March 24, 2009 | General

Historical Perspective: Screens & Air Classifiers

BioCycle March 2009, Vol. 50, No. 3, p. 20
The composting industry now has a fleet of screening and cleaning options designed specifically for compost, after decades of adapting agricultural, aggregate and wood equipment.
Rhodes Yepsen

SCREENING equipment for the composting industry has become widely available, with a variety of options – including trommels, star screens, air classifiers and disc screens – to fit specific applications. But early on, in the 1960s and 1970s, screens were often homemade or adapted from other industries. Charles Melhoff, founder of Wildcat Manufacturing Co., says that although they began making compost windrow turners for farmers in the mid 1970s, it wasn’t until the composting industry was more established in the 1980s, with municipal and commercial composters, that he saw the need to expand Wildcat into trommel screens.
“When Wildcat offered its first windrow turner in the mid 1970s, the composting market was small, primarily organic farmers who wanted better nutrients for their soil,” says Melhoff. “As landfills began to fill up in the 1980s, the composting industry evolved, with municipalities interested in how to better manage the waste stream. Farmers would spread the compost on their fields, but commercial and municipal composting operations wanted to screen and bag their product.”
Around the time that Wildcat was founded in South Dakota (1973), Jim McNelly started his potting soil business, Planet Earthworms (later renamed The NaturSoil Company), in Colorado. “There weren’t any screeners made specifically for compost, at least in Colorado, so I put together my own,” says McNelly, who now markets the NaturTech in-vessel composter. When his operation was small, he blended vermicompost, pine bark, peat and vermiculite in the back of a pickup truck with a shovel, then screened the soil mix using half-inch hardware cloth nailed to wheelbarrow handles. “I did this from about 1973 to 1975 to make extra money while at college, bagging about 1,000 8-pound bags in 1976,” says McNelly. “After graduation, I convinced the city of Northglenn, Colorado to start what must have been the first curbside collection program for grass clippings, receiving about 5,000 tons/year. I also expanded my vermicomposting operation, producing between 1 and 2 tons/week of castings by 1977.”
To service the larger volume, McNelly eventually put together a bagger and an automated vibratory screen at an abandoned dairy farm. “I bought a homemade sand screener, which was essentially a cannibalized 1932 Buick truck, with a 3- by 5-foot deck screen welded between the hood and bumper,” recalls McNelly. “I attached this to an old grain elevator, so my wife and I would scoop the potting mix into the screener, which emptied into buckets, conveyed up to the top of the grain hopper, down the chute of my bagger, into a 6-inch pipe, and then into a Levis jeans pant leg, into a bag. This was attached to a scale, and you’d squeeze the pant leg to stop the flow. Around 1978, I retired the pant leg, adding a commercial bag filler. I used this system until about 1983, when I left Colorado.”

