Optimal optics

Features - Sorting Equipment Focus

Ongoing maintenance is essential to ensure the optimal performance of optical sorters in a material recovery facility.

June 30, 2016

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Optical sorters have become an industry standard in material recovery facilities (MRFs). From higher resolution cameras and hyper spectral imaging that reveals the smallest of details, to higher throughputs and faster cameras to keep up with higher conveyor speeds, optical sorting technology continues to advance.

To keep these optical sorters at peak performance, though, comes the need for regular maintenance. With most MRFs that employ this technology using at least three optical sorters, depending on the size of the facility, operators have a lot to clean and maintain.

“Just like a car needs to be serviced and checked, so do optical sorters,” Jeff Fountain of Pellenc ST, headquartered in Pertuis, France, with U.S. offices in Fort Mill, South Carolina, says. “Dust, dirt, materials, temperature, humidity and human error can all be a factor in determining when a machine needs to be serviced.”


Within the last five years, optical sorting has evolved to meet clients’ needs, Fountain says. For Pellenc, this includes developing its NIR (near infrared) spectrometer to positively sort organics from municipal solid waste (MSW) streams.

The ability to differentiate among various plastics also is a necessary capability of optical sorters. At Pellenc and National Recovery Technologies (NRT), Nashville, Tennessee, plastics sorting capabilities have gotten a boost, contacts with the companies say.

At Pellenc, the ability to read hard-to-identify plastics, such as multilayer bottles, PET (polyethylene terephthalate) trays versus PET bottles and PVC (polyvinyl chloride) labels and PVC films, has increased over time. The company’s NIR and Vision sorters can identify a clear PET bottle with a full-sleeve colored plastic label around it, Fountain says. They can even identify the materials that make up that label.

NRT recently developed PET Boost technology that gives its optical sorter the ability to identify wet and thin-walled PET bottles more clearly.

“We also use transmissive technology, which places the container between the light source and the detection point for a signal 100 times stronger than reflective detection,” Matthias Erdmannsdoerfer, president of NRT, says.

Felix Hottenstein, sales director at MSS Inc., Nashville, says the resolution of sorters is improving over time—much like that of consumer cameras.

“We can see smaller particles, we can do the additional computer power that’s available to us and we can do advanced processing,” Hottenstein says. “The machine can do much more than it did four or five years ago.”

The elimination or reduction of external or moving parts on these systems is also a big change within the last five years. At Sesotec, headquartered in Schoenberg, Germany, and at Tomra Sorting Solutions, the Norwegian company that is represented in North America by Van Dyk Recycling Solutions (VDRS), Stamford, Connecticut, the elimination of external lights and multiplexers have created more reliable products, the suppliers say. Additionally, Tomra Sorting Solutions developed Autosort technology, which allowed for a flexible sensor and new sorting algorithms.

“This issue is the most important advantage in any MRF, clean or dirty,” says Alex Wolf, sales engineer with Van Dyk Recycling Systems/Tomra Sorting Solutions, referring to facilities processing single-stream recyclables as well as those recovering recyclables from processing municipal solid waste (MSW).


The materials optics are able to sort also have expanded over time. PET and HPDE (high-density polyethylene) bottles, aseptic containers and 3-7 plastics, along with paper and plastic film, can be sorted using this technology.

With that variety of materials, a certain number of optical sorters is necessary on the line. The typical MRF can contain anywhere from three to 16 machines, according to Wolf.

Sesotec’s North America Sales Manager Tracey Hartje, based in Bartlett, Illinois, says three sorters can work in parallel.

More optical sorters also can mean a higher percentage of purity for recovered recyclables.

“A typical recovery rate for one optical sorting stage doing polymer sortation would be 85 to 95 percent, depending on material quality, composition, preparation, presentation and throughput,” Hartje says. “Purities can reach up to 95 percent after one sorting stage.”

For the most part, most optical sorters can reach up to 99 percent purity, suppliers say, but that depends on the kind of material being sorted, the level of contamination and how the material is presented to the equipment. Depending on those factors, purity can be lower or higher.

“When there’s a high amount of residue, [purity] will drop to the high 80 percentiles,” Hottenstein says. “It really depends on the quality of the material—if everything is regulated, you’ll get 95 percent or above, no problem.”

With an expansion of material variety also comes the expansion of detection techniques. Most commonly, MRFs use color sorting and infrared detection.

“It’s gotten better compared to 10 years ago,” Hottenstein says. “The machines have gotten bigger and, because of the need to handle higher throughput, the performance has gotten better.”

In the infrared (IR) range, NRT’s SpydIR can detect material based on molecular composition, Erdmannsdoerfer says, while its ColorPlus sorter uses visible color. X-rays can detect an object by its atomic density, but those are less prevalent in MRFs.

Erdmannsdoerfer says, “Any of these technologies are used to automate any manual or enhance mechanical separation process downstream. Optical sorting really helps to sort at a high throughput and high precision where human capabilities reach their limits.”


For most MRFs, Wolf says, it begins to make sense to add an optical sorter when the stream reaches about 10 tons per hour.

For John Green, the founder of Green Machine LLC, Hampstead, New Hampshire, the standard is “if you need three or more people on the container sort line, you need an optical sorter.”

Jeff Fountain of Pellenc says MRF operators should consider several factors prior to switching to optical sorters, including the equipment’s purchase price versus paying an employee’s salary.

“Salaries for laborers are increasing more and more, and the turnover of laborers is always a problem for MRF managers,” Fountain says. “An optical sorter can easily be paid for by replacing a few of these salaries, and the equipment is much more reliable and efficient than any person.”

This is because, usually, the volumes of material that a MRF is sorting can be quite considerable, Erdmannsdoerfer says.

“In a typical scenario, with 2 percent of the material making up PET bottles, 8 tons per hour of input material would create 320 pounds of PET,” Erdmannsdoerfer says. “In this scenario, with average labor costs, an optical sorter would definitely make sense, financially, with a fast return on investment.”

Hottenstein says a MRF’s location also should be considered when contemplating the addition of optical sorting. MRFs near beaches, for example, may want to implement sorters, even at lower volumes.

“There are more PET bottles in the mix [at beaches],” he says. “During the months they are busy, an optical sorter for bottles can make sense even at 8 to 10 tons per hour,” Hottenstein adds.


Given the nature of the material being processed, keeping the device clean can be a challenge. Green says preventive maintenance for his company’s technology can include climate control system cleaning, sensor cleaning, air jet cleaning and adjusting belt tracking.

“This doesn’t mean that it has to be intensively cleaned daily, but of course, a light source or camera that is covered with dust and/or labels cannot perform at maximum,” Hartje says.

Several equipment suppliers in the industry usually suggest performing maintenance between shift changes as well as daily, weekly and monthly.

“These routine checks will allow the optical sorter to function for a life of 15 years or more,” Fountain says.

But daily, weekly and monthly service, as well as routine checkups and preventive maintenance, are only the beginning in terms of ensuring optical sorters’ optimal performance. The equipment’s effectiveness also is affected by the performance of the screens, magnets and eddy current separators in the MRF as well as by the burden depth of the material as it’s presented to the optical sorters.

“As long as you have good control over your stream and you keep your sorter maintained, you should be fine,” he says.

Getting to know your optical sorter also can help with keeping it working optimally, according to Wolf. He says VDRS offers a training school where MRF operators can send their maintenance personnel to learn about the company’s equipment.

“Understanding how the machines work does help maintenance a lot to understand what they need—and keep them performing as well,” he says.

The author is an assistant editor with the Recycling Today Media Group and can be reached at hcrisan@gie.net.