12 Waste Processing – Volume reduction

Dr. J. Rajesh Banu

Learning Objectives:

• To minimize the amount of solid waste material to be processed at the dumping site.
• To increase the efficiency of collection and disposal of solid wastes.
• To enhance the durability of dumping sites.
• Finally, to prepare the economic viability of solid waste management system.

Volume reduction

Volume reduction is a type of waste processing where the nature of the waste is altered physically. It is one of the essential steps in waste management system. Proper waste processing will improve the efficiency of solid waste management, facilitate recovery of resources and energy conversion. Volume reduction is defined as handling or processing of waste materials to reduce the volume of the waste. Volume reduction can be achieved by three means. They include

Mechanical volume reduction: By this process the density of the waste is increased through compaction. They are achieved by compaction process.

Thermal Volume reduction: It is a process by which the waste volume is reduced through thermal methods or temperature changes. The solid waste is converted into gas, liquid or inert solid residue. Eg. Incineration, Combustion and Gasification

Chemical Volume reduction: It is done by altering the chemical nature of the waste through the addition of chemicals or temperature changes. Egs. Pyrolysis, hydrolysis and chemical conversions.

Compaction

Waste compaction is the process of compacting waste to reduce its volume and densify the waste. In simple terms it is the process of pressing the waste to compact and increase the density. Compaction is done at three levels, namely in collection vehicles, transfer stations and disposal site. Garbage compactors and waste collection vehicles compress waste so that more waste can be loaded within the provided space. At the transfer station they are compacted into bales or cubes or pellets. Waste is also compacted again, more thoroughly, at the landfill to conserve valuable airspace and to extend the landfill’s life span.

Benefits of compaction

• The benefits of volume reduction by compaction
• Reduction in the amount of materials to be handled at the disposal site. Improved efficiency of collection and disposal of wastes.
• Feasibility analysis of waste management system.

Demerits of Compaction

• Recyclable material quality will be poor, when solid wastes get compacted.
• Once solid waste compacted, the separation or sorting process is too difficult.
• Food wastes and organic bio – degradable matters when compacted along with other waste will reduce the recycling value of paper and plastics.

Equipment used for compaction

The equipment used for mechanical compaction are classified as follows.

Based on the movement:

The compactors are categorized into two types based on their movement.

Stationary equipment

Stationary equipment is operated either manually or mechanically to collect the solid wastes and load them into the containers. The collection vehicle equipped with compaction mechanisms for compacting the solid wastes is called stationary compactors. Such compactors can be placed in residential, industrial and commercial areas. They are also placed in transfer stations. Stationary compactors can be classified according to their application as (1) light duty – used for residential and commercial solid wastes; (2) commercial or small industries; (3) large industries; (4) transfer station.

Movable equipment

This consists of wheel attached vehicle and tracked equipment used to dump the compacted solid wastes in a sanitary landfill.

Based on the pressure:

The compactors used at transfer stations are classified into 2 major categories based on the pressure used for compaction. They are low pressure (less than 7 kg/cm2 or 100 lb/in2); ii) high pressure (more than 7 kg/cm2 or 100 lb/in2)

Low – pressure (less than 7kg/cm2) compaction

Large sized container is required for low – pressure solid waste compaction. These low pressure compaction systems are used in many industries in association with material recovery such as paper and cardboards.

High – pressure (more than 7kg/cm2) compaction

In high – pressure compaction systems, specialized compactors systems are used to minimize the volume of solid wastes into small sections, cubes or bales of various sizes. Pulverized or shredded waste can be compacted to form logs or bales. The nature of waste determines the use of high – pressure compaction equipment. High pressure compacvtion units possess a capacity to compact waste upto 351.5 kg/cm2 or 5000 lb/in2 and the reduction range may vary from about 3 to 1 through 8 to 1.

Bailing equipment

Bales are another substitute to compaction equipment. It works under huge pressure, mostly 100 to 200 lb/in2. Through this equipment relatively low, compact bales of solid waste or recycled materials are generated. Bale size ranges from 48 x 30 x 42 in. up to 72 x 30 x 44 in. Likewise, the weight of the bales varies based on the solid waste composition and ranges between 1150 to 1800 lb for the light and high bales of crumpled cardboard. Moreover, baled solid wastes should be landfilled in “balefills”. Practically all kind of recycled materials like cardboard, paper, plastic, PETE bottles and aluminium cans be baled and sent to recycling industries. These kind of baled materials are easily load with the help of forklifts and can be economically shipped due to its large bulk density. Balers can be of two types: vertical and horizontal. Vertical balers are used in small scale operations and horizontal balers are used for large scale operations.

