by Iris von Kirschbaum and Gerd Michel
One of the negative results of growing prosperity worldwide has been an increase in waste generation from year to year. In response, policy makers and researchers are examining how best to decouple waste growth and economic growth. While work continues on waste reduction, waste recycling and alternative methods of disposal, the use of sanitary landfill sites continues to be one of the most important disposal methods in many parts of the world, not least because all other waste management measures still create residuals.
The importance of sanitation
In developing countries, the problem of uncontrolled dumps causes a significant danger for the surrounding environment and a health risk to the population. And this danger is growing more acute as more waste is exported to continents such as Asia and Africa. The installation of sanitary landfills can offer a valuable first step toward a solution.
 A typical landfill site |
In more mature economies sanitary landfills also continue to play an important role. For example, 55% of waste is currently sent to landfills in the US. While the number of landfills in the country is decreasing – in 1988 8000 landfills were active compared with only 1754 landfills in 2006 – the size of new landfills is generally much larger today and they are built to service a wider area. As a result the overall landfill capacity remains constant. Similarly in Europe today 50% of municipal waste management is landfilling.1
In both developed and developing countries sanitary landfill sites can be operated in such a way that danger to residents, and the environment, from leachate, odours, fire and explosion is almost entirely eliminated. And this depends in no small measure on effective landfill compaction. Reviewing the essential elements in landfill compaction is a valuable exercise, not least to ensure that money for new technology is well spent and landfills are efficiently managed.
Profitability of controlled landfill sites
The profitability of a controlled landfill site for the operator depends on the amount of waste it can accommodate – the more waste a landfill site can take, the higher its profitability. Therefore optimal compaction has become increasingly important. Up to 20 years ago bulldozers were used almost exclusively on mainly uncontrolled waste disposal sites. Their major task was, and still is, to distribute, push and spread the refuse. The low compaction levels achieved with those machines is in no way comparable to the performance of the refuse compactors used today.
 Refuse compactor on a landfill site |
The objectives of modern compaction are to decrease waste volume, reduce resettling of landfill waste and reduce leacheate and fire hazard caused by debris dispersal as well as vermin and pest nuisance. It also increases the stability and improves the tractability of a site. Tight compaction of the refuse is therefore desirable for a landfill site manager when considering constructional, ecological and economical aspects of his business.
Compaction density
The refuse density achievable is influenced by many different factors. On a waste disposal site the refuse structure is completely heterogeneous, biologically-active and a ductile formation, where a great number of individual parameters can cause change.
To define the density on a site, the operator must consider the time-related decomposition processes of the waste and the settlement resulting from refuse loads. Furthermore, the density will also be affected by the operating weight of the refuse compactor and the number of passes. Today’s practice is to ensure four to six passes over each thin layer, in order to achieve the right balance between a reasonable compaction value and reasonable operating costs.
Another important contribution to high compaction densities is the disposal method. The best way is a good crushing and mixing of the waste on a pre-compacted, firm sub-grade.
Refuse compactor technology
A refuse compactor has to distribute the refuse delivered by collecting vehicles and containers with its dozer blade and to destroy and crush non-homogeneous refuse (which can consist of household refuse and/or industrial refuse) so that the highest possible homogeneous mass is produced. Furthermore it has to compact the refuse to the highest possible density by performing several passes.
Choosing an appropriate machine weight is an important first step – Table 1 sets out some criteria to be considered.
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High reliability
The land tracks on which a refuse compactor rolls are mostly between 30 and 80 metres. The machine’s motions are characterized by continuous load changes between forward and reverse travel. Forward travel and distribution of the refuse require the maximum amount of pushing force. During backwards travel, the machine must often overcome inclines, with a high load on the machine’s drive system as a result of the downdrift force of waste depending on the inclination of the slope. Considering this, and the enormous traction of the strongly profiled steel wheels in the waste, it is obvious that the demands on the drive system for both forward and backward travel are extremely high.
The limited operating radius in the local disposal area as well as the working under load do not allow high travel speeds, but require high pushing ability at low speeds.
Protecting the refuse compactor against damages while it works, as well minimizing the entry of dust, small particles, slurry and other contamination is of the utmost importance.
Highest compaction
The compaction performance of a refuse compactor depends considerably on how the operating weight and/or the specific weight per wheel is transferred via the compacting wheels onto the refuse to be compacted. The initiation of a high specific pressure on the tooth tip still does not mean that a good compaction performance is achieved, even though extremely high specific pressures are obtained. The wheels of the compactor also ideally need to cover, in only a few passes, the whole track.
