The Large, The Small, The Clean And The Dirty: Equipping MRFs

November 1, 1994

9 Min Read
The Large, The Small, The Clean And The Dirty: Equipping MRFs

Erik E. Colville and Nancy J. McFeron

A materials recovery facility, or MRF, by any other name, would still be either clean or dirty. Of course, sorting recyclables is a dirty business, but the terms "clean" and "dirty" refer to the method of collection while the type of MRF is determined by a community's needs.

Clean MRFs handle commingled or pre-separated recyclables from curbside collection programs, drop-off sites or satellite recycling centers. Dirty MRFs process recyclables from a stream of raw solid waste and are sometimes used in areas with no curbside programs or in communities that are not interested in recycling. Selecting facility size, configuration and equipment differs greatly for the two basic types of MRFs, and it's important that the consultant recognizes these differences when designing a MRF.

A small, clean MRF processes less than 50 tons per day of recyclables and a large facility processes 200 to 300 tons per day. A small, dirty MRF processes less than 200 tons per day of mixed municipal solid waste and a large facility processes more than 700 tons per day.

More than 90 percent of the material entering a clean MRF is processed and made ready for sale. A dirty MRF recovers between five and 45 percent of the incoming material as recyclables, then the remainder is landfilled or otherwise disposed. Because the material entering a clean MRF typically weighs 50 to 100 pounds per cubic yard and the material entering a dirty MRF weighs about 350 pounds per cubic yard, MRF designs vary significantly.

However, the basic features of site planning are similar for both types of facilities. Each site plan should include a customer vehicle weigh scale and areas for queuing and maneuvering; recyclables shipping; vehicle parking; and outdoor storage of recyclables. The main building should include a tipping floor for dumping recyclables or mixed solid waste and areas for sorting materials and processing recyclables, interim storage of recyclable materials and warehouse storage for recyclables awaiting shipment. The facility also should include an employee area with restrooms and an optional office.

Squeaky Clean? A clean MRF's tipping floor is usually divided into separate areas for source-separated recyclables and commingled paper and containers. From the tipping floor, a rubber-tired loader pushes the recyclables onto incline conveyors. The conveyors feed sorting systems that separate the commingled recyclables by type of material, such as newspaper, mixed paper, corrugated cardboard, aluminum cans, glass bottles, plastic milk jugs, etc. A large MRF usually has one sorting system for paper and another for containers. Clean MRF sorting systems may be completely automated, strictly manual or, more commonly, a combination of the two.

Automated sorting systems typically sort containers by size, density and/or material chemical composition. A trommel, or rotating drum screen, sorts materials by size. The outside of the trommel consists of a screen with small holes that grow larger along the length of the screen. Small containers like aluminum and tin cans fall through the smaller screen holes, while plastic soda bottles and milk jugs pass through the larger holes.

Disc and vibrating screens also sort by size. The disk screen uses a series of parallel rotating shafts affixed with discs that are staggered from one shaft to the next. Between the shafts and the discs are openings where smaller materials fall as they pass over the surface of the rotating discs. Larger materials ride along the discs onto a conveyor. The vibrating screen's plate or grating shakes as materials pass over its surface. Particles that are smaller than the holes fall through the screen; larger pieces move across the screen and exit at the far end of the equipment.

In density sorting, the material is subjected to an air stream, also known as an air knife or air classifier. The air stream velocity is set so that lighter materials such as plastic or aluminum cans are blown away from heavier materials such as glass containers. Another type of density sorter, a ballistic separator, uses the momentum difference caused when moving materials of varying density. Through vibration, ballistic separators accelerate and separate the materials by density.

Magnetic equipment sorts material by chemical composition through removing ferrous metals or using eddy current separators to create an electrical current in aluminum materials that propels the aluminum away from other materials. Plastic detection technologies can identify and separate different plastics.

Consider the materials the MRF receives and the target materials that it will process and ship to market before choosing automated sorting technology. Since recycling markets can change almost weekly, it's important to research automated sorting equipment to ensure that the investment will reap long-term benefits. Look for flexible equipment that can be modified depending on market changes.

Manual sorting systems generally consist of flat conveyor belts where workers remove recyclables (or contaminants in a negative sort) by hand from the belt as they pass by. At a fully manual MRF, the conveyor belt for sorting containers may be as long as 50 to 100 feet to accommodate between five and 20 sorters. The length of the belt depends on the number of types of recyclables and the total amount of each type of recyclable being sorted, as well as an estimated sorting efficiency per worker for each type of recyclable.

The belt is typically three to six feet wide so that workers can reach the surface completely. Since all recyclables are removed manually, a change in the market would only call for retraining the workers, which would not dramatically affect the facility's operation.

