Passive composting is the simplest, lowest cost method, and it requires little or no management. The materials are simply stacked into piles and left to decompose over a long period of time. This method can produce objectionable odors due to anaerobic conditions and is not suitable for large quantities. Aerated piles are a more productive form of passive composting. Perforated pipes are placed within the pile, which supply the pile with oxygen and thus promote a faster rate of decomposition. Mixing the material well also speeds up the process. Blowers and chippers may be used to provide more efficient composting. Blowers force oxygen through the piles while chippers grind the materials to produce smaller particle size and provide for easier mixing. This method produces compost faster with minimal labor and costs. Costs are increased when blowers and chippers are used. Windrow composting involves long narrow piles, called windrows, which can vary in height and width depending on the materials and equipment available for turning. Windrows are turned or incorporate forced aeration for efficient composting. This method allows large quantities of waste to be composted. Windrows can range from three feet high for dense materials, to as high as twelve feet for lighter, more porous materials like leaves. The process starts as the materials are mixed together, with the yard waste and paper waste having been processed through a chipper and shredder, respectively. Water is added to aid in decomposition and then the waste is formed into windrows. Windrows are turned periodically to add oxygen, mix the materials, release excess heat, and expose all materials to the high interior heat that kills pathogens. When using forced aeration, materials must initially be mixed well for windrows because they are not regularly turned. When turning windrows to provide oxygenation, it may be necessary to turn daily or even several times a day to maintain sufficient oxygen levels. If objectionable odors develop, that is a signal that turning is required to provide increased aeration and reduce moisture content. Turning can be labor intensive depending on the equipment being used. Turning equipment can include front-end loaders, an old plow and a farm tractor, or specialty machines such as windrow turners. In addition to requiring turning equipment and a large area for the windrows, the operation will also need a source of water, dial thermometers and an oxygen meter. The instruments are placed in the windrow for monitoring temperature and oxygen content and are removed for turning. With frequent monitoring and essential turning, composting time can vary from weeks to a couple months depending on the material being composted. Once completed, the compost should be stored in large bins for further curing, screened, and either given away or sold. Larger particles that were screened are returned to the windrows. This method allows for large quantities to be composted in a relatively short period of time and produces a high quality product. However, this method requires a large land area, is labor intensive, and costs for equipment can be high. With in-vessel composting, the materials are composted within a container such as a tank or reactor. This method provides for total control and optimization of
69