Experiments using soil columns, lysimeters, and a field were conducted by permeation of outflow water from a septic tank. In experiments using soil columns, loam columns of over 25 cm in length gave removal percentages of over 9 0% for BOD, T-P, and coliforms. Lysimeters and a field registered removal percentages around 90% for BOD, T-P, and coliforms, but often below 50% for T-N. The field experiment was continued for 10 years without giving any indication of deterioration of quality of permeant water. A soil analysis in the field experiment revealed that the permeant water diffuses laterally 100 cm relative to the trench within the upper layer, with the diffusion range increasing to more than 300 cm with depths over 250 cm.

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