Moisture Effect on Compaction and Permeability in Composts

Abstract

The effects of initial moisture content and compressive loads on the compactability and air permeability of two compost mixes, biosolids (primary) + bark + sawdust + recycled compost, and cow manure are described. Biosolids compost in the moisture range of 42–57% was subjected to compressive stresses of 0–43.2 kPa and the degree of compaction and pressure drops in the flow range of 0.05–0.20 m/s were measured. The relationship between air permeability and total air-filled porosity at different moisture levels was described using the Kozeny-Carman flow model. Biosolids showed significant compressive behavior and lower permeabilities with increasing moisture content (42–57%). Cow manure had a high moisture-retaining capacity and the differences in compaction behavior and air permeabilities were small when moisture levels were varied (57–73%). The model described the relationship between air permeability and effective free air space with a high level of accuracy (R²> 0.95) at all moisture levels and compacted state. It was shown that in beds of moisture levels greater than 60% (wet basis), free air space less than 30% can be reached, and fan power requirements can increase greatly with increasing depth.