A variety of processing methods are available at present for handling solids wastes. Most have been in use in some modification for at least the last 50 years. The choice of processing method will depend not only on the type of waste but also on location, sources, quantity of waste, method of collection, public opinion and ultimately economics.
The major disposal methods in use are landfill and incineration. Of potential interest in some places are high pressure compaction and reclamation by recycling. Composting is also practised but has not been successfully applied although it does have potential. There are new processes and techniques appearing for waste disposal and for the first time an organised research and development effort has been mounted to look at solid waste disposal.
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Disposal methods could be discussed from the point of view of source – a brief summary of the most used methods for a variety of sources may be found in Table 5.7. This discussion will instead focus on the disposal methods most commonly in use today, landfill and incineration, followed by discussion of compaction, composting and some of the newer disposal techniques.
1. Sanitary Landfill:
Landfill is the most widely used method of waste disposal. There are many unauthorised dumps, but unfortunately only few are considered to be ‘truly’ sanitary. The remainder are unauthorised ones.
Sanitary landfill is a perfectly acceptable method of disposal of solids and provides for the ultimate disposal of many types of waste—exceptions are non-degradable materials such as plastic and aluminium which are placed in landfills. Other items material, toxic chemicals, are not allowed in landfills for safety. Where land is plentiful or marginal areas available for reclamation, sanitary landfills offer a number of advantages over other disposal methods including low initial and operating costs.
Other advantages and disadvantages are below:
Advantages:
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(i) Most economical method when land is available.
(ii) Low initial investment.
(iii) Complete and final disposal.
(iv) Short period of time from need to full operation.
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(v) Flexible daily capacity with same working force.
(vi) Reclamation of marginal land for recreational and other uses.
(vii) All types of waste are acceptable.
Disadvantages:
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(i) Lack of close-by suitable land in urban areas may make uneconomical.
(ii) Public opposition in, or near, residential areas.
(iii) Settling after completion on means continued maintenance.
(iv) Public nuisance and health hazard if not properly operated.
(v) Products of decomposition, methane and other gases, may create hazard.
(vi) Requires special practices for construction on completed fill.
Sanitary landfill is basically the dumping of wastes followed by compaction and the daily application of an earth cover. Several techniques are available, depending on the type of site available. The one constant in all operations is the daily earth cover, preferably a sandy loam, amounting to, usually, one part earth for every four parts refuse.
Proper site selection is as critical to a satisfactory landfill as is sound operation. Selection criteria include proper ground and surface water drainage to prevent pollution of the ground water drainage. Location in a drainage basin or near streams or lakes present special problems and should be avoided where possible.
Accessibility of cover material is an important consideration. The use of tidal areas and marshes presents special problems and is frowned upon now, as a misuse of this type of natural resource. Dry pits, abandoned quarries and certain types of canyons or depressions are satisfactory landfill sites.
The size of landfills is often restricted by the amount of land available. The capacity can be estimated with a fair degree of accuracy.
Much of the material in the sanitary landfill decomposes over a period of between three and ten years depending on climate, permeability of the cover, composition of the refuse and degree of compaction.
The decomposition in sanitary landfills is anaerobic as compared to aerobic degradation often found in other types of fill. Temperatures typically reach 120°F in the fill as a result of the degradation. The principal gas products are carbon dioxide and methane. The greatest gas production takes place in the first two years.
Ammonia and hydrogen sulphide are not problems in sanitary landfills although small amounts of these gases are produced. Odours resulting from the decomposition of putrescible material can be controlled by observing good operating practices; that is, covering the fill continuously and sealing surface cracks. Fire hazards and insects and vermin are not a problem, as compared to dumps, in a properly operated sanitary landfill although chemical control of the latter two is sometimes required.
Completed landfills are suitable for use as recreational facilities, airfields and parking areas; light industrial buildings may be erected on landfill. Building of residential structures on fill requires special precautions because of the potential hazards associated with the evolution of methane and other decomposition gases.
The cost of operating a sanitary landfill makes it an attractive means of disposal where land is available. The use of landfill will continue; however, its future, particularly in densely populated urban areas, is in doubt. Land is at a premium for this type of application close to urban centres. What land is available must be preserved for non-combustible material and ashes.
2. Incineration:
Incineration is essentially a method for reducing residue volume and at the same time producing an inert, essentially inorganic, solid effluent from material which is largely organic. In addition to the solid product a gas is produced consisting mainly of CO2, K2, O2 and N2 but containing other gaseous components in trace quantities depending on the type of material burned and the operating conditions. Incineration is not an ultimate disposal method in that the solid residue which is primarily an ash containing some metal must still be disposed of, usually as landfill.
The primary advantage is that it reduces the volume to be disposed of and results in a “clean” inert fill. For every 100 tons of material fed to the incinerator approximately 20 tons of residue result. The volume reduction is even more significant, often resulting in a 90% lower solids volume for organic materials.
The theory of incinerator operation is very simple. A unit is designed to expose combustible material to sufficient air at high temperature to achieve complete combustion. Combustion is usually carried out in deep fuel beds to ensure good contact of air and refuse.
