Commercial Composting Systems

There are four principal categories of composting systems:

• turned windrows,
• static piles,
• covered-aerated static piles,
• and in-vessel composting.

The choice of system selected depends on, the type and amount of organic material to be composted, the land area available and the sensitivity of the site to odours, the financial resources available, the desired end product and the time in which the material can remain in-process.

For turned windrow composting , the material to be composted is arranged in long rows (windrows) that are aerated by convective air movement, diffusion, and periodic mechanical turning that exposes the material to oxygen. The raw materials are mixed and aerated with front-end loaders or windrow turners. They are turned frequently during the initial period of high oxygen demand and heat generation and may be turned less frequently as the composting process proceeds. They may need to be turned as frequently as several times per week, depending on the material being composted.

Static piles are also formed in the shape of windrows but may be higher and wider since they do not need to conform to the size of a windrow turner. The term static piles refers to the fact that these piles are not turned or are turned infrequently (several times per year), generally with a front end loader. Static systems tend to be less expensive in terms of manpower and equipment than windrows, but require more land area because the material decomposes more slowly and stays on the site longer.

Covered-aerated, static-pile composting is useful for co-composting yard waste with sludge or manures. A forced aeration system is placed under the piles to maintain a minimum oxygen level throughout the composting mass. This aeration system usually consists of a series of perforated pipes or floors running underneath the pile connected to a pump that draws (negative pressure) or blows (positive pressure) air through the pile.

In-vessel , bin or closed-reactor composting takes place in partially or wholly enclosed containers in which environmental conditions are closely controlled. The principles of operation are essentially the same as for windrow and static pile systems in that the material is piled (in a container) and aerated by turning or forced air. In-vessel systems are more space efficient than the other options and have greater process controls. They are also much more expensive. Within the in-vessel category there are many proprietary systems.