Moisture in biofilters

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. Water availability is expressed by water activity or by water potential but is harder to measure than water content Water activities of 0.999 to 0.97, or water potentials of -0.2 to -3 bars (-10 to -300 kPa), support high microbial degradation rates in biofilters. Moisture availability decreases exponentially with moisture content, so keeping biofilter beds moist is important Maintaining proper moisture content is easier in hydrophilic (inorganic) media Hydrophobic (organic) media are difficult to rewet to the same condition as prior to drying.