BioCycle Magazine

Size Reduction Equipment Review

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Courtesy of Courtesy of BioCycle Magazine

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LAST YEAR, BioCycle was approached by CalRecovery, Inc. - a consulting engineering firm with offices in California and Leeds, England that specializes in integrated solid waste management - about publishing a North American edition of an updated composting technology book originally published in Italy in 2004 entitled La Tecnologia del Compostaggio (Composting Technology). The title of the new book is Modern Composting Technologies, and will be available in a few months. In anticipation of this publication, BioCycle initiated a series of articles for 2005 that highlight information from the book. These articles will review core design and operating principles of the various technology categories included in the new publication. They will appear in every other issue of BioCycle, starting with January 2005. This first installment covers grinders and shredders - the work-horses that reduce feedstocks to be processed into manageable (biologically and physically speaking) parts.


For most composting operations, particle size reduction is one of the first steps of the overall process. Reducing organic materials to 1- to 3-inches in size is optimal for the following reasons, states Modern Composting Technologies: 1) Increases the surface area of the material to be composted and therefore the area of contact with microbes; and 2) Obtains an average particle size similar to that desired for the end product.

Grinders and shredders are two distinct categories of size reduction equipment, even though the words frequently are used interchangeably. Chippers also get thrown into the vernacular as well. These three categories of equipment are defined (based in part on a “Reader's Q&A” in the April 2003 issue of BioCycle) as follows:
Chippers: These typically are used to cut pieces of wood into chips of various sizes. “Cut” is the key word. Most chippers have a set of knives mounted on a rotating disk or plate. As the disk rotates, the knives impart a slicing action, cutting the material into small pieces. In a typical landscaping chipper, limbs and branches are sliced piece by piece as they are fed manually through the chamber of the chipper. The pieces that emerge are relatively smooth and evenly shaped particles or chips. Chippers produce premium mulch products because of the relatively uniform shape and size of the chips.

Grinders: Grinders reduce particles in size by repeatedly pounding them into smaller and smaller pieces through a combination of tensile, shear and compressive forces. Nearly all grinders, including tub and horizontal feed grinders, rely on a hammermill as the pounding device. A hammermill has club-like projections - hammers - attached to a rapidly rotating drum (rotor). The high rotational speed (more than 1,000 revolutions per minute (rpm)) gives the hammers enough inertia to shred the material, notes Modern Composting Technologies. As the drum rotates, the hammers spin rapidly and smash against the material trapped inside the hammermill chamber until the pieces are small enough to pass through the discharge screen or grate. To be effective, the material being ground has to be somewhat rigid and brittle, although the hammers will eventually pulverize most anything. Particles coming out of a grinder look ragged, broken and smashed. The particles encompass a wide range of shapes and sizes (smaller than the screen opening).

Shredders: The term shredder is usually reserved for machines that tear particles apart (versus smash). The word “shear” is often added as an adjective, i.e., shear shredder. Compression forces are applied to a particle in offset planes to produce a shearing action. A common shredder in the recycling industry is a low-speed, high-torque shear shredder. This machine uses one or more rotating shafts, each with a set of cutting disks or knives mounted closely together on the shaft(s) that sits in a chamber at the bottom of a feed hopper. As the shaft rotates, the cutting devices pull the material down through the small spaces between the cutting disks/knives and the surrounding chamber. Many shredders use a pair of counter rotating shafts that draw the material down, forcing the pieces out between the two shafts. Particles produced by shredders tend to have an elongated shape.

Another type of size reduction mechanism, described in Modern Composting Technologies, is the Archimedean screw, which is similar to an auger found in mixers. This design was originally used in wagons to break up or mix animal feed. Notes the text, “The shattering/ mixing trailers used for the composting process are characterized by having from two to four Archimedean screws.” The size limit of materials to be size reduced is about 3-inches, yielding particle sizes of .75 to 2-inches. The hopper of the unit, “usually in the form of a truncated cone,” has a capacity that ranges from 13 to 26 cubic yards. Add the book's authors: “The size reduction process is discontinuous and the size of the product is determined by its residence time in the hopper. The rotational speed of the Archimedean screws is slow, on the order of several hundred rpm. Unloading the product generally is carried out in 5- to 10-minutes. Throughput capacity of this system is influenced by the type of starting material and its physical characteristics. Machines operated by means of a tractor's PTO (power take-off) can reach a throughput capacity of 65 cubic yards (cy)/hour; on the other hand, using a 400kW self-powered machine, the productivity increases to 200 cy/hour.”


