Composting In Schools

The Effect of Particle Size on Bioavailability

Tom Richard

Decomposition occurs primarily on or near the surfaces of particles, where oxygen diffusion into the aqueous films covering the particle is adequate for aerobic metabolism, and the substrate itself is readily accessible to microorganisms and their extracellular enzymes. Small particles have more surface area per unit mass or volume than large particles, so if aeration is adequate small particles will degrade more quickly. Experiments have shown that the process of grinding compost materials can increase the decomposition rate by a factor of two (Gray and Sherman, 1970). Gray et al. (1971) recommend a particle size of 1.3 to 7.6 cm (0.5 to 2 inches), with the lower end of this scale suitable for forced aeration or continuously mixed systems, and the upper end for windrow and other passively aerated systems.

A theoretical calculation by Haug (1993) suggests that for particles larger than 1 mm in thickness, oxygen may not diffuse all the way into the center of the particle. Thus the interior regions of large particles are probably anaerobic, and decomposition rates in this region are correspondingly slow. However, anaerobic conditions are more of a problem with small particles, as the resulting narrow pores readily fill with water due to capillary action. These issues are addressed more fully in the section on factors leading to anaerobic conditions.


Gray, K.R., and K. Sherman, 1970. Public Cleansing 60(7):343-354.
Gray, K.R., K. Sherman, and A.J. Biddlestone. Process Biochemistry 6(10):22-28.
Haug, R.T., 1993. Practical Handbook of Compost Eng'g. Lewis Publishers, Boca Ratan, FL. p.411.


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