BioCycle September 2005, Vol. 46, No. 9, p. 36
Dedicated mechanical mixers can help optimize composting parameters and improve materials handling efficiencies.
Nora Goldstein

A KEY to successful aerobic composting is getting started on the right foot. This includes an initial feedstock mix with a workable C:N ratio, moisture content, bulk density and structural integrity. It also includes having equipment available for efficient materials handling to avoid bottlenecks and to deal quickly with putrescible and odorous feedstocks. One piece of equipment that composting facility operators have found combines the need for the right mix with efficient materials handling are mechanical mixers. These units facilitate making adjustments to the mix (e.g., adding in various amendments and moisture) and yield a uniformity that benefits the composting process.
In some cases, applications are found for mechanical mixers that go beyond their essential purpose. For example, several years ago, BioCycle had an article on innovative collection vehicles for food residuals diversion (“Trucks With An Appetite,” January 2003). The article described a composting program at Purdue University that involved diversion of food residuals from campus dining halls. A truck-mounted Kuhn-Knight Industrial Reel Auggie compost mixer was married to an International truck tractor and fitted with a hydraulic cart tipper for a cost of about $50,000. The resulting vehicle served the triple purpose of collecting food, mixing in the amendments, and building compost windrows – essentially combining efficient materials handling of putrescible feedstocks with the opportunity to create an optimal initial mix in the windrow.
This segment of the Modern Composting Technologies article series takes a look at mechanical mixers. The article series, initiated in the January 2005 issue of BioCycle, is based on a new book of the same name, being published by BioCycle/The JG Press, Inc. and CalRecovery, Inc. Copies will be available in early October (see www.biocycle.net).
MIXING BASICS
Any discussion of mixing should start with a quick primer on what comprises an optimal composting process. Notes a section of Modern Composting Technologies dealing with process parameters: “It is clear that the evolution of a composting process depends not only on the correct composition of the organic matter, but also on the maintenance of the optimal operating and environmental conditions. The composting process involves a ‘three-variable system:’ air, water and organic matter. Under optimal composting conditions, the value of free air space should be between 35 and 50 percent…Free air space primarily depends on size distribution of the material to be composted; structural strength of the material particles; moisture content; and thickness of the layer of the material (pile height)…The volume of interstitial space is closely related to the bulk density of the material in the pile. To obtain a material with optimal porosity for composting, generally a bulk density of about 0.50 to 0.60 t/m3 is required. To achieve this density, it may be necessary to mix different types of materials.”
Modern Composting Technologies includes a short section on mixing. It notes that for composting sites using a static pile method with no agitation, “a homogenous mixture is necessary to achieve a proper evolution of the process throughout all of the piles and finally to produce a compost of homogeneous quality. For composting plants using mechanical turning, the initial homogenization of the biomass is not as much of a concern, since subsequent turnings help in achieving the desired degree of homogenization. However, in general, a reasonable mixing of different materials is achieved during the grinding operation or through use of a dedicated mixer (either stationary or mobile).”
Mechanical mixers include horizontal and vertical varieties. The latter are characterized by a cylindrical hopper (with a diameter of 2.0 to 3.0 m and a height of 1.5 to 2.0 m), in the middle of which a rotating vertical shaft with several toothed blades is installed. The typical operation of a mixer generally is discontinuous. Unloading of the mixed materials is carried out by a conveyor belt located at the base of the machine. As mentioned above, some mixers mounted on a truck discharge the mixed load directly into windrows.
MIXER OPTIONS
Scanning websites of mixer manufacturers, it is clear that some have their origins in the agricultural sector, used for mixing ingredients for animal feed and other farm applications. Several companies, e.g., Kuhn Knight, Rotomix and Supreme International, have used their core technologies to make mixers for the composting market. The following is an overview of some mechanical mixer options:
The Detachable Container & Compactor Corp. (DETCON) manufactures SSI 4-auger mixers that blend from top to bottom and side to side in a figure-8 pattern that homogenizes the material. The units have a planetary gear reducer to boost torque and mix heavy materials like biosolids, fly ash, wood chips and lime in roughly three to six minutes, notes the company’s literature. The models vary in capacity and configuration, e.g., stationary, trailer mounted, self-propelled, diesel or electric. All feature the same mixing unit design. The self-propelled mixer doesn’t need a separate power source to drive it and thus is lower to the ground than a truck-mounted mixer (which keeps the loading height low). It can be used at indoor composting facilities to mix feedstocks, then form windrows.