A combined system of automated and manual sorting usually begins with automated sizing and sorting and ends with manual sorting. The specific recyclables being sorted and the market for the recyclables dictate the configuration of a combination system.

In a clean MRF, once the recyclables are separated, they must be processed into materials for sale. The extent and type of processing are determined by the requirements of the marketplace and transportation costs. Generally, processing includes baling for paper, tin cans and plastic bottles; flattening or densifying for aluminum cans; granulating or perforating for plastic bottles; and crushing for glass bottles.

The baler is the workhorse - and the most expensive piece of equipment - at the MRF. The three types of balers include vertical balers for low-capacity applications, two-ram balers and single-ram balers. The two-ram baler can produce a bale of greater density than a single-ram baler and can switch to accommodate different types of materials quickly, but it bales in only one size. A single-ram baler can produce bales of any length, but takes more time between types of materials for baling.

A can flattener or densifier is selected based on its throughput capacity. Some end-users require flattened cans while others don't. The distance to the market also may determine the type of equipment since the can densifier increases the amount of weight that can be shipped per load.

For processing plastics, some markets prefer whole or perforated plastic bottles from which contaminants are removed prior to granulating; for example, incompatible bottoms are removed from plastic soda bottles. If the market does not dictate one method of processing, weigh the shipping cost of bins of granulated plastic against baled perforated plastic. The plastic perforator or granulator can be fed either manually or automatically, in batches or continuously.

Glass crushers come in a variety of throughput capacity and crushed glass (cullet) sizes. Some are batch-fed while others are continuously fed by a conveyor, either manually or automatically. Some MRFs (usually the larger ones) have a dedicated crusher for each glass color. Smaller MRFs usually use a single glass crusher and clean the unit between glass color changes.

Like a clean MRF, the sorting system at a dirty MRF can be automated, manual or a combination of both, but generally has only one sorting line rather than separate ones for paper and containers.

Down And Dirty In an automated system, the first sorting operation usually removes bulky or dangerous items, then the waste is screened to remove small grit. This screening operation also can remove aluminum and tin cans. Size-sorting equipment for a dirty material recovery facility should be built to withstand higher-impact loads and abrasion.

An air classifier, the only density sorting system commonly used at a dirty MRF, is used to split the solid waste stream into heavy and light fractions, which allows the other sorting operations to specialize in the most common materials found in the respective fraction. Equipment that sorts by chemical composition such as magnets and eddy current separators are commonly used in automated and manual dirty MRFs. Several plastic identification systems are also used in automated dirty MRFs.

Manual sorting follows automated sorting but, after the screening process, the waste travels down a flat conveyor belt and workers remove recyclables as they pass by. Sizing a sorting belt for a dirty MRF depends on the system capacity. Keep the burden depth on the belt low enough for workers to easily pull out recyclables.

In addition to balers and granulators, dirty MRFs employ other processing equipment such as shredders, pelletizers and compactors. A shredder reduces the volume and increases the uniformity of the residual solid waste not recovered for recycling - typically waste that is bound for a waste-to-energy facility. The most commonly used shredders include a shear shredder, in which slow-speed, counter-rotating discs slice the waste into small pieces; and an impact shredder, in which high-speed hammers pulverize the waste into small pieces.

Pelletizers receive shredded waste and extrude it under high pressure to form small pellets. The pellets are then used as fuel for a furnace or power plant.

Compactors are large balers that compress either shredded or unshredded solid waste into large bales weighing up to 29 tons. The compactor loads the bales into trailers for transport to a disposal site, which ensures the maximum legal load in the transport vehicle to reduce hauling costs.

Examine your collection system to determine your equipment needs. If a recyclables collection program has been established to collect clean, commingled recyclable materials, focus on the type of processing provided by a clean MRF. If you don't already have a collection program for recyclables, you should consider implementing a program or constructing a dirty MRF.

The second step is complex and situation-specific: either solicit proposals from vendors to build and operate the MRF or hire a firm to design a facility that you will construct and operate - or a combination of both. If you're uncertain, hire a consultant who has helped implement material recovery facilities both as merchant facilities and as custom designs.

A one-size-fits-all approach touted by a system vendor may not work in every situation. Recyclables and their markets, as well as solid waste, vary in different areas of the country, and a vendor's processing system must be flexible in order to function effectively. Weigh the factors carefully and seek help when necessary so that you and your customers won't be left holding the bag.

Stay in the Know - Subscribe to Our Newsletters
Join a network of more than 90,000 waste and recycling industry professionals. Get the latest news and insights straight to your inbox. Free.

You May Also Like