, the number of methods for the disposal of waste is also increasing. Some of the most popular ones are landfills, recycling, dumping and thermal methods. These days, with the advancements in technology, new methods for the disposal of wastes have been devised. Thermal methods of the disposal of wastes are one of the most prominent of them all. This refers to processes that involve the use of heat to treat waste. Listed below are descriptions of some commonly utilized thermal treatment processes.
Incineration is the most common thermal treatment process. This is the combustion of waste in the presence of oxygen. After incineration, the wastes are converted into carbon dioxide, water vapour and ash. This method may be used as a means of recovering energy to be used in heating or the supply of electricity. In addition to supplying energy incineration technologies have the advantage of reducing the volume of the waste, rendering it harmless, reducing transportation costs and reducing the production of the greenhouse gas methane. Public acceptance of incineration is retarded by the existence of a substantial number of old and inefficient (mainly batch-type) units which frequently operate substantially below their rated capacities, are visibly dirty in particulate emission, and cannot be economically modified to meet emission standards. A well-run, modern, large-capacity incinerator is not a major source of air pollutants. In an ordinary ton of mixed refuse, for example, sulphur constitutes as little as one-tenth of one percent.
Pyrolysis and Gasification
Pyrolysis and gasification are similar processes they both decompose organic waste by exposing it to high temperatures and low amounts of oxygen. Gasification uses a low oxygen environment while pyrolysis allows no oxygen. These techniques use heat and an oxygen starved environment to convert biomass into other forms. A mixture of combustible and non-combustible gases as well as pyroligenous liquid is produced by these processes. All of these products have a high heat value and can be utilised. Gasification is advantageous since it allows for the incineration of waste with energy recovery and without the air pollution that is characteristic of other incineration methods.
Open burning is the burning of unwanted materials in a manner that causes smoke and other emissions to be released directly into the air without passing through a chimney or stack. This includes the burning of outdoor piles, burning in a burn barrel and the use of incinerators that have no pollution control devices and as such release the gaseous by-products directly into the atmosphere (Department of Environmental quality 2006). Open burning has been practiced by some urban centres because it reduces the volume of refuse received at the dump and, therefore, extends the life of their dumpsite. Garbage may be burnt because of the ease and convenience of the method or because of the cheapness of the method. In countries where householders are required to pay for garbage disposal, burning of waste in the backyard allows the householder to avoid paying the costs associated with collecting, hauling and dumping the waste. Open burning has many negative effects on both human health and the environment. This uncontrolled burning of garbage releases many pollutants into the atmosphere. These include dioxins, particulate matter, polycyclic aromatic compounds, volatile organic compounds, carbon monoxide, hexachlorobenzene and ash. All of these chemicals pose serious risks to human health. The Dioxins are capable of producing a multitude of health problems; they can have adverse effects on reproduction, development, disrupt the hormonal systems or even cause cancer. The polycyclic aromatic compounds and the hexachlorobenzene are considered to be carcinogenic. The particulate matter can be harmful to persons with respiratory problems such as asthma or bronchitis, and carbon monoxide can cause neurological symptoms. The harmful effects of open burning are also felt by the environment. This process releases acidic gases such as the halo-hydrides; it also may release the oxides of nitrogen and carbon. Nitrogen oxides contribute to acid rain, ozone depletion, smog and global warming. Also, to being a greenhouse gas, carbon monoxide reacts with sunlight to produce ozone that can be harmful. The particulate matter creates smoke and haze that contribute to air pollution.
In Situ Thermal Treatment
In situ thermal treatment methods move or “mobilize” harmful chemicals in soil and groundwater using heat. The chemicals move through soil and groundwater toward wells where they are collected and piped to the ground surface to be treated using other cleanup methods. Some chemicals are destroyed underground during the heating process. Thermal treatment is described as “in situ” because the heat is applied underground directly to the contaminated area. It can be particularly useful for chemicals called “non-aqueous phase liquids” or “NAPLs,” which do not dissolve readily in groundwater and can be a source of groundwater contamination for a long time if not treated. Examples of NAPLs include solvents, petroleum, and creosote (a wood preservative).
In situ thermal treatment methods heat contaminated soil, and sometimes nearby groundwater, to very high temperatures. The heat vaporizes (evaporates) the chemicals and water changing them into gases. These gases also referred to as “vapors,” can move more easily through soil. The heating process can make it easier to remove NAPLs from both soil and groundwater. High temperatures also can destroy some chemicals in the area being heated.
In situ thermal methods generate heat in different ways:
• Electrical resistance heating (ERH) delivers an electrical current between metal rods called “electrodes” installed underground. The heat generated as movement of the current meets resistance from soil converts groundwater and water in soil into steam, vaporizing contaminants.
• Steam-enhanced Extraction (SEE) injects steam underground by pumping it through wells drilled in the contaminated area. The steam heats the area and mobilizes and evaporates contaminants.
• Thermal conduction heating (TCH) uses heaters placed in underground steel pipes. TCH can heat the contaminated area hot enough to destroy some chemicals.
These were some of the Thermal methods of waste disposal which are becoming famous these days. There is no denying that these methods are not as popular as the primitive ones and the reason behind this is that people feel the older methods are much more reliable. It is not possible or feasible to compare any of the methods. All the methods have advantages and disadvantages just like every coin has two sides.