Four bulking agents, pine shavings, mixed (long and chopped) grass hay, chopped grass hay and long (whole) wheat straw, were each mixed with pig slurry and tap water to obtain three moisture contents (MC) of 60, 65 and 70%. Quadruplets of each treatment were placed in laboratory composting vessels with a capacity of 105 l and a composting depth of 0.95 m. Using the air plenum at the bottom of each vessel, air was forced at apparent velocities of 0-0.002 m s−1through each compost mass to measure the air static pressure drop across the compost mass as a function of apparent air velocity. Airflow resistance values were measured for compost depths ranging from 0.55 to 0.85 m. Following this test, all mixtures were aerated for 21 days of composting without overturning. The static pressure measurement procedure was then repeated on all quadruplet mixtures. The air static pressure drop was found with respect to a packed bed under laminar flow, defined using the particle size distribution, porosity, depth and airflow channel characteristics of the compost material. Although MC affected the value of the airflow channel characteristics of the compost material, both the hay and straw demonstrated similar values, while shavings demonstrated values more variable and wider values for MC between 60 and 70%. There was a significant increase in airflow resistance after 21 days of composting, which supports the need for compost overturning to reestablish the material's structure and to restore the airflow channels or pores. © 2002 Silsoe Research Institute. Published by Elsevier Science Ltd. All rights reserved.
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