The companies below were interviewed about how and when they developed screening equipment for the composting and organics recycling industry. They are listed alphabetically.
Amadas: In 1963, Oliver K. Hobbs and James C. Adams formed Amadas Industries to produce a new generation of peanut harvesting and processing machinery. “In the late 1960s, when methods of burning wood waste were being outlawed, such as in teepee burners, Amadas began making screening equipment for organics recycling,” says Tiny Andrews, Industrial Sales Manager at Amadas. “We realized that our machine for classifying peanuts, which is basically a disc screen, could be adapted for screening these wood wastes, to make a marketable product out of what was previously being burned. The peanut classifier was made larger, and heavier duty, to handle the woody materials.”
Later, in 1989, Amadas was approached by a company in New Jersey to build a trommel screen. “They saw our other equipment and wanted us to make them a trommel screen,” says Andrews. “We made another trommel screen for a company in the Tidewater (Virginia) area, which is still being used. We decided not long after that to specialize in stationary trommels, and continue to custom build them today.”
Construction Equipment Company: “My father, Roger Smith, and I founded Construction Equipment Company (CEC) in 1981, in the aggregate industry,” says Gary Smith, President of CEC, based in Oregon. “We began building our own screen plants in the mid 1980s, and found they worked as well as anything in the organics recycling market. However, all screens at that time had difficulty with wet, sticky material, and in Oregon there is nothing but wet compost and log yard debris.”
The early design of CEC’s four-sided feed hopper with an exit hole, and high-angled screen, caused compost to bridge in the hopper and slide off the screen unsorted, recalls Smith. In response to these problems, CEC developed the Screen-it in 1991. “We eliminated the feeder problem, and introduced a 12-degree screen deck for additional retention time, with a ball deck cleaning system for the fine deck,” explains Smith. This model, he continues, set new standards for the industry, in terms of production expectations, as well as for ease of changing product sizing.
Doppstadt US: Doppstadt began manufacturing trommel screens for the organics industry in the mid 1980s. “Customers who purchased our grinders were often also looking for a reliable screen,” says Shane Donnelly, Sales Manager for DoppstadtUS. “Also, the market in Europe for organics processing was growing, with a demand for separating and processing green materials. Our trommel screen was developed with this booming market in mind.”
The Doppostadt trommel has a completely horizontal drum, using an auger instead of gravity to move product through the drum. “Because material isn’t lifted and dropped, spearing is practically eliminated, and dust is reduced,” says Donnelly.
One feature added about 10 years ago to the Doppstadt is a load-sensing device. “Just like other equipment, you need a consistent load to end up with a consistent end product,” explains Donnelly. “As the drum empties out, you get spearing. With load sensing, that doesn’t happen, because material is spread out over the drum consistently. As you put material in the hopper, the trommel adjusts the feeder, slowing as the material empties from the hopper. There are sensors in both the drum and in the hopper.” Doppstadt trommels have four separate hydraulic drives, allowing the speeds of conveyors and the drum to be changed independently.
Doppstadt used to offer a stand alone air classifier called the Taifun, but replaced it with an on-board air classifier. “Although it was efficient, the Taifun was expensive, so instead we now offer an air classifier as an optional feature built onto the trommel itself,” says Donnelly.
Hawker Corporation: Oren Posner of Eugene, Oregon started Lane Forest Products (LFP) in 1983 as a firewood supply business, and then expanded the company with his wife Susan in the late 1980s. In 1994, LFP began composting yard trimmings, and currently processes over 70,000 yards of finished compost and wood waste annually. LFP began manufacturing equipment in 1996, with the development of Blotech Blower trucks, a division that was later sold to Peterson Corp. “Plastic was contaminating our finished compost, which became apparent when we blew the compost onto landscapes,” says Susan Posner, President of Hawker Corporation, the manufacturing division of LFP. “Plastic was also affecting our ability to sell hog fuel to local cogeneration plants. So we applied our experience from the blower truck to developing an air classifier.”
The first Airlift Separator was created for LFP’s own use in 2003. After about a year, LFP decided to manufacture it commercially. “We realized we had created a solution for compost producers around the country with the same problem,” says Jimmy Smith, North American Manufacturing Representative for Hawker Corporation. He explains that they wanted to make an air classifier that was affordable, portable and easy to use. “The other classifiers on the market at the time were expensive, especially for small operators,” explains Smith. “But small operators deal with the same film plastic problems as large operators.”
Early Airlift Separators were mounted right on the screen, but after various hose configurations and nozzle designs were experimented with, the best was a stand-alone model with a single hose and round nozzle. One of the difficulties in removing plastic is that it tends to be buried in organic material. “We developed and patented a vibration system that mounts under the conveyor belt,” explains Smith. “A violent action of the vibration system shakes and loosens the material, thereby exposing the plastic to the airflow, thus solving the problem.”
Kompech USA: Komptech has a large line of screening and cleaning equipment, including trommel screens, star screens and air classifiers. “We offer a diverse line because composting and organics recycling applications are processing a variety of feedstocks into a variety of products, and need a specific machine to accomplish that efficiently,” says Todd Dunderdale, Director of Sales for Komptech USA. The trommel screens and Hurrikan air classifier were developed by Maschinenbau Farwick in Germany, and acquired by Komptech in 2003.
“Farwick made small trommels and lawn mowers in the 1960s, like the Euro Screen, which could be pulled by a small truck, and were used by landscaping companies,” he explains. “When Komptech took controlling stake in 1999, we began making bigger machines that could match the capacity of our other equipment. Moving forward, we began focusing on efficiency. For instance, the Mustang trommel now has an engine that can come out of the machine and be set apart, essentially making it a generator set that can be located away from dust, etc.”
Komptech’s air classifier, the Hurrikan, was also purchased from Farwick. “The Hurrikan was developed by Farwick over 15 years ago in Europe to remove plastic film contamination from food composting projects,” says Dunderdale. “Since then, we released the Hurrikan S, which is basically a double Hurrikan, for larger operations. We also have adapted this technology into our trommels with the Maxx Integral, and our full line of star screens.”