Cubing and pelleting equipment

Cubing and pelleting is a process that can be used to generate densified refuse derived fuels (dRDF) for combustion in incineration, gasification or pyrolysis techniques. The expenditure of the process can vary according to the following features namely when the dRDF has to be exported to a long distance, stocked or stored for more periods, or fired in existing boilers made for coal or wood. Cubing and pelleting equipment were operated under same principle. Waste paper RDF is ejecting through extrusion dies with an eccentric revolving press wheel (Figure 1). An entire cubing or pelleting techniques demands a shredder, a conveyor and a moisture control system (Figure 2). The cubes or pellets are joined by heat due to friction as the cubes or pellets are ejects. The cubes and pellets stored in dry condition, dRDF can be kept for months without decay. Normal cubes are almost 3 in by 1 in2. Pellets are circular, basically 0.5 to 0.75 in diameter by 0.5 to 1 in long.

Table 1 summarizes the various types of compaction equipments used in different locations
Table 1. Compaction equipment used at different locations

Figure 2. Bag or extruder

Criteria for the selection of compaction equipment

To confirm active processing, the following factors should be considered while choosing compaction equipment.

1. Characteristics of solid waste: Size, composition, bulk density and moisture content of the solid waste to be compacted.
2. Method of transporting, handling and feeding of solid wastes to the compactor equipment
3. Possible usages of compacted solid waste materials.
4. Design features such as the size of filling chamber, compaction pressure, compaction ratio, etc.
5. Operative characteristics such as energy requirements, maintenance requirement, ease of operation, reliability, noise output, air and water contamination regulator requirement.

Table 2. Factors governing selection of densification equipment

Compaction ratio:

The compaction ratio and the volume reduced during compaction are calculated as per the equation 1 and 2.

The compaction ration of the waste by equation 2, given below:

where Vi = volume of waste before compaction, m3

and Vf = volume of waste after compaction, m3

The relationship among the compaction ratio and the percent of volume reduction is depicted in Figure 3. Upto 80% volume reduction, the compaction ratio increase is very slow and gradual. Above which a gradual increase in compaction ratio occurs. To secure an increase from 80 to 90 %, an increase in compaction ratio from 5 to 10 is required. This analysis was important to assess the tradeoff analysis among compaction ratio and overall compaction ratio.

Another imperative factor that should be concluded is the final specific weight (density in metric units) of
the waste after compaction. Some regular curves for raw solid waste are presented in Figure 4 the asymptotic value used in expanding these curves in 1800 lb/yd3 , which is steady with values attained by using high pressure compactors. As a point of reference, the specific weight of water is 1685 lb/yd3.

When shredded wastes are compacted under the same scenario, the density may be up to 35 percent
greater than that of raw wastes, up to an induced pressure of 100 lb/in2.

The higher specific weight obtained under the applied pressure greater than 100 lb/in2 is not changed significantly by shredding. Perhaps the most significant fact to be noted in Figure 4 is that the primary raise in specific weight brought about by the application of pressure are mostly dependent on the initial specific weight of the waste to be compacted. This case is relevant when claims made by producers of compaction machinery are examined. The moisture content, which changes with region, is another variable that has a huge
reaction on the degree of compaction obtained. In some stationary compactors, arrangement is made to
introduce moisture, basically in the form of water, during the compaction mechanism.

For cubing and pelleting equipment, the two imperative achieved characteristics are the unit and bulk specific weights of the cubes and pellets. The unit specific weight is the specific weight of an individual cube or pellet. The bulk specific weight is an analysis of a permanent volume of cubes or pellets and is suggestive of the specific weight of cubes or pellets in the presence or absence of movement because it incorporate the void space caused by randomly packing cubes in a container. The cubing system has obtained unit specific weights of 1750 lb/yd3 for shredded newsprint. However, the bulk specific weight of the cubes was only 814 lb/yd3.

To made a trade – off analysis among compaction ratio and cost of compacted solids, the relationship between the percent of volume reduction and compaction ratio is very important. Moisture is another important parameter that needs to be considered. The moisture content will vary with respect to location. In certain cases, addition of moisture in the form of water to stationary compactors becomes essential while under processing.

Summary

In this lecture we learnt about mechanical volume reduction as an important step in waste processing. A
complete coverage on classification of compactors, types of compactors and selection criteria for choosing a best compactor has been provided. The significance and relation between volume reduction and compaction ratio has also been studied.

References

• George Tchobanoglous, Hilary Theisen, Samuel Vigil, “Integrated Solid Waste Management Engineering
  Principles and Management Issues”, (1993), referred page 570 – 577.
  http://nptel.ac.in/courses/120108005/module5/lecture5.pdf