For most communities, maximizing the utility of discarded materials while minimizing landfilled waste are important considerations.

By Greg Gesell

A key to optimizing solid waste management is to process the waste through a material recovery facility (MRF) and sort out selected materials. A MRF not only takes advantage of the many recoverable, valuable materials in the wastestream but also diverts the materials from the wastestream. A well-designed MRF can provide the highest recycling value for materials that have not been reduced or reused in other ways.

MRFs have evolved over time as different concepts and approaches have been applied to recycling programs. Some early attempts to recover materials were unsophisticated in nature, responding to perceived economic conditions, and were basically not much different from sorting through mixed waste. One example is that of the separation of clean concentrated loads of cardboard or metal on the tipping floor of a transfer station or waste-to-energy facility. Usually such efforts are very labor-intensive and experimental and are done by an operator who is trying to take advantage of a short-term spike in the value of specific materials. Most of the time, such efforts are not highly effective. Low recovery rates and poor product quality plague this approach. An exception may occur for certain commercial/industrial processes, where the value of key products may allow such a system to succeed.

Various MRF designs have been developed, and today these facilities can be designed for materials that arrive in various ways:

• Individual material streams, such as aluminum cans or newspapers, which may arrive separately and simply need screening to remove impurities and then bundling to consolidate them for shipping to market.
• Blue-bag systems, which combine various recyclables into a specially colored bag that can be separated from general MSW.
• Bin systems, which typically collect a number of recyclable materials together in a single bin or stream for transport to the MRF for separation and recovery.

A true single-stream MRF receives materials collected in a single container, which can be a bag or a bin. Single-stream systems generally have better community participation than systems requiring presorting of materials, because the effort expended by residents is minimal. Rather than separating the materials into multiple bins and finding the space to store the bins, the residents place all of the recyclable materials into a single container. The program’s acceptable materials can be listed on the bin or in a flyer. Collection is relatively quick and efficient. The trade-off is that the MRF itself becomes more complex than a facility designed for presorted streams.

Blue-bag approaches have been well received in some locations. If the bags containing recyclables are collected together with the trash, the first step is to retrieve these blue bags from the trash. Then the bags must be opened prior to introduction to the MRF and recovery of the materials inside. A disadvantage of this system is that the closed bags make it hard to prescreen for contaminants on the collection route or at the MRF facility prior to introduction to the facility.

A more common approach today is to combine all recyclables into one bin, which allows a quick visual check on the collection route as the first step in screening out undesirable materials. The program materials from bins, such as bottles, cans, newspapers, cardboard, and paper, are emptied into a separate collection vehicle and taken directly to the MRF for sorting. Although bin-based systems usually are more expensive, the increased efficiency and acceptance may quickly pay for the extra cost. A blue-bag collection system could function similarly, with the extra steps of retrieving the bags from the mixed waste and opening the bags.

General Features
Single-stream MRFs often offer the best combination of features for sorting materials. They may increase community acceptance by making the sorting process at the source somewhat easier than with the multi-bin approach. Most households and businesses are willing to do their part as long as the rules are not too cumbersome. At the same time, material quality can remain higher than with a mixed-MSW processing arrangement. The single-stream approach has been well accepted by the public in many cases, with participation reported to be greater than 70%. Although homeowners need to buy into the concept of recycling, the effort required of them is perceived to be manageable.

Designing an MRF hinges on understanding the owner’s intentions, the product characteristics of materials to be processed, and the cleanliness requirements of the potential markets. These should be carefully investigated. If the owner wants to be able to change the material mix routinely, the system has to be flexible. In some cases, the owner may need to explore potential markets or other key design requirements. If the types of customers or materials served vary from day to day or by season, the system has to be adaptable to these changes as well. Understanding these issues up front helps match the types of equipment and the facility arrangement to best meet the needs of the customer. Spending a little extra effort at this stage will greatly improve the outcome.

Some facilities serve a mixed residential and commercial/industrial waste shed. Careful review of collection routes may reveal that on certain days of the week the mix may be expected to shift because of an increase in certain types of customers on those routes. This may limit throughput at the MRF or require other operational adjustments on those days. This concern needs to be addressed to allow for sufficient margin in the design. Alternatively, it may be possible or desirable to modify collection routes.

Many waste sheds combine residential and industrial alike.

