The trends shaping MRF design

I have been reflecting on the past several years, thinking about where we are going in the world of material recovery facilities (MRFs). Our industry has accomplished a great deal since the COVID-19 pandemic, installing new plants and technology in the horse race to process and recover more recyclables with better quality. But the job is long from being done. The opportunities are exciting, and they seem to never stop.

This brings me to the subject of this article: What new trends am I seeing that are changing our landscape or are gaining in emphasis?

First, it is relevant to know that RRT Design & Construction designs and builds recycling facilities across the U.S. for the private sector and municipalities. The firm routinely is sought for advice and also co-created the annual MRF Operations Forum with the publisher of this publication. At the MRF Operations Forum, we have conversations with the leaders in this industry and gain additional insights. 

A number of trends have evolved rapidly over the past few years. They are reshaping the way we think about MRFs and also resulting in significant improvements to the bottom line: 

• Shaft-based screens with rotating discs/stars to separate paper from containers are no longer the standard.
• Optical sorting of paper has become the standard.
• Strategically rerunning or recirculating residue has become more common.
• Plastics are being optically sorted without manual quality control (QC) sorting.
• Eliminating the presort is becoming standard. 
• Only two paper grades commonly are recovered.
• Material handlers often are found feeding larger systems.

Screens no longer standard

Screens have a real and needed purpose at a MRF; however, the constant cleaning, high wear and safety issues have led some MRF designers to consider a different approach.

Although each manufacturer has given its version of these devices its own trade name, the important distinction is how differently they function compared with the screens used for the last 25-plus years when processing single-stream recyclables. The shafts are perpendicular to the material flow, as in disc or star screens, but the discs’ surfaces are helical, moving the material in two directions at the same time, perpendicular to the material flow in addition to being parallel. This unique movement creates the needed agitation to liberate and separate material by size without the wrapping issues commonly associated with screens of the past because the shafts are open-ended.

Rather than relying on a steep angle to separate material based on shape, these machines typically are installed horizontally, easing safety issues related to cleaning and eliminating the wear issues of rotating discs at high speed that apply frictional forces to move oversized or flat material vertically.

Just to be clear, disc/star screens still are used and will continue to be needed, but we are seeing their effectiveness shine farther down the line after the majority of the materials have been concentrated on their respective sort lines.

Today, we are seeing larger plants move away from using an OCC, or old corrugated containers, screen to these new types of screens with helical shafts to skim off OCC and any bulky material. Likewise, these new types of screens are replacing paper screens to separate paper from containers. Nevertheless, MRFs still have plenty of good reasons to use an OCC screen and paper screens, but we will see a continuing shift away from them. 

Although not a focus in this discussion, ballistic separators will continue to be used as they also remedy the challenges associated with the old-style paper screens but with a limitation on capacity. Their application is valuable as a final step to remove tramp paper and plastic film prior to optically sorting containers.

Optically sorting paper

Less than 10 years ago, the effectiveness of using optical sorters to sort and clean paper enough to produce a sorted residential papers and news (SRPN) grade and a mixed paper grade was a subject of debate. That debate is over.

Optical sorters are working remarkably well in this application, automating the sorting of paper into a marketable grade. Depending on the quality of the optical sorter and how material is presented to the unit, manual sorting to produce a fiber grade is becoming a thing of the past for the larger plants that can cost-justify the automation.

The top plants are so well-designed and working so well that the practice of “trimming” or “cleaning” the outside surface of a bale also is waning.

The labor savings involved in using optical sorters to produce fiber grades are huge, plus automation eases the effects of increasing throughput.

Strategically recirculating residue

No technology is 100 percent effective when dealing with the heterogeneous nature of single-stream material. Some MRFs were adding “last chance” conveyors for manual sorters to capture what was missed. Then robots were added because the task aligns well with the technology’s capabilities. However, the variety of material that needs to find its way into the right bin is challenging given how spaced apart the bins are.

Rerunning all the residue on the original system was not the answer as it would reintroduce all the contaminants that the system already worked hard to remove. The true solution was to add another optical sorter that is highly programmable with excellent efficiency at the end of the line to process all the residue and very discriminately separate specific targeted materials (typically all the missed containers) and eject them with high purity. The residue is not rerun, but the portion that is valuable and previously was missed is recovered as a clean mixed stream and then reintroduced upstream into the system. Because of this additional loading, all of the optical sorters are sized accordingly.

