Another method of treating municipal solid waste is composting, a biological process in which the organic portion of refuse is allowed to decompose under carefully controlled conditions. Microbes metabolize the organic waste material and reduce its volume by as much as 50 percent. The stabilized product is called compost or humus. It resembles potting soil in texture and odour and may be used as a soil conditioner or mulch.
Composting offers a method of processing and recycling both garbage and sewage sludge in one operation. As more stringent environmental rules and siting constraints limit the use of solid-waste incineration and landfill options, the application of composting is likely to increase. The steps involved in the process include sorting and separating, size reduction, and digestion of the refuse.
Sorting and shredding
The decomposable materials in refuse are isolated from glass, metal, and other inorganic items through sorting and separating operations. These are carried out mechanically, using differences in such physical characteristics of the refuse as size, density, and magnetic properties. Shredding or pulverizing reduces the size of the waste articles, resulting in a uniform mass of material. It is accomplished with hammer mills and rotary shredders.
Digesting and processing
Pulverized waste is ready for composting either by the open windrow method or in an enclosed mechanical facility. Windrows are long, low mounds of refuse. They are turned or mixed every few days to provide air for the microbes digesting the organics. Depending on moisture conditions, it may take five to eight weeks for complete digestion of the waste. Because of the metabolic action of aerobic bacteria, temperatures in an active compost pile reach about 65 °C (150 °F), killing pathogenic organisms that may be in the waste material.
Open windrow composting requires relatively large land areas. Enclosed mechanical composting facilities can reduce land requirements by about 85 percent. Mechanical composting systems employ one or more closed tanks or digesters equipped with rotating vanes that mix and aerate the shredded waste. Complete digestion of the waste takes about one week.
Digested compost must be processed before it can be used as a mulch or soil conditioner. Processing includes drying, screening, and granulating or pelletizing. These steps improve the market value of the compost, which is the most serious constraint to the success of composting as a waste management option. Agricultural demand for digested compost is usually low because of the high cost of transporting it and because of competition with inorganic chemical fertilizers.
Sanitary landfill
sanitary landfill
sanitary landfill
Construction of a sanitary landfill.
Encyclopædia Britannica, Inc.
Land disposal is the most common management strategy for municipal solid waste. Refuse can be safely deposited in a sanitary landfill, a disposal site that is carefully selected, designed, constructed, and operated to protect the environment and public health. One of the most important factors relating to landfilling is that the buried waste never comes in contact with surface water or groundwater. Engineering design requirements include a minimum distance between the bottom of the landfill and the seasonally high groundwater table. Most new landfills are required to have an impermeable liner or barrier at the bottom, as well as a system of groundwater-monitoring wells. Completed landfill sections must be capped with an impermeable cover to keep precipitation or surface runoff away from the buried waste. Bottom and cap liners may be made of flexible plastic membranes, layers of clay soil, or a combination of both.
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Constructing the landfill
The basic element of a sanitary landfill is the refuse cell. This is a confined portion of the site in which refuse is spread and compacted in thin layers. Several layers may be compacted on top of one another to a maximum depth of about 3 metres (10 feet). The compacted refuse occupies about one-quarter of its original loose volume. At the end of each day’s operation, the refuse is covered with a layer of soil to eliminate windblown litter, odours, and insect or rodent problems. One refuse cell thus contains the daily volume of compacted refuse and soil cover. Several adjacent refuse cells make up a lift, and eventually a landfill may comprise two or more lifts stacked one on top of the other. The final cap for a completed landfill may also be covered with a layer of topsoil that can support vegetative growth.
Two methods of constructing a sanitary landfill. (The top and bottom liners and the leachate collection systems are not shown.)
Two methods of constructing a sanitary landfill. (The top and bottom liners and the leachate collection systems are not shown.)
Encyclopædia Britannica, Inc.
Daily cover soil may be available on-site, or it may be hauled in and stockpiled from off-site sources. Various types of heavy machinery, such as crawler tractors or rubber-tired dozers, are used to spread and compact the refuse and soil. Heavy steel-wheeled compactors may also be employed to achieve high-density compaction of the refuse.
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The area and depth of a new landfill are carefully staked out, and the base is prepared for construction of any required liner and leachate-collection system. Where a plastic liner is used, at least 30 cm (12 inches) of sand is carefully spread over it to provide protection from landfill vehicles. At sites where excavations can be made below grade, the trench method of construction may be followed. Where this is not feasible because of topography or groundwater conditions, the area method may be practiced, resulting in a mound or hill rising above the original ground. Since no ground is excavated in the area method, soil usually must be hauled to the site from some other location. Variations of the area method may be employed where a landfill site is located on sloping ground, in a valley, or in a ravine. The completed landfill eventually blends in with the landscape.
