The right turn: Choosing the proper tool from the wide range of compost turners

Generating high-quality compost requires proper aeration of the organic material, so employing the right turning equipment is crucial. However, such equipment represents one of the largest fixed costs in composting. With a bewildering variety of equipment out there, what should purchasers of turning equipment look out for? by Virginia Nelson Compost is much more than just a fertilizer. It stores nitrogen, phosphorous and potassium as slow-release nutrients, readily available to plants, as well as offering a wide range of micronutrients. Organic matter added to soil improves the soil tilth, water retention and nutrient conversion. Composting temperatures are able to kill pathogens and destroy viability of weed seeds, thus increasing crop yields and reducing herbicide requirements. Compost also enhances a plant’s natural disease-suppression abilities and provides a range of micro-organisms to the soil. While it is difficult to put credible figures on the value of the industry and its constituent parts - such as equipment purchasing, land costs, construction, end-product worth and application services - it is clear that the industry is growing. Compost organizations and the public are driving public policymakers to make commitments regarding the recycling and composting industry. Also, support for compost is mounting as more of the organics sector looks to the compost industry to restore depleted agricultural soils without using pesticides and herbicides, and as confined feeding operations use composting as a way of handling manure and animal mortalities. Compost generation Composting is the controlled biological decomposition of organic material into a product called humus. Typically, the organic material is placed in windrows or channels to be composted. Windrow composting is achieved by piling organic matter in long rows to compost, primarily outdoors with or without a roof. Channel composting is achieved by placing organic matter in long concrete channels to compost, primarily inside a building. Composting is accomplished in two main stages: an active composting and a curing stage. The active composting stage involves three sub-stages: an initial stage lasting 1-3 days when mesophilic micro-organisms degrade constituents such as sugars, starch and proteins, during which compost temperatures rise rapidly a high-rate thermophilic stage, lasting 10-100 days, in which temperatures rise above 40°C and fats, hemicellulose, cellulose and some lignins are degraded and pathogens are destroyed a stabilization stage, lasting 10-100 days, during which the temperature declines and further degradation of cellulose, hemicellulose and lignins occurs. While composting is a natural process, it requires proper conditions to occur rapidly, minimize odour generation, and prevent nuisance problems. Many controllable factors affect composting, and these include pH, carbon-to-nitrogen ratio, particle size and porosity of the initial organic material. In many cases the composting material in windrows or channels is mixed with a mechanical turner. The machinery for turning the windrows is a key component to produce quality compost, yet it is also the source of one of the largest fixed and operating costs. Turning compost Like most living things, micro-organisms need food, oxygen and water to survive, and they produce heat when they are thriving. Therefore the oxygen concentration, moisture content and temperature need to be controlled throughout the composting process to produce a top-quality product. Mechanical turning of the equipment replenishes oxygen concentration, aids in temperature control, produces a homogeneous product and, in many cases, facilitates water addition. Active aeration introduces air by mechanical means. Temperatures above 65°C can be detrimental to the micro-organisms responsible for composting. When temperatures reach these damaging levels, the composting material should be turned to facilitate heat loss and water should most likely be added to the material to allow the microbes to control their temperature through evaporation. Temperatures may fall, indicating that the material may not contain enough oxygen for the microbes, so turning the material adds oxygen and may re-establish pore space. During this high-rate thermophillic stage, the material most likely will need to be turned once to twice per week. Product quality requirements The sale of compost is directly related to the quality. The compost must be free of weed seeds and pathogens. Studies indicate that the viability of weed seeds and pathogens are destroyed after two weeks of temperatures above 55°C. The compost must be mature; it must not reheat if moisture is added and nitrogen must be present in an organic form. The compost should not produce any foul odours or emit greenhouse gases. The foreign matter content of compost must be negligible to ensure there is no risk to people, animals, or equipment. Compost should be homogeneous and most often rich in colour with a texture similar to loam or potting soil. The particle size of the compost should provide good porosity so that, when added to soil, it allows for great root growth and moisture retention. It is imperative to provide the microbes with an environment that allows them to flourish to achieve top-quality compost. The tools of the trade There are two different ways to introduce air into the windrows of composting material: actively or passively. Active aeration introduces air by mechanical means, either by turning and mixing the media or by forcing air into ducts beneath the composting media. Passive aeration requires a perforated pipe to be placed beneath a static windrow, which introduces air into the media through natural convection. Experiments conducted to compare active and passive aeration of windrows reported superiority of active aeration over passive aeration in terms of compost quality, volume and mass reduction, and water retention. However, higher initial costs are associated with active aeration, either by mechanical mixing or