Moisture in biofilters

Abstract

This paper discusses 1) different expressions of moisture content and availability in porous media, 2 ) the range of water availability which is optimal for microbial activity in biofilters, and 3) contrasts water behavior in hydrophobic and hydrophilic biofilter media. MOISTURE AVAILABILITY AND MOISTURE CONTENTThe energy of water retention by a biofilter medium increases negatively and exponentially as the water content decreases. Organisms must overcome this energy in order to absorb the water and utilize it for their metabolism. Only part of the total moisture is available to microbes. Although some organisms can utilize water that is unavailable to others, in general the rate of metabolic activity decreases with decreasing water availability. In biofilters, the metabolic rate is the rate of pollutant removal and degradationThe unavailable fraction of water, or bound water, is roughly defined here as that held within living cells and in dead cells and molecular-size interstices of organic and inorganic media. A large fraction of water in organic media is bound water and hence unavailable. The bound water fraction of inorganic biofilter medkd is generally less. Biofilter microorganisms must obtain their moisture from the fraction of available water-water retained in larger pores (ca. >3 um diameter) and in the >3 um meniscuses between adjacent particles. The distinction between bound and available water depends on the organisms involved and the desired metabolic rate. Bacteria are more moisture-tolerant than higher plants and less moisture-tolerant than fungi [5-81. For organic biofilter media, 50-60% moisture con- amount and nature of the bulking agent. The 50-60% range is fairly general because most organic media and bulking agents have about the same bulk density and the same degree of bound and available water. Inorganic media, since they are aluminosilicates, also have the same bulk density so their range of satisfactory moisture content depends on the particle (pore) size distribution. I MATRIC POTENTIAL AND ACTIVITY OF WATERWater availability is defined by its activity or potential, which are essentially synonymous. Activity is commonly used when the water is being emphasized and the matrix retaining the water is ignored; potential is more commonly used when the matrix is being considered. The activityipotential is the sum of the water's osmotic activity and the matric potential. Osmotic activity is d u e to dissolved solutes in the water. Since biofilters usually contain low-solute water, the osmotic potential is small, ~0 . 1 bar, relative to the matric potential and will be disregarded here. Matric potential is due to the pore sizes and configurations of the medium.The water's potential/activity is the fugacity, or relative escaping tendency, of water. The activity of the water equals the relative humidity of the air and ranges from 0 (total dryness) to 1 (the activity of a free water surface of pure water, or when the biofilter pores are completely water-saturated). Activity is a convenient term fo...

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