Chipping, shredding and grinding systems on the market have various feed mechanisms, including conveyors and buckets. With tub grinders, some facilities load material directly into the unit with a bucket loader. The following information was excerpted from Modern Composting Technologies: Horizontal feed units typically include the mobile surface of the hopper, the roller feeding unit, the hammer rotor and the counter blades. Some equipment uses an upper conveyor belt instead of a feeding roller. The feeding apparatus usually is mounted ahead of the rotor. The toothed rollers on the feeding unit continuously dose the material into the shredding system, as well as compress the material to allow for better rotor action. This set-up is generally adopted on machines having a chain conveyor belt that brings the material towards the rotor, situated at the bottom of the loading hopper.

With tub grinders, the contact between the incoming material and the size reduction system takes place at the bottom of the hopper, where an opening brings material into contact with the hammermill. With most units, the size-reduced product is discharged via a conveyor/elevator belt. A magnetic separator can be mounted on the discharge belt to remove ferrous material.


As with just about any equipment selection decision, there are tradeoffs between the options available. For example, grinders are thought to be best suited for preparing feedstocks for composting because the pounding and smashing creates cracks and splinters in the particles, leaving many microsurfaces for microorganisms to colonize. However, facilities that receive a variety of feedstocks may want to evaluate shredders, as they are known to be more capable than grinders and chippers to effectively process both rigid and flexible items. Other considerations include throughput capacity, i.e., tons or cubic yards/hour that can be chipped, ground or shredded; energy consumption; maintenance requirements (i.e., frequency of hammer replacement or resurfacing); and cost.

Because the screen size determines how quickly ground material is “released” from the grinder, the screen ends up having a great influence on the capacity of the machine, says Modern Composting Technologies: Machines equipped with this type of apparatus work continuously and hence have a higher productivity than discontinuous ones. For a normal product size of .75 to 2-inches and operating on yard trimmings, more than 200 cy/hour can be reached with an installed power of 250 kW, and about 325 cy/hour with 350 kW.

The book describes shredders as a “toothed rotor grinding apparatus” where the rotor “operates at very low rotational speeds (around 30 rpm) and can be equipped with a hydraulic device that adjusts the rotational speed as a function of the resistance offered by the processed material. In this manner, the installed power can be fully exploited without obstruction occurrences.” These machines operate on a continuous basis, with throughput of about 160 cy/hour when power reaches 300 kW.
While perusing websites to get the most updated information for the equipment section that follows, we came across worksheets on the West Salem Machinery website ( that are a good tool for evaluating size reduction equipment needs. The Wood/Bark Grinding Application Worksheet includes the following data entry categories: Species of wood (e.g., oak, fir, mixed hardwoods); Amount of material to be processed (measured by either average or maximum based on lbs. or kg/hour or cubic feet or cubic meter/hour); Desired end product size (e.g., 2-inch or 50 mm minus); End product use (e.g., mulch, bulking agent, boiler fuel, particleboard furnish, etc.); Method of infeed to wood grinder - conveyor, loader, hand feed; Power available (e.g., 3-phase, 60 cycle, 480 volt); and Machine usage (8 hours/day, 24 hours/day). Website visitors fill out this information and then submit it to West Salem for review and feedback.

The Morbark website includes an article titled “The Ground Rules for Buying A Grinder.” Included are case studies of various wood waste and yard trimmings processing facilities. The article provides the following guidance: “To simply reduce whole trees and big limbs, a whole tree chipper probably is best because it can quickly handle tree length material. Throw stumps, root balls, brush, pallets and yard waste into the mix, however, and a grinder, not a chipper may be necessary. Whole tree chips can be marketed in some areas, but if you want to manufacture today's popular shredded landscape mulch, a hammermill style grinder may be the better choice.”


There are many manufacturers in the size reduction equipment category. A number of these products can be seen and/or read about regularly in the pages of BioCycle, as well as at our conferences. Visits to the companies' websites (see directory for addresses) provide specifications on models available. Complementary equipment, such as mulch colorizers, also is noted on the website. The following (presented in alphabetical order) provides an overview of size reduction systems frequently used at composting and mulch production facilities.

Allu Group: ALLU markets a Screener Crusher that is mounted on wheeled loaders, excavators, etc. The Screener Crusher has a rigid steel frame, onto which horizontally rotating screening and crushing drums have been mounted. All the drums rotate to the same direction. The blades of the drums crush, screen, aerate, pulverize and force the material through the drums; big and hard fragments are left in the bucket. The base machine provides the unit's hydraulic power (use requires two-way hydraulics system). Transmission of power from motor to the drums is via chains and chain wheels. The rotating direction of the drums is always the same but the hydraulics need the ability to work in reverse so that any material blocking the drums can be loosened.

