Abstract
In Ryukyu Islands, to reduce the red soil runoff, a settling basin equipped with an underdrain and a filter layer was proposed and it is expected that finer soil particles are trapped in the filter later by filtration. However, rapid clogging in the filter layer caused by trapped soil particles which shortens filtration duration is a problem to be solved. In this paper, filtration experiment was conducted using double filter layer composed of a large artificial semi-cylindrical filter material in the upper and quartz sand in the lower, to study an extent of filtration duration compared to the single filter layer of quartz sand. Clay suspension of three levels of particle size (maximum diameter of 70, 10, 6 μm) and two levels of suspension concentration (1,500 and 4,500 mg L-1) was put into the single and the double filter layer and temporal changes of effluent flux, water pressure profile were measured. The time when effluent flux decreases to 70 % of the initial flux was regarded as filtration duration. The results showed that in cases of the clay particle diameter of 70 μm and 10 μm, effluent flux decreased rapidly in the single filter layer caused by clogging at the surface and less clay particles were trapped. In contrast, in the double filter layer, effluent flux decreased gradually, and many clay particles were trapped both in the upper filter layer and on the surface of the lower filter layer. Filtration duration of double filter layer was about 4 ∼ 26 times longer than that of the single filter layer. In case of the clay particle diameter of 6 μm, clay particle size was too small to be trapped in the upper filter layer and the particles were trapped inside the quartz sand layer. In this case, filtration duration of double filter layer was about only 1.6 times longer than that of the single filter layer. From these results, the double filter layer composed of a large filter material in the upper and the sand filter in the lower is a promising method to extent filtration duration in case that the particle size in the suspension is large compared to the pore diameter in the lower filter.