The typical approach for MRF projects is to make the equipment suppliers responsible for the detailed design of the individual process equipment. Use of their proprietary equipment gives them the flexibility to adjust the design from facility to facility without the engineer having to become intimately knowledgeable about every support detail, chute design, and cable tray. Different vendors may have proprietary equipment or preferred arrangements that are equally acceptable for the task, though they might look entirely different. Vendors provide guarantees for their equipment performance. Clients, on the other hand, may have strong preferences regarding certain types of equipment. Meetings with the various potential vendors furnish additional information, such as the requirements for building size, support utilities and interface points. A specification is then developed to provide standards for various types of equipment, effectively establishing system quality. A clear understanding of what the system is expected to accomplish helps everyone achieve the desired outcome.

Understanding the In-Feed Range
The key to any MRF design is the anticipated source of the materials to be processed. Commercial/industrial streams will likely be concentrated with certain types of products, such as plastics, metals, cardboard, and wood or other fiber (paper) materials depending on the types of facilities or industry served. The variation of these percentages from route to route, as well as from day to day or season to season, should be determined. Phone books are an example of a seasonal recyclable accepted at many facilities. Sometimes presorted loads that require minimal screening and baling will be a major part of the in-feed material. If certain sources provide in-feed material that is heavily concentrated with certain recyclables, this material may need to be mixed with other materials or kept separate and require design of a bypass or shortened processing system to maximize processing efficiency.

Understanding the range of in-feed composition is critical. The mix of in-feed materials may be highly variable. If the materials to be processed are primarily from residential sources, more used beverage cans and containers can be anticipated, although community specifics can vary considerably. It is also possible for the in-feed stream to change; notable is the large increase in bottled-water containers in recent years. Local specifics may require a few changes in the capacity of systems designed to separate these types of materials.

Facility Layout
Different equipment vendors use their own specially designed equipment to separate in-feed components automatically. Cardboard and paper products are typically separated first using specialized screens. Containers can be split off from the wastestream by means of gravity, air separation, continuous magnets, and eddy-current magnets. Some materials can be positively sorted, meaning that they are selectively pulled from other materials on the conveyor. This may involve mechanical sorters pulling items off a conveyor or equipment, such as by means of a magnet to collect ferrous cans. Other materials can be negatively sorted, meaning that contaminants are removed, leaving only the desired material. Cardboard can be positively sorted by a screen, followed by negative sorting to remove potential contaminants from the concentrated cardboard prior to baling. Negative sorting may also be used in cases where a facility is designed to receive one or more recyclables in concentrated loads. This material may be handled separately in a system that bypasses much of the sorting equipment. Contaminants may be screened and removed prior to baling.

When MRFs are privately operated, there may be more flexibility to select sources of materials. The suppliers, in-feed material mix, and level of processing can be adjusted as material quality or market prices dictate. It may pay to sort plastics by type, or it may not be worth the effort, depending on the markets available. Given this variability, processing capability and flexibility must be high. When MRFs are publicly owned, too, the flexibility to adjust the program as conditions warrant may be desirable.

Equipment
The pieces of equipment needed for sorting depend on the materials the facility is designed to handle. All facilities use conveyors of various types. Depending on the service, conveyors can look substantially different from one another. As needed for the task, some are flat and slide on plates to facilitate sorting; others are troughed, ribbed, or cleated, and others are chain conveyors. Variable-speed drives and reversible motors may be used in some cases. Quality belting material is required with a heavy-duty and costly design because it is difficult to build in redundancy.

Opening multiple shifts may improve revenue performance.

One of the first steps usually is to separate the fiber materials from the containers, such as cans and jars. Disc and star screens are often used to separate out the cardboard and paper, which tend to ride over the top of the screen and are carried to a conveyor while the containers fall between gaps in the screen onto another conveyor. Sometimes the screens are combined with air classifiers to help with material separation. Slanted screens or conveyors may also be used to allow containers to roll off the side while fiber is carried along the main screen or conveyor.

Trommels, which are large, rotating drums with holes, or other types of screens can be used for similar applications. The smaller items, such as containers, fall through the holes while the larger items, such as cardboard, pass through the drum. Trommels can also be fitted with bag openers or other sections to help with sorting.

Drum and belt magnets are used to separate magnetic ferrous containers from other components of the in-feed material. Eddy-current separators remove aluminum beverage cans from other materials. Rapidly pulsing electric current induces a magnetic field in the cans and any other electrically conducting materials while having no effect on the nonconductive items, such as plastic containers.