Controlling by-products
Organic material buried in a landfill decomposes by anaerobic microbial action. Complete decomposition usually takes more than 20 years. One of the by-products of this decomposition is methane gas. Methane is poisonous and explosive when diluted in the air, and it can flow long distances through porous layers of soil. If it is allowed to collect in basements or other confined areas, dangerous conditions may arise. In modern landfills, methane movement is controlled by impermeable barriers and by gas-venting systems. In some landfills the methane gas is collected and recovered for use as a fuel.
A highly contaminated liquid called leachate is another by-product of decomposition in sanitary landfills. Most leachate is the result of runoff that infiltrates the refuse cells and comes in contact with decomposing garbage. If leachate reaches the groundwater or seeps out onto the ground surface, serious environmental pollution problems can occur, including the possible contamination of drinking-water supplies. Methods of controlling leachate include the interception of surface water in order to prevent it from entering the landfill and the use of impermeable liners or barriers between the waste and the groundwater. New landfill sites should also be provided with groundwater-monitoring wells and leachate-collection and treatment systems.
Importance in waste management
In communities where appropriate sites are available, sanitary landfills usually provide the most economical option for disposal of nonrecyclable refuse. However, it is becoming increasingly difficult to find sites that offer adequate capacity, accessibility, and environmental conditions. Nevertheless, landfills will always play a key role in solid-waste management. It is not possible to recycle all components of solid waste, and there will always be residues from incineration and other treatment processes that will eventually require disposal underground. In addition, landfills can actually improve poor-quality land. In some communities properly completed landfills are converted into recreational parks, playgrounds, or golf courses.
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Recycling
recycling: role in solid-waste disposal
recycling: role in solid-waste disposal
The role of recycling in solid-waste disposal.
Encyclopædia Britannica, Inc.
recycling: automobile
recycling: automobile
Learn how automobiles are recycled, including the uses of various parts.
Contunico © ZDF Enterprises GmbH, Mainz
recycling
recycling
Learn why garbage is a valuable resource.
Contunico © ZDF Enterprises GmbH, Mainz
Separating, recovering, and reusing components of solid waste that may still have economic value is called recycling. One type of recycling is the recovery and reuse of heat energy, a practice discussed separately in Incineration. Composting can also be considered a recycling process, since it reclaims the organic parts of solid waste for reuse as mulch or soil conditioner. Still other waste materials have potential for reuse. These include paper, metal, glass, plastic, and rubber, and their recovery is discussed here.
Separation
Before any material can be recycled, it must be separated from the raw waste and sorted. Separation can be accomplished at the source of the waste or at a central processing facility. Source separation, also called curbside separation, is done by individual citizens who collect newspapers, bottles, cans, and garbage separately and place them at the curb for collection. Many communities allow “commingling” of nonpaper recyclables (glass, metal, and plastic). In either case, municipal collection of source-separated refuse is more expensive than ordinary refuse collection.
In lieu of source separation, recyclable materials can be separated from garbage at centralized mechanical processing plants. Experience has shown that the quality of recyclables recovered from such facilities is lowered by contamination with moist garbage and broken glass. The best practice, as now recognized, is to have citizens separate refuse into a limited number of categories, including newspaper; magazines and other wastepaper; commingled metals, glass, and plastics; and garbage and other nonrecyclables. The newspaper, other paper wastes, and commingled recyclables are collected separately from the other refuse and are processed at a centralized material recycling facility, or MRF (pronounced “murf” in waste-management jargon). A modern MRF can process about 300 tons of recyclable wastes per day.
At a typical MRF, commingled recyclables are loaded onto a conveyor. Steel cans (“tin” cans are actually steel with only a thin coating of tin) are removed by an electromagnetic separator, and the remaining material passes over a vibrating screen in order to remove broken glass. Next, the conveyor passes through an air classifier, which separates aluminum and plastic containers from heavier glass containers. Glass is manually sorted by colour, and aluminum cans are separated from plastics by an eddy-current separator, which repels the aluminum from the conveyor belt.
