From Transfer Station to MRF

One of the basic laws of physics concern’s the ability of a substance to change from a gas to a liquid and to a solid depending on its physical circumstances. Increase the pressure acting on the substance, and it changes from a gas to a liquid and then to a solid. Increase its temperature, and it melts from solid to liquid and then further vaporizes into a gas. Water, which can be vapor, liquid, or ice is a perfect example.

Waste processing facilities can also achieve three different operational states. They can be simple transfer stations, receiving waste from local waste haulers so it can be consolidated into one large semitrailer for long-distance hauling to a landfill. They can be simple, material recovery facilities (MRFs) receiving presorted streams of recyclables for baling and shipping to various end users. And they can be more complicated MRFs that actually perform this separation of recyclables onsite from a single stream of collected waste while utilizing sophisticated machinery and dedicated manpower.

The same facility can often be used for all or any of the above operations. Pressure and temperature aren’t the drivers of these facility phase transitions. Operational costs and market opportunities can justify the retrofitting of a transfer facility to a MRF or the reorganization of one type of MRF into another.

Transfer Station Design and Siting Requirements
What is a transfer station and why are they used? A transfer station is the focal point of multiple collection-truck routes where the accumulated waste can be transferred to a larger transport vehicle for economical long-distance hauling to a final disposal site. The development of transfer stations mirrors the solid waste industry’s general trend toward fewer but larger landfills. Such landfills, in the first place, tend to be located in remote areas away from population centers. Having fewer of them will proportionally increase the effective distance from curbside to disposal working face.

Transfer stations also provide inherent cost advantages. The cost of hauling is measured in dollars per ton x mile. That is, a collection vehicle, with a maximum capacity measured in tons, has a typical operating cost per mile traveled. If this collection truck has a maximum payload of 6 tons and a direct hauling cost of $3 per mile, its transport costs would be $0.50 per ton x mile. At a similar operating cost of $3 per mile, a trailer truck with a payload capacity of 24 tons would have a transportation cost of $0.125 per ton x mile (or one fourth of the costs of direct hauling by the collection trucks).

Suppose however, that the waste handling company builds a transfer station whose total amortized capital costs and operating costs are equivalent to a fixed cost of $12 per ton.

BEM = FC ÷ (CHC – THC)

where

BEM = Breakeven mileage
FC = Fixed costs of the transfer station
CHC = Collection truck hauling costs
(equivalent to the cost savings)
THC = Trailer hauling costs
(or the variable costs)

The equation works out as follows:

BEM = ($12 per ton)
÷ [($0.50 per ton x mile)
– ($0.125 per ton x mile)]

BEM = 32 miles

But this is not a fixed equation. With economies of scale, longer hauling distances would have to be served by larger transfer stations. And with increased size comes lower per-ton annual facility costs, which will simultaneously lower the breakeven point while increasing cost savings (or profit margins).

So given the anticipated annual tonnage of waste to be handled by the transfer station, how should it be designed? That depends in large part on the type of transfer station configuration. The simplest design utilizes a combination of a tipping floor and an open-topped transfer trailer parked in a lower bay. Waste haulers dump their waste onto the tipping floor, from which it is pushed into the top of the transfer trailer by a front-end loader, or else the waste haulers use the tipping floor to back their trucks up to the edge of the bay for direct dumping into the transfer trailer.

A more complicated design utilizes a surge pit. This is an intermediate level between the tipping floor and the transfer trailer bay that allows for the temporary storage of peak waste loads. This reduces the number of transfer trailers required. Furthermore, the surge pit sidewalls can be used for additional compaction. A dozer or front-end loader can push the waste piles up against a sidewall, achieving a higher degree of compaction. The increase in waste density increases the effective payload of the transfer truck (without having to utilize greater carrying volumes), and further decreases operating costs.

Variations of the surge pit configuration utilize hydraulic rams, precompactors, or even bailers to achieve results. Hydraulic rams are installed in the surge pit sidewalls and drive the waste pile into the back of a specially designed trailer. Precompactors are set inside a cylinder and create a high-density waste log that is pushed into the back of a trailer by means of a live floor. Bailers are fed waste from the tipping floor and create dense bricks of waste that are placed by forklift into a transfer trailer or on the back of a flatbed truck.