Other equipment can be incorporated depending on need. Examples include screen types other than those mentioned above, glass crushers, bag breakers, shredders, and air classifiers. Although a well-controlled in-feed stream can allow increased automation, some manual inspection or sorting is still required. Sorting starts on the tipping floor and continues at various points up to the balers. Presorting personnel located in front of the mechanical equipment watch for materials that might cause damage downstream or are a safety concern. They also may break apart bundles and bags. Sorters generally are used to positively sort materials by pulling the desired material(s) from the mix at the sorting station.

Balers, commonly the final processing step at MRFs, compact the sorted materials into bales, which are then loaded into trailers or containers for shipment to the market. Highly compacted bales allow maximum road weights, thereby keeping transportation costs down.

Future Trends
Many concepts have been tried in the past, and innovations will continue. Recycling costs are high, and ways to reduce these costs will likely push future designs. Economies of scale tend to increase facility capacity. Owners generally want the capability of operating around the clock if in-feed material is available. Operating two or more shifts may also allow a facility to improve its revenue performance without major capital investment. This demands a heavy-duty processing system with as much redundancy as practical.

Automation also is increasing. The more sophisticated equipment typically increases capital costs but may pay for itself quickly by reducing operating costs. Optical sorters for everything from glass types to plastic grades to fiber-product cleanup are being implemented at modern MRFs. Automation is most effective on well-defined and single-stream systems where efforts are vigilant to prevent contaminants. This is not always a characteristic of the local waste shed. If the in-feed material is not well defined or limited in scope, then a less sophisticated, more labor-intensive, positive-sort system may make more sense.

Markets can be highly variable, and having a better-quality product may keep the product from being rejected when demand decreases. In the future, material quality will be increasingly important. If a premium can be demanded for a cleaner product, operators will strive for the higher performance. On the other hand, it may sometimes be more cost-effective to create an intermediate product, such as mixed plastics, which can be shipped to a specialty facility for separation.

Versatility and redundancy also help operators stretch budgets. In the future, more systems will have the ability to change processing speeds or make equipment adjustments to allow more material to be handled without additional variable costs.

Local opportunities may allow continued innovation even though the industry is mature. MRFs usually are expensive to operate on a per-ton basis. Therefore, facility operators will likely concentrate on newspaper, aluminum beverage cans, and cardboard, which are normally the higher-value products. Local markets, however, can make other products worthwhile. For instance, if a local market or incentive is available for glass recovery, considerable value can be obtained by diverting this heavy material from the landfill.

Community programs may include some newer or specialty materials, which can be received and processed at MRFs. For example, electronics recycling may increase in the future. Certain items, such as cathode ray tubes, can be handled on the tipping floor and processed separately. Other materials, such as cell phones or circuit boards, can be processed and treated according to market requirements. MRFs for construction and demolition materials also can work in some settings. Recycled concrete, wood, gypsum, and steel can be beneficial and avoid landfill disposal of significant quantities of material.

Areas of Special Attention
Each MRF project will have its own set of questions to address. A common issue is employee health and safety. Quality of life is deemed increasingly important, and it is difficult to find workers willing to perform some of the tasks required. This may be an incentive for increased automation, although it is not likely that all workers will ever be eliminated. Sorters and other operators still are required, and providing them with adequate ventilation, heating, and cooling is important. Dust suppression can be achieved with a light misting system. In some cases, this can benefit the products as well.

Needle sticks and exposure to other MSW hazards are a concern at a MRF, just as they are for hauling operations or any other aspect of waste handling. To protect personnel and avoid damaging equipment or contaminating materials, it is important to provide for careful screening of the in-feed material. The dirtier and more varied the in-feed material is, the more critical this becomes. Operator training, personal protective equipment, and community education are first lines of defense, but other steps can be taken in the design to minimize concerns. A large tipping floor with plenty of space to presort material is helpful. At least two days' worth of storage capacity is desirable even if not expected to be used. This allows plenty of room to mix, sort, and evaluate materials before feeding. Rather than loading directly into the in-feed conveyor, it generally is better to remove any undesirable materials with the front-end loader before they cause any damage.

