Composting Materials Needed:

Table 1. Guidelines for composting- major factors.

Reasonable Range Preferred Range
Nutrient balance, C/N 25:1 – 40:1 30:1 – 35:1
Water Content 45-65% w.b. 50-60% w.b.
Particle Size 0.8-1.2 cm (1/8-1/2 inch) Depends on Material
Porosity 30-50% 35-45%
Bulk Density <640 kg/m3 (1100lb/yd3)
pH 5.8-9.0 6.5-8.0
Oxygen Concentration >5% > 10%
Temperature (110-155oF) (140-150oF)

Material Mix: Carbon: Nitrogen Ratio

The proper composting mixture requires both carbon and nitrogen at the proper Carbon/Nitrogen ratio. A proper C/N ratio will result in a composting process that generates little odor, yet offers an environment where microorganisms can flourish. Generally an initial C/N ratio that is 25:1 to 40:1 is satisfactory. Most “compostable” animal materials have a C/N ratio that is too low to compost properly on their own, usually below 10:1. In order to compost these materials, amendments that contain high levels of carbon must be added. Plant materials such as wood chips, peanut hulls, sawdust and shavings have a high C/N ratio and are ideal for composting.

Water Content & Porosity

Like all living things, microorganisms need water. To encourage their growth and rapid composting, water content of the mixture should be 50 to 60% (wet basis). It is critical to the process that this moisture content be maintained from the very beginning and throughout the composting process. The forced aeration floor design compensates for excess moisture by allowing it to leach out of the materials and drain to the front and back of each bin then draining to a storage tank, where it is stored and aerated and returned to newly built piles as the primary source of moisture.

Determining the level of moisture present in each pile is not as complicated as one might think. Take a small sample of material from each pile. If the mixture feels moist, and when a handful is squeezed only one or two drops of moisture is released, the mixture has adequate water content. Low moisture content significantly slows down the composting process. Since the bins are always covered, leachate in the initial stages and clean water in second stage may need to be added to avoid process inhibition.

Microorganisms that are encouraged to grow in a compost pile are aerobic, or require oxygen. Open spaces (porosity) must be maintained to allow air to penetrate and move through the pile providing oxygen. Approximately 25% of the pile volume should be small open spaces. The addition of forced aeration greatly improves the availability of fresh oxygen for microbial activity.


“The typical static pile composting process will generate and regulate its own temperature. As the pile heats up, warm air within the mixture will rise and move out of the pile, while fresh air will be drawn in to replace it. This process exhausts CO2 created in the pile, and maintains an aerobic environment for the microorganisms.”

By forcing fresh air up through the pile, exhausting the CO2, aerobic environments can be maintained even in the denser environments. Forced aeration composting allows the grower to somewhat regulate temperature by controlling the amount of aeration, generating greater microbial activity and therefore higher processing temperatures.

The following figure is a schematic showing the process followed for static pile composting. The bin compost pile is an inconsistent mixture with multiple layers of material (the waste being composted) having a low C/N ratio, a high moisture content, and nearly zero porosity surrounded by a material (the composting medium) with a high C/N ratio, low moisture levels, and good porosity. The feedstock and the composting medium are layered into the pile and no mixing is required in this process till after the high microbial decomposition stage –high temperatures – of composting has occurred. What remains is mixed and moved to second stage for additional composting.

Material flow in a typical composting system utilizing forced aeration. Mixing of the carbon and nitrogen sources into a composting mix and surrounding the feedstock with the mix is critical in industrial composting operations where large volumns of waste are being processed.

Cross-Sectional View of Composting in a forced aeration bin for waste management. The layering process is critical to getting consistent decomposition as fast as possible. Thin layering of the feedstock surrounded by a very wet layer of sawdust and litter mix allows the process of decomposition to begin immediately after the filling of the bins takes places. The pile is not turned until the material has finished the first phase of composting where temperatures reach 140°F or greater.

The above figure is a cross section of the compost bin. The decomposition process in static pile composting typically is anaerobic (lacking oxygen) in and around the feedstock, which generates odors in turning. Forced aeration minimizes this anaerobic zone by continually purging CO2 and replacing it with fresh oxygen-rich ambient air. Surrounding the layers of mortality with a high carbon source and frequently aerating the compost pile has the potential to accelerate the decomposition significantly faster than static pile composting, up to three times faster. For animal mortalities ranging from deer, poultry to swine, this period is generally 30 to 45 days after the bin is filled and capped off, for food products typically 15-30 days are required. After this time the compost is moved to a second stage bin where it is allowed to compost for an additional time period of 15 to 30 days. The secondary pile should then be turned and placed in the storage facility for storage of another 60 days. Moving and turning the pile from first stage to second stage and then to storage breaks up highly compacted areas and mixes the contents of the pile, leading to more uniformity in the finished compost.