Several types of configurations are used to achieve contact; these include concurrent flow of fuel and air-under fire, counter current flow of fuel and air-overfire, flow of fuel and air at an angle to each other—crossfeed and combinations of these.
The combustion is basically the same for all methods in that at the ignition front oxygen is rapidly consumed in the reaction O2 + C → CO2 and if oxygen is depleted CO2 + C → 2 CO. Therefore sufficient oxygen must be available to obtain complete combustion; usually this is provided by adding additional air in the chamber above the fuel.
Incinerators are typically operated with about 50 to 150% excess air in order that the gas temperatures do not drop below that required for good odour-free combustion; this is usually in the 1700-2300° F range. Recent trends have been to go to the higher part of this range while old units often operate at 1500° F or below. The effect of excess air on gas composition is summarised in Table 5.9 for a typical refuse.
Trace components in the incinerator-start gas include some SO2 and NOx. The former depends on the sulphur in the refuse and is typically around 0.01 to 0.02%. Nitrogen oxide is generally formed in combustion processes and depends on the amount of excess air and to some degree the operating temperature of the incinerator. Other trace components can be found in the off-gas and are summarised in Table 5.10.
Their presence or absence is very much dependent on the type of refuse incinerated and the operating conditions. Incinerator residue composition ranges are shown in Table 5.11. A typical ash and slag chemical analysis is shown in Table 5.12.
Particulate matter is also present in the stack gas and is removed by the usual techniques. Solids residue from incinerators will vary widely with the type of feed and incinerator operating conditions.
Incineration can effectively be divided into local, on site, and central methods. The basic principles are the same but the applications vary considerably. Central incineration facilities handle refuse from many sources and a wide variety of feeds. Local incinerators handle either special feeds, onsite, such as industrial or hospital wastes, or serve a particular small location such as an apartment house. Size is not necessarily a criterion although generally central incineration facilities have capacities in excess of 100 tons per day.
A typical incineration facility will have a capacity ranging from 100 to 1200 tons per day with individual furnaces usually limited to a 300 ton per day rating. Most large incinerators today are continuous-feed rather than batch design because operation is more controlled and easier.
In addition the absence of the heating and cooling cycle results in lower maintenance and a higher capacity per investment dollar. Air pollution control is improved significantly in continuous-feed incinerators as compared to batch plants.
A large central incineration facility is schematically shown in Fig. 5.1.
It can be divided into five areas:
(i) The receiving section which includes the weigh station, storage hopper and bucket crane.
(ii) The furnace—which includes the charging hopper, stokers, furnace chamber and air feed system.
(iii) The effluent gas treating facilities.
(iv) The ash handling system.
(v) The cooling water system.
The particular system shown does not have provision for waste-heat recovery; very few systems incorporate this at present.
At the present time control of particulate matter in the effluent gas is the most critical problem in incinerator design and operation. A typical modern facility will include either a wet scrubber or a spray chamber followed by solid separation in a multi cyclone separator for primary off-gas cleaning. Often this is followed by passage of the gas through either a baghouse filter or electrostatic precipitator.
These methods can achieve up to 99% removal of particulate matter which will meet the code requirement of 0.1 pound particulate per 1000 pounds of flue gas at 50% excess air in almost all cases. The costs for this required dean-up are significant.
Odour control is achieved by providing adequate time (0.5 sec) in the combustion chamber at temperatures above 1500°F. As incineration temperatures in modern units are between 1800 and 2200°F this poses no problem.
The investment and operating costs for incinerators are high and to date have been one of the major deterrents to wider use.
1. Feed section
2. Feed chute
3. Grate
4. Furnace
5. Residue hopper
6. Secondary combustion chamber and down-pass flue volume
7. Final burning and settling chamber volume
8. High pressure opposed spray curtain
9. Fly-ash sluiceways
10. Sequential cyclone collectors
11. Induced-draft fan
12. Bypass flue
13. Provision for added filters or precipitators.
On-site incinerators have a number of distinct applications. They are of particular value in reducing the volume of domestic refuse that must be collected and have therefore been very popular in larger apartment houses as well as more rural areas where collections are infrequent or non-existent.
The use of on-site incinerators for reducing domestic refuse has been discouraged recently because of the potential pollution problems associated with inadequate design and operation. The outdoor or backyard incinerator is essentially an open refuse burner producing smoke, odours and large amounts of airborne particulate matter. It is inexpensive and therefore is still used in rural areas. In urban areas, outdoor burners are totally unsatisfactory and their use is illegal in most areas.
Indoor home burners have in general proved almost as unsatisfactory as the outdoor incinerator. Most indoor burners today use auxiliary fuel to improve their operation.
There are generally three types of auxiliary fuel-fired home incinerators:
(i) High-BTU input.
(ii) Dehydrating.
(iii) After-burner.
The first two units essentially provide auxiliary fuel so that sufficient heat is available to dry and ignite the wet refuse. They tend to have smoky gas discharges, with the typical odours associated with the burning of garbage, because their gas flue temperatures are usually below the 1300°F minimum required for odour-free combustion. The after-burner incinerator provides a secondary flame in the stack to ensure proper combustion of the off-gases; this will usually reduce odour and smoke.
