Avoiding Problems in Handling Solid Waste

Bulk-solid waste materials present unique handling challenges. Presented here is a discussion on how to navigate those challenges, along with some characterization strategies to avoid them

Across industry sectors, solid waste materials are common and highly varied. The complexities of material handling in solids waste processes often evade scrutiny until operational implementation. Challenges associated with material handling for solid wastes result from the typologies and physical properties of the materials, including variable particle sizes, extreme shapes and high moisture content. These challenges turn into significant barriers when storing, classifying, drying, transporting and carrying out other processes with the solid waste materials. To avoid these barriers, performing special characterization tests of the material is essential to inform the process design and the operations, thereby minimizing operational risks. This article outlines the main challenges presented by solid waste materials, as well as strategies to overcome the challenges and improve processes involving solid wastes.

Typologies of solid wastes

Solid wastes are extremely varied in terms of sources and are often present as a mixture of multiple different types of wastes. The volume and breadth of solid wastes generated by human activity have increased rapidly, accelerating after the industrial revolution. Disposing of the waste in the natural environment is not an option, and disposal in landfills is becoming less favorable as time goes on, because of the risks posed by the wastes to nature and human health. The situation calls for additional processing steps or recycling the solid wastes. Because of their diversity, solid wastes can be wildly different in terms of particle sizes, shapes, densities and ingredients. To better utilize waste materials, handling processes must cope with the variations in material properties, and be able to address the challenges associated with different types of solid wastes. The following are some of the common solid-waste materials that recycling and reuse processes might encounter.

Biomass waste. Biomass waste comprises an extensive group of solid wastes commonly generated from agriculture, food processing and forestry. It can become extremely challenging in terms of handling when massive quantities of biomass materials are involved in large-scale processes, such as electricity generation. Biomass wastes can include herbaceous and agricultural materials (wheat straw, barley straw) material, wood (eucalyptus, pine, oak, poplar), forest residues, wastes from farms and food processes and biowastes from municipal sources. The appearance of the solid wastes is commonly light (low bulk densities) with extreme particle shapes. Biomass wastes often contain a high level of water — sometimes up to 60 to 70% of the total mass internally or externally.

Municipal solid waste. Municipal solid waste (MSW) is one of the most common wastes, usually with an incredibly large quantity available. Because of different human activities involved, the MSW generated can be from households (55–80%), market or commercial areas (10–30%) or sewage sludge (10–15%). MSW typically consists of kitchen waste, yard waste, papers and cardboard, plastic and rubber, metals, glass, electronic waste and inert materials, which can be from dwellings, industrial structures, streets, institutions and many others. Therefore, MSW has extremely wide diversity with significantly different physical and chemical characteristics. The compositions of MSW depend on where and when it is collected. Household wastes can contain food waste, plastics, wood, metals, papers, rubbers, leather, batteries, inert materials, textiles, paint containers, demolition and construction materials, as well as many other wastes. MSW solids can have very wide particle-size ranges and extreme particle shapes with mixtures of varied solid densities. The MSWs can be dry, but some special MSWs may have high water content or even high levels of oil and other hazardous substances.

E-scrap waste. Electronic waste (also called e-scrap) has increased rapidly in recent years, and has become the fastest-growing waste stream in industrialized areas. Each year, over 50 million metric tons of e-scrap wastes are produced globally, and this total increases by about 3–4% each year. Due to the relatively short lifecycles of electronic products, recovery of valuable metals and critical materials from e-scraps is of high economic interest. Incineration of the waste is a common method. Compared to other wastes, such as biomass and MSW, e-scrap contains a high percentage of heavy metals, including those found in printed circuit boards (PCBs), metal wires, monitors, motors and frames, and plastics from the casings of the equipment. Therefore, e-scrap can be diverse and complex in terms of size, shape and density, and can contain varied hazardous and non-hazardous ingredients. E-scrap commonly contains more than 50% steel, 13% copper and aluminium, and about 21% of plastics and other substances.

Plastic waste. Polymers have been widely used since their invention, and dealing with large quantities of plastic waste disposal has become an environmental challenge. Plastics appear in single-use products, such as plastic bottles, used vehicle tires, as well as in parts of other solid wastes, such as MSW and e-waste. Plastic wastes have special mechanical and physical properties, such as elastoplasticity, extreme sizes and shapes (like thin films or long strings), and complex chemical compositions. Often, plastic waste is lightweight and sometimes can have a substantial water content. Although the plastic wastes have less variations in particle density, the difference in size and shape can be significant.

Solid-waste handling challenges

A typical process for energy recovery from solid wastes is shown in Figure 1, including waste collection, drying, primary crushing, classification, secondary crushing and pelletization of the refuse or injection for direct combustion. In the process, the wastes can come from varied sources and need to be delivered to the process plant as bulk material mixtures of different waste types (Figures 2a and 2b). Figure 2c shows examples of wastes in different sizes and shapes, and with high water content (Figure 2d). Handling such wastes requires process designers to answer fundamental questions, as many options are available for common bulk-solid materials, but these techniques may be not suitable for solid wastes.

