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- 18 - <br /> <br />(oxygen-free) treatment produces methane gas and humus. Aerobic <br />(oxygen-rich) treatment produces a sterilized humus or compost. <br />These biological treatment methods can reduce the weight of solid <br />waste by 50 percent. Aerobic treatment, or composting, can be <br />applied to several different kinds of solid waste including leaf <br />waste, source-separated organics such as food waste, and mixed <br />waste. <br /> <br />Existing systems for biological treatment and recovery of solid <br />waste include the following technologies: <br /> <br />a. Back Yard Composting <br /> <br />This is a biological process of composting using small bin <br />containers where the leaves, grass clippings and other garden <br />and yard wastes are stored for a period of one to two years. <br />An alternative method involves frequent turning of the mate- <br />rial and adding water to accelerate the process which, can be <br />reduced to as little as six weeks. <br /> <br />b. Windrowing <br /> <br />This is a biological process of composting similar to back- <br />yard composting only on a larger scale using heavy equipment. <br />Windrowing involves aerobic decomposition through frequent <br />turning of long rows of organic material. Windrow turning can <br />be accomplished through a variety of methods utilizing special <br />machinery. <br /> <br />c. Aerobic Chambers <br /> <br />A variety of aerobic chambers 'are used throughout Europe to <br />compost solid waste. While this compost technology is more <br />capital intensive than windrowing, aerobic chambers speed up <br />material processing because moisture, oxygen and temperature <br />levels can be controlled. Two of the most notable processes <br />are the Dano drum process developed in Europe and the Metro <br />process developed in Texas. <br /> <br />d. Clay Digester <br /> <br />This is an experimental technology that requires shredding of <br />all solid waste. The shredded wastes are placed in a clay- <br />lined cell that is designed to hold one year's worth of <br />waste. The bottom of the cell is sloped to one end so that <br />leachate can be easily collected. A clay cap is placed on the <br />waste to prevent oxygen from entering the cell. Perforated <br />paper is placed in the cells and pumps are used to extract <br />methane and carbon dioxide from the waste material. Water <br />pipes are also placed in the cell to increase moisture content <br />and increase methane yield. After 10 years of methane re- <br />covery, the material in the cell could be aerated and then <br />distributed as a humus. With. such a management practice, <br />cells could be reused and would provide an alternative to <br />landfill expansions. <br /> <br /> <br />