Bandit Industries, Inc.: Bandit's “Beast Recyclers” size reduction equipment features its patented cuttermill that allows materials to be cut, split or ground while providing a multitude of options for end products. The company notes that the controlled, regulated cutting system of the mill maximizes available horsepower, limits fuel consumption and reduces vibration. The Model 4680, the most recent addition to the product line, processes approximately 500 cy/hour of material, depending on what is being processed. It can handle stumps and logs 40-inches in diameter and greater. Most of the material is cut to the proper size during the initial breakdown and not in the screening chamber, allowing material to quickly flow through the screen, ultimately maximizing production.

Continental Biomass Industries: CBI markets a full range of size reduction equipment, varying from portable horizontal grinders (Freedom Hog) to C&D processing lines (featuring the product names “Annihilator” and “Grindall”). Its Magnum Force Series 4000 Wood Hog is specifically designed for a high throughput of yard trimmings and wood waste, yielding a fine ground mulch. Features of the 4000 series include a patented upswing rotor with replaceable hammers and tips, 860-1030 HP CAT electronic engine, built-in prescreener, radio controlled, hydraulic swing-away anvil for tramp metal and quick screen changes. The 6000 horizontal hog series has similar features but is designed to handle larger fractions of material.

DuraTech Industries: DuraTech describes its Model 2009 Industrial Tub Grinder as “efficient and economical for grinding jobs that do not require high horsepower. It is powered by a 275 HP electronic diesel engine that meets Tier II emissions standards and is connected to the heavy duty mill by a power train self-adjusting, microprocessor controlled clutch.” The hammermill unit is equipped with a rotary self-cleaning screen and a constant and clean supply of air to the radiator and engine. There is an optional tub cover to protect employees from projectiles, as well as an optional magnetic roller and aluminum chute that removes metal from the finished product. Other models of tub grinders are available.

EarthSaver Equipment, Inc.: EarthSaver specializes in equipment for processing wood and green waste. It sells replacement parts, as well as brokers a wide variety of size reduction equipment. The company also manufactures its own tub grinder, available in two models. In addition, it provides engineering services for parts that need to be modified or redesigned. “Our redesigns have primarily been in the area of high-wear parts such as hammers, screens and rollers but we are open to any ideas or suggestions you may have,” notes the website. Available wear parts include hammers, tips and rods, screens, cutter bars, rotors (as well as separate shafts and plates), conveyor parts, hydraulic components and tub grinder parts.

Fecon: The BA Wood Hog grinders and the MZA horizontal shredders are Fecon's two primary size reduction products for composting and mulch production facilities. The BA Hog has a 40-inch by 62-inch feed opening with slanted sidewalls to facilitate feeding, and a 62-inch hammermill with reversible cutting tools. The units are powered by either a 600 HP or 800 HP Caterpillar engine. Feedstocks that can be processed include yard trimmings, wood waste, C&D, food waste and stumps. The horizontal shredders are “designed specifically for composting.” They have a continuous floor feeding mechanism and can shred up to 50 tons/hour of mixed yard trimmings. Free swinging, one-piece hammers swing through comb-like counter cutters to achieve size reduction. The shredder has a loading height of less than 6-feet, which the company notes “substantially reduces the size and cost of the loader required.”

Komptech/Norton Environmental: Norton Environmental is the North American distributor of Komptech Farwick organics processing equipment. It markets two size reduction units, with product names that rival CBI's Annihilator. Crambo is a dual shaft low-speed, high-torque shredder that employs two crusher rollers rotating in opposite directions, pressing the material against a cutting comb on a screening basket located underneath. The company adds that the Crambo is resistant to damage from harmful contaminants due to its slow moving shredding tools. The Terminator is a stationary shredder designed to process mixed waste, wood, industrial waste, etc. It has an “intelligent” hydraulic system with constant power control that enables independent adjustment of the shredder process depending on the feedstock. This allows for a continuously adjustable shredding gap between the drum and the counter-scraper (resulting in greater throughput for certain feedstocks).
Morbark: Like some other companies that manufacture size reduction equipment, Morbark started out in the lumber industry, designing a portable pulp wood debarker. Today, the line includes tub and horizontal grinders, hand-fed and industrial chippers and slow-speed shredders, as well as colorizing equipment that works in tandem with the grinders. There are seven tub grinder models. The 950, for example, is a mobile unit with power options up to 250 HP. It has a hydraulic tub tilt that provides direct access to the hammers and the screen. The wood hogs (horizontal grinders) have power options ranging from 250 to 1,000 HP. The 3600 model, for example, goes from 325 to 500 HP. There is a hinged door to provide access to hammers, tips and grates. Morbark's high-torque shredder, the Predator, is designed to process C&D debris, railroad ties and other waste streams unsuitable for high-speed grinding.

