PART 3: Different Methods of Sorting & Preparing Waste

Why is Sorting Waste Important?

Waste recovery facilities employ various techniques to sort and prepare waste, often incorporating machinery to enhance efficiency. The waste sorting process typically begins manually at the household level, where residents separate their waste into different categories.

Sorting waste prior to treatment or disposal is essential as it influences the selection of the most suitable disposal methods for specific waste streams, ultimately reducing the volume of waste directed to landfills.

The following are some of the most common methods for sorting and preparing waste:

Manual Separation

In manual separation, large items like wood, rocks, and long fabrics are removed by hand before mechanical processing commences. This process usually involves the use of sorting belts or tables, making hand-picking the most widely used technique for managing municipal solid waste (MSW). Notably, it is the sole method capable of extracting PVC plastics.

Although manual separation is labor-intensive and time-consuming, it enables a more thorough division of waste, facilitating the recovery of recyclable materials.

Air Separation

This method employs fans to generate an upward stream of air, allowing lighter materials to be blown upwards while denser items fall. Light materials such as paper and plastic bags are then directed into a separator to be collected. The effectiveness of air separation is influenced by the power of the air currents, the manner of material introduction into the column, and moisture content, as excess water can cause materials to clump or become too heavy.

Size Reduction

Two primary instruments are utilized in size reduction processes: hammer mills and shear shredders. Hammer mills consist of rotating sets of steel hammers that break down waste materials, while shear shredders are effective for tougher items like tires, mattresses, and certain plastics. Regular maintenance is required due to wear and tear on the hammers. Both methods are energy-intensive and require careful attention to maintenance. Hammer mills are particularly used for shattering fragile items like fluorescent light bulbs and batteries.

Hammer Mill:

Shear Shredder:

Another approach to size reduction is using a centrifugal shredded system, which can decrease the original volume of waste by up to 80% and reduce weight by about 50%. Notably, this process helps to minimize not just the size but also the moisture content of the waste.

Trommel Screening

A trommel screen, or rotary screen, is a mechanical unit designed to sort materials, predominantly in solid-waste processing. It features a perforated cylindrical drum set at an angle. As materials spiral down the inclined drum, lifting bars help to elevate and drop objects, aiding in the separation process. Smaller materials fall through the screen holes, while larger pieces exit at the other end. In the municipal solid waste sector, trommel screens are utilized to categorize solid waste sizes, improving the quality of fuel-derived solid waste by removing undesirable inorganic materials.

Drying

The drying process is essential for lowering moisture levels in waste, preventing the creation of leachate—harmful liquids that can contaminate water resources when waste is improperly stored in landfills. Dried waste materials tend to become biologically inactive and are easier to store, resulting in a more uniform refuse-derived fuel (RDF). If any waste is partially decayed, drying methods such as sun exposure or hot air drying may be employed.

While air drying may be limited in the UK, it can still be considered in other regions as part of feasibility assessments. The specifics of the drying process can vary significantly based on available investment and land resources. Solar drying is often not feasible in rainy seasons, leading many facilities to operate below capacity during that time and redirecting waste to landfills. Mechanical drying requires considerable energy, which can make RDF production unviable unless supported by significant government initiatives.

Ferrous Metal Separation (Magnetic Separation)

This stage employs electro-magnets that can be activated or deactivated to assist in extracting collected ferrous metals. However, this method cannot capture all metals, especially non-ferrous types that do not respond to magnetic fields, like stainless steel, copper, and aluminum. Small magnetic items can remain buried in non-magnetic substances, complicating recovery, whilst larger magnetic pieces might inadvertently pull unwanted waste such as paper and food materials.

Non-ferrous Metal Separation (Eddy Current Separator)

Eddy current separators, or non-ferrous separators, separate non-magnetic metals from other materials by generating swirling currents within a conductor, influenced by changing magnetic fields. This technology enables continuous and automated handling of high volumes of materials, enhancing operational efficiency. Consistent material flow, maintained by vibrating feeders or conveyor belts, is crucial for ensuring effective separation.

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