Municipal solid waste composting
Municipal Solid Waste (MSW) composting is a rapidly growing method of solid waste management in the United States. MSW includes the residential, commercial, and institutional solid waste generated within a community. MSW composting is the process by which the organic, biodegradable portion of MSW is microbiologically degraded under aerobic conditions.
During the process of degradation, bacteria are used to decompose and break down the organic matter into water and carbon dioxide , which produces large amounts of heat and water vapor in the process. Given sufficient oxygen and optimum temperatures, the composting process achieves a high degree of volume reduction and also generates a stable end product called compost that can be used for mulching, soil amendment, and soil enhancement. As a form of solid waste management, MSW composting reduces the amount of waste that would otherwise end up in landfills.
Although composting has been practiced by humans for centuries, the concept of composting mixed solid waste as a form of large-scale solid waste management is still in an early stage of development in the United States. MSW generally consists of a mixture of organic compostable materials such as food waste and paper and inert, nonbiodegradable materials such as plastics and glass. The introduction of non-compostable materials may pose problems in materials handling during the composting process and also hinder the formation of a uniform, homogeneous compost. Therefore, in order to practice MSW composting as a form of waste management, the composting system must be designed to remove the non-compostable materials either by presorting and screening or by sifting and removal at the end of the process.
The three most common methods of MSW composting are closed in-vessel, windrow, and static aerated pile composting. The method of choice depends on the volume of waste to be composted and the availability of space for composting. In the closed in-vessel method, the MSW is physically contained within large drums or cylinders and all necessary aeration and agitation is supplied to the vessel. In windrows, the MSW is heaped in long rows of material approximately four to seven feet high. Air and ventilation are supplied by physically turning over the piles with mechanical windrow turners. In static aerated compost piles, the MSW piles are not physically agitated, rather air is supplied and excess heat is removed by a system of sensors and pipes within the pile. In all cases, the goal is to ensure a steady, optimum rate of composting by providing adequate oxygen and ventilation to remove excess heat and water so that microbiological action is not impaired. When there is insufficient air supply, microbial action is unable to fully decompose the waste and the piles become anaerobic and unpleasant odors and putrefaction may result. Strong neighborhood complaints against odors is the single most common reason for the failure and shutdown of composting plants.
In the United States, numerous, small-scale, pilot projects have demonstrated the feasibility of MSW composting for townships and municipalities. However, there are relatively few operations that successfully carry out MSW composting on a large, commercial scale. The operational large-scale facilities are located mainly in Florida and Minnesota, two states that have traditionally shown interest in innovative waste management options.