Storage and flow. Solid wastes are commonly stored in open yards or landfilled directly before any processing. The solids will need to be collected and transported to a process plant so the wastes can be either burned for heat or recycled. Temporary storage and transport between the storage sites are essential. Because of the varied size, shape, density and moisture of the wastes, a major question in the design of such a process will be: can the wastes be stored and discharged using a common storage silo? The answer is likely to be ‘no,’ because of the special particle shape, significant size difference and high moisture content.

Size reduction and classifying. Size reduction is likely essential in any waste-handling process. Shredding or grinding are the most popular techniques, but the challenge with using shredders is the feeding rather than the grinding. In particular, for solids with special particle shapes, such as the metal wire shown in Figure 2c, moving the wires into the shredder is impossible because of their extremely long length in one dimension. Trimming the wires to a shorter length is the only solution before they can be processed using a common shredder. In the size-reduction process, extreme size variation can be another challenge, because shredders are normally designed according to a top working size. A further barrier can be the high moisture in solid wastes, which can allow water to accumulate in certain areas of the process and cause rust damage.

Barriers for waste classification are similar to those for size reduction, including variation of particle size, special particle shapes and high moisture. Variation in particle solid densities can be a benefit in material classification, but can become a challenge in situations where the solids have similar solid density or when one of the components of the mixture has a particle shape that allows nesting of particles.

Effective drying process. Most solid wastes are wet or contain a high level of moisture. Drying the solid waste materials may be essential prior to any further processes, such as combustion or pyrolysis. However, because of the material-handling challenges, common drying methods are generally not effective, and drying costs can be very high due to the lack of solids-handling efficiency.

In effective solid-waste drying, one of the challenges is maintaining good air permeability of the solids, so water vapor can be removed easily by air circulation. However, smaller solid-particle sizes reduce the air permeability significantly. On the other hand, most organic solid wastes contain a high level of internal moisture, which is not easily evaporated if the particles are too large. Therefore, controlling the particles to optimized sizes can be critical for drying wastes effectively and efficiently. This may be the greatest challenge, especially for materials with extreme shapes.

Mechanical and pneumatic conveying. The process of transporting solid wastes is also subject to the previously mentioned material characteristics (size variations, extreme shapes, high moisture contents). Mechanical conveyors (belt, chain or screw conveyors) are commonly used for solid waste handling, although sometimes pneumatic conveyors or dumper trucks are also used for processed wastes. In practice, belt conveyors are suitable for most solid waste materials, but it can be difficult to load the materials and to keep the belt clean if the waste is dumped onto the belt. Some materials can generate a high proportion of fines and dusts. Screw conveyors can be troublesome when transporting waste materials with extreme shapes. Chain and bucket conveyors are likely to see jams or blockages if the solid wastes fall on the chains.

In principle, pneumatic conveyors can be effective with any solids, if the solid particles are not overly cohesive or too large compared to the pipe size. However, the challenge in using pneumatic conveying as a transport method is feeding solids into the pipeline. Feeding can be affected by characteristics of the solids, such as particle size, shape and flowability. Pneumatic conveying of solid wastes is less popular not only because of challenges associated with feeding, but also because of the high energy consumption required.

Key concerns of conveying solid wastes include the top particle size and extreme particle shapes, although water content can sometimes be problematic. If the materials are damp or waterlogged, the solids will become very cohesive and easily adhere to equipment surfaces. Accumulation of fines and dust can also cause friction, leading to heat buildup and the potential for fire in the conveyor. Spillage and wind lift-off of the solids are also serious concerns, as small particles can be picked up by the wind and lead to high dust emissions.

Fire, explosion and dust emissions. In waste handling, fire and dust explosions can cause great financial losses, and dust emissions are also hazardous to human health and to the environment. The fines and dusts can be generated from drying and size-reduction processes, or can be the result of high-speed solids transport. All increase dust emissions. Dust emissions from waste handling can contaminate other parts of the process and fine solid particles can increase cohesiveness of the waste material. Dust emissions most often occur at transfer points in the material-conveying process, such as through openings or from storage silos with a large drop height.

Dust emissions can create high risks of fire and explosion if the materials are combustible with low to moderate water content, or if the materials can generate heat by themselves, such as organic waste with moisture. High-risk areas for fire include undisturbed static storage, such as silos, stockpiles and flat stores, especially with self-heating materials. Most solid wastes are combustible and organic, and many also emit flammable gases. Fire and explosion risks also depend on the concentration of the combustible materials (dust), as well as ignition and oxygen levels.

Characterization of solid waste

To avoid handling challenges, proper material characterizations for the design of the process are essential. Important characteristics of the solids to be assessed include particle size and particle-size distributions, particle shape, solid density, water content, cohesiveness of the ingredients and of the mixture, as well as flowability and classification of the materials.

