Open Pile, Windrow Method - This simplest, most common form of composting consists of constructing a small open pile (a minimum volume of one cubic yard for smaller, onsite applications is recommended), or elongated pile ("windrow") that can be static (unturned/unaerated), or turned and watered at regular intervals to create the moist, aerobic environment necessary to hasten decomposition. Most large-scale facilities in the U.S. use the open windrow method, and windrows or piles are aerated with a front-end loader or windrow turner. Windrows are typically oriented perpendicular to the site's slope to minimize ponding and improve drainage. Open piles or windrows are best for landscape residuals such as leaves, grass and chipped brush, agricultural byproducts like manure and crop culls, but can be used for composting vegetative food scraps and other more volatile materials as well. Depending on how it's managed and what equipment is utilized, this method can be the least expensive, and suitable for a wide range of feedstocks.
Bin or Bay System - A good onsite system for community gardens, small farms and equestrian facilities. A typical bin or bay set-up consists of three one-cubic yard bins side-by-side to facilitate management and movement of material through composting cycles. But any number of bays can be built. Depending on the size, composting material in this system can be static, moved or turned by hand, moved or turned with a tractor or front-end loader, or aerated by a single or multiple blowers. Each bay is an individual batch, and once one is full, the next one is filled as the first composts. Material can be left in its original bin to complete composting, or moved from one to the next. A common progression is beginning with the first bay and loading it until full. Once full, the material is moved to bay 2 and bay 1 is reloaded with fresh material. Once bay 1 is full, material from bay 2 is moved to bay 3, bay 1 material moved to bay 2, and bay 1 is reloaded. With this sequence bay 1 receives active feedstock, bay 2 is actively composting, and material in bay 3 is curing. Bins and bays may be covered to help prevent vectors.
Aerated Static Pile, or ASP System - Generally utilized at large composting facilities composting food scraps or other more volatile feedstocks, ASP systems allow for better process control, can save time through reduced physical aeration with a front-end loader or windrow turner, and help maximize site capacity. Some systems require moving material from a high rate of aeration phase to a lowered aeration phase. Aeration can be positive (air pushed through piles), negative (air pulled through piles and typically moved through a biofilter) or a combination of both. Piping can be above ground or below, with the former much less expensive but the latter greatly reducing the time it takes to build and breakdown composting zones, as above ground systems require removing piping from within the piles and resetting it when a new zone is built.
Covered Systems - Typically used in conjunction with ASP composting, but also used with open windrows, covered systems utilize a proprietary breathable fabric cover (usually a geotextile membrane) to help reduce moisture loss in arid regions, prevent excessive moisture in areas with extreme rainfall, and reduce odors and emissions. A cap of finished compost (bio cap) may also be used in lieu of fabric covers, and some air districts in California require an approved fabric cover or bio cap.
In-Vessel Systems - These systems can be aerobic or anaerobic, and in-vessel units are entirely enclosed and may include components and controls for physically moving and mixing feedstocks, as well as aeration, moisture and temperature. The enclosed process controls air-borne emissions and biofilters are typically used to scrub VOCs and other gases from exhaust streams. Preprocessing of feedstock using shredders or grinders may precede the in-vessel process to ensure more rapid and uniform decomposition, and post-composting/curing may be done outside of the vessel in an open windrow. In-vessel anaerobic digestion operates at slower decomposition rates and lower temperatures than aerobic composting and may need additional post processing to achieve pathogen reduction and produce stable compost-like product.
In-vessel composting systems allow great control of the composting process, but are more expensive than other systems. In-vessel units can be small, simple and compact for onsite composting operations, medium-sized for grocery or food distribution centers, and large and more technologically advanced for centralized programs.
In-vessel digestion involves an anaerobic process in which microbes offput methane and other gases. In-vessel digestion methods are often proprietary and vary amongst technology providers. Anaerobic digestion primarily generates methane gas for energy production. Two types of anaerobic digestion common in municipal projects include High Solids (HSAD) and Low Solids (LWAD) Anaerobic Digestion facilities. Sizes also range from small-scale to large. Some in-vessel processes include biomass conversion and fermentation to produce chemicals and/or alcohol-based fuels. In-vessel digestion operates at slower decomposition rates and lower temperatures than aerobic composting and may need additional post processing to achieve pathogen reduction and produce stable compost-like product.