Reuse
Recovered broken glass can be crushed and used in asphalt pavement. Colour-sorted glass is crushed and sold to glass manufacturers as cullet, an essential ingredient in glassmaking. Steel cans are baled and shipped to steel mills as scrap, and aluminum is baled or compacted for reuse by smelters. Aluminum is one of the smallest components of municipal solid waste, but it has the highest value as a recyclable material. Recycling of plastic is a challenge, mostly because of the many different polymeric materials used in its production. Mixed thermoplastics can be used only to make lower-quality products, such as “plastic lumber.”
In the paper stream, old newspapers are sorted by hand on a conveyor belt in order to remove corrugated materials and mixed papers. They are then baled or loose-loaded into trailers for shipment to paper mills, where they are reused in the making of more newspaper. Mixed paper is separated from corrugated paper for sale to tissue mills. Although the processes of pulping, de-inking, and screening wastepaper are generally more expensive than making paper from virgin wood fibres, the market for recycled paper should improve as more processing plants are established.
Rubber is sometimes reclaimed from solid waste and shredded, reformed, and remolded in a process called revulcanization, but it is usually not as strong as the original material. Shredded rubber can be used as an additive in asphalt pavements, and discarded tires may be employed as swings and other recreational structures for use by children in “tire playgrounds.” In general, the most difficult problem associated with the recycling of any solid-waste material is finding applications and suitable markets. Recycling by itself will not solve the growing problem of solid-waste management and disposal. There will always be some unusable and completely valueless solid residue requiring final disposal.
MECHANICAL COMPOSTING
Category: Composting
Though manual methods are preferable in countries where labour is comparatively cheap, mechanical processes are preferred (Gotaas 1956) where higher labour costs and limitations of space exist. In 1922, Becari in Italy patented a process using a combination of aerobic and anaerobic decomposition in enclosed containers. The first full scale plant was established in 1932 in the Netherlands by a non profit utility company-VAM using Van Maanen Process in which raw refuse is composted in large windrows, which are turned at intervals by mobile cranes moving on rails. The Dano Process was developed in Denmark in 1930. Several other processes were subsequently developed using different methods of processing of solid waste using different designs of digester.
14.6.1 Unit Processes
A mechanical composting plant is a combination of various units which perform specific functions. Fig.14.3 gives a general flowchart of a mechanical compost plant.
Solid waste collected from various areas reaches the plant site at a variable rate depending upon the distance of collection point. As the compost plant operates at a constant rate, a balancing storage has to be provided to absorb the fluctuations in the waste input to the plant. This is provided in a storage hopper of 8 to 24 hours storage capacity, the exact value depending upon the schedule of incoming trucks, the number of shifts and the number of days the plant and solid waste collection system works.
The waste is then fed to a slowly moving (5metres/minute) conveyor belt and the non-decomposable material such as plastics, glass, metals are manually removed by labourers standing on either side of the conveyor belt. The labourers are provided with hand gloves and manually remove the material from the moving belt (the thickness over the belt is kept less than 15cms) and the removed material is stored separately.
The metals are then removed from the waste by either a suspended magnet system(Fig.14.4a) or a magnetic pulley system (Fig.14.4b). Majority of the metals are recycled at the source itself and hence are not contained in the waste.
Magnetic removal of metals hence is not very efficient and therefore not used is India.
14.6.1 Unit Processes
A mechanical composting plant is a combination of various units which perform specific functions. Fig.14.3 gives a general flowchart of a mechanical compost plant.
Solid waste collected from various areas reaches the plant site at a variable rate depending upon the distance of collection point. As the compost plant operates at a constant rate, a balancing storage has to be provided to absorb the fluctuations in the waste input to the plant. This is provided in a storage hopper of 8 to 24 hours storage capacity, the exact value depending upon the schedule of incoming trucks, the number of shifts and the number of days the plant and solid waste collection system works.
The waste is then fed to a slowly moving (5metres/minute) conveyor belt and the non-decomposable material such as plastics, glass, metals are manually removed by labourers standing on either side of the conveyor belt. The labourers are provided with hand gloves and manually remove the material from the moving belt (the thickness over the belt is kept less than 15cms) and the removed material is stored separately.
The metals are then removed from the waste by either a suspended magnet system(Fig.14.4a) or a magnetic pulley system (Fig.14.4b). Majority of the metals are recycled at the source itself and hence are not contained in the waste.
Magnetic removal of metals hence is not very efficient and therefore not used is India.
In developed countries glass and metals are present in larger concentration and are removed by using ballistic separators. In these units, the waste is thrown with a large force when different constituents take different tragectories and get separated (Fig.14.5). This unit is energy intensive and due to smaller content of glass and metals in Indian municipal solid waste, it is not used in India.