The overall size and capacity of the transfer station also depends on whether the station design is based on the rate at which waste can be unloaded from the collection trucks, or if it is based on rate that transfer trucks can be loaded. The first is a function of the collection vehicle payloads, the areas of the dumping space, hours per day of operation, time to offload a collection truck, and a “peaking factor” (the ratio of the peak number of trucks versus the average number of trucks during a 30-minute portion of the workday). The second is determined by the transfer trailer capacity, the number of trailers than can be loaded simultaneously, hours per day of operation, the time to load each trailer and the time needed to position and park each trailer. Each design is indirectly affected by queuing requirements (into and out of the station), available operating equipment, staffing, and so on.

The tipping floor is the heart of the transfer station operation. Usually, a base minimum area of 4,000 square feet (about 21 yards by 21 yards) is required. In addition to this base area, an additional 20 square feet should be added for each ton of waste received each workday. At 100 tons per day, a transfer station would require a tipping floor with an area of at least 6,000 square feet. Most transfer stations are designed with potential future expansion in mind. The overall site will require that area set aside for access roads, sound and sight buffers, perimeter security, employee parking, additional office and storage space, and truck queuing (if truck backups are not allowed out onto the city streets).

The siting of a transfer station depends on several factors, both practical and required by regulations. First of all, the site property has to have enough area to enclose all of the functions described above. Public opposition has an indirect but critical affect on the siting process. The inconvenience to the site’s neighborhood from increased truck traffic and transfer operations must be balanced by job and economic opportunities provided by the transfer business as well as by the increased tax revenues it will generate.

Also, the site must minimize transport costs by being in a central location that minimizes both short-distance waste hauling and long-distance waste transport. Finding such a location is often more art than science, and it is often difficult to find one spot that meets all these criteria. Transportation costs are further minimized by being adjacent to such major transportation routes as freeways. Vehicle and truck compatibility on local roadways is also a concern.

Exclusionary siting criteria set by local, state, and federal regulations are often similar to that for landfills and other waste management operations. These usually preclude siting a transfer station in wetlands, floodplains, wildlife habitats, and parks, or near hospitals or domiciles. Urban locations will also have to deal with zoning requirements that will limit transfer station operations to areas zoned industrial.

MRF Design and Siting Requirements
MRFs are subject to similar but different design and siting requirements. The function of the MRF is almost the mirror image of the transfer station. The purpose of the transfer station is to consolidate various wastestreams into one central location where they can be combined for a single, long-haul transport route for eventual disposal. The purpose of a MRF is to take those same wastestreams, divide them up further, and recombine them by their constituent materials to create multiple piles of similar types of recyclables for eventual resale.

There are two basic types of MRFs: multiple stream and single stream. Oddly enough, it is the multiple-stream MRF that is the simpler of the two. These kinds of MRFs rely on the homeowner, business, or other originator of municipal solid waste to source-separate its waste by type prior to putting it out for collection. Waste generators normally place one or more key types of recyclables (newsprint, office paper, cans, or plastic bottles) into one or more segregation bins for separate pickup. These separated items arrive at a multiple-stream MRF as individual streams of presorted materials. This makes the sorting and collating of the materials a relatively simple process that usually requires only manual sorting by laborers. The only necessary pieces of equipment normally required by multiple-stream MRFs are conveyor belts to move and carry the material and balers to compact and bind the separated waste types prior to shipment.

Single-stream MRFs are far more complex in their inner workings. These facilities receive a single wastestream (from multiple hauling trucks, of course, but each depositing the same mix of collected municipal solid waste), which is then separated at the MRF by mechanical means. Since these MRFs receive messy loads of waste as opposed to neatly separated recyclables, they are often referred to as “dirty MRFs” (while multiple-stream MRFs are nicknamed “clean MRFs”).

The amount and types of equipment utilized at single-stream MRFs depends on both the wastestream and the anticipated resale value of the recycled materials. Magnetic separators remove ferrous metals from the wastestream by means of attraction generated by an electromagnet.

Similarly, eddy-current separators remove nonferrous metals (mostly aluminum cans and foil) by using induced magnetic fields generated by rapidly spinning magnetic rotors. For large, lightweight, nonmetallic objects, there are disc screens that induce a wavelike motion that carries large objects (like cardboard boxes) to the top of the wastestream for easy removal.

