For separating the nutrient-rich material—compost, mulch and soils—trommel screens play a pivotal role for optimizing production. There are several factors to consider when choosing the right model for a facility.
By Duwayne Bonhorst
Communities throughout the U.S. are implementing recycling programs to extend the life of the nation’s waste management facilities. Green waste programs have been a centerpiece of those efforts for decades. However, until recently, many green waste programs have only focused on collecting yard and wood waste material efficiently, with the majority of profits coming from disposal costs. Today, savvy green waste facilities are flipping that old business model by incorporating technology and science to create renewable energy and nutrient-rich organic material that can be sold at a premium price.
For separating the nutrient-rich material—compost, mulch and soils—trommel screens play a pivotal role for optimizing production. There are several factors to consider when choosing the right model for a facility:
• Desired production rates
• Machine design
• Customization
• Support from the manufacturer and dealer
Production rates can be affected by machine design. Trommel screens are engineered to size material. Smaller material (fines) falls through the screen, while larger material (overs) gets pushed out the back. Along with facility location and the type of material being processed, the machine design is just as important as its size when it comes to swapping out screens or drums and performing routine maintenance.
To begin choosing the right size trommel screen for an operation, start looking at the screen size. Most operations are looking for screens between an 1/8 inch to 3 inches (.3 cm to 7.6 cm), and production rates can vary greatly depending on size selected. Smaller screens produce a finer material for reselling. A company that is producing soil will have substantially more overs than if they are producing mulch and require a lot more material to be run through the screen to amass the same volume of end product.
The length of the drum on the trommel screen is another important consideration for production rates. Longer drum lengths give the material more time to separate fines from overs.
Design Considerations
Thinking about where a trommel screen will be working is also essential for selecting the right model. Some models will have conveyers off to the side while others are designed with conveyers only off the front and back. Space becomes a factor for companies with smaller yards, which is pretty common when working in an urban environment. For these locations, a trommel screen with a conveyer out to the side may be a better fit. For similar production rates on larger, more open yards, the Vermeer TR626 trommel screen with its 6-foot (1.83 m) drum and ability to be outfitted with a 180-degree arc radial stacking conveyor to increase the amount of material being stacked on the ground should also be considered. Other machine design considerations include the hopper dump height, operator controls and ease of serviceability.
Hopper height and size, along with the ability to adjust as needed, are crucial to how efficiently a trommel screen will run. The size of the hopper dictates how much material can be added at a time, but the hopper’s height will impact cycle times, setup time and spillage when adding material. When working with lower hopper heights, operators are not having to raise their buckets as high. This can help keep cycle times short, as well as reduce the chance of materials spilling. With a lower hopper height, the ramp to feed the machine does not have to be built, which can also save time during site setup.
Screen Customization
It is important for a trommel screen to be customizable for any unique local regulations or material types. For example, some cities may have strict dust regulations, so a dust cover may need to be added to help keep dust levels down. If a facility works with a lot of wet material, an extra set of brushes can help reduce the amount of time spent cleaning screens.
Drum Options
The industry offers two configurations of trommel screens—a tensioned screen drum or an auger drum. Trommels with a tension screen drum use a lift-and-throw action to separate material. As compost is cycled through the drum, the smaller material passes through the holes of the screen while larger material exits at the end of the trommel. The slope of the machine dictates the rate at which the material flows through the drum as it is tumbled. Using tensioned woven-wire screen panels allows the use of smaller gauge wire to increase the total open screen area, which helps increase production. However, tension screens are more susceptible to material spearing and more vulnerable to heavy contaminates like large rocks and concrete.
Trommels with an auger drum separate material using tumble-and-roll action, which reduces the chance of material spearing the screens. However, auger drums have less open surface area and are more prone to having wet material build up in the drum, and it can be more expensive to change product size due to needing an additional drum.
Additional Customization
It is important for a trommel screen to be customizable for any unique local regulations or material types. For example, some cities may have strict dust regulations, so a dust cover may need to be added to help keep dust levels down. Or if a faciltity works with wet material a lot, an extra set of brushes can help reduce the amount of time spent cleaning screens.
Support
Finally, when shopping for a trommel screen, manufacturer and dealer support matters. A trommel screen is likely not the only piece of equipment in a facility’s fleet. So, it is crucial to select partners that understand the industry and can help a facility operate as efficiently as possible through every step of production process. | WA
Case Study: Operational Benefits
At the Balls Ford Road Composting Facility operated by Freestate Farms in Manassas, VA, yard waste from approximately 150,000 households and 8,000 local businesses is collected. The facility has a permitted capacity of 50,000 tons (35.4 mT) per year but are presently handling around 30,000 tons (27.2 mT) of material annually with four Vermeer TR626 trommel screens sorting material.
According to Jeffery Morton, operations manager for Freestate Farms, processing, handling and screening are vital in creating quality end products. “Incoming material gets run through our grinder, then stacked in windrows and turned regularly to make sure there are adequate oxygen levels,” he said. “That part of the process usually takes between six and eight months. After that, we screen off the larger material, leaving us with nutrient-rich compost and soils that are used by growers and consumers. The larger material is then turned into nutrient-dense mulch or added back into the composting piles.”
The experienced team at Freestate Farms understands how screening material impacts its value and what someone will pay for it. “Screening takes time and is often viewed as a bottleneck in the composting process, but it doesn’t have to be that way,” explained Jeffery. “After trying several different models of trommel screens on the market, we discovered that the Vermeer TR626 was the right fit for our operations. It’s convenient to operate, it’s reliable and it consistently produces no matter what the moisture content is outside.”
“We’ve always found that this Vermeer trommel produces more per hour than any other machine in its price range,” Jeffery explained. “We consistently produce close to 65 yards (59.4 m) per hour. It’s also efficient with fuel and easy to maintain, which helps keep our running costs low.”
Sam Morton, Freestate Farms trommel operator and site manager, said the TR626 is convenient to use, and it does a fantastic job of removing unwanted material from the finished product. “It outperforms anything I’ve operated before. Also, replacing and swapping out screens is easy to do, too,” he explained.
Balancing equipment costs and operating expenses helps Freestate Farms manage its fixed expenses. Of course, fixed expenses are only part of the equation. Composting facilities need to have a demand for the products they are producing. Jeffery and Sam both give the TR626 high marks in that department. “Word of mouth has driven up demand for the compost we produce,” said Jeffrey. “Businesses and homeowners know we produce an excellent product, so we never find ourselves in a spot wondering what we are going to do with all the material we take in. Instead, we hustle every day to try to keep up.”
Duwayne Bonhorst is a Product Specialist for Vermeer Environmental Solutions (Pella, IA). Since 2000, Duwayne has traveled the world to help compost manufacturers, waste handling facilities and soil producers with trommel screen and compost turner with training and operation. Duwayne can be reached at [email protected].
This article contains third-party observations, advice or experiences that do not necessarily reflect the opinions of Vermeer Corporation, its affiliates or its dealers. Testimonials and/or endorsements by contractors in specific circumstances may not be representative of normal circumstances experienced by all customers.
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