Science and Engineering

Rapidly Degrading Substrate

Tom Richard

The oxygen content at any location in a composting pile reflects a balance between oxygen supply and oxygen consumption. Rapidly degrading substrates like grass clippings or food scraps consume oxygen much more rapidly than leaves or digested sewage sludge. Oxygen consumption is a function of substrate characteristics (C/N ratio, bioavailability, moisture, particle size, and other issues -- described in the section Getting the Right Mix) and environmental conditions (temperature, moisture, oxygen concentration, and pH -- described under chemistry and physics in the background information section).

Oxygen uptake rates measured in compost vary widely, from less than 1 to over 10 g O2/kg volatile solids per hour (Haug, 1993). Rates for a variety of substrate materials under typical environmental conditions are detailed in the section on biodegradation rates and oxygen uptake (coming soon) .

To provide for the increased oxygen demand of a rapidly degrading substrate, oxygen supply must also be increased. In forced aeration systems this can often be accomplished by increasing the blower size or operating frequency. With passive systems, any restrictions on oxygen transport such as inadequate porosity or excess moisture must be reduced. A final alternative, which is commonly practiced with grass clippings and food scraps, is to reduce the pile size.


Haug, R.T. 1993. The Practical Handbook of Compost Engineering. Lewis Publishers, Boca Raton. 717 pp. Return to citation in text.


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