IN TODAY'S modern composting world, there is “windrow composting,” a composting methodology, and then there is “composting in windrows,” typically as the second phase of a different composting methodology, such as in-vessel, or enclosed aerated static piles. In the former, all composting from start to finish is
Bdone in windrows, and is typically conducted outdoors. In the latter, windrow composting may also include forced aeration, and is frequently conducted indoors (in an aeration building). In either case, windrow turning equipment is typically employed to optimize the composting process.
In a windrow, temperature control and oxygen levels are managed via mechanical agitation. Pile temperature and oxygen level need to be taken by a site operator with hand-held monitoring tools. Pile turning introduces oxygen, accelerates physical degradation of feedstocks and provides an opportunity to adjust the moisture content to the optimum level. Many windrow turners have a watering attachment, which enables moisture to be added to the pile while turning. Generally speaking, the total composting time can be managed by the aggressiveness of the turning regime. More frequent turning breaks particles down more quickly, and provides an opportunity to optimize composting conditions, thus accelerating the process. This enables a windrow composting facility to increase its annual throughput capacity.
This overview of windrow turning equipment is the second in BioCycle's Modern Composting Technologies series, based on information in a new book by that name to be published by BioCycle/The JG Press, Inc. this spring. It builds on past articles in BioCycle that have presented comprehensive information on windrow turners. (See Advances In Windrow Turning, July 2001; Windrow Turner Origins And Fundamentals, August 2001; and Innovations In Windrow Turners, July 2003.)
Turning machines generally work windrows by facing them head-on. Some of the turners work the entire windrow in a single pass while others process only a part of the windrow and have to perform more than one pass to finish the task. Windrow turners can be categorized in several ways, e.g. by turning mechanism, orientation to the windrow, power source, mode of travel. However, the defining feature of the turner is the mechanical action as that is what agitates the compost. Modern Composting Technologies categorizes windrow turners based primarily on their construction. This breakdown is as follows:
Working Apparatus: Horizontal rotor or drum; Toothed conveyor belt; Archimedean screw device; and Mill combined with elevator belt.
Driving Gear: Self-propelled; and Pulled by a tractor.
Product Translation: Longitudinal translation; and Lateral translation.
Windrow facilities with straddle turners (a turner which goes over the top of the pile) are limited in pile height by the height of the turner. Other turner technologies, e.g., elevating face, perform the turning function from the side and therefore pile height is less of a constraint. Generally speaking, however, to optimize the windrow composting process, pile height typically is limited to 10 to 13 feet. In many cases, materials are premixed prior to being formed into windrows. However, operations may layer the materials to be composted (e.g., placing source separated organics, biosolids or manure on a bed of ground yard trimmings, wood chips or sawdust) then mix them with the turner. To control release of odors when putrescible or odorous feedstocks are being composted, some facility managers create the windrows and then wait for a few days or a week before the first pile turning. In some cases, the windrows are covered with a layer of ground yard trimmings, which acts as a biofilter during this initial stage.
In an effort to increase efficiency and improve facility economics, turner manufacturers have developed equipment for larger windrows, as well as equipment that reduces aisle space (thus increasing site capacity without increasing the actual footprint). For example, compared to ten years ago, more tractor-assisted turners are single-pass straddle style, as opposed to double-pass units. While the double-pass units allow wider windrows, single-pass machines require a driving aisle on only one side of a windrow. Aisle space is eliminated (or drastically reduced) by self-powered straddle turners and especially elevating face and front-mounted auger-style turners. For this reason, many manufacturers now offer self-driven units.
CATEGORIES OF TURNERS
Straddle Type: The most common turning machines for windrow composting are self-propelled and in particular are of the straddle type with longitudinal movement of the product. The turning apparatus generally consists of a horizontal rotor (also referred to as a drum), perpendicular to the direction of travel, including two coils that convey the material to the center of the shaft, where some devices in the shape of shovels are mounted and are used to propel the material towards the rear of the turner. The turning apparatus is in a windrow-shaped housing. The rotor sits slightly above ground level; the housing above the rotor encloses the compost as the turner agitates and reshapes the windrow. Some rotors feature paddles mounted on the edge. The presence of deflectors, in some cases equipped with screws, helps to convey the material to the turning apparatus, with the possibility of processing wider windrows.
