April 21, 2004 | General

Reader's Q&A

BioCycle April 2004, Vol. 45, No. 4, p. 14

Q: I heard about the “chimney effect” and its role in composting. Please let me know what to do to keep the “chimney” working properly.
A: Particle size is crucial to the structure of the windrow pile and is a major factor in allowing a natural air exchange to take place, veteran composter Bill Farrell of Florida explained some years ago. For proper air exchange to occur, the pile must be constructed so that a “chimney effect” takes place within the pile. This chimney effect allows air to be drawn into the bottom and then vented through the top of the pile providing the necessary oxygen for microbial activity. In a properly constructed pile, this venting is apparent when the area along the ridge is giving off visible vapors.
Larger particles need to be balanced with smaller ones. Too many fine particles will close off air channels and squeeze out oxygen. Too many large particles will bring too much air into the pile, cooling the interior. Either condition will slow down the decomposition.
As decomposition progresses, the pile will collapse on itself, reducing the windrow size and closing off air channels within the structure – reducing the chimney effect. This chokes off the venting, which you can see by the drop in the visible vapors. These indicators are a signal to the site operator that the pile needs to be aerated. The usual time for this to take place is approximately seven to ten days or as much as two weeks after windrowing, but will vary depending on the material used, Farrell points out.
To add fuel to the “biological fire,” Tim Haug – author of The Practical Handbook of Compost Engineering and a BioCycle Editorial Board member – emphasizes the importance of feedstock preparation. “Almost everything that happens once the operator mixes the substrates together, is determined by this feed conditioning process,” Haug stresses, citing these three reasons: 1) To create the necessary porosity or free air space to make sure there is proper moisture in the starting material and proper particle size. If you receive wood waste that is large, it is not going to do very much because the kinetic rates are too slow; 2) To make sure there are sufficient nutrients like nitrogen and phosphorus to support the biological growth; and 3) To make sure there is sufficient energy in the mixture to do the work that the composting must do.
Haug gives the example of conditioning an energy poor material such as wastewater biosolids that have been dewatered to 20 percent solids. “That solid material can be broken down into an ash or an inert and an organic fraction with biodegradable and nonbiodegradable components. Not all of the organics received in any substrate are going to be biodegradable in the time frame allowed for composting. To make a compost with 60 percent solids, tremendous evaporation of water must occur. One source of heat to support evaporation must come from the biodegradable volatile solids.”
Among the scenarios Haug offers is to add more fuel to support the “‘biological fire.” That fuel comes in the form of additional biodegradable volatile solids such as sawdust.
By adding the sawdust, this theoretical mixture of biosolids and amendment is only about 32 percent solids, he adds. It is energy conditioned but not friable enough to be put into a windrow. One solution is to add more amendment, but it would take significantly more to get to about 40 percent solids. Another way to go is to supply only what is needed to close the energy balance, perhaps by recycling in the finished compost to provide the additional structural conditioning needed to go from 30 to 40 percent solids. The point to remember is that composting is a heat engine and you need to make sure you have enough energy to do the job. With wet substrates, that is extremely important. Conversely, with energy rich material, the major thing to do is add water, Haug concludes.
Readers are invited to send BioCycle their experiences with improving the “chimney effect” in composting as well as their results with feedstock conditioning and maintaining the “biological fire.” E-mail: along with your questions for this department.

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