The Composting Process

Bulletin 792

What structures and equipment are required for composting?

Investment in structures and/or equipment for the composting process can range from practically none to completely enclosed buildings with automated handling, computerized process control and monitoring systems.

The Com-Til Facility in Columbus, Ohio, composts biosolids. Wood chips, recycled compost and sawdust are added as bulking agents. The facility handles 50,000 wet tons of biosolids each year-25 dry tons per day. One eighth of the volume produced is sold as bagged compost and the remainder as bulk compost. The facility features an open concrete pad. Compost is piled 10- to 12-feet high on perforated plastic tile to achieve updraft aeration.

Static piles

• Compost ingredients are mixed and piled outdoors or under a roof in a windrow.
• The size and shape of the windrow is designed to optimize the effects of natural ventilation.
• Aeration can be enhanced with fans blowing through plastic tile or by piling compost on a wood chip base.

Windrow

• Windrows can be remixed or turned with a front-end loader or a mechanical device designed for that purpose.
• Aeration can be provided either by natural ventilation or a fan system.
• Piling on hardened surfaces facilitates handling and is essential in wet climates.

The Greater Lake County Composting Plant in Mentor, Ohio, composts anaerobically digested biosolids. Sawdust and woodchips are used as bulking agents. The plant handles 33 wet tons of biosolids per day (8-9 dry tons). The composting facility is a three-acre enclosed building with a concrete floor. A turning machine is used to stir windrows daily. The windrows are 4 to 4 1/2 feet high by 12 feet wide. Downdraft aeration is used and air exits through a biofilter consisting of compost, bark, and woodchips in which microorganisms destroy compounds responsible for odors.

Reactor vessels

• The vessels consist of concrete bins with false floors or air plenums installed in the bottom.
• Height is limited to about ten feet. Excess height leads to excessive settling. Compost that is too dense cannot be properly aerated.
• The width is determined by design requirements for turning and loading equipment. A 20-foot width has worked well in many units but some are 100 feet wide.
• The length of the vessel can be based on anticipated capacity, available space, and turning and loading requirements.
• Automated mixing and conveying equipment are normally a part of the system.
• Reactor vessels operate well in open buildings (roof only) or in enclosed buildings.
• Temperature, oxygen level and moisture are often regulated under feedback control.

Other techniques

• There are many variations of the above methods described in composting literature. The ultimate design for a system depends on needs and financial resources.