Because a drum turner straddles the windrow (turning the entire width of the windrow in a single pass), the windrow width must conform to the length of the drum (although a model is available which turns a windrow in two passes, with the drum extending to the middle of the windrow on each pass). Some drum turners are pulled by a tractor or loader traveling alongside the windrow. These units require an aisle between windrows or pairs of windrows for the tractor to travel; the aisles are unproductive space that reduces capacity of the site. Therefore, many models of drum turners are self-propelled and self-powered.
The dimensions of the windrows that can be turned with this type of machine vary with available power, and obviously can be larger with self-propelled machines. The most powerful self-propelled machines, equipped with 250 to 300 kW engines, are rated with a productivity of 3,300 to 3,900 cu yd/hour; turners with engines in the range of 150 to 200 kW have a throughput capacity of 2,000 to 2,600 cu yd/hour. Some models of self-propelled straddle turners can be equipped with an irrigation system that is used to maintain the proper moisture content of the composting material. The hose reel can be mounted on the chassis or can be pulled by the machine itself.
The machines pulled sideways by a tractor are suitable for small- to medium-size composting facilities and are used primarily in the agricultural sector for the treatment of animal wastes or of biomass produced in the food industry. The turning device can be driven by PTO or can be self-powered.
Most models move material toward the center of the windrow. A few use an auger to achieve this movement. Manufacturers of straddle type turners include Allu, Backhus, Frontier, HCL, Komptech, Midwest Bio-Systems, Resource Recovery Systems of Nebraska, Scarab and Wildcat.
Auger: This turning machine uses an auger, or auger-like paddles, to lift and move the windrows. A few auger turners have a shaft with a continuous flight, like a screw. Others achieve a similar turning effect using a set of large, slanted paddles, arranged in a helical fashion. Auger-type turners usually mount on the front of a skid-steer loader, or wheel loader. The turning mechanism for most models is powered by the tractor or loader's hydraulic power transmission system. As auger turners move down the length of the windrow, it shifts the material to one side, nearly eliminating the need for space between windrows. Manufacturers of auger turners include Brown Bear and Scat. (Brown Bear, the primary manufacturer of this type of turner, sells self-contained, self-powered models.)
Elevating Face: The elevating face (or elevating belt) turner is essentially a wide backwards-sloping conveyor with cleats or teeth that lift the compost up and over itself. The teeth help to break apart bags but otherwise do not reduce particle size. Additional conveyors on the back of the turner place the compost in the desired location. Models include tractor- or loader-powered or self-propelled. Self-propelled models minimize aisle space and can turn an entire windrow or a slice of a large pile. Scat Engineering is the only manufacturer of this type of turner in North America. Modern Composting Technologies includes a directory of equipment suppliers in Europe that offer this turner design.
With an elevating face turner, the presence of a conveyor equipped with front track broach plates and a horizontal shaft is very useful. The horizontal shaft, in fact, helps reduce belt fatigue and consequently reduces severe wear on the belt. The plates, instead, help to reduce the width of the conveyor belt keeping the same width of track which translates into a lower cost. Elevator type turning machines also can be equipped with two horizontal shafts: the upper one is primarily used to convey the material to the lower shaft, which is the main turning device. This solution, combined with the use of deflectors of increased size, enables processing windrows of bigger height.
When this type of turner does not have the front conveyor included, it can result in better oxygenation of the biomass when compared to Archimedean screw ones. This relatively superior performance is due to the increase of the contact time between the biomass and the ambient air. At present, the market offers self-propelled turning machines with a working width of 3.3 yd, which are capable of processing windrows about 3.3 yd high. The installed power reaches 150 kW for productivity of about 2,400 cu yd/hour.
An interesting application is the utilization of a mill conveyor, of the type used for the extraction of corn silage from silos. The relatively high rotational speed of the rotor and the presence of many sharp blades on its periphery reduce the work of the belt to mostly transportation of the material.