For well-run and well-designed large plants, we can see residue rates drop, reducing disposal costs while recovering more valuable material for more revenue, all without using more labor.

Optically sorting plastics without QC

When selecting higher quality optical sorters combined with well-designed plants that truly present material well to the scanning and ejection components of an optical sorter, aggressive operators readily can eliminate the need for manual sorters to achieve mill specifications for prohibitives in container bales of polyethylene terephthalate, high-density polyethylene and polypropylene. This approach has yielded reduced staffing needs and the resulting savings in operating costs.

Eliminating the presort

Standard MRF design for decades has incorporated a manually staffed “presort” at the start of the system for the removal of trash, bulky metals, mixed rigid plastic and any other items that would be problematic downstream. It is difficult for any manual sorter to perform this task.

However, with the advent of different screening options that tolerate these items, the trend is to eliminate presort stations by using automation to concentrate the large objects, thereby reducing the burden depth of the materials as they are presented downstream, fully liberated onto a postsort conveyor. This shift has resulted in safer operations and gives the sorter far greater opportunity to identify and remove the items while the plant runs at higher throughput given the reduction in the burden depth. This trend is reducing workplace injury, improving employee retention and increasing plant uptime.

Only two paper grades commonly are recovered

For single-stream MRFs, the paper portion always has been the higher percentage of the material composition and the mainstay of any program.

In 2010, I wrote an article with RRT Vice President David Weitzman that still is cited today because of its impact. We forecast the fall-off of newspapers as the dominant portion of paper to be processed at a MRF given the phasing out of daily newspapers, necessitating an end to building MRFs that focused on producing what was then called ONP No. 8. Within 10 years, the ONP grade was gone, replaced by what we call the SRPN grade.

During this transition, single-stream MRFs focused on recovering OCC and mixed paper, but operators thought that having the flexibility to also produce SRPN was important, so sort lines were designed with added equipment and space for this task. The reality today and the trend going forward is to produce only two grades, OCC and mixed paper, with a focus on meeting the mill specs for minimizing prohibitives, such as plastic film and thermoforms, while also ensuring valuable containers are not present in the paper bales.

Along with this trend and the application of automation to achieve these objectives, the overlay trend has been to reduce the amount of OCC (and other brown grades, such as kraft) in the mixed paper given the difference in market value.

With all this said, dual-stream MRFs are different. Because the containers are separately collected, the paper stream tipped on the floor already arrives free of most nonpaper materials. Dual-stream MRFs are able to continue producing three grades of paper by recovering an SRPN grade. Even though the percentage of the total recovery is modest, these MRFs enjoy the higher value for SRPN compared with mixed paper, so we see this continuing for a number of years until all newspapers are virtually gone.

The role of material handlers

Thinking about other trends, the wheel loader also comes to mind. For decades, these machines have been used to transfer tipped loads from the floor onto the infeed conveyor or metering storage bin. Their limitations are quite apparent when operators start using material handlers to load material onto larger systems.

Delivered recyclables have a high tendency to stratify, with heavy material migrating to the bottom of transfer trailers as well as during the shuffling of material on the tipping floor. Tipped loads can be clumped, wet, frozen and filled with undesirables. Loaders generally are not very efficient in blending and metering material and declumping and breaking up frozen loads given the less-precise shape of their buckets compared with a grapple attachment on a material handler when dealing with heterogeneous materials.

However, using a material handler can help MRF operators achieve a relatively uniform feed rate in terms of average density and volume, which helps ensure a steady production and recovery rate by controlling the burden depth throughout the system and the deviation of the expected material composition.

Additionally, the identification and removal of undesirable material on the tipping floor is accomplished more easily using the material handler, leading to safer operations and fewer jams.

Material handlers are more common today, and that will continue to grow over time.

The shift to lower operating costs

The recycling industry has been busy reconstructing older MRFs and building new ones that take advantage of modern technologies that can be applied strategically in the MRF’s design while also considering changes to the composition of incoming material.

The net result has shifted MRF operations toward higher recovery rates, far lower operating costs through labor reductions and increased uptime by eliminating bottlenecks to yield greater throughputs. The trend is to replace or upgrade every MRF that was built 15 or more years ago with the approaches noted above.

The financial stability realized from these investments is notable, especially when the facility is well-designed and constructed for the next wave of trends that are certain to happen.

The author is a professional engineer and the president of RRT Design & Construction, Melville, New York. He can be reached at negosi@rrtenviro.com.