forced aeration.1 The rotating drum The most common type of windrow turner is the rotating drum, which turns windrows 3-4 metres wide by 1-2 metres high. A compost turner has a rotating drum which mixes and turns the organic material. photo: earthsave Click here to enlarge image Rotating drums are equipped with a variety of equipment including augers and solid flails in an auger configuration. The flails work in two ways: they lift the material and throw it backward, while the material from the outside of the windrow is drawn toward the centre. The rate of rotation of the drum can vary from 150 to 800 rpm depending upon the machine. A protective hood or shield, connected to the rotating drum, covers the windrow to prevent material from being thrown from the windrow, and in some cases to house nozzles used to apply water to the material while turning. Equipment is available to turn windrows as large as 6 metres wide by 2 metres high. These units most often straddle the windrow and are either self-propelled or pull-type models. The pull-type units are typically driven by either the tractor PTO or a hydraulic motor. Manufacturers of pull-type equipment include Wildcat, Topturn, Aeromaster, HCL, Rotaerator, Doppstadt, Sandberger and Scat, amongst others. Self-propelled machines are also available from companies such as Wildcat, Double T, Topturn, Aeromaster, Frontier, Backhus, Scarab, Behler, Allu, Komptech, Sandberger and KW. In the event of a breakdown, most machines can be removed from a windrow. For example, the Aeromaster and HCL pull-type composting equipment can be completely lifted from the windrow in seconds. And the Brown Bear compost turner utilizes a rotating drum that moves the all the material to one side, thus allowing for easier maintenance if the machine fails while turning the windrow. The elevating face Another style of windrow turner is an elevating face, used in both Scat and Transform composting equipment. Powerful teeth on an inclined conveyor break up and lift the composting material. Scat is designed for windrow composting; therefore, adjustable vanes at the top of the elevating face are used to reconstruct the windrow as the material falls. The Transform compost turner, used in channel composting, moves the material along the channel with each turning. However, other channel composting equipment, such as the RotoKing and Backhus tunnel turner, move material along the channel while using a rotating drum and paddles. The elevating face is a style of windrow turner that uses powerful teeth on an inclined conveyor that together to break up and lift the composting material. photo: scat Click here to enlarge image A combination of these two styles has become common. For example, both the Buhler and the Sandberger use rotating drums to turn the composting material and a conveyor to lift and reshape the material forming the windrow. On some models the conveyor can be repositioned to load the composted material into a truck for transport. Many turners now incorporate a feature allowing the addition of moisture to the composting material. photo: aeromaster Click here to enlarge image The addition of moisture to composting material, especially outdoors, can be difficult. Many turners, including the Aeromaster and Sandberger, have included this option into some of their models. Some turning equipment has been installed with screens and crushing drums to deal with rocks and other undesirable objects in organic waste. photo: allu Click here to enlarge image Some industries, such as land reclamation, require aggressive equipment because the material may include rocks and other inert objects that could cause considerable damage to the turners previously mentioned. In response to this, technologies have emerged such as the Allu bucket, mounted with two to four screening and crushing drums. Once the Allu has a bucket full of material, the hydraulics that rotates the drums is turned on, allowing the material to pass through the drums. The type of drums, counter blade, and rotation speed used determines the size of the processed material. The rotation of the drums can be reversed to loosen material. Factors to consider when purchasing a windrow turner With the wide range of windrow turners available, choosing the most suitable machine is often difficult. Since most windrow turners are designed to cover a variety of conditions, each machine has different advantages and features. The best turner for one may not necessarily be the best for another. No single turner will best fill the needs for all applications. Choosing a windrow turner is therefore a compromise among the advantages, features and costs of the different machines. The following will help determine which windrow turners are more suitable for individual applications. Turning mechanisms/processing system There are a variety of processing systems available. The different systems affect required horsepower, capacity and particle size. Horsepower:The design of the turning mechanism affects the horsepower required to turn the windrow. Systems that simply lift and toss the material generally require less horsepower than systems that move the windrow. Capacity:The travelling speed and size of the windrow turner affects the capacity of compost produced. Particle size: Organic material may contain lumps that are reduced by turning the windrow. The different processing systems produce a range of particle sizes. Typically, turners that have a ‘mixing’ effect break up larger particles to produce smaller, more consistent particles. Turner adjustments/options Manufacturers are producing windrow turners with multiple adjustments and options. Height adjustments: Many windrow turners allow the operator to adjust the height within a specified range. A wide range of height adjustments allows a greater variation of windrow heights to be turned with one turner. Ease of height adjustment must be considered. Windrow shape:As a turner passes through a windrow, there are features that reshape the windrow. Some turners allow adjustments to be made so the user can modify the windrow shape created. Drum-style