Peterson Pacific Corp.: Peterson has been manufacturing equipment for the wood processing and recycling industries since 1982. It started out with chipping plants for the pulp and paper industry. In 1991, the company added recycling equipment, including horizontal feed grinders. The HC 2400 line features portable grinders. The 2400-B, when equipped with a 450 HP engine, can process up to 240 cy (70 tons)/hour of yard trimmings and up to 270 cy (41 tons) of scrap dry wood. With the new 580 HP engine, the unit can process up to 272 cy (79) tons of yard trimmings and 337 cy (51 tons) of dry scrap wood. The mill hog has 24 fixed hammers with replaceable tips. The HC 2410 is a track-mounted grinder that can get into areas inaccessible to wheeled grinders.

Powerscreen: Powerscreen (a Terex company) recently introduced the Powershredder 1800, designed to process yard trimmings, wood, C&D debris and other industrial and municipal waste. The low-speed, high-torque shredder is equipped with a range of knives, hooks and combs to size reduce feedstocks. Optional add-ons include an overband magnet for removal of tramp metals and two different types of breaker bars that can be installed below the shafts for further size reduction. The unit is mobile.
Rotochopper: A unique feature of Rotochoppers horizontal grinding system is that it can color wood as it grinds. Its patented Power Application System delivers a mix of water, colorant, odorants or fertilizer directly into the grinding chamber. The wood and liquid are grinding and mixing at 2000 rpm. The system also can be used with only water to minimize dust generation. Rotochopper has mobile and stationary units. Its stationary diesel 256/266 wood waste processor can grind up to 400 cy/hour and is used for trees and brush, pallets, bark, food waste, municipal solid waste, paper products, etc. The company also has a mobile chip processor/colorizer (CP 118) that has a 5-yard hopper, a high-speed grinding mill (with up to 40 cy/hour capacity) and the coloring system that operates simultaneously. Its low 6-foot, 2-inch height enables use of a skid steer loader to drop feedstocks into the hopper.

U.S. Manufacturing, Inc.: USM makes hammermill components for both original equipment manufacturers and replacement parts. The product line includes precision-machined swing and fixed hammers; forged, infused tungsten carbide tips; and blended alloy, hammermill grates. It also manufactures tub grinder wear parts and will do custom welding if the part needed is not available. USM's website lists about 10 brands of equipment for which it manufactures wear parts. Engineers at the company use computer aided drafting, which allows them to calculate how changes in design affect the internal working of the grinders. It also enables them to maintain precise measurements to assure proper fit and function of the replacement parts.

West Salem Machinery: WSM has been manufacturing wood processing equipment since 1947. Its horizontal feed grinders - diesel and electric-powered, mobile and stationary -range in power capacity from 100 to 1200 HP. On its hammermills, the hammer and screen configurations, combined with operating speeds of 1800 to 2600 rpm, have processing capacities ranging from one to 50 tons/hour. Notes the WSM website: “Our recent focus has been on supplying complete recycling systems to produce specific fiber products such as mulch, boiler fuel, chipboard furnish, playground chips, wood flour, and compost.” The company introduced equipment for C&D processing. The Brute Wood Recycler is a horizontal hog with a “massive” powered feed mechanism designed for C&D processing and similar types of mixed feedstocks. More recently, it introduced the Series 1600 horizontal slow- speed, high-torque shredder.

Nora Goldstein is an editor of Modern Composting Technologies, as well as Executive Editor of BioCycle. Luis Diaz is president of CalRecovery, Inc. in Concord, California. He is a coauthor of Modern Composting Technologies, to be published in spring 2005 by BioCycle/The JG Press, Inc.


Allu Group
861 Main St., Hackensack, NJ 07601

Bandit Industries, Inc.
6750 Millbrook Rd., Remus, MI 49340

Continental Biomass Industries
22 Whittier St., Newton, NH 03858

Duratech Industries International, Inc.
P.O. Box 1940
Jamestown, ND 58402-1940

EarthSaver Equipment, Inc.
P.O. Box 7325, Kalispell, MT 59904

Fecon, Inc.
3460 Grant Dr.
Lebanon, OH 45036

Komptech/Norton Environmental
6200 Rockside Woods Blvd.
Independence, OH 44131

Morbark Inc.
P.O. Box 1000, Winn, MI 48896

Peterson Corp.
P.O. Box 40490, Eugene, OR 97404

Powerscreen USA
11001 Electron Dr., Louisville, KY 40299

Rotochopper, Inc.
217 West St., St. Martin, MN 56376

West Salem Machinery Co.
P.O. Box 5288, Salem, OR 97304

U.S. Manufacturing, Inc.
1707 21st St., Eldora, IA 50627

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