The aerated static pile method takes the piped aeration system a step further, using a blower to supply air to the composting materials. The blower provides direct control of the process and allows larger piles. No turning or agitation of the materials occurs once the pile is formed. When the pile has been properly formed and if the air supply is sufficient and the distribution is uniform, the active composting period will be completed in approximately three to five weeks.
With the aerated static pile technique, the raw material mixture is piled over a base of wood chips, chopped straw, or other very porous material. The porous base material contains a perforated aeration pipe. The pipe is connected to a blower, which either pulls or pushes air through the pile.
The initial height of piles should be 5-8 feet high, depending on the material porosity, weather conditions, and the reach of the equipment used to build the pile. Extra height is advantageous in the wintertime to retain heat. It may be necessary to top off the pile with 6 inches of finished compost or bulking agent. The layer of finished compost protects the surface of the pile from drying, insulates it from heat loss, discourages flies, and filters ammonia and potential odours generated within the pile.
Two forms of aerated static piles are common: individual piles and extended piles.
Individual piles are long triangular piles with a width (10-16 feet, not including the cover) equal to about twice the pile height. The aeration pipe runs lengthwise beneath the ridge of the pile. Individual piles hold a single large batch of material or a few batches of roughly the same recipe and age (within three days, for example). Individual piles are practical when raw materials are available for composting at intervals rather than continuously.
Since the pile does not receive additional turnings, the selection and initial mixing of raw materials are critical. Otherwise, poor air distribution and uneven composting occur. The pile must have good structure as well as to maintain porosity through the entire composting period. This generally requires a fairly stiff bulking agent such as straw or wood chips. Wood chips are commonly used for composting sewage sludge by this method. Because of their large size, wood chips pass through the process only partially composted. They are usually screened from the finished compost and reused as bulking agents for an additional two or three cycles. Since straw decomposes over the composting period, a pile with straw as an amendment can gradually lose structure. This is partially compensated by the drying which takes place as composting proceeds. Other possible bulking agents and amendments for static pile composting include recycled compost, peat moss, corn cobs, crop residues, bark, leaves, shellfish shells, waste paper, and shredded tires. Uncomposted material like shredded tires and mollusc shells must eventually be screened from the compost and reused. To obtain good air distribution, manure or sludge must be thoroughly blended with the bulking agent before the pile is established.
The required airflow rates and the choice of blowers and aeration pipe depend on how aeration is managed - that is, how the blower is controlled. The blower can be controlled in several different modes. It can be run continuously or intermittently. In the latter case, the control mechanism can be either a programmed time clock or a temperature sensor.
The airflow rates are based on the dry weight of the primary raw material, such as sludge or manure. They should take into account the presence of typical amendments like wood chips, straw, and compost. In practice, it may be necessary to adjust the timer cycle, pile size, or blower, to suit the specific conditions and materials. For static pile composting, the air can be supplied in two ways: a suction system with
the air drawn through the pile or a pressure system with the blower pushing the air into the pile. Suction draws air into the pile from the outer surface and collects it in the aeration pipe. Since the exhaust air is contained in the discharge pipe, it can be easily filtered if odours are occurring during the composting process.
With positive pressure aeration, the exhaust air leaves the compost pile over the entire pile surface. Therefore, it is difficult to collect the air for odour treatment. If better odour control is desired, a thicker outer layer of compost can be used. Pressure aeration provides better airflow than suction aeration, largely because of the lack of an odour filter. The lower pressure loss results in greater airflow at the same blower power. Therefore, pressure systems can be more effective at cooling the pile and are preferred when temperature control is the overriding concern.