Kristin Smith

Kristin is a member of the Recycling Today staff.

Features

Processing for power

Mixed Waste Processing

Mixed-waste processing facilities are becoming more prevalent as a way to improve recycling rates and provide materials for waste conversion processes.

April 8, 2015

Those who manufacture and install equipment used in material recovery facilities (MRFs) are seeing an increased interest in mixed-waste processing designs. While the reasons vary from customer to customer, interest in further organics recovery or alternative energy are often the main drivers. California’s requirement of 75 percent diversion by 2015, for example, has made mixed-waste processing an appealing proposition as it enables a larger amount of material to be diverted from the entire waste stream. Conversely, single-stream recycling only includes recyclables collected from those who choose to participate in a recycling program, resulting in a much cleaner stream of recyclables.

Unlike the “dirty MRFs” of 30 years ago, which were designed to capture high-value recyclables from the waste stream, accounting for about 30 percent diversion, the mixed-waste processing facilities of today are designed to recover as much as possible from the waste stream, 70 percent or more.

Single-stream MRFs start out with cleaner streams of material than do MRFs processing MSW, but technology designed specifically for mixed-waste processing facilities are successfully producing salable commodities.

Chris Hawn, North American sales manager for Machinex, Plessisville, Quebec, says when it comes to mixed-waste MRFs or single-stream MRFs, “There is not a one-size-fits-all solution. Some of the components can cross over between the applications, but there is not a single technology that is a silver bullet.”

Machinex has installed several processing lines in the last couple of years, and of these Hawn observes, “Each one has a different application with a different desired result.”

One system was designed with the capability of removing source-separated organics (SSO) in biodegradable blue bags from the stream while using the separating equipment to capture any recoverable recyclables from the remaining stream as an added benefit. Another installation was designed as a refuse-derived fuel (RDF) preparation facility that required removal of undesirable contaminants and drying of the material. “Of course there is always the customer that simply believes that a one-bin solution is the best answer and the MRF to focus on recoverable recyclables is the only driver,” says Hawn.

In the system designed for removal of SSO, components of a typical single-stream facility were removed or replaced by what Hawn describes as “similar technology with better operational benefits while not sacrificing efficiency.” He notes, “I would suggest that a system designed for MSW can be engineered such that processing single stream is not a problem. In reverse, a system designed for single stream is not as easy of a transition to MSW unless the customer is prepared to sacrifice the operations, maintenance and increased wear and tear.”
 

The differences

Dirk Kantak, director of sales of San Diego-based CP Group, says the typical mixed-waste processing facility starts with a trommel screen after the presort to split the waste into different fractions by size and weight. Meanwhile, single-stream systems typically begin with an old corrugated container (OCC) screen following the presort. MSW systems also have an increased use of air separation systems compared with single-stream systems.

Major differences between processing materials in a mixed-waste MRF versus a single-stream MRF happen at the front-end. John Green, president of Green Machine, Hampstead, New Hampshire, says the presorting at the mixed-waste MRF occurs to “remove nasty materials.”

Bag opening devices and manual presorts remove bulky nonrecyclable items dangerous to the downstream equipment. Air drum separation separates the heavy materials from the light fraction. The light fraction is then delivered to a “more traditional single-stream-like system,” but Green says, “It is highly contaminated and requires a great deal of additional labor to inspect the material in order to make current markets.”

The heavy fraction undergoes additional screening to remove fines, glass and further remove ferrous metals and to identify compostable and aggregate materials for recycling.

“Mixed waste MRFs have to be ready for anything, including the kitchen sink,” remarks Brian Wells, sales operations manager for Eugene, Oregon-based Bulk Handling Systems (BHS). “The most significant challenge when processing MSW is conditioning the waste stream to allow for efficient recovery of the available recyclables.”

Wells continues, “In single-stream, the material is mostly predictable and the vast majority of what goes into a plant should come out as a recyclable commodity. In mixed-waste processing, we can still recover a very high percentage and achieve best-in-class diversion rates, but additional equipment is required due to the challenges the material provides.”

Wells says most single-stream plants in operation today would struggle to efficiently process mixed waste. “The contamination levels are simply beyond what those facilities were designed to effectively handle,” he says.

While Wells contends that processing systems can handle both MSW and single-stream, it all comes back to the conditioning steps — removing the organics and inert materials prior to the final sort. Many mixed-waste processing plants designed by BHS incorporate its Debris Roll screen (DRS) and Nihot Single Drum separator (SDS) to eliminate those types of materials. The DRS removes the fines and organics while the SDS classifies the stream according to density.

Wells says the next steps in the mixed waste processing system are also used in single-stream systems — the BHS polishing screen to separate 2D materials from 3D materials and then NRT optical sorters for final recovery.

“The result is exceptionally high recovery and product quality, previously unheard of in the old world of ‘dirty MRFs’. These are advanced, modern production facilities and using that old moniker really does them a disservice,” stresses Wells.

Van Dyk Recycling Systems (VDRS), Stamford, Connecticut, applies a variety of technologies to handle the variability of the waste, according to Brian Schellati, VDRS director of business development, including Walair density separators, Lubo AWS (antiwrapping) screen and Titech sensor-based sorters.
 

Quality concerns

Concerns about quality with regard to the commodities being marketed from mixed-waste processing facilities do not come as a surprise to MRF equipment manufacturers.

“We heard these same concerns during the rise of single-stream recycling—mills and processors were worried about contamination from a mixed stream,” recalls Wells. “As single stream became more prevalent, those voices died down because the technology performed. Increased participation rates and efficiencies won out as we expect to be the case with MSW processing.”

Hawn agrees that quality concerns are valid. “Trying to simply put a positive spin on it is not doing the industry any service.” He says, although the container processing does not have as much concern for quality, the fiber stream is a different story. “In the scenario where household organics are loose in the stream, there is no question that fiber quality suffers.”

The input percentages also differ greatly between a single-stream MRF and a mixed waste MRF. That being the case, Schellati says recovery percentages from both types of facilities are about the same.

Schellati’s colleague Wilfred Poiesz, western region vice president for VDRS, explains that mixed waste processing facilities achieve 70-80 percent recovery of all recyclables while enjoying 100 percent of MSW in the recovery process before landfilling. He says these facilities “easily achieve high recovery of hard plastics, metals and nonferrous metals.” Recovery of film plastics is possible, he says, but relatively expensive.

“The best strategy is to extract mixed film, then sort the high-quality clean film,” says Poiesz. The remaining material would make up a low-grade mix or can be used as a fuel application. Paper can be separated at multiple levels of the system to produce multiple grades, including clean fiber, wet fiber (drying required) and a fraction to be used in the organics, which is composted or used in anaerobic digestion.

Hawn expresses a bit of skepticism in the idea that conversion technologies are the main driver for mixed-waste processing facilities. While he says there is plenty of talk regarding conversion technologies and alternative energy, “the commercialization is limited and aside from mass-burn there is very little long-term experience in this market.” He adds that some recent installations have not yet added conversion technologies to the back-ends of their systems to fully prove themselves.

While the U.S. moves further into the mixed-waste processing space, Schellati points out that in Europe, mechanical-biological treatment plants, often called MBTs, are effectively using a combination of conversion technologies to minimize residuals.

“The preprocessing systems and separation technology that we supply for these conversion technologies is already commercially proven, but the challenge is just getting both technologies working together on the same scale and have them be financially viable.”



The author is a managing editor of Renewable Energy from Waste and can be reached at ksmith@gie.net.

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