In-Vessel Composting Technologies
Many gardeners make compost in their back yards, in relatively small quantities. When a municipality or large company wants to make compost from thousands of tons of organic materials (such as yard waste and biosolids from wastewater treatment systems), large industrial facilities are built that use what is called in-vessel composting technology.
In-vessel agitated-bed composting takes place in long concrete bays or troughs, within an enclosed custom structure, a large building similar to an airplane hangar. Compost material is moved about 12 feet per day down the bays, which are usually about 220 feet long, by large agitators (similar to rototillers) than run on rails set into the tops of the bay walls. It usually takes about three weeks for the material to be transformed from a smelly waste to odorless compost. Every garden needs lots of compost, and this is a way to make it from material that otherwise would be landfilled in a dump or burned in a waste incinerator.
Several factors are closely controlled to produce Class A compost. These include temperature, moisture, and oxygen content (aeration). Some in-vessel systems have sensors at intervals down the length of the bays to monitor these factors.
This built-in monitoring, if any, is often supplemented by periodic manual checks by a worker who walks down the bay with a device that measures temperature, moisture, and/or oxygen content. This serves as a backup to the automatic system and as a check on its accuracy.
If the compost becomes anaerobic (without oxygen) most beneficial biological organisms are killed. Ammonia is produced which creates an environment that is harmful to people who are working in the building to maintain the system, and is corrosive to equipment and the structure itself.
Need for Forced Air
When moisture or nitrogen levels climb beyond a certain point, not even the aerating effect of the agitators (which turn the compost like a rototiller does, pulling air into and through it) is sufficient to prevent an anaerobic condition from occurring. For this reason in-vessel composting uses a forced-air system of buried perforated pipes to push air through the material and ensure adequate oxygenation of the compost materials. The passage of air through the material can also be used to cool it if temperatures climb above the desired level. Forced-air systems can push air up through the compost or suck air down through it. These are known respectively as positive-flow or negative-flow systems.
Positive-Flow Forced Air
Positive-flow aeration uses huge fans to push air through the compost, like a giant version of a home hot-air heating system. The air coming out of the compost is often very humid, may smell very unpleasant, and must be exhausted from the building by a separate air extraction system. In some operations, this humid air is exhausted through a network of perforated pipes buried deep under a biofilter (a thick layer of compost, sand, and wood chips) which filters out objectionable odors in the vicinity of the building. If this is not done, neighbors downwind of the plant will soon object--a factor that has forced the closure of some facilities.
Negative-Flow Forced Air
Negative-flow aeration uses large fans to pull air down through the compost, which results in lower ammonia concentrations and diminished fog levels within the building. It has been found, in one experiment, to produce less effective cooling of the compost than a positive-flow system. Leachate (the liquid that leaks out of the compost materials) must be collected more frequently. As with positive-flow systems, the used air has to be filtered in some way to remove objectionable odors.