The waste is thus subjected to size reduction when the surface area per unit weight is increased for faster biological decomposition. Size reduction also helps in reducing fly breeding in the decomposing mass. This is commonly carried out either in Hammermills or Rasp mills. Hammermills are high speed (600-1200 revolutions per minute) compact machines but consume large energy (Fig.14.6). Rasp mills are slow moving large units that require lesser energy (Fig.14.7). The capital cost of a hammer mill is less but its operating cost is more than that of a rasp mill mainly due to the larger energy requirement as well as more frequent replacement / retipping of hammers.
The stabilisation is carried out in open windrows provided over flagstone paved or cement concrete paved ground. These windrows are turned every 5 days to ensure aerobic decomposition. Various types of equipment such as front end loaders/windrows reshifters are used for turning of windrows.
At the end of the 3 to 4 weeks period, the material is known as green or fresh compost wherein the cellulose has not been fully stabilised. It is hence stored in large sized windrows for 1-2 months either at the plant or the farms. At the end of the storage period, it is known as ripe compost. It may be sometimes subjected to size reduction to suit kitchen garden and horticulture requirements.
The waste is thus subjected to size reduction when the surface area per unit weight is increased for faster biological decomposition. Size reduction also helps in reducing fly breeding in the decomposing mass. This is commonly carried out either in Hammermills or Rasp mills. Hammermills are high speed (600-1200 revolutions per minute) compact machines but consume large energy (Fig.14.6). Rasp mills are slow moving large units that require lesser energy (Fig.14.7). The capital cost of a hammer mill is less but its operating cost is more than that of a rasp mill mainly due to the larger energy requirement as well as more frequent replacement / retipping of hammers.
The stabilisation is carried out in open windrows provided over flagstone paved or cement concrete paved ground. These windrows are turned every 5 days to ensure aerobic decomposition. Various types of equipment such as front end loaders/windrows reshifters are used for turning of windrows.
At the end of the 3 to 4 weeks period, the material is known as green or fresh compost wherein the cellulose has not been fully stabilised. It is hence stored in large sized windrows for 1-2 months either at the plant or the farms. At the end of the storage period, it is known as ripe compost. It may be sometimes subjected to size reduction to suit kitchen garden and horticulture requirements.
What is Mechanical Composting?
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As an alternative to windrow composting, it is possible to produce humus within 5-7 days using mechanical systems often the composted material is removed and cured in open windows for an additional period of about 3 weeks. Once the solid waste has been converted to humus, they are ready for the third step of product preparation and marketing.
This method of composting is carried out in different vessels:
(i) horizontal plug flow reactor
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(ii) vertical continuous flow reactor
(iii) rotating drum.
Process microbiology
Aerobic composting is a dynamic system in which bacteria, actinomycetes, fungi, and other biological forms are actively involved. The relative predominance of one species over other depends upon the constantly changing available food supply, temperature and substrate conditions. In this process, facultative and obligate aerobic forms of bacteria, actinomycetes and fungi are most active. Mesophilic forms are predominant in the initial stages which soon give way to thermophilic bacteria and fungi except during the final stages of composting when the temperature drops, actinomycetes and fungi are confined to 5-15 cm of outer surface layer. If turning is not carried out frequently, increased growth of actinomycetes and fungi in the outer layer imparts a typical greyish white colour.
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Attempts are yet to be made to identify the role of different organisms in the breakdown of different materials. Thermophilic bacteria are mainly responsible for breakdown of protein and other readily biodegradable organic matter. Fungi and actinomycetes play an important role in the decomposition of cellulose and lignin.
Design consideration for composting (aerobic) process
Particle size
For optimum results the particle size in the range of 25-75 mm.
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Seeding
Composting time can be reduced by adding partially decomposed compost (1-5%) or sewage sludge.
Mixing /tuning
To prevent drying, caking and air channeling, materials in the process of being composted should be mixed or turned on a regular schedule.
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Moisture content
It should be in the range of 50-60% during composting process.
Temperature
The optimum temperature for biological stabilization is between 45-55° C.
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Carbon nitrogen ratio
The initial carbon-nitrogen ratio between 35-50 is optimum for composting process. At lower ratios and higher pH levels nitrogen is in excess and will be given off as ammonia. At higher ratios nitrogen will be the limiting nutrient.
pH
The pH should be maintained within 8.5 to minimise the loss of nitrogen in the form of ammonia.
Control of pathogens
At the end of composting process temperature should be maintained between 60 and 70°C for 24 to destroy pathogenic organisms