The company’s star screens were purchased from BG Recyclingmaschinen in 2003. “Star screens are the next generation of screening for the composting industry, capable of processing a much larger volume than trommels, and with the ability to change product sizing in seconds, while the machine is running,” notes Dunderdale. “Star screens have historically had trouble with material build up on the stars, which leads to wear and tear. Komptech has solved this problem with its patented Cleanstar technology, with a metal tab on each star that cleans the shaft in front of it, and fingers on the stars next to it.”
McCloskey: Paschal McCloskey started McCloskey International, Ltd. in 1985. He found that many producers at the time were using aggregate equipment to screen compost, which wasn’t very effective, and therefore saw a niche for developing a trommel screen for compost. “Our early machines were quite different, with drums raised about 14 or 15 feet in the air, and fines dropping straight on the ground,” says Barry Greenaway of McCloskey. “These older machines didn’t have enough power for large drums, as they were limited by weight. Also, it was dangerous to fix and maintain a drum that high up.”
The machine was redesigned with a larger drum for increased production, varying sizes of holes for finer material, and lower to the ground. “Our trommels became five or six times heavier, but also that much more productive,” explains Greenaway. “Anyone can build a bigger machine, but ours has convenience and operational efficiency, particularly with our innovative on-board radial stacker. Previously, you had to move the whole machine to stockpile.”
After establishing itself in the trommel market, McCloskey expanded with a full line of vibratory screeners. Some newer features offered on its trommels include bag rippers and vacuum separators.
Screen USA: Screen USA traces its roots back to 1944 in West Virginia, when the company was called Southern Steel Products. “We were originally in structural steel fabrication, but once we moved to Atlanta in 1973, I diversified into screening equipment for the organics industry,” says Rick Cohen, President of Screen USA. “We had been involved with aggregate screeners, but I saw the need for small and medium-sized equipment for the soil, compost and mulch industry. Our first commercial screen for the organics industry was offered in 1980, model number CF 35D1, which was a portable shaker screen.”
Cohen explains that most feeders on screens at that time were narrow, which leads to bridging in the hoppers. To solve this problem, he designed a three-foot wide dray chain feeder, with straight hopper sides. “Next we developed a trommel screen, because we saw the need for cleaner material, and the ability to handle more moisture than a shaker screen was capable of handling,” he says. “However, as particle size got smaller and moisture levels got higher, such as with biosolids and manure composts, the trommel also had difficulty. So we developed a star screen to effectively tap those markets.”
Screen USA teamed up with a Canadian company that was making star screens for the peat industry, but the prototype failed at first. “Peat has high moisture, but it doesn’t stick like compost, so the screen plugged up instantly,” recalls Cohen. “We went back to the drawing board and redesigned it with a special spacer compound between the stars, and a cleaning system.” The revised model was released in 1992, and featured combs that continuously cleaned the stars. The combs spin at a slower speed than the star shafts, cleaning different segments of the stars on each rotation.
“Most of our customer base is still Mom and Pop operations,” says Cohen. “We’ve had more and more demand for smaller machines, even as everyone else in the industry is making bigger equipment.”
West Salem: West Salem Machinery Co. (WSM) has been manufacturing screening equipment for the sawmill industry since the 1960s. It diversified into equipment for soils and compost in the 1980s, when the sawmill industry declined. “It was really a carrot and the stick scenario, with air quality concerns resulting in bans on teepee burning and incineration, which necessitated a change, and at the same time wood producers were stepping up to the plate, developing markets for waste products,” says Mark Lyman, President of WSM. “Screening equipment was a necessary part of creating these markets, whether for fiberboard, mulch, fuel or compost.”
WSM offers four different types of screening machines for different applications: oscillating, vibrating, trommel and disc screens. “We didn’t have to make any major adaptations to fit the composting industry, because our equipment was already heavy duty, developed for 24/7 mill operations,” says Lyman. “Our equipment improvements often come from customer requests, such as for different disc profiles and screen configurations. Other changes come about through field observation, where we learn how our equipment could better fit a specific application.”
More important than the way the equipment has changed over the years, notes Lyman, is the way that WSM has changed as a company. “We evolved from being just an equipment manufacturer, to being a complete systems provider, working with customers to set up an efficient combination of machines for their operation. Our mission is to add value to fiber products. We do this by providing a processing system that reduces material handling costs.”
Wildcat: Charles Melhoff founded Wildcat Manufacturing in 1973 as a farm repair shop in South Dakota. “We made some farm equipment, bail carriers, snow blowers, things like that,” says Melhoff, who sold the company to the Pat O’Neil family in 2001. “In the mid to late 1970s we developed the windrow turner, and then in the late 1980s we introduced a trommel screen, primarily for municipal and commercial customers.”
Tim O’Hara, currently Sales & Service Manager of Wildcat (which was acquired by Vermeer Mfg. Co. in 2007), notes that the company’s trommel screens have changed with the market over the years. “Most notably, we moved the conveyor to each end of the machine instead of coming out of the rear corner, to prevent material cross-contamination,” says O’Hara. “Customers have requested better service access, which we accommodated with large service doors. When responding to different market needs, Wildcat’s primary change has been developing specialized screen panels.”
Manufacturing trommel screens is no longer a matter of taking a unit out on a demo and closing the sale, he adds. “Each customer has site-specific challenges that need to be understood by the sales team before the right piece of equipment can demonstrate its true potential. Wildcat equipment has become more efficient, and innovations have been introduced, such as screening and coloring in one-step, or adding specialized stainless steel screens for certain materials.”
A keen observer of composting trends, Jim McNelly notes that it has only been in the last 10 to 15 years that screening equipment for compost has really evolved in the U.S. “We’ve had to refine our own technologies for the composting industry,” he says. “Compost producers today have better screens, but just like in the past, they need to find the sweet spot between presizing and separation to reduce blinding. Each operation is different, and the producer still needs to understand how to manage a specific material for screening, finding the right moisture and the best way to meter material into the screener.”

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