Numerous on-site incinerators are operated with satisfactory results for the reduction of industrial wastes. These facilities are usually specially designed to handle one type of refuse. Typical materials that are incinerated include plastics, rubber, wood scrap and paper. The economics of waste recovery are changing so that often these materials are no longer burned.
For example wood chips and sawdust at sawmills are often sent to paper mills or composition board producers as feed, where formerly they were burned. Insulated copper wire and automobiles are incinerated to remove the organic components prior to recovery of the metal. Special liquid wastes such as water containing organics are incinerated. The special applications are numerous, including the incineration of radioactively contaminated wastes.
In addition to the more traditional incinerators, whether rectangular or cylindrical, special designs are employed in industrial waste disposal. For example, shredded plastic as well as “white water” from paper mills is incinerated in a fluidised-bed combustion chamber.
Industrial sludge is being burned in a rotary kiln by some industries. Solid cyanide waste in automobile plants is put into solution and then burned while aluminium chloride sludge from a petrochemical operation can be burned to produce HCl and alumina.
Although hauling to landfill sites is the present disposal method for many industries, on-site incineration of industrial wastes will receive wider use for waste disposal where recycle is not possible and volume is sufficient, in excess of 500lb/day to justify an installation. Proper design and operation of an incineration facility in an industrial setting is easier than in a dwelling.
Usually trained people are available to maintain the equipment, enabling the operation to meet pollution control requirements. Special, centralised, privately owned and operated, incineration facilities are being built to compete with on-site facilities. These operations will have the economy of size in their favour but haulage and variability of feed may more than negate this advantage.
Hospital wastes are now commonly being disposed of in on-site incinerators. To eliminate the possibility of spreading infection wastes should be promptly incinerated. This is best done in an on-site facility.
3. Compaction:
The reduction of waste volume is receiving considerable attention in an effort to reduce collection costs; compaction is one of the favoured methods to achieve this reduction. High pressure compaction has been developed to provide a high density product suitable as an essentially inert fill or even as a building material. Using this product as a base covered with a minimal earth cover, the Japanese have reclaimed land from tidal areas having a water depth of 10 feet.
The Japanese process collects refuse and subjects it to three stages of compression with the final main press exerting 3000 psi on the refuse. The resulting bale is usually wrapped in chicken wire and coated with asphalt for ease of handling and to prevent crumbling and/or leakage.
Studies by the Japanese indicate that the high pressure squeezing and resultant elevated temperatures decrease the BOD from 6,000 in the raw refuse to 200 in the product. Similarly the COD of 8,000 (which compares to about 14,000 in U.S. refuse) was reduced to about 150. Inspection of the interior of the bale shows a homogeneous, plastic-like mass.
The bale will not support vermin, rodents or insects and is essentially odour free even if it is not protected by an asphalt coating. The only other product of the compaction is waste liquor which amounts to 5% of the feed in Japan and will probably be about 3% with U.S. refuse because of its lower moisture content.
This system appears to be of particular interest where long hauls are required to dispose of waste. In addition it is attractive where it is desirable to reclaim tidal lands or obtain prompt use of landfill areas because the highly compacted fill eliminates the usual hazards and nuisances associated with the usual landfill operations.
4. Composting:
Composting is the biochemical degradation of organic material to yield a sanitary soil supplement. Anaerobic composting has been practised in Asia and is the process by which sanitary landfills degrade refuse.
Composting while not economical now could prove more attractive as public opinion moves toward an attitude which requires that wastes returned to the earth be compatible with the environment.
Composting has one overriding advantage; it is the only process which provides for recycling of organic residue. The process can handle garbage and other organic refuse (but not plastics) as well as sewage sludge and industrial waste from certain operations such as saw and paper mills. The primary disadvantages are cost, the need for fairly large areas for final outdoor curing, a slight odour associated with a composting plant, and lack of a market for the product.
Composting is practised in several forms. Traditionally rows of refuse, shredded or ground, four to six feet high, are exposed to the environment and turned regularly. This is known as the “wind-row” method and is still used. Complete composting can be achieved in 10 to 14 days if seeding with compost is employed but often takes four to six weeks.
Mechanically aided aerobic composting is carried out in a number of processes. Among the more prevalent are the Dano process, the Earp-Thomas Multi-Bactor compost tower, and a number of cell-type stated tower systems. Decomposition takes place under aerobic conditions with the micro-organisms supplied by seed compost.
Typical operating temperatures reach 130 to 140°F. Material is held in the unit from one to six days depending on the process. This is usually followed by an open air curing. A new plant in Schweinfurt, Germany using the Caspari-Brikollare process, produces briquettes in which form the compost is stored until it is to be used.
Raw materials suitable for aerobic composting will be finely ground (coarse for windrowing) and have a maximum carbon to nitrogen ratio of 50 to 1. It is important that good dispersion of air be achieved and that the moisture level be maintained between 50 and 60%.
Recycling of between 1 and 10% of active compost enhances the composting process by minimising the time required for sufficient micro-organisms to develop. The yield from composting is about one volume for every three volumes of feed; the weight yield is between 30 and 40%.