Particle size and shape. For solid wastes, particle size and shape can be extremely varied from type to type, depending upon the ingredients in the waste. It can be difficult to define a standard particle size for waste solids due to the irregular shapes. For example, a common definition of particle diameters is hard to determine when particle orientation if the particles have an extreme dimension.

According to the dimensions of particle sides, solids can be classified into three groups [2]: Class 1 is rounded particles with roughly equal dimensions for all sizes, without cohesion; Class 2 is similar-shaped particles, but with significant cohesiveness; and Class 3 is extreme-shaped particles. This classification of the type of solid waste is important and useful when using particle physical properties to design effective handling. Any Class 3 materials, because of their extreme particle shape, will present vastly greater problems in material handling.

Water content. Solid wastes, such as biomass waste, food waste, sewage sludge from municipal wastes, and so on, contain high moisture. High water content in bulk solids has a huge impact on material physical and bulk properties, as well as the processability [3]. Among the direct impacts of the high moisture content is to increase the cohesiveness of the solids, which hinders flow in solids storage. Water in the solids, which can be internal and external, creates challenges not only with drying, but also handling.

Flowability and compaction. Flowability of solids is an important characteristic for material handling, and can be a significant barrier for solids-handling process performance. Because solids can sustain stresses to variable thresholds according to their structure and stress history with a degree of elastic deformation, failure to move (or flow), when it does occur, is rarely uniform and may result in two common failures of solid flow in hoppers or silos (Figure 3a). Even if the solids do flow through the storage vessel, there are two typical flow patterns occur: core flow and mass flow. The flow patterns depend to a great extent on the material properties and vessel geometry (Figure 3b). For solid wastes, material flowability is significantly more complicated because of the highly varied particle size and shape, high cohesiveness (due to moisture), high compressibility and varied bulk density under loading stress

There are a variety of conventional techniques that can quantify ‘flowability’ for solid materials, such as Dr. Andrew Jenike’s shear-cell test, the Carr Index, the tri-axial cell for investigating the strength and deformation behavior of soils at high stresses and the direct shear-cell test [4]. These conventional techniques work well for most Class 1 and Class 2 solids, but are not suitable for Class 3 materials. Many solid wastes fall in the Class 3 category because of their extreme particle shapes (including long, stringy particles or shredded sheet materials). For Class 3 materials, when the material is compressed vertically, particles flatten during compression rather than moving relative to each other. The compression creates strong interlocking forces between the particles, and the solids will become stuck in any storage space where they are under stress.

Best practices

To avoid the difficulties in obtaining efficient plant performance with waste-material handling, the approach must always be “know your enemy first.” That is to say, invest time and money to obtain a thorough understanding of the handling and processing challenges of the particular waste stream to be used. Only after a thorough understanding is established should equipment types be selected and detailed designs made to ensure the process design and equipment are able to cope with the solid’s behaviors. Variations in waste streams can be significant, so obtaining significant numbers of test samples is always recommended for material characterizations in order to understand these variations. Conventional characterization methods for handling properties can be suitable for Class 1 and 2 materials, but will not work for Class 3 materials because of the extreme particle shape.

Best practices for handling the waste materials are as follows:

1. Representative samples of the solid waste material must be comprehensive, especially for extremely varied particle size and shape. The same name does not mean the same materials.

2. Any Class 3 materials will need special characterizations in terms of flow and handling.

3. Any materials with high moisture will create extra hazards, such as self-heating, water-logging and high cohesiveness.

4. Materials with extreme shapes will have a high tendency to interlock and to be highly compressible.

5. Large tolerances for varying properties must always be included in the design because experience shows that the waste material will always show more variations than expected.

6. For solid wastes, it is almost impossible to avoid fire, explosion and dust emissions risks.

7. Professional characterizations and designs are strongly recommended, especially for “first-of-a-kind” plants in a series. 

Edited by Scott Jenkins

 

References

1. Klinghoffer, N. B., and Castaldi, M. J. (Eds.), “Waste to energy conversion technology,” Elsevier, 2013.

2. Blott, S.J., and Pye, K., Particle shape: a review and new methods of characterization and classification. Sedimentology, 55(1), pp. 31–63, 2008.

3. Jung, H., Lee, Y. J. and Yoon, W. B., Effect of moisture content on the grinding process and powder properties in food: A review. Processes, 6(6), 69, 2018.

4. Tsunakawa, H., and Aoki, R., Measurements of the failure properties of granular materials and cohesive powders. Powder Technology, 33(2), pp. 249–256, 1982.

 

Author

Tong Deng is a senior industrial consultancy engineer at the Wolfson Centre for Bulk Solids Handling Technology and a senior lecturer at the School of Engineering, University of Greenwich, U.K. (Email: [email protected]) He obtained a Ph.D. in particle dynamics and instrumentation in erosion tests in 2001 and started his career in the tribology of powders and bulk solids handling technology. Deng’s expertise is in erosive wear testing, electrostatics of particulate solids, characterization of material bulk properties and pneumatic conveying. His research interests include impact dynamics, powder flow and pneumatic conveying.