The following points highlight the three types of waste treatment. The types are: 1. Thermal Treatment 2. Thermochemical Conversion Methods 3. Biological Waste Treatment.
Type # 1. Thermal Treatment:
Thermal treatment is a process that involves the use of heat to treat waste.
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Descriptions of some commonly utilised thermal treatment processes are listed below:
i. Open Burning (Uncontrolled Burning):
In the open burning method, burning of solid waste 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 process includes the burning of outdoor piles, burning in a burn barrel and the use of incinerators which have no pollution control devices and as such release the gaseous by products directly into the atmosphere.
Open- burning has been practiced by a number of 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 house holders 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 adverse effects on all living beings, monuments 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.
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 carbon, sulphur and nitrogen. Oxides of sulphur and nitrogen contribute to acid rain, ozone depletion, smog and global warming. In addition to being a green-house gas carbon monoxide reacts with sunlight to produce ozone which can be harmful. The particulate matter creates smoke and haze which contribute to air pollution.
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ii. Incineration:
Incineration is the process in which the burning of solid waste is takes place in large furnace. In this process recyclable material is segregated and the non-recyclable material is burnt. At the end of the process ash is left, some ash flies out with the hot air. This is called fly ash. The remaining ash has high concentration of dangerous toxins such as toxins of heavy metals. Disposing this ash poses a problem.
At present incineration is considered the last resort and is used mainly for treating hospital (infections waste). Incineration is the most common thermal treatment process which takes place at high temperature. This is the combustion of waste in the presence of oxygen. After incineration, the wastes are converted to 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 green-house gas methane.
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iii. Plasma Arc Furnaces:
Plasma arc furnaces are used for incineration of hazardous waste. High temperature is applied in plasma arc, these are electrically neutral, stream in gaseous form consisting of positively charged particles, by the electron beam to this. Pressure of this plasma arc is responsible for its temperature, transport properties and permitting the distractive capacity.
The temperature rises to 10000°C. The waste gets decomposed within milliseconds at extremely high temperature. The major advantage of this technique for decomposition of waste material is without generation of secondary combustion products, the produced flue gases cleanup easily.
iv. Wet Oxidation:
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In this technique of waste degradation wet oxidation is applied at high temperature. The principle of this process is that any waste material that can burn and also can be oxidised in the presence of water at 250°C-700°C. Produced sludge is exposed at high temperature and pressure in a reaction chamber (260°C, 1000-1700 psi).
After increase in pressure (1000-1700 psi) the air is injected in presence of water to enhance oxidation. Solid waste reacts with the oxygen and the products in solid form can be separated from liquid by using grit removal, precipitation, settling, vacuum filtration or concentrate by dehydration.
Type # 2. Thermochemical Conversion Methods:
Thermochemical conversion methods consist of as per following:
i. Pyrolysis
ii. Thermal liquifaction
iii. Carbonisation and Torrefaction.
i. Pyrolysis:
Pyrolysis is the process in which thermal degradation of solid waste (wood) in the absence of oxygen. It enables biomass to be converted to a combination of solid char, gas and a liquid bio-oil. Pyrolysis process is of two types on the basis of time and temperature utilised in the process. They can be commonly categorised as ‘fast pyrolysis’ and ‘slow pyrolysis’.
The products of pyrolysis are generated in roughly equal proportions with slow pyrolysis. Bio-oil is the yield of fast pyrolysis, can be as high as 80 per cent of the product on a dry fuel basis. Bio-oil can act as a liquid fuel or as a feedstock for chemical production. Different bio-oils production processes are under development, including fluid bed reactors, ablative pyrolysis, entrained flow reactors, rotating cone reactors, and vacuum pyrolysis.
Any solid waste is heated at high temperature in the absence of oxygen can be degraded. Word ‘biomass pyrolysis’ is generally associated with the processes involving bio-oils and liquid chemical fuel production.
Fast/flash pyrolysis is performed at high temperature (400-600°C) for very short time (0.6-3.5 seconds). The shorter time exposure of high temperature to the organic matter (e.g., waste) in fast/flash pyrolysis process results in increased significance of heat and mass transfer, and phase transition along with chemical reaction kinetics. Long residence times (few minutes to hours) and lower temperature range (205-360°C) favour charcoal formation.
In this process reactors like fluidised bed reactors use smaller particle size and high temperature to achieve very fast heat transfer, leading to minimised char formation. The low thermal conductivity of biomass particles is very well exploited in such reactors where pellets of biomass are pressed against heated surface, resulting formation of vapour as well as exposing unaffected inner surface.
