Factors Leading to Anaerobic Conditions

Tom Richard

Significant release of anaerobic odors from a composting system is usually a symptom that some important management factor has been neglected or misunderstood. This discussion is an attempt to enhance your understanding of the conditions which can lead to odor formation, in the hopes that they can be avoided or at least minimized in the future.

The most common factors which result in anaerobic odors are:

1) excess moisture,
2) inadequate porosity,
3) a rapidly degrading substrate, and
4) excessive pile size.

All of these factors make it more difficult for oxygen to penetrate throughout a pile before it is depleted, or allow airflow to short-circuit around large zones which become anaerobic. One of the mechanisms of oxygen transport is diffusion, which is function of the concentration difference between the outside air (21% oxygen) and the oxygen concentration in the interior of the pile (if anaerobic, zero). In a passively aerated or windrow system, diffusion is assisted by natural convection, but that assistance is probably limited to the upper and outer parts of the pile (Miller et al., 1989). Even with a forced aeration system, the blower's effects are mostly confined to large pores, and diffusion is needed to drive oxygen movement from the large pores into small pores and the aqueous films surrounding particles.

If we examine the equations governing oxygen diffusion in air, we see that diffusivity increases with temperature, and decreases with increasing pressure. Under conditions typical of composting systems, the gas phase oxygen diffusion coefficient can range from 0.20 cm²/sec to 0.28 cm²/sec .

While the diffusion coefficient can vary depending on the conditions in the gas phase, even more dramatic effects are seen when the oxygen has to diffuse through water rather than air. In water at 60°C, the oxygen diffusion coefficient is approximately 4.8 x 10^-5 cm²/sec, almost 4 orders of magnitude smaller than that in air. Because the impact of moisture is so dramatic, excess moisture is the most common factor leading to anaerobic conditions.

Reference

Miller, F.C., E.R. Harper, and B.J. Macauley. 1989. Field examination of temperature and oxygen relationships in mushroom composting stacks - consideration of stack oxygenation based on utilization and supply. Australian Journal of Experimental Agriculture 29(5):741-750.


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