Sorting Systems: Picking the Right One for Your Facility

The technology for automated separation of waste is derived from mining and forestry applications. Trommels and vibratory screens were originally utilized to separate mine slag from ore. Rotating disc separators were first used in separating materials in pulp and paper mills. These machines have been adapted to use in separation of valuable recyclables from heterogeneous wastestreams. As part of this technical evolution, these machines have been further modified in the development of new machines, such as air clarifiers and finger screen separators, specifically utilized in waste recycling. The size, capital cost, production rates, and type of material to be processed will determine which sorter is appropriate for your facility.

Facility Types

Sorting equipment can be found in a wide variety of industrial, mining, logging, and waste management operations. Though occasionally found at landfills, sorting machinery is used primarily at material recovery facilities (MRFs) and transfer stations.

MRFs are industrial sites designed for the sorting and separating of MSW into component wastestreams. Typical MSW consists of the following materials each year prior to recycling:

After recycling, the makeup of MSW is as follows:

(Source: EPA Solid Waste Fact Book)

As can be seen, the bulk of recycled materials are paper products, metals, wood, and glass. Recycled yardwaste, though representing a large recycling category, is primarily for composting purposes and is not covered by this article. A MRF’s primary function is the physical separation of the individual wastestreams to provide recycled raw materials. The reclaimed products are baled and sold to secondary material markets. MRFs located in large industrial cities may be designed to process more than 1,000 tpd of material. In less populated rural areas, a MRF might process only 100 tpd. Larger MRFs tend to utilize a highly automated mechanical process while smaller facilities rely on simpler processes or even manual sorting. Most MRFs in the United States are sized somewhere between these two levels of operation. Sorting operations are the heart of a MRF, making bulk recycling operations possible.

Transfer stations perform an entirely different role. They exist to minimize transportation costs in low-population-density areas where long haul of waste loads becomes uneconomical. Acting as a hub for local waste collection routes, the transfer station receives MSW from individual waste trucks, which deposit the waste on tipping floors. From the tipping floors the waste is pushed or lifted onto long-haul trucks for economical transportation to a regional landfill.

Sorting is a secondary concern for transfer stations. Often, a kind of prerecycling operation is collocated at the transfer station. At these facilities, certain high-value end products, such as corrugated cardboard and newsprint, are separated prior to loading onto the haul trucks. For these facilities, there is a preference for specialized sorters with lower throughput capacity.

Overall Wastestream Management System

Commingled MSW enters the MRF at the tipping floor. Typically the accumulated waste is pushed into hoppers or loaded onto conveyor belts using front-end loaders. Conveyors (slider-belt, chain-driven, roller-type, steel-belt, or rubber-belt) carry the waste to the next stage: the sorting system. This may consist of something as simple as a manual sorting platform or a multistage commingled waste processing system designed to automatically remove several different types of recyclables. Shredders and pulverizers may be used prior to separation to help sort material by size. The extracted products are then sent through densifiers, can flatteners, and baling systems before stockpiling. Of all the processing and recycling stages, sorting is the heart of the operation.

Types of Sorters

Curbside Presorting. Sorting, like charity, should begin at home. It is fairly common for most urban and suburban households to source-separate particular types of waste. Most often these are glass, plastic jugs and containers, newsprint, and corrugated cardboard. The waste-removal company or agency typically provides specialized receptacles for these recyclables. Separate trucks round up this material independent of bulk waste removal. The collected material might then be taken directly to recyclable stockpiles at the MRF or transfer station. While not as capital-intensive as automated sorting systems, the effectiveness of curbside separation is inconsistent since it depends on the voluntary compliance of households. By diverting recyclables prior to process, however, the workload required by the automated and manual sorting systems is greatly reduced. This capital savings in plant equipment needs to be balanced against the operating costs of a separate recyclables truck collection fleet.

Sorting Platforms and Stations are typically designed for manual removal of recycled waste. Waste is fed to a sorting platform on a conveyor picking belt. Simple conveyor belts include steel belts, roller chain belts, PVC-style belts, flat belt sliders, and troughers. As the waste material is fed onto the conveyor belt, vibratory motion typically is used to spread the waste out onto the belt for ease of observation. Manual picking stations line one or both sides of the moving conveyor belt. Each picking station usually is devoted to one type of recyclable material with appropriately sized collection bins. While this system requires less capital investment upfront, it is labor-intensive with proportionally higher operating costs. The quality of the materials recovered and the consistency of the productivity might suffer from labor fatigue and the monotony of repetitious work. Whenever it is justified by large quantities of incoming waste, avoidance of expensive landfill tipping fees, or attractive prices on the recycling marker, an automated system is preferred.

Specialized Fiber Sorting Systems are utilized for each major type of recycled paper waste: corrugated cardboard, newsprint, and stiff containers. The main design concept and operating principle of all screeners is to allow the removal of valuable recyclables negatively off the end of the conveyor system. This greatly reduces the need for labor-intensive removal by positively picking the material out of the wastestream, though a few quality-control pickers are typically needed to inspect the material and remove miscellaneous contaminants. Corrugated cardboard separators utilize a relatively simple screening operation. The larger corrugated containers are conveyed across the screen, while office paper, newsprint, and smaller contaminants fall through the screening surface. Bulk sorting devices further clarify the wastestream by removing other paper fiber and mixed containers. Additional refinement of the wastestream can be achieved by using a high grader system designed to remove chipboard, junk mail, and other small contaminants from incoming residential fiber material. An average fiber sorter measures 22.5 x 14 x 11 ft. high and weighs 9 tons. Power requirements are 10-15 hp driven by 208-, 230-, 380-, 415-, or 575-volt three-phase power. Typical production capacity is 15 tph.