Dry biomass, small particle size, short residence times, moderate-to- high temperatures, and rapid quenching of pyrolysed vapour will favour fast pyrolysis. The lack of predictive kinetic constants for fast pyrolysis is due to its unsteady state nature as the biomass complexity requires multistage thermal decomposition with production of substantial quantities of highly unstable compounds. These factors have great impact on the design of a fast pyrolysis system which should rapidly heat biomass to desired temperature as well as quickly quench down the products.
ii. Thermal Liquefaction:
Thermal liquefaction is very much similar to pyrolysis process in simplified comparisons. Both processes differ in operating parameters, requirement of catalyst, and final products. Liquefaction produces mainly liquid and some amounts of gaseous components at temperature and pressure ranges of 250-350°C and 700-3000 psi, respectively, in the presence of alkali metal salts as catalyst. To facilitate the overall process, liquefaction may require supplemental CO and H2 as reactants.
The mechanisms of liquefaction reactions lack sufficient description about role of catalysts. The catalysts hydrolyse the cellulose, hemi- celluloses, and lignin macromolecules into smaller micellar-like fragments, which are further degraded to smaller compounds. In such type of degradation reactions like dehydration, dehydrogenation, deoxygenation, and decarboxylation takes place.
In comparison to Torrefaction/carbonisation, thermal liquefaction can provide liquid fuels in line with petroleum products along with several high value chemicals. In biomass thermochemical conversion, liquefaction, could not be successful at commercial scale.
The possible factors that limit the liquefaction commercialisation could be the lower overall yield of oil (between 20-55% w/w) compared to contemporary options like pyrolysis, inferior oil quality (heavy tar like liquid), higher reaction temperature, pressure, requirements of catalysts and other reactants (CO, propanol, butanol, and glycerine).
iii. Carbonisation and Torrefaction:
Carbonisation is used for biochar formation whereas, Torrefaction is a thermal treatment to convert biomass into more efficient form of energy source containing less moisture and high fixed carbon, for the purpose to produce chemical feedstock which reduces the associated transportation costs, and torrefaction are closely related processes. In carbonisation, the biomass is thermochemically treated at the temperature range between 200°C to 318°C in the absence of oxygen. There will be complete conversion of biomass into biochar in this carbonisation process.
Thus, the product gains much higher energy density than the raw biomass, which lowers the transportation cost of the carbonised biomass due to reduction in volume of biomass (waste). In the case of torrefaction, there is partial degradation of the biomass (particularly the hemicellulose), giving off various types of volatiles resulting in brittle, dried and more volatile free solid product.
Carbonised or torrefied biomass has favourable characteristics such as, hydrophobic nature, similar to properties as coal, easy to crush, grind or pulverise. The end-products contains condensable gases such as water vapour, formic acid, acetic acid, furfural, methanol, lactic acid, and phenol. Non- condensable gases such as carbon dioxide, carbon monoxide and small amounts of hydrogen and methane are also obtained.
Thus, carbonisation and torrefaction processes are used for the conversion of biomass into more promising forms of energy source and to reduce the associated transportation costs. Although it is efficient form of energy, still it is not in competition with petroleum fuels in transport sector.
Gasification:
Concept and Principle:
Gasification is the process of converting fuels from solid in nature to gaseous fuel. It is very similar to pyrolysis but it is not simply pyrolysis. Pyrolysis is only one of the steps in the conversion process. Combustion with air and reduction of the product of combustion, (water vapour and carbon dioxide) into combustible gases, (carbon monoxide, hydrogen, methane, some higher hydrocarbons) and inert, (carbon dioxide and nitrogen) are other steps of this process.
The process leads to generation of a gas with some fine dust particles and condensable compounds termed as tar, if this produced gas is to be used in internal combustion engines, particle size must be restricted to less than about 100 ppm each.
Gasification is a complete thermal breakdown of the biomass particles into a combustible gas, volatiles and ash in an enclosed reactor known as gasifier, in the presence of any externally supplied oxidising agent (air, O2, H2O, CO2, etc.). Amount of oxidising agents is depends on stoichiometric amount of oxidising agent. Stoichiometric amount is the theoretical amount of air or any other oxidising agent required to burn the fuel completely.
Gasification is a two-step, endothermic process. It is an intermediate step between pyrolysis and combustion. During the first step the volatile components of the fuel are vaporised at temperatures below 600°C by a group of complex reactions. In this step of the process, no oxygen is needed.
Volatile vapours include hydrocarbon gases, hydrogen, carbon monoxide, carbon dioxide, tar and water vapour. Char and ash are the by-products of the process which are not vaporised. Char is made up of fixed carbon. In the second step, char is gasified through the reactions with oxygen, steam and hydrogen. Residual unburned char is combusted to release the heat needed for the endothermic gasification reactions.
Gas, char, and tars are the main gasification products. Gasification products, their composition and amount are strongly influenced by gasification agent, temperature, and pressure, heating rate and fuel characteristics. These fuel characteristics are composition of fuel, water content and granulometry. Gaseous products formed during the gasification may be further used for heating or electricity production. The main gas components are CO2, CO, H2O, CH4, H2 and other hydrocarbons.
Combustible gas, produced during gasification can be cleaned and used for the synthesis of special chemical products or for the generation of heat and/or electricity. Specific hydrogen – carbon monoxide mixtures are known as water gas, cracked gas, and methanol synthesis gas or oxo-synthesis gas.
