Myths & Facts #7 What makes granular filter outperform GT with respect to fine particle removal

In general, nonwoven geotextile (GT) has very high porosity (85 to 95%) whereas woven fabrics has mostly less than 40% in POA (percentage opening area) according to Giroud (1996). Therefore, the two types of GT also differ by their specific surface of fiber per unit area (Sa) of GT. For instance a specific area (Sa) of woven GT with POA=10% may have 4.3 m2/m2 whereas the Sa of a nonwoven GT with porosity, n=0.9 and thickness of 2.8 mm would be 38 m2/m2.

Porosity is closely related to geotextile density (mass per total volume) and fiber specific length (fiber length per unit area). These parameters are altogether indicative of how tight the fabrics is interwoven in a micro-structure, which have been found to be related to the time-rate of the clogging process and its acceleration observed at high specific length GT (Faure and Kehila, 1998).

The porosity of GT seems playing an important key role in controlling the ultimate degree of filter clogging by fine particles. In the case of GT where the pore space is large while the specific area is small, the probability of fine deposition or adsorption on the fibers will be very low. Therefore, within a GT filter the flow rate will be relatively high and contact time will be very short between migrating fine particles and a fiber. Compared to granular filter which has a lower porosity and a larger specific area (e.g. n=0.3, Sa=463 m2/m2 for a 74mm thick layer), the deposition/adsorption rate of fine particles onto GT fibers can be considered relatively negligible (Reddi et al, 2000, Xiao and Reddi, 2000).