The Kuhn Knight website notes it specializes “not only in building mixers but also in mounting them on trucks.” Its truck-mounted Reel-Auggie, available with capacities from 174 to 420 cubic feet, has a higher discharge point, which often allows the use of the slide tray. The mixing unit has poly wipers that assist the reel and augers in mixing and ensure complete cleanout; adjustable spring-loaded crossbars relieve excessive pressure. The large upper auger moves material rearward, while the lower auger mixes by moving material forward and provides a fast steady feed flow to the discharge. Kuhn Knight also sells the Botec 4-Auger TMR Mixers, which have a choice of a double right-angle gearbox drive or a GTD (gear tooth belt drive). The Botec box design (with capacities from 360 to 900 cubic feet) requires less horsepower to operate and eliminates the need for a hydrostatic drive in most applications. Finally, there is a vertical mixer (Verti-Maxx) with bull gear drive, available in mixing capacities from 424 to 1125 cubic feet. Units also are available as batch, stationary mixers.
The Roto-Mix units use a rotary design that requires less horsepower as it lifts and blends material in a tumbling action that does not pack material and helps to introduce air into the mix, notes the company’s website. The design resembles a paddle wheel, which facilitates the addition of water evenly into the mix. There are cross augers on the side that move material for even mixing end to end. The company offers truck-mounted, trailer and stationary mixers. There is an optional 120-inch long folding stacking conveyor. Mixer capacities range in size from 420 cubic feet to 920 cubic feet. Mixer lengths are from 166 feet to 232 feet. There are “easy access” inspection doors to the completely enclosed oil-bath drive and an external single point grease bank for ease of maintenance.
The SEKO line of mixers and other composting equipment was recently introduced into the North American market by North 40 Compost Technologies in Rockwell, North Carolina. (There is more information on SEKO of Italy in Modern Composting Technologies.) The SAM 4 Series of the Bio-Chopper-Mixers are equipped with low RPM rotors with chopping blades that can be remotely controlled via computer. During the chopping process, the material is also actively mixed. The Constellation Series offers a high output, continuous cycle design manufactured with special wear resistant materials. These units can process large quantities of materials such as wood waste (100/300 m3/hour according to the models and products used).
Supreme International Limited manufactures a full line of vertical feed industrial series processors designed to cut and mix various waste commodities such as food residuals, biosolids, green waste, mortalities and wood waste (up to 2-inches in diameter) in any combination. Enviro Processor units with vertical screw designs can be customized to fit operators’ individual requirements, e.g., choices of interference knife packages, varied lengths of belt conveyors for windrowing of finished mixture and various stainless steel liner options to accommodate more acidic commodities. Mixers in various size ranges are available in pull type, stationary and the more mobile truck mounted units. There is a platform on the units to view mixing operations.
The accompanying directory provides contact information for the mixing systems just described. Websites include specific details on the various models available.
MIXING SYSTEMS
Detcon
5039 Industrial Road,
P.O. Box 2249
Farmingdale, NJ 07727
732-938-2211
Kuhn Knight, Inc.
1501 W. Seventh Avenue
Brodhead, WI 53520
608-897-2131
www.kuhnknight.com
Roto-Mix
2205 E. Wyatt Earp
Dodge City, KS 67801
620-225-1142
www.rotomix.com
SEKO
c/o North 40 Compost Technologies
P.O. Box 1330
Rockwell, NC 28138
336-499-5881
www.n40compost.com
www.sekospa.com
Supreme International Ltd.
Box 6450 STN MAIN
Wetaskiwin, AB
Canada T9A 2G2
780-352-6061
www.supremeinternational.com