Challenges to the landfill machinery operator
Even having chosen the optimal refuse compactor for the given size, structure and kind of waste to be compacted, the driver contributes considerably to the landfill productivity. His (or her) comfort and safety must be taken into consideration in all circumstances, as he is subjected to permanent strain due to the extreme operating conditions. Access to the machine at the landfill site can be difficult depending on the actual position. The operator is also exposed to physical vibrations and noise, dust and odours. The operator works on a small radius travelling forwards and backwards where the machine has to overcome inclines. His overall safety on the landfill site is dependant on good all-round visibility from the machine. Also he is exposed to down-drift forces depending on the inclination of slopes. A larger risk, depending on the condition of the landfill site, is that the machine can sink into the waste and, in rare cases, unequally compacted soil can cause the machine to tip over.
 A Bomag compactor in action, showing the optimal coverage provided by the second pass |
To minimize these factors, the working area – the cabin – must offer the operator the best possible comfort. Considerations are safe access to and from the cabin, simple and ergonomic controls, good visibility, low oscillating loads, low noise level as well as low irritation caused by dust, and of course the possibility to close the windows and to adjust the inside temperature. The machine needs to have a low centre of gravity for maximum drive safety and stable roll-over protection.
But the best protection for the driver and best investment for the profitability of the landfill site still rests on thorough training about the work site in order to achieve the best compaction, effective utilization of the machine and therefore the highest and most efficient performance of both employees and machinery.
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Dr Iris von Kirschbaum is Head of Communications and Gerd Michel is Product Manager of Heavy Equipment at Bomag GmbH
e-mail: iris.kirschbaum@bomag.com, gerd.michel@bomag.com
Reference
1. Don’t waste waste – it is a resource by ISWA; European figures from the Institute for Public Policy Research; USA figures from the EPA courtesy of World Resources Institute.
Case study: learning from Bomag’s experience
For more than 30 years Bomag, based in Boppard, Germany, has designed refuse compactors especially for the rough operation expected on landfill sites. The company offers a wide range of refuse compactor types, from 21 to 55 tonnes of operating weight. Examining its offerings in a little more detail provides a valuable insight into the mechanics of landfill compaction.
Performance
Under the operating conditions of limited rolling tracks, high pushing ability at low speeds are still achieved by the Bomag hydrostatic drive system.
 Hydrostatic drive system |
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In Bomag refuse compactors, protection against damage and the entrance of dust, tiny particles, slurry and other contamination is achieved by a fully-closed frame concept. This consists of a front and a rear frame linked together by an articulation joint which also fulfils the task of oscillation. This frame concept is only possible with the hydrostatic drive system, as there are no drive shafts or oscillating axles.
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Compaction
High compaction depends on the way the weight of the machine is transferred onto the refuse. Bomag uses polygonal disc wheels – the discs form a wedge, in which the refuse is pressed. Due to this wedge formation the forces are not just applied vertically but also partly in a horizontal direction. This results in a kind of kneading of the refuse. An impact effect is created by the shape of the segments which form a polygon on the circumference of the wheel, under which the refuse is destroyed and crushed. A high rate of compaction is achieved by the combined performance of high static load, the basic weight, and the dynamic behaviour of the compacting wheels. The offset of front and rear wheels by half of the polygon disc guarantees that through the wheels an extreme kneading and squashing effect is produced and this guarantees high compaction.
 A fully closed frame concept |
With one pass an optimum compaction coverage is achieved under the wheel track width. A completely compacted track is achieved with twice-repeated pass with offset machine tracks. This low number of passes saves time and thus results in higher economic efficiency.
 Compaction effect of a polygonal wheel disc |
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Comfort and safety for the operator
Driver comfort and safety contribute considerably to productivity. All machines offer easy access to and from the cabin. In order to reduce vibrations and impact loads the driver’s seat is pneumatically suspended, the cab is elastically supported and available with air conditioning. All operating controls are in easy reach. Fully adjustable electronic joystick steering makes it easy to operate the machine. Large windows provide excellent all-round visibility, supported by an optional ‘back-eye’ – a small camera showing the rear area behind the machine.
 Overlap of front and rear wheels |
All Bomag compactors have a low centre of gravity for maximum drive safety under any waste site conditions. The stable roll-over protection structure is integrated into the frame construction and fulfills the ROPS/FOPS requirements. And, importantly, the company offers training for operators on the work site.