turners use flail configurations and guards to shape the windrow, conveyor styles use deflecting vanes and guards, while other styles use operator control. Retractable mechanism: Some turners are able to lift the turning mechanisms completely out of the windrow at any time. This allows the operator to test from the centre of the windrow, view what is happening in the middle, and remove the turner in the event of a breakdown without destroying the windrow. Windrow height and width:The maximum height and width of the windrow turned varies among the turners. Consider how the windrows will be formed because this will influence the turner selected. Water addition: An option available is a method of water and inoculation addition while turning the windrow. A tank either mounted on or pulled behind the turner supplies the liquid to a delivery system near the turning mechanism. Turners have been designed to deal with different windrow conditions, including height, width, shape, turning method and moisture requirements. photo: frontier windrow turner Click here to enlarge image null Quality The turner can influence the temperatures reached and sustained as well as the particle breakdown, which affect the quality of the compost. Temperature: The temperature in the windrow is affected by many things, one of which is the windrow turner. The turner affects the temperature by the shape of the windrow, size of the windrow and mechanical mixing action. Particle size: The mechanical action of the windrow turner determines the physical particle breakdown. Generally, rotating drum style turners break the material down smaller and faster than conveyor styles and front-end loaders. End-use: Small, consistent particles are more aesthetically pleasing than large lumps and inconsistent particle sizes. The end-use of the finished compost, for personal use versus for sale, can determine the turner chosen. Economics null Industry trends No two composting operations are the same, and each requires composting equipment unique to its operation. Companies manufacturing composting equipment are moving away from standardized equipment to machinery designed for specific operating conditions and requirements. Scarab now provides numerous configurations possibilities to their customers. These include options related to horsepower range, drive system and overall machine size. A variety of composting turning equipment is available for composters to choose from; however, it’s not easy to choose one, nor is it cheap to acquire it. Size is an interesting issue - compost operators have always had a challenge to match the size of the windrows to the size of a machine available on the market. One company that has addressed this challenge is Doppstadt. Its compost turners will turn a windrow independent of the windrow size or shape. Two cutting discs break up and lift the composting material onto a conveyer which reconstructs the windrow beside the turner. This design allows larger windrows to be turned in more than one pass. Although multiple passes may be necessary to turn all of the material in the windrow, the conveyor has a lateral extension to allow for proper windrow reconstruction. There is significant momentum in the composting industry worldwide. Composting organizations, such as the European Compost Network, UK Composting Association, US Composting Council, and Composting Council of Canada, are working toward quality assurance programmes, promoting research and providing information. Through these and other organizations, there has been a strong focus on waste diversion from landfills, primarily food waste. As consumers demand safer products and as the organic farming and foods industry grows, so does the composting industry. Compost has become a product compatible with organic farming principles. As composting increases in popularity, its beneficial impact on our environment will be remarkable. Virginia Nelson is Project Manager at AgTech Centre, Canada.e-mail: virginia.nelson@gov.ab.ca Note 1. Larney, F.J., A.F. Olson, A.A. Carcamo, and C. Chang. 2000. Physical changes during active and passive composting of beef feedlot manure in winter and summer. Bioresource Technology 75: 139-148. To comment on this article or to see related features from our archive, go to www.waste-management-world.com and click the ‘Forum’ tab. Composting manure An important source of compost is animal manure. Mismanagement of animal manure can have a substantial impact on our water, soil and air resources. The environmental risks of applying manure to land include nitrate and phosphorous leaching from manure into water sources, pathogens contaminating water sources, odour nuisance from land application, and nutrient loading due to the cost of transporting nutrients to further fields or the lack of sufficient land base. Composting can address environmental sustainability, health concerns and odour problems while adding value to and improving the transportability of manure to offset the costs of management practices. Composting commonly reduces the mass and volume of manure by 30%-50%, which reduces the hauling costs. The composted material is in a stabilized state, which is less prone to processes that produce odours or greenhouse gases during storage and spreading. Composting uses a high carbon-to-nitrogen ratio (C:N), common in highly bedded manure, by reducing the amount of ammonia loss to atmosphere. Turning - an energetic process The organic material at the beginning of the composting cycle requires more energy to turn than the product at the end of the process. The style and design of the turner impact on the maximum horespower, the time required to turn the material, and therefore the overall energy required to turn the material. In 2002, Alberta Agriculture, Food and Rural Development investigated the capacity of a variety of styles of windrow turners and concluded that: increasing the speed of the drum on drum-style turners increases the energy required elevating-face turners and turners of the slow rotating drum style require the least overall energy to turn raw material or finished compost.