Water availability is a major determinant of pollutant degradation rates by microbes in biofilters. Water content is an indirect index of availability because much water in organic biofilter media is bound too tightly in the particles to be available for microbial metabolism.
Available water is primarily in the contacts between particles.
The relationship of water availability to moisture content depends on the size and shape of the particles and the composition of the medium. Expressing water content on a volumetric basis avoids the confusion of wet weight vs, dry weight basis and rationalizes the differences between organic and inorganic media.