-3- 



The forced aeration system has several advantages over the windrow 

 method. It requires less land and less manpower because the pile is left 

 undistrubed for the first 21 days. In addition, higher and more uniform 

 temperatures are maintained throughout the pile because a layer of finished 

 compost acts as insulation. There is little odor production due to a me- 

 chanical aeration system (Hoyle, Tanner and Assoc, Inc., 1976). 



In Durham's forced aeration process, four^.nch diameter perforated 

 plastic pipe is laid out on the ground in a 10 foot by 50 foot rectangle, 

 then connected through a water trap to a 1/3 horsepower cage fan by a simi- 

 lar solid pipe (Figure 1). The perforated plastic pipe is then covered by 

 an eight -inch pad of wood chips which extends five feet beyond the pipe on 

 all sides. This pad acts as an air distribution system, prevents clogging 

 of the pipe, and absorbs some moisture. The wet sludge (about 20 percent 

 solids) is thoroughly mixed with wood chips at a ratio of three parts chips 

 to one part sludge by volume. Several sludge to chip ratios have been used, 

 but this ratio produced the best results. This mixture is then placed on 

 the chip pad and covered with about a foot of screened compost for insula- 

 tion purposes. The finished pile is about 20 feet wide by 60 feet long by 

 12 feet high and contains 60 cubic yards of wet sludge and 180 cubic yards 

 of wood chips. During the first 12 to 14 days, air is drawn through the 

 composting pile and exhausted into a small pile of finished compost for 

 odor absorption. Durham has had good success by putting the fan on a timer 

 so that the fan runs about five minutes every half hour. The optimum length 

 of time for fan operation can be determined by noting the percent oxygen 

 readings and adjusting the time accordingly. Temperatures are consistently 

 between 60 and 85 degrees Celsius during this phase, even in winter. After 



