Get it right from the start

Many factors can influence a material recovery facility’s (MRF’s) throughput—the composition of the inbound material stream, the speed and efficiency of optical sorting or the number of manual sorters involved. Operators can upgrade optics or install robotics to boost efficiencies.

But improving throughput also involves evaluating tipping floor management and how well materials are fed to the system. While sometimes overlooked, these factors are vital to ensuring smooth operations.

During the 2021 MRF Operations Forum, which took place online in October, Mark Henke, senior recycling manager at Phoenix-based Republic Services, and David Marcouiller, executive vice president of sales engineering at Plessisville, Quebec-based Machinex, offered some key takeaways for operators to consider for tipping floor operations and feeding their systems.

Designing the tipping floor

When designing or making changes to a MRF’s tipping floor, Marcouiller advised facility operators to seek outside input from equipment suppliers “as early as possible in the process.”

He said, “It’s usually with great pleasure that we’ll assist you, give you some references, conceptualize and share good insights with you.”

Marcouiller added that MRF operators should be ready to provide their suppliers and any other design consultants with basic details: What is traffic flow like around the building? What is the site plan? If the building is in place, where will the tipping floor be? If the building is not yet in place and it’s a new MRF, what shape does the operator want the tipping floor to be? What kinds of materials will be processed? Will the MRF operate one shift or two? Finally, what are the MRF’s plans for growth?

“All of these things are very important to designing [a MRF],” Marcouiller said.

“Early collaboration is key,” he continued. “We like to have brainstorming sessions with your building contractors and your operator. If you’re a municipality and you already have an operator, we’d like to get involved with them, understanding what their preferences are and how they operate. If you’re a private [company], it’s always good to sit with your current staff and find out the lessons learned from the past and share those ideas.”

Once these basic questions are answered, best practices should be kept in mind when designing a tipping floor.

The shape of a tipping floor can vary: It can be linear, square or L-shaped. Marcouiller said these different shapes all have pros and cons, but ultimately the MRF’s site plan or property will determine the layout that makes the most sense.

Space also might be determined in part by the site plan and property; however, Henke said MRF operators will want to ensure the tipping floor has more space than it needs in case of downtime or future growth. He suggested ensuring the tipping floor can handle at least two full days of material volume.

“You need that capacity on your tip floor so you can have enough space so you don’t go outside the confines of the tip floor, which obviously can lead to a compliance issue,” Henke said. “We often think maybe this is overlooked or doesn’t get the attention it needs during the design phase.”

It might cost more to build a larger tipping floor but, Marcouiller said, “A lot of times this will serve you well in the future when you have extra capacity for minimal upfront capital investment.”

Additionally, Henke said MRF operators should plan to include multiple access doors to the tipping floor in case multiple loads arrive at the same time.

“You always seem to get material in during about an hour to an hour-and-a-half period of time,” he said because haulers will dispatch their trucks at a certain time. “It would be great if they could alternate that, so we have more of a distribution of the trucks as they come in, but we do have to manage that rush. And that’s why we have multiple access doors.”

He added that MRFs that use walking floors in the tipping area should dedicate one access door solely to the walking floor to help manage the flow.

One final consideration would be whether to incorporate an on-site trailer tipper. Although they eat up space and add to the overall design cost, Henke said they offer more flexibility for unloading.

“A tipper can come in and within five to 10 minutes tip loads in a dedicated location, and they’re out of the way of other inbound trucks,” Henke said.

“They increase the initial expenses upfront,” he continued. “But if you talk to people who operate [on-site trailer tippers], they figure that within a couple of years they do pay for themselves because of reduced times on-site.”

Feeding the system

Once the tipping floor has been designed, a MRF operator needs to determine how to load and feed the processing system.

For loading, operators should know the system’s speed and choose a loader that will adequately deliver the volume of material required at the rate required. MRF operators can use a front-end loader equipped with a bucket or a material handler equipped with a grapple. While pros and cons are associated with both approaches, Henke said, “A grapple gives you a little more or better payload. There’s less spillage. It also allows you to work the material quite well on the tip floor.”

He added that grapples allow an operator the dexterity to pick through incoming material, enabling the operator to “choose some of the heavier items that you see within the stream that you don’t want to introduce to your drum feed unit or your process downstream.”

A MRF’s drum feeder should be designed to consistently meter the flow of the entire system. These feeders also tend to fluff material that comes in. As the loader operator works with the material on the tipping floor, he or she should provide the drum feeder with a good, homogenous mixture. This will ensure better introduction of the materials downstream in the system, Henke said.

“If you underfeed the system, you’re not utilizing your end components to their maximum capacity. If you overfeed the system, it’s very important to understand that you’re also overfeeding your opticals or your eddy currents at the end,” he explained, adding that this is one reason why opticals or eddy currents might not be performing well at an existing MRF.

“You want a good, consistent feed,” he added. “Work with your tip floor operator and make sure he or she is consistently feeding it to a certain level.”

A successful system

Tipping floor management is key when feeding the entire system. A big part of tipping floor management means knowing the inbound material stream, which will dictate what methods work best.

Henke said operators need to know the basic characteristics of materials ending up on the tipping floor: Is it wet? How much residue comes in? What is the blend of commercial to residential materials?

He noted a MRF’s material stream and characteristics likely will change over time. In the past, for instance, old corrugated containers (OCC) made up only 10 percent to 15 percent of the typical inbound stream. Today, he said, a MRF might receive 20 percent to 25 percent OCC.

“[Material streams] keep changing on us,” he said. “A cubic yard of residential material today probably weighs a little less than it did just a few years ago.”

Knowing the characteristics of that inbound material stream will help determine a MRF’s labor needs for the tipping floor and for quality control later in the system, while knowing the weight of the inbound material stream will help to determine what operating speed makes the most sense for feeding the system. Henke advised MRF operators to weigh some cubic yards of material on the tipping floor to determine that average.

“If you have a 30-cubic-yard-capacity system and you’re rated at 30 tons per hour and material weighs 300 pounds per yard, you are achieving 30 tons per hour if the drum feed empties at about six minutes, which is a typical flow rate,” he explained. “However, if everything else stays the same and you run that same 30 yards and it’s only 250 pounds per yard, you’re actually going at less than 25 tons an hour. That all comes into play in your throughput capacity.

“So, understand the characteristics of the material,” Henke concluded. “It changes over time.”

This article first appeared in the February edition of Recycling Today, Waste Today’s sister publication. The author is associate editor for the Recycling Today Media Group and can be reached at msmalley@gie.net.