Gypsum Board Recycling Systems are specialized sorters utilized to remove drywall from construction and demolition (C&D) debris and break it down to remove its gypsum core. Scrap gypsum board is loaded in the in-feed hopper and carried through the in-feed metering system, which delivers an even flow into the gypsum separator component. A flexible impact system removes the paper facing from the gypsum board and breaks down the gypsum core into valuable-size material. Second-stage removal of ferrous materials from the gypsum is accomplished with a trommel and magnetic separator combination. The trommel separates fine gypsum from coarse gypsum.

Disc-Type Sorters utilize rotating discs to impart a wavelike motion into the material stream. The wave motion raises larger objects to the top of the incoming waste mass, causing smaller objects and particles to settle to the bottom. Disc sorting usually is combined with screening to allow separation of smaller objects and debris and/or decks to separate larger objects. Disc-type sorters tend to jam if overloaded with debris and waste containing large amounts of small objects. Most come with a variable-speed drive option, however, that allows the operator to adapt to different types of corrugated cardboard and paper. This ensures the even flow of material over the screening sections. An average disc-type sorter measures 30 x 8 x 10 ft. high and weighs 15 tons. Power requirements are 5-10 hp driven by 208-, 230-, 380-, 415-, or 575-volt three-phase power. Typical production capacity is 30 tph.

Magnetic Belt Separators are utilized to directly remove ferrous materials from the wastestream. They can be either floor-mounted or suspended by support beams over a moving conveyor belt. Magnetic pulls of 15 in. or greater can be achieved. The magnetic belt separator moves like a conveyor belt, carrying the materials to stripper magnet for controlled discharge. A stainless steel section on existing conveyor installations is required for maximum magnet effectiveness. The power source for the system is electrical: 208/230V single phase or 208/230/460V three phase, housed in a NEMA 4 (watertight) enclosure.

Eddy-Current Separators are designed to separate conductive but nonferrous metals from lightweight commingled waste. This is usually performed near the end of a commingled separation-system process. For example, eddy-current separators are useful for separating aluminum from plastic mix. They work through the principle of high-frequency oscillatory magnetic fields, which induce an electric current in the conductive object. The oscillating fields can be adjusted to optimize separation. This electric current generates a magnetic field, which causes the object to be repelled away from the primary magnetic field. Conductive particles are fed either directly into the separator’s rotating drum or onto a belt enveloping the drum. Aluminum, brass, copper, magnesium, and zinc can be separated from nonmetallic materials such as glass, paper, plastic, rubber, and debris. They are also used to separate computer and electronic scrap.

Trommel Screens are rotating drums that primarily use a combination of rotation and screening to clarify MSW, construction debris, turnings, demolition lumber, paper, ferrous, and nonferrous scrap. Diameters can range from 2 to 16 ft., while lengths run from 8 to 80 ft. Trommels are typically driven by a trunnion wheel or a double-strand roller chain. The tumbling motion created by the rotating drum shakes loose smaller particles that exit through the screen, leaving behind the materials to be recycled.

Screening Units combine vibratory action with screen separation. MSW, C&D debris, greenwaste, and wood products are fed onto the screen, and the vibratory action causes the smaller particles to fall through and separate from the larger, recoverable materials. Stacking screens of assorted opening sizes into double and triple decks allows for multiple separation of various-size materials. Separated material is then deposited onto conveyor belts and stackers that deposit it into containers or stockpiles.

Portable Screening Units are available for separation of excavation spoil, clearing and grubbing debris, and C&D debris. The object is to remove dirt, sand, rock, and other small, abrasive contaminants prior to further processing downstream. This removal significantly reduces subsequent wear and tear on this machinery.

Debris Roll Screens are derived from disc-screen designs. Disc screens were originally used in the wood-products and pulp and paper industries but were found to be inadequate for bulk waste recycling because of excessive jamming. A debris roll screen utilizes a shape and configuration that allows it to process MSW, greenwaste, biomass debris, C&D debris, wood chips, compost, and aluminum. The debris roll screen uses oval-shape rollers to create a wave action in the incoming waste. This agitation releases smaller materials through screen openings and operates without vibration or blinding. Debris roll screens also come in portable units, which are used primarily to prescreen greenwaste and C&D debris by removing dirt, rock, sand, and other abrasive materials prior to being processed by size-reduction machinery. Debris rolling screens also can be designed to remove objects in the 3- to 4-in.-minus range, allowing for the removal of organics, printer cartridges, and aluminum cans.

Finger Screen Vibratory Classifiers are an alternative to rotary trommels or disc-type screening devices. Solid waste material cascades over a series of slotted finger elements that successfully classify the incoming wastestream. The finger screen vibratory design avoids the catching or hang-ups that occur in conventionally perforated wire-mesh screening equipment. The classifiers can be used for C&D debris, commingled waste, paper classification, and removal of metal or glass from mass-burn bottom ash. They also have no rotating shafts that require periodic production stops to remove wound material.

Destoner Dry Classifiers utilize a combination of vibratory action and high-velocity air streams. Destoners fluidize and stratify material according to the differences in their terminal velocities. They can handle high volumes of commingled materials, shredded MSW, auto scrap “fluff,” biomass fuel, and refuse-derived fuel. Heavy items such as glass, metals, stones, and dirt can be efficiently removed by this jam-proof unit, which has no moving parts to wear or maintain.

Air Clarifiers allow for automatic and continuous recovery of uniform-quality, thin plastic