Well-designed presort stations are important to allow material to be previewed before it gets into the major equipment components. One or two presorters will pick off large metal objects, computer components, liquid or foodwaste containing cans or jars, drums, used chemical containers and other objects that residents and haulers may think are recyclable but that may cause problems downstream. The presorters can also cut ties and open bags so the automated processing equipment can reliably do its job. Because increased product quality raises value, it pays to design in the capability to preview materials. Newspaper is particularly susceptible to contamination. Several different grades of newsprint can be generated. If the equipment is well designed and the feedstock is not contaminated, a higher grade of paper can be produced and thus a higher price paid.

Maximizing load density reduces cost and thus improves economics. This is particularly important for plastics, which are lightweight. Perforators and flatteners help increase bale density, but even with this feature, bales of plastics tend to have low density.

Public relations is generally a major concern for a county or city solid waste administrator. A negative image discourages active public involvement in solid waste issues. As a result, less quality material is available for the facility. Product revenue and tonnage suffer, while the cost per ton processed escalates. Reversing this trend can be difficult, but a well-designed and well-operated MRF can play a major role in projecting a better image. For the administrator, this is an opportunity to present a positive recycling report for the city, even if some of the costs are high.

Often, the best way to boost recycling awareness is through public education. MRFs can be used to showcase waste responsibility. Adding features to ensure that busloads of active grade school students and adult visitors are safe and comfortable usually is well worth the investment. At a minimum, this involves barriers and hazard-free tour routes where operations can be monitored from a safe location. Air-conditioned observation galleries with large windows and perhaps video monitors are especially useful. Administration areas need to have a conference room for orientations, video presentations, and meetings.

Siting Issues
Many different types of sites can be acceptable for a MRF. Transportation costs are increasing and often drive the choice of location. A way to lower these costs is to reduce the distance the incoming material must travel to the facility.

Good access to major roadways is a plus. Usually the facility size is not so large that there are major traffic-congestion issues with the extra truck traffic, although proposed sites must be screened for these concerns. Access to rail could be beneficial but is not necessary; direct rail access is not a significant advantage given the relatively small tonnages processed by a MRF. What may be more important is having bale and container sizes designed to fit intermodal container specifications, so the materials can be loaded onto rail cars or barges offsite.

In most cases, it is preferential not to compact or handle in-feed material any more than is necessary because of the potential for contamination and glass breakage as well as the cost. Greater vehicle utilization must be weighed against material quality.

Locating the facility at a brownfield site sometimes can be advantageous. Reuse of an existing site and perhaps even an existing building, which may be located near the source of materials, can be a way to clean up an eyesore and increase community acceptance.

Still, when siting a MRF—as is the case with any solid waste facility—questions dealing with traffic, noise, vectors, odor, and other issues will need to be addressed. Environmental justice claims and not-in-my-backyard protests can make siting difficult.

If an MRF is combined with a new transfer station, landfill, or waste-to-energy facility, the tonnage processed by these other operations typically will govern the acreage and buffers needed for the facility. In these cases, the MRF usually is a small addition, requiring just a few more acres of land and some thought for traffic patterns and site usage.

Another possibility is to add a MRF to an existing or retired transfer station, landfill, or waste-to-energy facility. Assuming adequate space is available, this is probably the easiest, most economical siting solution because the permitting agencies and the public have already accepted the existing site for solid waste operations. Often some facilities can be shared, including roadways, parking, scales, administration and locker rooms, rolling stock, and possibly tipping floors or other site features.

Use of the common site may mean that collection routes can remain unchanged and the impact on traffic is minimal; however, there may also be drawbacks. For example, drivers or tipping-floor operators may have a greater challenge getting material to the desired tipping location. At a landfill, unstable soils or landfill-gas issues may need to be addressed. Each facility must be able to function without negative effects from the other.

Optimal Results
The many advantages of single-stream MRFs include their positive visibility, effectiveness in enlisting public participation, and generation of high-quality recycled products for a reasonable construction and operating price.

Successful design of single-stream MRFs requires a thorough understanding of the client’s desires and plans as well as the characteristics of the in-feed material before deciding on the best means of processing. Cleaner and more defined in-feed materials allow for more automation.

A desire for the flexibility to change in-feed–material mixes and/or process a dirtier material may require a more versatile technology and more sorters to allow the system to adapt to the desired materials.

Innovation on the part of the various vendors will continue to allow single-stream MRFs to develop and become more cost-effective. In the end, management of the various issues influencing design can result in a showcase facility for the community.

Greg Gesell is a senior mechanical engineer with HDR in Omaha, Nebraska.

MSW - March/April 2006