Currently, this type of machine also is available with automatic drive for track operations, and is mainly used in the agricultural-animal raising field for composting animal wastes. These units have a working width of about 2.2 yd, and have shown to be suitable for track or windrow utilization in small- and medium-size plants. A hydraulic-electrical type also is available with an installed power of 15 kW, for propulsion and for operation of the turning mechanism.
In the general design of turning machines with inclined elevator belts, a self-propelled machine equipped with double inclined elevator belts and Archimedean screw vertical apparatus has recently appeared in the market that makes it capable of processing windrows with a flattened top with a maximum height of 3.3 yd.
Trapezoid: Trapezoid turners rest on the sloping side of a large windrow or pile (which naturally forms a trapezoidal shape). They are characterized by having one or two rotors, with inclined or horizontal axis, whose role is to remove vertical layers of the material from the existing windrow and convey them laterally to form a new windrow. As the turner moves along the side, rotating plates or teeth shave off a layer of the compost. The conveyor moving the compost determines the location and height of the new windrow. The plates or teeth on the turning mechanism can be changed to provide more or less shredding action. These machines can be self-propelled or pulled by tractor, PTO driven or self-powered. Backus is a manufacturer of trapezoid turners.
An important advantage of this solution is the possibility to operate on rectangular windrows with a virtually “unlimited” width. This type of machines also can feature a turning device with two horizontal axis rotors, generally equipped with blades. The capacity of these machines is relatively high, reaching about 2,000 cu yd/hour for a self-propelled unit with 180 kW of available power.
IMPROVEMENTS IN TURNING MACHINES
The changes and innovations to windrow turning equipment have been numerous, driven by the evolving needs and trends in the composting industry as well as manufacturers' efforts to improve machine performance. The modifications have resulted in turners that are more varied, durable, require less maintenance, provide the operator more control and more comfort, produce a desired effect from the turning action, accommodate large or different sizes of windrows, travel and transport more easily, and perform additional functions like watering, materials handling and covering piles with fabric.
The present selection of turners can deal with windrows that are smaller and larger than the previous norms. Nearly all of the manufacturers have widened their product selection. Those who originally offered tractor-assisted models now have self-propelled units, and vice-versa.
Operator control and comfort seem to have become a priority in the designs of turners. Cabs of self-propelled turners are now air conditioned and heated; some incorporate systems for filtering the air. Models assisted by tractors also have evolved to accommodate operator comfort. Advances have given operators more control from within the cab and improved the ergonomics of the equipment controls.
In an effort to increase efficiency, many facility operators continually are trying to process more compost on a site of a given size. Turner manufacturers have moved in the same direction, developing equipment for larger windrows and also equipment that reduces aisle space. To manage on difficult sites, more manufacturers offer four-wheel drive and the option of wheels or crawler tracks (or a combination). In keeping with the trend towards space (and labor) efficiency, self-driven turners have been designed to have a short turning radius. Some models essentially pivot rather than turn.
A number of innovations have taken place concerning the mechanics of turning the material to be composted. For example, several manufacturers have changed the design of the paddle to achieve a specific effect, e.g., to introduce more air, pulverize the feedstocks less, require less power, reduce paddle wear, increase throughput or exchange the contents of the windrow from the inside out. Other innovations have made compost turning more thorough or improved the operator's control. Innovations in drum operation include variable and low drum speed (rpm), adjustable height, reversible rotation, and the ability to lift the drum out of the windrow at any point.
Other innovations have addressed the mechanical components of the machine. For example, several manufacturers have converted the drum power transmission on some or all of their models from hydraulic to belt drives (as the power requirement increases, hydraulic power becomes less efficient). Other modifications include improvements in radiators to deal with the dust and debris, automatic reversing fans, digital load controllers (adjusts speed based on drum pressure), enclosed hydraulic system, automatic tensioning system for belt drive, improved couplings at the gear box and PTO connection, improved bushings, stainless steel hose ends and connectors, heavier chains, and enclosed drum bearings. Several manufacturers now use off-the-shelf components that can be replaced easily.
