In the windrow process, dewatered sludge mixed with a bulking agent is formed in long parallel rows or windrows. The width of a typical windrow is 2 to 4.5 m (6 to 14 ft) at the base and the height is 1 to 2 m (3 to 6.5 ft). Depending on the characteristics of the equipment used for mixing and turning of the windrows, the cross section of the pile may be triangular or trapezoidal. Windrow composting is commonly performed at open outdoor sites. However, in areas of significant precipitation, it may be desirable to provide a roofed structure to cover the windrows.
The windrows are turned periodically to expose materials to the air and loosen the materials for ease of air movement through the materials. It also helps to reduce moisture. A frond-end loader or a dedicated turning machine can be used for turning the windrows (see Figure 7.4).
Bulking agents may include the recycled composted sludge, and external agents such as wood chips, sawdust, ground bark, straw, yard waste, or rice
hulls. Enough bulking agent is added to the dewatered sludge to obtain the mixture solids content of 40 to 50%. Higher solids content may impede bacterial activity and limit the rate of decomposition. Bulking agent greatly improves the porosity of the mixture, which in turn improves the aeration characteristics. External bulking agent is a good source of carbon for the biological decomposition. The ideal carbon-to-nitrogen ratio is in the range 25 : 1 to 35 : 1. Bulking agent also increases the structural integrity of the mixture and thus its ability to maintain a properly shaped windrow.
The composting period for a windrow process is about 21 to 28 days, although this is climate and feed stock dependent. Typically, windrows are turned every 4 to 5 days. Temperature in the central portion of the windrow reaches as high as 65°C as a result of the decay process. The temperature should be maintained at or above 60°C for optimum biological activity. Temperature in the outer layer is considerably lower and may reach ambient conditions. During winter conditions, temperature in the center portion may reach only 50 to 60°C, which would prolong the composting period.
Aerobic conditions are difficult to maintain throughout the windrows. If anaerobic conditions exist, offensive odor will be released when the windrows are turned. Some facilities provide a trench with a perforated pipe under each windrow for aeration with a fan to maintain aerobic conditions within the materials.
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