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Temperature and oxygen profiles evolution in windrows were monitored in windrows located at a commercial composting plant. Controlled turning of the windrows was executed using a turning machine. Measurements were made from the onset of turning, when temperature and oxygen were uniform throughout, onward, to evaluate the development of the relevant profiles. Temperatures rose and oxygen declined soon after turning. In fresh active windrows, a temperature maxima at a depth of 50-70 cm was detected, with temperatures as high as 65°-70°. The outward layers were cooler due to the effect of the ambient air temperature. Inward cooler core region seemed to be due to limited oxygen availability and the resulted restricted metabolic activity. No temperature maxima were observed as windrows approached stability. At this stage, the temperature of the core was as high as that of mid-layers. Oxygen, assumed to be in equilibrium with the atmosphere at the instance of turning, dropped down rapidly and reached steady levels within a period of about 4 hours. Oxygen concentration was lowered with depth, demonstrating a linear gradient down to about 70 cm depth. These data iterated the notion that oxygen is an important limiting factor in windrow composting. For any given point, high temperature may be a limiting factor. However, it seems that in properly managed windrows, high temperature is not limiting overall, due to an intrinsic correction feed back mechanism. If temperature in the hottest mid-layers gets to a level where microbial activity is hampered, less oxygen is consumed as it moves toward the core. Thus, more oxygen can penetrate into the core and the overall activity is not affected. It should be stressed that oxygen and temperature are not the only factors affecting composting rates. Other factors such as moisture, nutrients, substrate and porosity do have an important role.
Introduction
Windrow composting (i.e. piles with no active aeration) is the most common method to prepare compost from organic wastes (Stentiford 1996). With the increased environmental and health demand to properly handle organic solid waste all over the world, and the relatively low investment for windrow composting, it is expected that this technology will further expand. Raising the efficiency of this method can save costs and area requirements.
Composting is an exothermic process and thus,...