Removal of small objects that contaminate the wastestream is performed by rotating trommels. These are large, rotating, perforated canisters that allow fines to escape. Weight separation can be performed by air classifiers that carry lightweight objects to the top of a chimney stack for easy removal. Similarly, air knives use high-pressure air blasts forming sheets of airflows.

And, finally, glass color separators use light spectrophotometry to separate glass and plastic by color.

MRF operations should emphasize both quantity and quality. They should maximize the recovery of the greatest quantity of the highest quality (as measured by market value) materials. For a MRF, these recovery processes are mechanical or manual in nature. There is no biological digestion or thermal combustion performed at these facilities. The recovered material must have physical characteristics (or be processed to achieve these characteristics) allowing them to meet resale specifications on the scrap market.

Like transfer stations, MRF operations begin at the tipping floor. Like the size of a transfer station floor, the tipping floor at a MRF should be sized to handle anticipated incoming waste quantities. However, the MRF tipping floor should have enough surge capacity to hold at least two days’ worth of waste receipts and either be recessed or enclosed with short walls in order to ensure waste containment.

One special problem faced by MRFs is the effect of glass breakage. Shattered glass bottles and glass shards result from the dumping of waste onto a hard concrete floor. Broken glass has little or no market value and can cause wear and tear (or even serious damage) to components of the waste-processing equipment.

Transfer stations are not concerned with broken glass, since it all gets pushed into a semi-trailer for hauling and disposal anyway.

A radical method for dealing with broken glass is to replace the floor with a large moving belt that directly caries the incoming waste to the first stage of the recovery process. Instead of sizing a tipping floor to manage anticipated waste volumes, the conveyor belt is designed with enough width and operating speed to provide the same equivalent operational area. No heavy equipment, such as front-end loaders, is required to move or load the waste. However, the conveyor belt is more expensive, prone to breakdowns, and lacks the surge capacity needed to manage additional loads.

The area required for actual processing depends on the type and number of equipment utilized for the recovery process. This in turn, depends on the amount of waste and its anticipated constituents. Figure 1 indicates the average constituents of municipal solid waste by weight.

If the tipping floor is the heart of the MRF operation, the conveyor-belt system provides its arteries. These belts are by necessity, heavy-duty (with multi-ply construction) with raised sides to prevent waste from falling off. The waste passes under or through blades or screws that rip open waste bags to expose their contents. Belts can be integrated directly with processing equipment (like magnetic belt separators).

Belts can also be set at inclined angles, connecting operations at different levels throughout the facility. This allows for the effective utilization of a relatively small building footprint by setting operation on different floors and levels. Transfer stations, on the other hand, can only operate at a single level.

Such operations usually (but not always) follow a similar pattern. The operational stages should be sequenced so that the material easiest to remove is extracted from the wastestream first, followed by material that is more difficult to remove. By removing waste in preceding stages, the more difficult extraction processes are made easier to perform. This is done until the most difficult material to sort and remove is all that is left of the wastestream at the end of the process.

In many communities, yardwastes are no longer included in standard waste pickups. By keeping waste collected at restaurants, stores, and similar commercial operations separate from other waste-hauling operations, foodwaste can be reduced to insignificant quantities.

With the organics removed from the wastestream, the easiest materials to extract are the ferrous metals that can be separated by means of an overhead magnet or magnetized conveyor belt. Once the ferrous metals have been removed, the nonferrous metals can be removed by means of an eddy-current separator. Once the metals have been removed, paper and cardboard can be processed by the appropriate combination of disc screeners and air clarifiers to remove the amount justified by current market demand. This leaves plastic and glass, the least valuable portions of the wastestream.

Plastic is difficult to recycle, because there are literally dozens of different types of plastics used in commercial packaging. Glass is varied by color, requiring another step in the recycling process, and is in much less demand than metals or cardboard.

Once the desired materials have been separated and stockpiled, they must be prepared for shipment. The last stage of the operation is done by compactors and balers. Balers bind up the compacted material in twine or wire to ensure that it keeps its shape during transport. Balers and compactors are sized to the quantity of material being processed and to the amount of pressure required to achieve proper compaction. By compacting and bailing the material, subsequent transportation costs can be greatly reduced.