Gasification is a unique technology that can even convert waste (from MSW to agricultural or crop residues, like coconut shells, rice husks, straw, wood residues, bagasse, etc.), to a useful and high quality energy source. It is known how complicated the disposal of any kind of waste is, now-a-days due to environmental regulations and legislations. Separation of the noxious substances from the fuel gas is the advantage of the gasification process prior to the combustion.
Biomass Gasification Technology:
Biomass has been considered as a major energy source, prior to the discovery of fossil fuels like coal and petroleum. It is widely used in rural communities of the developing countries for their energy needs in terms of cooking and limited industrial use. Even it is not currently popular in developed countries. Biomass, which is in solid form, can be converted into gaseous form through gasification process.
Biomass gasification has also received much attention in recent times. The solid biomass is converted to simplified products like CO and H2, in the optimised concentrations of oxygen and H2O at temperatures 800°C which is completely distinct from gasification via., anaerobic digestion.
The final products are syngas, CO2, NOx, SOx, and ash/metal slag (quantity will depend upon the type of the waste- municipal, agricultural, or wood biomass). CO and H2 mixture is known as syngas. It has multiple applications such as fuel cells, synthetic fuel, and chemical feedstock. As for as technology is concern, gasification is an excellent method of extracting bioenergy free from N, P, S, Cl and metals contamination from diverse biomass types without further treatment or upgrading.
Type # 3. Biological Waste Treatment:
Composting is the decomposition of organic matter under controlled aerobic conditions by the action of microorganisms and small invertebrates. There are a number of composting techniques being used now days. These techniques include- in vessel composting, windrow composting, vermicomposting and static pile composting. The composting process is controlled by making the environmental conditions optimum for the micro-organisms involved in decomposition of waste.
The rate of compost formation is controlled by the composition and constituents of the materials, i.e., their Carbon/Nitrogen (C/N) ratio, the temperature, the moisture content and the amount of air. It is natural process of decomposition of organic waste and yield manure or compost which is rich in nutrients.
Composting is a biological process in which microorganisms like fungi and bacteria convert degradable organic waste in to humus like substance. This humus is rich in carbon and nitrogen content and it is a best medium for growing plants. Municipal Solid Waste (MSW) contains 32-40% organic matter in India. This waste can be recycled by compositing.
This method is clean, safe and can significantly reduce the amount of disposable garbage. Organic fertilisers have advantages over chemical fertilisers as they increase the ability of the soil to hold water, making it easier to cultivate. Vermicompost also contains good amount of nutrient.
For the growth of microorganisms require carbon as an energy source and nitrogen for the synthesis of some proteins. If the correct C/N ration is not achieved, then application of the compost with either a high or low C/N ratio can have adverse effects on both the soil and the plants. A high C/N ratio can be maintained by dehydrated mud and a low ratio corrected by addition of cellulose.
Moisture content present in waste is greatly influences the composting process. The micro-organisms require the moisture content to perform their metabolic functions. If the waste becomes too dry the composting is not good. If there is too much moisture then it is possible that it may displace the air in the compost heap depriving the organisms of oxygen and drowning them.
In this process high temperature is required for the elimination of pathogenic organisms. However, if temperature is too high, as >75°C then the organisms will not survive, those are necessary to complete the composting process. Optimum temperatures for the composting process are in the range of 50-60°C. Aeration is another a very important factor and the quantity of air need to be properly controlled during this process of composting.
If oxygen is not sufficient, the aerobic micro-organisms will begin to die and will be replaced by anaerobes. The anaerobes are undesirable since they will slow-down this process, produce odours and also produce the highly flammable methane gas. In this process, aeration can be done by churning the compost.
Anaerobic digestion uses biological processes to decompose organic waste. However, where composting can use a variety of aerobic micro-organisms, which must require air, anaerobic digestion uses anaerobic bacteria to decompose the waste. Aerobic respiration, typical of composting, results in the formation of CO2 and H2O. While the anaerobic respiration results in the formation of CO2 and CH4.
In addition to generating the humus which is used as a soil conditioner, anaerobic digestion is also used as a method of producing biogas which can be used to generate electricity. This process require nutrients such as nitrogen, phosphorous and potassium, also requires the pH be maintained around 7 and the alkalinity be appropriate to buffer pH changes, temperature should also be controlled.
iii. Integrated Solid Waste Management:
Integrated solid waste management is an overall approach to create sustainable systems that are economically affordable, socially acceptable and environmentally effective. This integrated solid waste management system requires the use of a range of different treatment methods. Collection and sorting of the waste is the key to the functioning of such a system.
It is important to note that no one single treatment method can manage all the waste materials in an environmentally effective way. Thus all the available treatment and disposal options must be evaluated equally and the best combination of the available options suited to the particular community chosen. Effective management plans are needed to operate in such a ways which best meet current social, economic, and environmental conditions of the municipality.