Uni and bi-directional dewatering behaviour of open geotextile containers used for filtration of waste slurries

The simultaneous analysis of filtration mechanisms under vertical, horizontal, and bi-directional flow conditions has not yet been clarified for geotextile-dewatering involving different grain-sized slurries, therefore it is still not obvious which condition is dominant as particle sizes vary. In this paper, the differences associated to horizontal versus vertical filtration are investigated based on solids retention and effluent volume through the geotextiles. Three geotextile materials (two woven and a nonwoven) are separately employed for the filtration of three slurries with d50 ranging from 55 μm to 410 μm using an open dewatering unit. To facilitate the formation of a permeable filter cake on the inside of the geotextile surface, a range of particles with grain size distribution indicating internal stability was chosen for the slurries. The geotextile/slurry combinations targeted possible unfavorable retention conditions wherein only the particles of approximately the same size as the largest opening size of the filter could be retained (O90/d85≥ 1.0). A probabilistic model based on the geotextile pore size distribution curve and number of constrictions is added to the laboratory investigation to assess the geotextile retention reliability and confirms that for most cases slurry particles of all sizes can pass through the filters. Nonetheless, filtration efficiencies greater than 90% and control of particle loss are reached relatively fast for almost all experiments. In other words, it is a range of particles retained at the beginning of filtration tests that stimulates filter cake formation and governs further retention.