Siting requirements for MRFs differ from those of transfer stations. First, a MRF should ideally be located near the various users of the recycled materials while also minimizing the distance from waste collection routes. In fact, a hybrid arrangement is possible, in which a MRF located near its customers receives long-haul waste from a transfer station located closer to the waste collection sources.

The area required for a MRF also varies somewhat from that of a transfer station. In general, a MRF will need between 1,000 and 4,000 square feet of operational area per ton per hour flow-through capacity. For example, a 150-ton-per-hour facility will typically require a minimum 160,000 square feet of operational area (tipping floor, processing equipment, and vehicle and labor operations). Outside the facility, another 100,000 square feet will be required for scales, truck queuing, and other exterior operations. Add another 250,000 square feet for parking rolling stock, 30,000 square feet for employee parking and 150,000 square feet for buffer areas, and the overall facility requiring 690,000 square feet.

Design Similarities and Differences
Transfer station operations are optimized by the “get a bigger hammer” approach. Finesse is not a concern at a transfer station. If increased capacity is required, the facility must expand its operational area (especially its tipping floor and truck-queuing lengths) and increase the number of front-end loaders.

There is basically a one-to-one relationship between area and capacity. However, since a given tract of land has a limited amount of acreage, there is an upper limit on the amount of waste that can be handled by any given transfer station. MRFs, on the other hand, require some relatively sophisticated planning and forethought. Their profitability is far more sensitive to market signals and scrap pricing. Greater-capacity equipment does not necessarily require proportionally increased floor space. In fact, greater efficiencies (as measured by tons per hour for a given operational area) can be achieved by optimizing facility layout and rationalizing the process sequence.

So in contrast with transfer stations, MRF capacity is more a function of brains over brawn.

These factors affect the possibility of retrofitting a transfer station into a MRF of equivalent capacity. More often than not, a MRF created by a conversion of an existing transfer station will have the potential for a higher capacity than the original facility. While this is presumably a good thing, there has to be a match between the amounts of waste generated by sources utilizing the facility. It may not be economical to acquire and refit an entire transfer facility into a MRF whose processing capacity greatly exceeds the available supply of waste. In many cases, it may make better financial sense to convert only part of a transfer station over to MRF operations.

As with all commercial real estate ventures, what matters for both MRFs and transfer stations is location, location, location. In both cases, the preferred location is one that minimizes transportation costs both coming and going (all other factors being equal).

Assuming that a transfer station or MRF is located in the same community with the same waste sources, it would be a relatively simple matter to pick a location that minimizes the hauling costs to either kind of facility. In both cases, it is waste collection trucks (and sometimes specialized recycling trucks). The problem lies in the fact that the ultimate disposal site (landfill) that is to receive the waste from a transfer station may not be located in the same region as scrap customers receiving recyclables from a MRF.

This factor affects the overall economics of a proposed MRF operation and, with it, the decision to convert to a MRF in the first place. Fortunately, the ultimate shipping point (the landfill) for waste hauled from a transfer station will tend to be much further away from the urban area where the facility and its potential scrap resale customers are located. Instead of shipping bulk waste to a landfill way out in the countryside, trucks hauling recyclables from a converted MRF will typically have a shorter travel distance to industrial users of scrap materials.

The general layout of a transfer station is actually pretty barebones. Its enclosed space has a tipping floor, a pit to park the semitruck, and not much else. The floors can have somewhat varying configurations depending whether or not onsite compaction or bailing of the waste mass is being performed.

MRFs, however, can have complicated floor plans that have several levels instead one recessed pit.

Oddly enough, the fact that MRFs have potentially more complicated design plans actually gives them greater flexibility than the much simpler transfer station layout. So a converted MRF and its processing equipment can more easily be fitted into a simple transfer station floor plan. Some ingenuity may be required to fit everything and lay out all the necessary conveyor belt connections, but it can be done.

So it usually makes economic sense to convert all or part of an existing transfer station into a material recovery facility, provided the capital costs of the retrofit can be offset by reduced hauling costs and the resale price of the recycled materials. The exact configuration and extent of the proposed conversion will depend on which recyclables have high market demand along with t