Maintenance is a very important issue for operators of facilities. The wear and tear on turner components is very high. Many of the modifications discussed in the previous paragraph were driven by maintenance concerns. Other maintenance improvements are more structural in nature. For example, machine parts, particularly drums and paddles, have been strengthened, either by increasing their thickness or by switching to stronger and/or more durable materials. Reducing the time and cost of maintenance operations also has been an objective. Paddle designs have been changed on some machines to allow faster removal.
PRODUCING MORE VERSATILE MACHINES
Perhaps the most significant development with windrow turners is that they are now more versatile than before. This multitasking is probably best exemplified by the ability to add water to windrows while turning. Although watering systems have long been rigged onto turners, only in the last ten years have they been integrated into the turner design. Nearly all turner manufacturers now offer water spray nozzles and associated plumbing on their turning equipment, with water supplied via a tank or a hose reel. Tanks (towed behind) are more common on smaller turners. Larger turners are more likely to accommodate a hose reel that unwinds and winds as the turner travels. The watering system can be used to suppress dust generated during turning when a full dose of water is not needed. The watering system also can be used to add inoculants to the windrows.
Manufacturers have started to take advantage of the materials handling potential of windrow turners. Some types of turners (auger and elevating face) have always been able to move materials, as well as turn them. However, by adding conveyors and other devices, the materials handling capabilities of all types of turners have been enhanced. Depending on the model and the arrangement of conveyors, a turner can place compost to either side or behind itself and discharge at various heights.
A growing interest in sharing equipment among several sites has produced a niche for machines that are easily transported. The concurrent trend toward the use of large equipment makes this particularly challenging (especially if the operator wants to avoid securing a permit for an “oversized” vehicle). Nevertheless, most turner manufacturers have filled this niche well, improving the transportability of their existing equipment, or developing new equipment with frequent transportation in mind. For example, Frontier manufactures a self-propelled straddle-type turner that can be transported without a separate trailer. The wheels of the unit pivot 90° so the turner can be towed sideways by a truck. Similarly, several smaller, tractor-assisted, turner models are “self-trailering”; the drum frame swings up into a vertical position, leaving a narrow base that can be towed on the road. However, the final height of the trailer may limit its road status.
TURNING MACHINES FOR SMALL SCALE FACILITIES
Machines with a relatively simple design can be used in composting plants with relatively minor operational demands. Some of these turning machines include those that are pulled by a tractor and those in the shape of buckets. The use of a turning bucket that can be attached to a tractor by means of a three point hitch and operated by the tractor's PTO offers a very practical solution. This arrangement can also be used for mixing different types of materials.
This type of equipment mounts at the bottom of the hopper to a high speed conveyor belt that discharges the product laterally, allowing better contact of the composting mass with atmospheric oxygen and hence an acceptable oxygenation of the biomass. Hopper capacities vary from 2.6 to 4 cu yd, with productivity indicated to be around 130 cu yd/hour.
A suitable substitute to the units described in the previous paragraphs but having a lower productivity is the mobile bottom bucket. This type of bucket allows for a better aeration of the material.
Nora Goldstein is an editor of Modern Composting Technologies, as well as Executive Editor of BioCycle. Luis Diaz and George Savage are with CalRecovery, Inc. in Concord, California. Diaz is a coauthor of Modern Composting Technologies, to be published in spring 2005 by BioCycle/The JG Press, Inc.
861 Main St.
Hackensack, NJ 07601
P.O. Box 193
Allamuchy, NJ 07820
Brown Bear Corp.
P.O. Box 29
602 Ave. of Industry Park
Corning, IA 50841
P.O. Box 7325
Kalispell, MT 59904
Frontier Industrial Corp.
P.O. Box 700
Lyons, OR 97358
HCL Machine Works
15142 Merrill Ave.
Dos Palos, CA 93620
6200 Rockside Woods Blvd.
Independence, OH 44131
28933-35 E St.
Tampico, IL 61283
Systems of Nebraska/
Route 4, 511 Pawnee Dr.
Sterling, CO 80751
& Leasing, Inc.
HCR 1, Box 205
White Deer, TX 79097
Scat Engineering, Inc.
2255 Little Wall Lake Rd.
Blairsburg, IA 50034
Hwy. 81, Box 523
Freeman, SD 57029