Soil filter clogging: Physical, chemical, and biological mechanisms

Soil filters are used in all major geotechnical and geoenvironmental subsurface infrastructure systems. The function of a filter is to provide proper soil retention and adequate permeability. In general, design of soil filter systems was largely based on a comparison of the particle sizes of the soil filter and the base soil. However, when the soil filters are expected to perform both filtration and drainage functions simultaneously, the permeability changes of the filters become important. Soil filters might be successful in preventing the erosion of the base soil, but they might undergo significant reductions in permeability due to the accumulation of fine particles. Based on a thorough literature survey, it is clear that the effects of physical, chemical, and/or biological factors need to be incorporated to understand the clogging mechanisms in soil filters and/or drainage layers. The broad objective of this study is to address the effect of physical, chemical, and biological factors on soil filter clogging mechanisms using results from experimental investigations. A mathematical model available to predict physical clogging of soil filters was used in this study to validate some of the experimental findings. In general, the permeability reduced by more than one order of magnitude, even when the migrating particles were smaller than the majority of the soil filter pores. The concentration of particles in the pore stream affected the rate at which the permeability reduced. A higher ionic strength of the influent suspension led to a faster reduction in permeability. The maximum specific growth rate of the microorganisms, the initial substrate concentration, the initial concentration of microorganisms in the system all affected the rate of permeability reduction. Results show that self filtration of the filter, which is prominent at higher flow rates, may itself lead to a 20–70% reduction in the permeability.